Material Testing Techniques

MATERIAL

TESTING

Presented By: Muhammad Atique Atlas Honda Ltd. Muhammad Arif Alamgir Group Of Industries. Mohsin Javed Pakistan Spring Ltd.

Project Advisor: Mr. Muhammad Ali GCT. Railway Road Lhr.

PRESTON

UNIVERSITY LAHORE CAMPUS

PRESTON UNIVERSITY, LAHORE CAMPUS I

IN THE NAME OF ALLAH THE MOST

BENEFICIENT AND THE MOST

MERCIFUL…!

Special Dedications To:

The Holy Prophet Hazrat Muhammad (SAW).

Our Parents.

Our Teachers.

PRESTON UNIVERSITY, LAHORE CAMPUS II

PREFACE The objective of Material Testing continues to be the presentation of the entire scope of material testing in industries in a single comprehensive volume. Thus the book starts with a continuum description of ferrous & non-ferrous metals, their major mechanical properties and then considers the major mechanical property tests for these materials as well as the equipment used to perform these tests with their standards. Such as

• Hardness test • Fatigue test • Creep test • Tensile test • Bend test • Compression test • Ultrasonic test • Radiographic test etc.

As an aid to the students, numerous illustrative examples have been included through out the book. We can make our manufacturing more standardized and with quality by following up the different tests standards, mentioned in this book.

Muhammad Atique Muhammad Arif Muhsin Javed

PRESTON UNIVERSITY, LAHORE CAMPUS III

ACKNOWLEDGEMENT

All thanks are dully on Almighty Allah, most gracious, most merciful who enabled me to complete this project. I would like to express my thanks for the many useful comments and suggestions by Muhsin Javed and Muhammad Arif who have rendered a great help to me in completion of this project. Their help enabled me to surmount the problems faced from the inception to the completion of this project.

It was my heartiest desire to select a project, which has common and vast practical applications. I am especially indebted to our Project Advisor Muhammad Ali Sb being much kind toward me by awarding such opportunity. He is well known for assigning such projects. Which inspire the students to work upon them and create new dimensions. His inspiring guidance , constant encouragement , generous help , advices and remarkable suggestions have been immensely valuable. I have gained tremendously from the vast reservoirs of knowledge, which he possesses and I feel that these few lines of acknowledgement reflect only a fraction of the gratitude.

Muhammad Atique

PRESTON UNIVERSITY, LAHORE CAMPUS IV

Chapter 1 Ferrous Metals 1.1 Introduction 1.2 Carbon steel 1.3 Alloy steel 1.4 Stainless steel 1.5 Tool steel 1.6 ASLA steel 1.7 Steel for strength 1.8 Iron based super alloys Chapter 2 Non-Ferrous Metals

1 1 4 5 8 9 10 11

CONTENTS

2.1 Introduction 12

12 15 16 19 20 24 26 28

2.2 Aluminium 2.3 Beryllium 2.4 Copper 2.5 Magnesium 2.6 Nickel 2.7 Refractory Metals 2.8 Titanium 2.9 Zirconium

PRESTON UNIVERSITY, LAHORE CAMPUS V

Chapter 3 Mechanical Properties 3.1 Introduction 3.2 Hardness 3.3 Brittleness 3.4 Malleability 3.5 Ductility 3.6 Elasticity 3.7 Toughness 3.8 Density 3.9 Strength 3.10 Stiffness 3.11 Fatigue 3.12 Creep 3.13 Stress 3.14 Strain 3.15 Terms for behavior of materials 3.16 Stress-Strain diagram 3.17 Hooke’s law Chapter 4 Applications To Materials Testing

30 30 30 30 31 31 31 31 31 32 32 36 38 39 40 40 43

4.1 Introduction 45

46 46 49 50 52 53 55 57 58 60 61 62 63 64 66

4.2 Destructive Test 4.3 Fracture Toughness Test 4.4 Spark Test 4.5 Bending Test 4.6 Hardness Test

• Brinell Hardness Test • Knoop Test • Rockwell Test • Shore Test • Vickers Test

4.7 Compression Test 4.8 Fatigue Test 4.9 Flexure Test 4.10 Jominy End-Quench Test 4.11 Impact Test

PRESTON UNIVERSITY, LAHORE CAMPUS VI

4.12 Torsion test 67 68 73 75 76 77 78 80 82 83 84

4.13 Tensile Test 4.14 Creep Test 4.15 Charpy Test 4.16 Izod Test 4.17 Non-Destructive Test 4.18 Ultrasonic Testing 4.19 Liquid Penetrant Testing 4.20 Radiographic Testing 4.21 Magnetic Particle Testing 4.22 Magnetic Flux Leackage Test Chapter 5 Material Testing Equipment 5.1 Introduction 85

85 86 88 89 92 92 93

5.2 Brinell tester 5.3 Rockwell tester 5.4 Riehle tester 5.5 Barcol tester 5.6 Ernst tester 5.7 Universal hardness tester 5.8 Micro Vickers hardness tester

________________________________________

PRESTON UNIVERSITY, LAHORE CAMPUS VII

FERROUS METALS

CHAPTER

ONE FERROUS METALS

1.1 INTRODUCTION

As the most abundant of all commercial metals, alloys of iron and steel continue to cover a broad range of structural applications.

Iron ore constitutes about 5% of the earth's crust and is easy to convert to a useful form. Iron is obtained by fusing the ore to drive off oxygen, sulfur, and other impurities. The ore is melted in a furnace in direct contact with the fuel using limestone as a flux. The limestone combines with impurities and forms a slag, which is easily removed.

Adding carbon in small amounts reduces the melting point (2,777°F) of iron. All commercial forms of iron and steel contain carbon, which is an integral part of the metallurgy of iron and steel. Manipulation of atom-to-atom relationships between iron, carbon, and various alloying elements establishes the specific properties of ferrous metals. As atoms transform from one specific arrangement, or crystal lattice, to another, strength, toughness, impact resistance, hardness, ductility, and other properties are altered. The metallurgy of iron and steel is a study of how these atomic rearrangements take place, how they can be controlled, and which properties are affected.

Topics on ferrous Metals:

• Carbon Steel • Alloy Steel • Stainless Steel • Tool Steel • HSLA Steel • Steel for strength • Iron based super alloys

1.2 CARBON STEEL

Carbon steel, also called plain carbon steel, is a malleable, iron-based metal containing carbon, small amounts of manganese, and other elements that are inherently present. Steels can either be cast to shape or wrought into various mill forms from which finished parts are formed, machined, forged, stamped, or otherwise shaped.

Cast steels are poured to near-final shape in sand molds. The castings are then heat treated to achieve specified properties and machined to required dimensions.

Wrought steel undergoes two operations. First, it is either poured into ingots or strand cast. Then, the metal is reheated and hot rolled into the finished, wrought form. Hot-rolled steel is characterized by a scaled

PRESTON UNIVERSITY, LAHORE CAMPUS 1

FERROUS METALS

surface and a decarburized skin. Hot-rolled bars may be subsequently finished in a two-part process. First, acid pickling or shot blasting removes scale. Then, cold drawing through a die and restraightening improves surface properties and strength. Hot-rolled steel may also be cold finished by metal-removal processes such as turning or grinding. Wrought steel can be subsequently heat treated to improve machinability or to adjust mechanical properties.

Carbon steels may be specified by chemical composition, mechanical properties, method of deoxidation, or thermal treatment (and the resulting microstructure).

Composition

Wrought steels are most often specified by composition. No single element controls the characteristics of a steel; rather, the combined effects of several elements influence hardness, machinability, corrosion resistance, tensile strength, deoxidation of the solidifying metal, and microstructure of the solidified metal.

Effects of carbon, the principal hardening and strengthening element in steel, include increased hardness and strength and decreased weldability and ductility. For plain carbon steels, about 0.2 to 0.25% C provides the best machinability. Above and below this level, machinability is generally lower for hot-rolled steels.

Standard wrought-steel compositions (for both carbon and alloy steels) are designated by an AISI or SAE four-digit code, the last two digits of which indicate the nominal carbon content. The carbon-steel grades are:

• 10xx: Plain carbon • 11xx: Resulfurized • 12xz: Resulfurized and rephosphorized • 15xx: Nonresulfurized, Mn over 1.0%

The letter "L" between the second and third digits indicates a leaded steel; "B" indicates a boron steel. Cast-carbon steels are usually specified by grade, such as A, B, or C. The A grade (also LCA, WCA, AN, AQ, etc.) contains 0.25% C and 0.70% Mn maximum. B-grade steels contain 0.30% C and 1.00% Mn, and the C-grade steels contain 0.25% C and 1.20% Mn. These carbon and manganese contents are designed to provide good strength, toughness, and weldability. Cast carbon steels are specified to ASTM A27, A216, A352, or A487.

Microalloying technology has created a new category of steels, positioned both in cost and in performance between carbon steels and the alloy grades. These in-between steels consist of conventional carbon steels to which minute quantities of alloying elements -- usually less than 0.5% -- are added in the steelmaking process to improve mechanical properties. Strength and hardness are increased significantly.

Any base-grade steel can be microalloyed, but the technique was first used in sheet steel a number of years ago. More recently, microalloying has been applied to bar products to eliminate the need for heat-treating operations after parts are forged. Automotive and truck applications include connecting rods, blower shafts, stabilizer bars, U-bolts, and universal joints. Other uses are sucker rods for oil wells and anchor bolts for the construction industry.

Mechanical properties

Cast and wrought products are often specified to meet distinct mechanical requirements in structural applications where forming and machining are not extensive. When steels are specified by mechanical properties only, the producer is free to adjust the analysis of the steel (within limits) to obtain the required properties. Properties may vary with cross section and part size.


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Mechanical tests are usually specified under one of two conditions: mechanical test requirements and no chemical limits on any element, or mechanical test requirements and chemical limits on one or more elements, provided that such requirements are technologically compatible.

Method of deoxidation

Molten steel contains dissolved oxygen -- an important element in the steelmaking reaction. How this oxygen is removed or allowed to escape as the metal solidifies determines some of the properties of the steel. So in many cases, "method of deoxidation" is specified in addition to AISI and SAE chemical compositions.

For "killed" steels, elements such as aluminum and silicon may be added to combine chemically with the oxygen, removing most of it from the liquid steel. Killed steels are often specified for hot forging, carburizing, and other processes or applications where maximum uniformity is required. In sheet steel, aging is controlled by killing -- usually with aluminum. Steels intended for use in the as-cast condition are always killed. For this reason, steels for casting are always fully deoxidized.

On the other hand, for "rimmed" steels, oxygen (in the form of carbon monoxide) evolves briskly throughout the solidification process. The outer skin of rimmed steels is practically free from carbon and is very ductile. For these reasons, rimmed steels are often specified for cold-forming applications. Rimmed steels are often available in grades with less than 0.25% C and 0.60% Mn.

Segregation -- a nonuniform variation in internal characteristics and composition that results when various alloying elements redistribute themselves during solidification -- may be pronounced in rimmed steels. For this reason, they are usually not specified for hot forging or for applications requiring uniformity.

"Capped" and "semikilled" steels fall between the rimmed and killed steels in behavior, properties, and degree of oxidation and segregation. Capped steels, for example, are suited for certain cold-forming applications because they have a soft, ductile, surface skin, which is thinner than rimmed-steel skin. For other cold-forming applications, such as cold extrusion, killed steels are more suitable.

Microstructure

The microstructure of carbon and alloy steels in the as-rolled or as-cast condition generally consists of ferrite and pearlite. This basic structure can be altered significantly by various heat treatments or by rolling techniques. A spheroidized annealed structure would consist of spheroids of iron and alloy carbides dispersed in a ferrite matrix for low hardness and maximum ductility, as might be required for cold-forming operations. Quenching and tempering provide the optimum combination of mechanical properties and toughness obtainable from steel. Grain size can also be an important aspect of the microstructure. Toughness of fine-grained steels is generally greater than that of coarse-grained steels.

Free-machining steels

Several free-machining carbon steels are available as castings and as hot-rolled or cold-drawn bar stock and plate. Machinability in steels is improved in several ways, including:

• Addition of elements such as lead (the "leaded" steels such as 12L13 and 12L14), phosphorus and sulfur (the "rephosphorized, resulfurized" steels such as 1211, 1212, or 1213), sulfur (the "resulfurized only" steels such as 1117, 1118, or 1119), and tellerium, selenium, and bismuth (the "super" free-machining steels)

• Cold finishing • Reducing the level of residual stress (usually by a stress-relieving heat treatment) • Adjusting microstructure to optimize machinability


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1.3 ALLOY STEEL

Steels that contain specified amounts of alloying elements -- other than carbon and the commonly accepted amounts of manganese, copper, silicon, sulfur, and phosphorus -- are known as alloy steels. Alloying elements are added to change mechanical or physical properties. A steel is considered to be an alloy when the maximum of the range given for the content of alloying elements exceeds one or more of these limits: 1.65% Mn, 0.60% Si, or 0.60% Cu; or when a definite range or minimum amount of any of the following elements is specified or required within the limits recognized for constructional alloy steels: aluminum, chromium (to 3.99%), cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium or other element added to obtain an alloying effect.

Technically, then, tool and stainless steels are alloy steels. In this chapter, however, the term alloy steel is reserved for those steels that contain a modest amount of alloying elements and that usually depend on thermal treatment to develop specific properties. With proper heat treatment, for example, tensile strength of certain alloy steels can be raised from about 55,000 psi to nearly 300,000 psi.

Subdivisions for most steels in this family include "through-hardenable" and "carburizing" grades (plus several specialty grades such as nitriding steels). Through-hardening grades -- which are heat treated by quenching and tempering -- are used when maximum hardness and strength must extend deep within a part. Carburizing grades are used where a tough core and relatively shallow, hard surface are needed. After a surface-hardening treatment such as carburizing (or nitriding for nitriding alloys), these steels are suitable for parts that must withstand wear as well as high stresses. Cast steels are generally through hardened, not surface treated.

Carbon content and alloying elements influence the overall characteristics of both types of alloy steels. Maximum attainable surface hardness depends primarily on carbon content. Maximum hardness and strength in small sections increase as carbon content increases, up to about 0.7%. However, carbon contents greater than 0.3% can increase the possibility of cracking during quenching or welding. Alloying elements primarily influence hardenability. They also influence other mechanical and fabrication properties including toughness and machinability.

Lead additions (0.15 to 0.35%) substantially improve machinability of alloy steels by high-speed tool steels. For machining with carbide tools, calcium-treated steels are reported to double or triple tool life in addition to improving surface finish.

Few exact rules exist for selecting through-hardening or surface-hardening grades of alloy steels. In most cases, critical parts are field tested to evaluate their performance. Parts with large sections -- heavy forgings, for example -- are often made from alloy steels that have been vacuum degassed. While in a molten state, these steels are exposed to a vacuum which removes hydrogen and, to a lesser degree, oxygen and nitrogen.

Alloy steels are often specified when high strength is needed in moderate-to-large sections. Whether tensile or yield strength is the basis of design, thermally treated alloy steels generally offer high strength-to-weight ratios. For applications requiring maximum ductility, alloys with low sulfur levels (<0.01%) can be supplied by producers using ladle-refining techniques.

In general, wear resistance can be improved by increasing the hardness of an alloy, by specifying an alloy with greater carbon content (without increasing hardness), or by both. The surface of a flame-hardened, medium-carbon steel, for example, is likely to have poorer wear resistance than the carbon-rich case of a carburized steel of equal hardness. Exceptions are nitrided parts, which have better wear resistance than would be expected from the carbon content alone.

For any combination of alloy steel and heat treatment, three factors tend to decrease toughness: low service temperature, high loading rates, and stress concentrations or residual stress. The general effects of these


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three conditions are qualitatively similar, so low-temperature impact tests (to -50°F) are useful for many applications as toughness indicators under various service conditions and temperatures.

Fully hardened-and-tempered, low-carbon (0.10 to 0.30% C) alloy steels have a good combination of strength and toughness, both at room and low temperature. Care must be taken in heat treatment of certain alloy-steel grades, however, because toughness may be decreased substantially by temper brittleness -- a form of embrittlement developed by slow cooling through the range of 900 to 600°F, or by holding or tempering in this range.

When liquid quenching is impractical (because of the danger of cracking or distortion, or because of cost), various low-carbon nickel or nickel-molybdenum steels in the normalized-and-tempered condition can be used for low-temperature service.

Wrought alloy steels (and carbon steels) are classified by a series of AISI and SAE numbers that designate composition and alloy type. Letters, which are used in addition to the four-digit designations, include the suffix "H," used for steel produced to specific hardenability limits (which allows wider composition ranges for certain alloying elements), and the prefix "E," which indicates a steel made by the basic electric-furnace method. Other specifications, such as those issued by ASTM, specify minimum properties for critical structural, pressure-vessel, and nuclear applications.

ASTM specifications classify cast alloy steels by relating the steel to the mechanical properties and intended service condition. Chemical analysis is secondary. There are ASTM specifications for general use such as A27 or A148 when mechanical properties are critical. For low-temperature service, A352 or A757 is recommended when toughness is important. For weldability, A216 is specified when fabrication is critical, and for pressure service, A217 or A389 is recommended when a number of properties are important. Still other ASTM alloy steels are available for special applications. Other specifications such as SAE J435 are used for cast steels in automotive applications. A summary of steel-casting specifications is available from the Steel Founders' Society of America, Des Plaines, Ill.

1.4 STAINLESS STEEL

One of the features that characterize stainless steels is a minimum 10.5% chromium content as the principal alloying element. Four major categories of wrought stainless steel, based on metallurgical structure, are austenitic, ferritic, martensitic, and precipitation hardening. Cast stainless-steel grades are generally designated as either heat resistant or corrosion resistant.

Austenitic wrought stainless steel are classified in three groups:

• The AISI 200 series (alloys of iron-chromium-nickel-manganese) • The AISI 300 series (alloys of iron-chromium-nickel) • Nitrogen-strengthened alloys

Carbon content is usually low (0.15% or less), and the alloys contain a minimum of 16% chromium with sufficient nickel and manganese to provide an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy.

Nitrogen-strengthened austenitic stainless steels are alloys of chromium-manganese-nitrogen; some grades also contain nickel. Yield strengths of these alloys (annealed) are typically 50% higher than those of the nonnitrogen-bearing grades. They are nonmagnetic and most remain so, even after severe cold working.

Like carbon, nitrogen increases the strength of a steel. But unlike carbon, nitrogen does not combine significantly with chromium in a stainless steel. This combination, which forms chromium carbide, reduces the strength and corrosion resistance of an alloy.


FERROUS METALS

Until recently, metallurgists had difficulty adding controlled amounts of nitrogen to an alloy. The development of the argon-oxygen decarburization (AOD) method has made possible strength levels formerly unattainable in conventional annealed stainless alloys.

Austenitic stainless steels are generally used where corrosion resistance and toughness are primary requirements. Typical applications include shafts, pumps, fasteners, and piping in seawater and equipment for processing chemicals, food, and dairy products.

Ferritic wrought alloys (the AISI 400 series) contain from 10.5 to 27% chromium. In addition, the use of argon-oxygen decarburization and vacuum-induction melting has produced several new ferritic grades including 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. Low in carbon content, but generally higher in chromium than the martensitic grades, these steels cannot be hardened by heat treating and are only moderately hardened by cold working. Ferritic stainless steels are magnetic and retain their basic microstructure up to the melting point if sufficient Cr and Mo are present. In the annealed condition, strength of these grades is approximately 50% higher than that of carbon steels.

Ferritic stainless steels are typically used where moderate corrosion resistance is required and where toughness is not a major need. They are also used where chloride stress-corrosion cracking may be a problem because they have high resistance to this type of corrosion failure. In heavy sections, achieving sufficient toughness is difficult with the higher-alloyed ferritic grades. Typical applications include automotive trim and exhaust systems and heat-transfer equipment for the chemical and petrochemical industries.

Martensitic steels are also in the AISI 400 series. These wrought, higher-carbon steels contain from 11.5 to 18% chromium and may have small quantities of additional alloying elements. They are magnetic, can be hardened by heat treatment, and have high strength and moderate toughness in the hardened-and-tempered condition. Forming should be done in the annealed condition. Martensitic stainless steels are less resistant to corrosion than the austenitic or ferritic grades. Two types of martensitic steels -- 416 and 420F -- have been developed specifically for good machinability.

Martensitic stainless steels are used where strength and/or hardness are of primary concern and where the environment is relatively mild from a corrosive standpoint. These alloys are typically used for bearings, molds, cutlery, medical instruments, aircraft structural parts, and turbine components. Type 420 is used increasingly for molds for plastics and for industrial components requiring hardness and corrosion resistance.

Precipitation-hardening stainless steels develop very high strength through a low-temperature heat treatment that does not significantly distort precision parts. Compositions of most precipitation-hardening stainless steels are balanced to produce hardening by an aging treatment that precipitates hard, intermetallic compounds and simultaneously tempers the martensite. The beginning microstructure of PH alloys is austenite or martensite. The austenitic alloys must be thermally treated to transform austenite to martensite before precipitation hardening can be accomplished.

These alloys are used where high strength, moderate corrosion resistance, and good fabricability are required. Typical applications include shafting, high-pressure pumps, aircraft components, high-temper springs, and fasteners.

Cast stainless steels usually have corresponding wrought grades that have similar compositions and properties. However, there are small but important differences in composition between cast and wrought grades. Stainless-steel castings should be specified by the designations established by the ACI (Alloy Casting Institute), and not by the designation of similar wrought alloys.

Service temperature provides the basis for a distinction between heat-resistant and corrosion-resistant cast grades. The C series of ACI grades designates the corrosion-resistant steels; the H series designates the heat-resistant steels, which can be used for structural applications at service temperatures between 1,200


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and 2,200°F. Carbon and nickel contents of the H-series alloys are considerably higher than those of the C series. H-series steels are not immune to corrosion, but they corrode slowly -- even when exposed to fuel-combustion products or atmospheres prepared for carburizing and nitriding. C-series grades are used in valve, pumps, and fittings. H-series grades are used for furnace parts and turbine components.

Galling and wear are failure modes that require special attention with stainless steels because these materials serve in many harsh environments. They often operate, for example, at high temperatures, in food-contact applications, and where access is limited. Such restrictions prevent the use of lubricants, leading to metal-to-metal contact -- a condition that promotes galling and accelerated wear.

In a sliding-wear situation, a galling failure mode occurs first, followed by dimensional loss due to wear, which is, in turn, usually followed by corrosion. Galling is a severe form of adhesive wear that shows up as torn areas of the metal surface. Galling can be minimized by decreasing contact stresses or by the use of protective surface layers such as lubricants (where acceptable), weld overlays, platings, and nitrided or carburized surface treatments.

Test results from stainless-steel couples (table) indicate the relatively poor galling resistance of austenitic grades and even alloy 17-4 PH, despite its high hardness. Among the standard grades, only AISI 416 and 440C performed well. Good to excellent galling resistance was demonstrated by Armco's Nitronic 32 and 60 alloys (the latter were developed specifically for antigalling service).

Recent research findings prove that adding silicon to a high-manganese, nitrogen-strengthened austenitic stainless alloy produces a wear-resistant stainless steel. Wear and corrosion resistance are still considered unavoidable trade-offs in stainless, but the new formula promises to resist both conditions.

Beating corrosion is the number one reason for choosing stainless. But in cases where parts are difficult to lubricate, most stainless steels cannot resist wear. Under high loads and insufficient lubrication, stainless often sports a type of surface damage known as galling. In critical parts, galling can lead to seizure or freezing, which can shut down machinery.

Designers typically get around galling by using cast alloys or by applying a cobalt facing to stainless parts. Either way, the fixes can be expensive and may pose new problems that accompany the hard-facing process. These include maintaining uniform facing thickness and ensuring proper adhesion between facing and substrate. A new stainless formula aims to sidestep these difficulties by offering an alternative to expensive wear-resistant materials.

In search of a cost-effective alternative, researchers at Carpenter Technology, Reading, Pa., looked at elemental effects of silicon, manganese, and nickel on galling resistance of nitrogen-strengthened, austenitic stainless steels. Results of an initial test program determined that silicon was a catalyst for galling resistance, while nickel and manganese were not.

The silicon levels in a recently developed gall-resistant stainless alloy are between 3 and 4%. Silicon levels must remain lower than 5% to maintain the proper metallurgical structure. In addition, too much silicon decreases nitrogen solubility. To maintain strength, higher amounts of costly nickel would need to be added.

Researchers can now define optimum composition limits for a gall-resistant stainless steel. To prove the new steel's validity, properties such as galling, wear, and corrosion are evaluated and compared with commercially available stainless steels. Four alloys, a gall-resistant austenitic alloy called Gall-Tough, another austenitic alloys with higher nickel and manganese content (16Cr-8Ni-4Si-8Mn), and Types 304 and 430 stainless steels are included in the comparison.

Results show the galling threshold for gall-resistant stainless is over 15 times higher than that of conventional stainless steels. In addition, gall-resistant stainless withstands more than twice the stress


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without galling compared to the 16Cr-8Ni-4Si-8Mn alloy. Yet, the new formula sacrifices only a slight amount of corrosion resistance.

For strength and hardness, both gall-resistant stainless and the 16Cr-8Ni-4Si-8Mn alloy beat Types 304 and 430 alloys. The new alloy also shows a uniquely high ultimate tensile strength, possibly due to martensite formation during tensile testing. Ductility for all four alloys is excellent. These findings indicate that gall-resistant alloys can economically bridge the gap between corrosion, galling, and metal-to-metal wear resistance.

1.5 TOOL STEEL

The same properties that qualify tool steels for tools and dies are also used for other parts that require resistance to wear, stability during heat treatment, strength at high temperatures, or toughness. Tool steels are increasingly being used for mechanical parts to reduce size or weight, or to resist wear or high-temperature shock.

Tool steels are metallurgically "clean," high-alloy steels that are melted in relatively small heats in electric furnaces and produced with careful attention to homogeneity. They can be further refined by argon/oxygen decarburization (AOD), vacuum methods, or electroslag refining (ESR). As a result, tool steels are often specified for critical high-strength or wear-resistant applications. Because of their high alloy content, tool steels must be rolled or forged with care to produce satisfactory bar products.

To develop their best properties, tool steels are always heat treated. Because the parts may distort during heat treatment, precision parts should be semifinished, heat treated, then finished. Severe distortion is most likely to occur during liquid quenching, so an alloy should be selected that provides the needed mechanical properties with the least severe quench.

Tool steels are classified into several broad groups, some of which are further divided into subgroups according to alloy composition, hardenability, or mechanical similarities.

Water-hardening, or carbon, tool steels, designated Type W by AISI, rely solely on carbon content for their useful properties. These steels are available as shallow, medium, or deep hardening, so the specific alloy selected depends on part cross section and required surface and core hardnesses.

Shock-resisting tool steels (Type S) are strong and tough, but they are not as wear resistant as many other tool steels. These steels resist sudden and repeated loadings. Applications include pneumatic tooling parts, chisels, punches, shear blades, bolts, and springs subjected to moderate heat in service.

Cold-work tool steels, which include oil and air-hardening Types O, A, and D, are often more costly but can be quenched less drastically than water-hardening types. Type O steels are oil hardening; Type A and D steels are air hardening (the least severe quench), and are best suited for applications such as machine ways, brick mold liners, and fuel-injector nozzles. The air-hardening types are specified for thin parts or parts with severe changes in cross section -- parts that are prone to crack or distort during hardening. Hardened parts from these steels have a high surface hardness; however, these steels should not be specified for service at elevated temperatures.

Hot-work steels (Type H) serve well at elevated temperatures. The tungsten and molybdenum high-alloy hot-work steels are heat and abrasion resistant even at 600 to 1,000°F. But although these alloys do not soften at these high temperatures, they should be preheated before and cooled slowly after service to avoid cracking. The chromium grades of hot-work steels are less expensive than the tungsten and molybdenum grades. One of the chromium grades H11, is used extensively for aircraft parts such as primary airframe structures, cargo-support lugs, catapult hooks, and elevon hinges. Grade H13, which is similar to H11 is usually more readily available from suppliers.


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High-speed tool steels -- Types T (tungsten alloy) and M (molybdenum alloy) -- make good cutting tools because they resist softening and maintain a sharp cutting edge at high service temperatures. This characteristic is sometimes called "red hardness." These deep-hardening alloys are used for steady, high-load conditions rather than shock loads. Typical applications are pump vanes and parts for heavy-duty strapping machinery.

Other grades, called special-purpose tool steels, include low-cost, Type L, low-alloy steels, often specified for machine parts when wear resistance combined with toughness is important. Carbon-tungsten alloys (Type F) are shallow hardening and wear resistant, but are not suited for high temperatures or for shock service.

Type P mold steels are designed specifically for plastic-molding and zinc die-casting dies. These steels are seldom used for nontooling components.

Many steel mills have formulated their own special-purpose tool-steel alloy. Such alloys may not match a specific AISI designation and must be specified by trade name. Special-purpose tool steels may be superior to the standard grades when used as intended, but they should be specified only after careful evaluation of mechanical properties, heat-treat behavior, and availability in comparison with the standard grades.

1.6 HSLA STEEL

Those steel alloys known as high-strength low-alloy (HSLA) steels provide increased strength-to-weight ratios over conventional low-carbon steels for only a modest price premium. Because HSLA alloys are stronger, they can be used in thinner sections, making them particularly attractive for transportation-equipment components where weight reduction is important. HSLA steels are available in all standard wrought forms -- sheet, strip, plate, structural shapes, bar-size shapes, and special shapes.

Typically, HSLA steels are low-carbon steels with up to 1.5% manganese, strengthened by small additions of elements, such as columbium, copper, vanadium or titanium and sometimes by special rolling and cooling techniques. Improved-formability HSLA steels contain additions such as zirconium, calcium, or rare-earth elements for sulfide-inclusion shape control.

Since parts made from HSLA steels can have thinner cross sections than equivalent parts made from low-carbon steel, corrosion of an HSLA steel can significantly reduce strength by decreasing the load-bearing cross section. While additions of elements such as copper, silicon, nickel, chromium, and phosphorus can improve atmospheric corrosion resistance of these alloys, they also increase cost. Galvanizing, zinc-rich coatings, and other rust-preventive finishes can help protect HSLA-steel parts from corrosion.

Grades known as "improved-formability" HSLA steels (sheet-steel grades designated ASTM A715, and plates designated ASTM A656) have yield strengths up to 80,000 psi, yet cost only about 24% more than a typical 34,000-psi plain-carbon steel. Because these alloys must compete with other structural metals such as AISI 1010 steel and aluminum, they must be as inexpensive as possible. However, formulating and rolling a steel that meets this cost requirement is not easy, and the finished product presents a number of trade-offs. For example, the increase in strength from 35,000 to 80,000 psi may be accompanied by a 30 to 40% loss in ductility.

Improved-formability HSLA steels were developed primarily for the automotive industry to replace low-carbon steel parts with thinner cross-section parts for reduced weight without sacrificing strength and dent resistance. Typical passenger-car applications include door-intrusion beams, chassis members, reinforcing and mounting brackets, steering and suspension parts, bumpers, and wheels.

Trucks, construction equipment, off-highway vehicles, mining equipment, and other heavy-duty vehicles use HSLA sheets or plates for chassis components, buckets, grader blades, and structural members outside the body. For these applications, sheets or light-gage plates are specified. Structural forms (alloys from the


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family of 45,000 to 50,000-psi minimum yield strength HSLA steels) are specified in applications such as offshore oil and gas rigs, single-pole power-transmission towers, railroad cars, and ship construction.

In equipment such as power cranes, cement mixers, farm machinery, trucks, trailers, and power-transmission towers, HSLA bar, with minimum yield strengths ranging from 50,000 to 70,000 psi is used. Forming, drilling, sawing, and other machining operations on HSLA steels usually require 25 to 30% more power than do structural carbon steels.

Most HSLA alloys have directionally sensitive properties. For some grades, formability and impact strength vary significantly depending on whether the material is tested longitudinally or transversely to the rolled direction. For example, bends parallel to the longitudinal direction are more apt to cause cracking around the outside, tension-bearing surface of the bend. This effect is more pronounced in thick sheets. This directional characteristic is substantially reduced in HSLA steels that have been treated for sulfide shape control.

1.7 STEEL FOR STRENGTH

Developed primarily for high-strength applications, these steels are usually heat-treated alloys that provide strengths at least equal to those of as-rolled steel. Heat-treated constructional alloy steels and the ultrahigh-strength steels are used in applications where high strength can be converted to a weight-saving advantage over other steels.

High-yield-strength, quenched-and-tempered constructional alloy steels are usually heat treated at the mill to develop their properties so they require no further heat treatment by the fabricator. Although these heat-treated alloy steels are available in all conventional product forms, they are most common in plate products. Some grades are also available as abrasion-resistant (AR) modifications. In these conditions, high hardness is the desired property, with some toughness being sacrificed. Over 20 types of these proprietary high-strength alloy steels are produced. Some have been developed to combine improved welding characteristics along with high strength. Most have good impact properties in addition to high strength. An example of the high-yield-strength grades in this class is HY-80/100, which is used for naval vessels. This material combines high strength and toughness with weldability.

Ultrahigh-strength steels start with grade 4340 and are modifications of this alloy. When these steels are used for aerospace components, they are usually produced by the vacuum-arc-remelt (VAR) process. Steels commonly considered to be in the ultrahigh-strength category have yield strengths greater than 180,000 psi. They are classified into several broad categories based on chemical composition or metallurgical-hardening mechanisms.

Medium-carbon alloy steels are generally modifications of grade 4330 or 4340 (usually with increased molybdenum, silicon, and/or vanadium). These grades provide excellent hardenability in thick sections.

Modified tool steels of the 5% Cr, 1% Mo, 1% V hot-work die-steel variety (H11 modified, H13) provide the next step in increased hardenability and greater strength. Most steels in this group are air hardened in moderate to large sections and, therefore, are not likely to distort or quench crack. Structural uses of these steels are not as widespread as they once were, mainly because of the development of other steels costing about the same but offering greater fracture toughness.

Maraging steels contain 18% nickel, along with appreciable amounts of molybdenum, cobalt, and titanium, and almost no carbon. These alloys can be strengthened significantly by a precipitation reaction at a relatively low temperature. They can be formed and machined in the solution-annealed condition but not without difficulty. Weldability is excellent. They can be heat treated to 250 to 300-ksi yield strength with a simple 900°F aging treatment. Fracture toughness of the maraging steels is considerably higher than that of the conventional high-strength steels.


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Maraging steels are used in a variety of high-performance applications, and most extensively in aircraft and tooling components.

The 9% Ni, 4% Co alloys were designed to provide high strength and toughness at room temperature as well as at moderately elevated temperatures -- to about 800°F. Weldability and fracture toughness are good, but the alloys are susceptible to hydrogen embrittlement. These steels are used in airframes, gears, and large aircraft parts.

1.8 IRON BASED SUPER ALLOYS

Iron, nickel, and cobalt-based alloys used primarily for high-temperature applications are known as superalloys. The iron-based grades, which are less expensive than cobalt or nickel-based grades, are of three types: alloys that can be strengthened by a martensitic type of transformation, alloys that are austenitic and are strengthened by a sequence of hot and cold working (usually, forging at 2,000 to 2,100°F followed by finishing at 1,200 to 1,600°F), and austenitic alloys strengthened by precipitation hardening.

Some metallurgists consider the last group only as superalloys, the others being categorized as high-temperature, high-strength alloys. In general, the martensitic types are used at temperatures below 1,000°F; the austenitic types, above 1,000°F.

The AISI 600 series of superalloys consists of six subclasses of iron-based alloys:

• 601 through 604: Martensitic low-alloy steels. • 610 through 613: Martensitic secondary hardening steels. • 614 through 619: Martensitic chromium steels. • 630 through 635: Semiaustenitic and martensitic precipitation-hardening stainless steels. • 650 through 653: Austenitic steels strengthened by hot/cold work. • 660 through 665: Austenitic superalloys; all grades except alloy 661 are strengthened by second-

phase precipitation.

Iron-based superalloys are characterized by high temperature as well as room-temperature strength and resistance to creep, oxidation, corrosion, and wear. Wear resistance increases with carbon content. Maximum wear resistance is obtained in alloys 611, 612, and 613, which are used in high-temperature aircraft bearings and machinery parts subjected to sliding contact. Oxidation resistance increases with chromium content. The martensitic chromium steels, particularly alloy 616, are used for steam-turbine blades.

The superalloys are available in all conventional mill forms -- billet, bar, sheet, and forgings -- and special shapes are available for most alloys. In general, austenitic alloys are more difficult to machine than martensitic types, which machine best in the annealed condition. Austenitic alloys are usually "gummy" in the solution-treated condition and machine best after being partially aged or fully hardened.

Crack sensitivity makes most of the martensitic steels difficult to weld by conventional methods. These alloys should be annealed or tempered prior to welding; even then, preheating and postheating are recommended. Welding drastically lowers the mechanical properties of alloys that depend on hot/cold work for strength.

All of the martensitic low-alloy steels machine satisfactorily and are readily fabricated by hot working and cold working. The martensitic secondary-hardening and chromium alloys are all hot worked by preheating and hot forging. Austenitic alloys are more difficult to forge than the martensitic grades.

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NON-FERROUS METALS

CHAPTER

TWO NON-FERROUS

METALS 2.1 INTODUCTION Nonferrous metals offer a wide variety of mechanical properties and material characteristics.Nonferrous metals are specified for structural applications requiring reduced weight, higher strength, nonmagnetic properties, higher melting points, or resistance to chemical and atmospheric corrosion. They are also specified for electrical and electronic applications. Material selection for a mechanical or structural application requires some important considerations, including how easily the material can be shaped into a finished part and how its properties can be either intentionally or inadvertently altered in the process. Depending on the end use, metals can be simply cast into the finished part, or cast into an intermediate form, such as an ingot, then worked, or wrought, by rolling, forging, extruding, or other deformation process. Although the same operations are used with ferrous as well as nonferrous metals and alloys, the reaction of nonferrous metals to these forming processes is often more severe. Consequently, properties may differ considerably between the cast and wrought forms of the same metal or alloy.

To shape both nonferrous and ferrous metals, designers use processes that range from casting and sintered powder metallurgy (P/M) to hot and cold working. Each forming method imparts unique physical and mechanical characteristics to the final component.

Topics on Nonferrous Metals

• Aluminum • Beryllium • Copper • Magnesium • Nickel • Refractory Metals • Titanium • Zirconium

2.2 ALUMINIUM

Though light in weight, commercially pure aluminum has a tensile strength of about 13,000 psi. Cold working the metal approximately doubles its strength. In other attempts to increase strength, aluminum is alloyed with elements such as manganese, silicon, copper, magnesium, or zinc. The alloys can also be strengthened by cold working. Some alloys are further strengthened and hardened by heat treatments. At subzero temperatures, aluminum is stronger than at room temperature and is no less ductile. Most


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aluminum alloys lose strength at elevated temperatures, although some retain significant strength to 500°F. Besides a high strength-to-weight ratio and good formability, aluminum also possesses its own anticorrosion mechanism. When exposed to air, aluminum does not oxidize progressively because a hard, microscopic oxide coating forms on the surface and seals the metal from the environment. The tight chemical oxide bond is the reason that aluminum is not found in nature; it exists only as a compound.

Aluminum and its alloys, numbering in the hundreds, are available in all common commercial forms. Aluminum-alloy sheet can be formed, drawn, stamped, or spun. Many wrought or cast aluminum alloys can be welded, brazed, or soldered, and aluminum surfaces readily accept a wide variety of finishes, both mechanical and chemical. Because of their high electrical conductivity, aluminum alloys are used as electrical conductors. Aluminum reflects radiant energy throughout the entire spectrum, is nonsparking, and nonmagnetic.

• Wrought aluminum

A four-digit number that corresponds to a specific alloying element combination usually designates wrought aluminum alloys. This number is followed by a temper designation that identifies thermal and mechanical treatments.

To develop strength, heat-treatable wrought alloys are solution heat treated, then quenched and precipitation hardened. Solution heat treatment consists of heating the metal, holding at temperature to bring the hardening constituents into solution, then cooling to retain those constituents in solution. Precipitation hardening after solution heat treatment increases strength and hardness of these alloys.

While some alloys age at room temperature, others require precipitation heat treatment at an elevated temperature (artificial aging) for optimum properties. However, distortion and dimensional changes during natural or artificial aging can be significant. In addition, distortion and residual stresses can be introduced during quenching from the solution heat-treatment cycle. These induced changes can be removed by deforming the metal (for example, stretching).

Wrought aluminum alloys are also strengthened by cold working. The high-strength alloys -- either heat treatable or not -- work harden more rapidly than the lower-strength, softer alloys and so may require annealing after cold working. Because hot forming does not always work harden aluminum alloys, this method is used to avoid annealing and straightening operations; however, hot forming fully heat-treated materials is difficult. Generally, aluminum formability increases with temperature.

Recently developed aluminum alloys can provide nearly custom-engineered strength, fracture toughness, fatigue resistance, and corrosion resistance for aircraft forgings and other critical components. The rapid-solidification process is the basis for these new alloy systems, called wrought P/M alloys. The term wrought P/M is used to distinguish this technology from conventional press-and-sinter P/M technology. Grades 7090 and 7091 are the first commercially available wrought P/M aluminum alloys. These alloys can be handled like conventional aluminum alloys on existing aluminum-fabrication facilities.

Other significant new materials are the aluminum-lithium alloys. These lightweight metals are as strong as alloys now in use and can be fabricated on existing metalworking equipment. Although impressive structural weight reductions (from 7 to 10%) are possible through direct substitution, even greater reduction (up to 15%) can be realized by developing fully optimized alloys for new designs. Such alloys would be specifically tailored to provide property combinations not presently available. Producing an alloy that will provide these combinations is the object of second and third-generation low-density alloy development efforts.

Operating economy is still an important consideration in vehicle design despite fluctuating fuel prices. Downsizing to save fuel has reached its practical limits; now, reducing the weight of individual components is taking over. One significant change being implemented by designers of automobiles and


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military vehicles today is converting driveshafts, radiators, cylinder heads, suspension members, and other structural components to aluminum.

• Cast aluminum

Aluminum can be cast by all common casting processes. Aluminum casting alloys are identified with a unified, four-digit (xxx.x) system. The first digit indicates the major alloying element. For instance, 100 series is reserved for 99% pure aluminum with no major alloying element used. The second and third digits in the 100 series indicate the precise minimum aluminum content. For example, 165.0 has a 99.65% minimum aluminum content. The 200-900 series designate various aluminum alloys, with the second two digits assigned to new alloys as they are registered. The fourth digit indicates the product form. Castings are designated 0; ingots are designed 1 or 2.

Letter prefixes before the numerical designation indicate special control of one or more elements or a modification of the original alloy. Prefix X designates an experimental composition. The material may retain the experimental designation up to five years. Limits for the experimental alloy may be changed by the registrant.

Commercial casting alloys include heat-treatable and nonheat-treatable compositions. Alloys that are heat treated carry the temper designations 0, T4, T5, T6, and T7. Die castings are not usually solution heat treated because the temperature can cause blistering.

Permanent-mold casting technology involves several variations having to do with how the metal gets into the mold cavity. Initially, molds were simply gravity filled from ladles, in the same manner as sand molds. Subsequently, low pressure on the liquid-metal surface of a crucible was used to force the metal up, through a vertical tube, into the mold cavity. This refinement produces castings with higher mechanical properties and is more economical than gravity filling because extensive gates and risers are unnecessary.

More recently, the process was modified to use a low level of vacuum drawn on the mold cavity, causing atmospheric pressure to force the molten metal up into the mold. This process variation, together with controlled and rapid solidification, increases properties further because it produces castings that are almost entirely free of porosity.

Although both variations improve properties and speed casting cycles, the added equipment complexities limit the casting size that can be handled. Consequently, all three permanent-mold processes are in use today, turning out aluminum castings weighing from less than one pound to several hundred pounds.

Aluminum matrix composites: Metal matrix composites (MMCs) consist of metal alloys reinforced with fibers, whiskers, particulates, or wires. Alloys of numerous metals (aluminum, titanium, magnesium and copper) have been used as matrices to date.

Recent MMC developments, however, seem to thrust aluminum into the spotlight. In the NASA space shuttle, for example, 240 struts are made from aluminum reinforced with boron fibers. Also, aluminum diesel-engine pistons that have been locally reinforced with ceramic fibers are eliminating the need for wear-resistant nickel-cast iron inserts in the automotive environment.

Fabrication methods differ for both products. Monolayer tapes in the space shuttle struts are wrapped around a mandrel and hot isostatically pressed to diffusion bond the layers. For the pistons, a squeeze-casting process infiltrates liquid metal into a fiber preform under pressure. Other fabrication methods for MMCs include: hot pressing a layer of parallel fibers between foils to create a monolayer tape; creep and superplastic forming in a die; and spraying metal plasmas on collimated fibers followed by hot pressing.


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• Superplastic aluminum

Superplastic forming of metal, a process similar to vacuum forming of plastic sheet, has been used to form low-strength aluminum into nonstructural parts such as cash-register housings, luggage compartments for passenger trains, and nonload-bearing aircraft components. New in this area of technology is a superplastic-formable high-strength aluminum alloy, now available for structural applications and designated 7475-02. Strength of alloy 7475 is in the range of aerospace alloy 7075, which requires conventional forming operations. Although initial cost of 7475 is higher, finished part cost is usually lower than that of 7075 because of the savings involved in the simplified design/assembly.

2.3 BERYLLIUM

Among structural metals, beryllium has a unique combination of properties. It has low density (two-thirds that of aluminum), high modulus per weight (five times that of ultrahigh-strength steels), high specific heat, high strength per density, excellent dimensional stability, and transparency to X-rays. Beryllium is expensive, however, and its impact strength is low compared to values for most other metals. Available forms include block, rod, sheet, plate, foil, extrusions, and wire. Machining blanks, which are machined from large vacuum hot pressings, make up the majority of beryllium purchases. However, shapes can also be produced directly from powder by processes such as cold-press/sinter/coin, CIP/HIP, CIP/sinter, CIP/hot-press and plasma spray/sinter. (CIP is cold-isostatic press, and HIP is hot-isostatic press.) Mechanical properties depend on powder characteristics, chemistry, consolidation process, and thermal treatment. Wrought forms, produced by hot working, have high strength in the working direction, but properties are usually anisotropic.

Beryllium parts can be hot formed from cross-rolled sheet and plate as well as plate machined from hot-pressed block. Forming rates are slower than for titanium, for example, but tooling and forming costs for production items are comparable.

Structural assemblies of beryllium components can be joined by most techniques such as mechanical fasteners, rivets, adhesive bonding, brazing, and diffusion bonding. Fusion-welding processes are generally avoided because they cause excessive grain growth and reduced mechanical properties.

Beryllium behaves like other light metals when exposed to air by forming a tenacious protective oxide film that provides corrosion protection. However, the bare metal corrodes readily when exposed for prolonged periods to tap or seawater or to a corrosive environment that includes high humidity. The corrosion resistance of beryllium in both aqueous and gaseous environments can be improved by applying chemical conversion, metallic, or nonmetallic coatings. Beryllium can be electroless nickel plated, and flame or plasma sprayed.

All conventional machining operations are possible with beryllium, including EDM and ECM. However, beryllium powder is toxic if inhaled. Since airborne beryllium particles and beryllium salts present a health hazard, the metal must be machined in specially equipped facilities for safety. Machining damages the surface of beryllium parts. Strength is reduced by the formation of microcracks and "twinning." The depth of the damage can be limited during finish machining by taking several light machining cuts and sharpening cutting tools frequently or by using nonconventional metal-removal processes. For highly stressed structural parts, 0.002 to 0.004 in. should be removed from each surface by chemical etching or milling after machining. This process removes cracks and other surface damage caused by machining, thereby preventing premature failure. Precision parts should be machined with a sequence of light cuts and intermediate thermal stress reliefs to provide the greatest dimensional stability.

Beryllium typically appears in military-aircraft and space-shuttle brake systems, in missile reentry body structures, missile guidance systems, mirrors and optical systems, satellite structures, and X-ray windows. The modulus-to-density ratio is higher than that of unidirectionally reinforced, "high-modulus" boron,


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carbon, and graphite-fiber composites. Beryllium has an additional advantage because its modulus of elasticity is isotropic.

2.4 COPPER

Copper conducts electricity at a rate 97% that of silver, and is the standard for electrical conductivity. Copper provides a diverse range of properties: good thermal and electrical conductivity, corrosion resistance, ease of forming, ease of joining, and color. In addition, however, copper and its alloys have relatively low strength-to-weight ratios and low strengths at elevated temperatures. Some copper alloys are also susceptible to stress-corrosion cracking unless they are stress relieved.

Production of CopperCopper ores often contain very low concentrations of the metal. Because of this, many stages of the production process focus on eliminating impurities. The ore is crushed and milled before entering a flotation chamber, in which copper will concentrate at the top while unwanted fragments sink. Next, the concentrate, now called charge, will enter a reverberatory furnace, where more impurities are removed. During smelting, waste gases are removed, and the material forms a molten pool of copper and iron, called the matte, at the bottom of the furnace. The orange layer of impure metal on top of the matte is slag, which is drained off while the copper matte continues on to a converter. Molten copper from the converter is cast and must be refined once more by electrolysis before it is ready for use in the manufacture of products such as electrical wire and utensils.

Copper and its alloys -- the brasses and bronzes -- are available in rod, plate, strip, sheet, tube shapes, forgings, wire, and castings. These metals are grouped according to composition into several general categories: coppers, high-copper alloys, brasses, leaded brasses, bronzes, aluminum bronzes, silicon bronzes, copper nickels, and nickel silvers.

Copper-based alloys form adherent films that are relatively impervious to corrosion and that protect the base metal from further attack. Certain alloy systems darken rapidly from brown to black in air. Under most outdoor conditions, however, copper surfaces develop a blue-green patina. Lacquer coatings can be applied to retain the original alloy color. An acrylic coating with benzotriazole as an additive lasts several years under most outdoor, abrasion-free conditions.

Although they work harden, copper and its alloys can be hot or cold worked. Ductility can be restored by annealing or heating incident to welding or brazing operations. For applications requiring maximum electrical conductivity, the most widely used copper is C11000, "tough pitch," which contains approximately 0.03% oxygen and a minimum of 99.0% copper. In addition to high electrical conductivity,


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oxygen-free grades C10100 and C10200 provide immunity to embrittlement at high temperature. The addition of phosphorus produces grade C12200 -- the standard water-tube copper.

High-copper alloys contain small amounts of alloying elements that improve strength with some loss in electrical conductivity. In amounts of 1%, for example, cadmium increases strength by 50%, with a loss in conductivity to 85%. Small amounts of cadmium raise the softening temperature in alloy C11600, which is used widely for printed circuits. Tellurium or sulfur, present in small amounts in Grades C14500 and C14700, has been shown to increase machinability.

Copper alloys do not have a sharply defined yield point, so yield strength is reported either as 0.5% extension under load, or as 0.2% offset. On the most common basis (0.5% extension), yield strength of annealed material is approximately one-third the tensile strength. As the material is cold worked or hardened, it becomes less ductile, and yield strength approaches tensile strength.

Copper is specified according to temper, which is established by cold working or annealing. Typical levels are: soft, half-hard, hard, spring, and extra-spring. Yield strength of a hard-temper copper is approximately two-thirds of tensile strength.

For brasses, phosphor bronzes, or other commonly cold-worked grades, the hardest available tempers are also the strongest and represent approximately 70% reduction in area. Ductility is sacrificed, of course, to gain strength. Copper-beryllium alloys can be precipitation hardened to the highest strength levels attainable in copper-base alloys.

The ASME Boiler and Pressure Vessel Code should be used for designing critical copper-alloy parts for service at elevated temperatures. The code recommends that, for a specific service temperature, the maximum allowable design stress should be the lowest of these values as tabulated by the code: one-fourth of the ultimate tensile strength, two-thirds of the yield strength, and two-thirds of the average creep strength or stress-rupture strength under specified conditions. Silicon bronzes, aluminum brasses, and copper nickels are widely used for elevated-temperature applications.

All copper alloys resist corrosion by fresh water and steam. Copper nickels, aluminum brass, and aluminum bronzes provide superior resistance to saltwater corrosion. Copper alloys have high resistance to alkalies and organic acids, but have poor resistance to inorganic acids. One corrosive situation encountered, particularly in the high-zinc alloy, is dezincification. The brass dissolves as an alloy, but the copper constituent redeposits as a porous, spongy metal. Meanwhile, the zinc component is carried away by the atmosphere or deposited on the surface as an insoluble compound.

Copper melts at about 1083° C (about 1981° F), boils at about 2567° C (about 4753° F), and has a specific gravity of 8.9. The atomic weight of copper is 63.546.


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• Designating alloys

Originally developed as a three-digit system by the U.S. copper and brass industry, the designation system for copper-based alloys has been expanded to five digits preceded by the letter C as part of the Unified Numbering System for Metals and Alloys (UNS). The UNS designations are simply an expansion of the former designation numbers. For example, Copper Alloy No. 377 (forging brass) becomes C37700. Numbers C10000 through C79900 are assigned to wrought compositions, and numbers C80000 through C99900 to casting alloys.

The designation system is not a specification; rather, it is a method for identifying and defining the chemical composition of mill and foundry products. The precise requirements to be satisfied by a material and the temper nomenclature that applies are defined by the relevant standard specifications (ASTM, Federal, and Military) for each composition.

There are approximately 370 commercial copper and copper-alloy compositions. Brass mills make wrought compositions in the form of rod, plate, sheet, strip, tube, pipe, extrusions, foil, forgings, and wire. Foundries supply castings. The following general categories apply to both wrought and cast compositions.

• Coppers, high-copper alloys

Both wrought and cast compositions have a designated minimum copper content and may include other elements or additions for special properties.

• Brasses

These alloys contain zinc as the principal alloying element and may have other designated elements. The wrought alloys are comprised of copper-zinc alloys, copper-zinc-lead alloys (leaded brasses), and copper-zinc-tin alloys (tin brasses). The cast alloys are comprised of copper-zinc-tin alloys (red, semired and yellow brasses), manganese bronze alloys (high-strength yellow brasses), leaded manganese bronze alloys (leaded high-strength yellow brasses), and copper-zinc-silicon alloys (silicon brasses and bronzes).

• Bronzes

Wrought bronze alloys comprise four main groups: copper-tin-phosphorus alloys (phosphor bronzes), copper-tin-lead-phosphorus alloys (leaded phosphor bronzes), and copper-silicon alloys (silicon bronzes). Cast alloys also have four main families: copper-tin alloys (tin bronzes), copper-tin-lead alloys (leaded and high-leaded tin bronzes), copper-tin-nickel alloys (nickel-tin bronzes), and copper-aluminum alloys (aluminum bronzes).

• Copper-nickels

These are either wrought or cast alloys containing nickel as the principal alloying element.

• Copper-nickel-zinc alloys

These are known as nickel silvers, from their color.

• Leaded coppers

These are cast alloys containing 20% lead or more.


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2.5 MAGNESIUM

As the lightest structural metal available, magnesium's combination of low density and good mechanical strength results in a high strength-to-weight ratio.

Because of their low modulus of elasticity, magnesium alloys can absorb energy elastically. Combined with moderate strength, this provides excellent dent resistance and high damping capacity. Magnesium has good fatigue resistance and performs particularly well in applications involving a large number of cycles at relatively low stress. The metal is sensitive to stress concentration, however, so notches, sharp corners, and abrupt section changes should be avoided.

Magnesium parts are generally used from room temperature to about 200°F or, in some cases, to 350°F. Some alloys can be used in service environments to 700°F for brief exposures.

Magnesium is widely recognized for its favorable strength-to-weight ratio and excellent castability, but deeply ingrained misconceptions often prevent designers from specifying it as a die-cast material. However, what is true of magnesium as a generic material is not true of today's die-casting alloy. The new high-purity alloy, combined with advances in fluxless, hot-chamber die-casting processing, has altered the traditional guidelines for evaluating the cost and performance of magnesium die castings.

Fabrication: Magnesium alloys are the easiest of the structural metals to machine. They can be shaped and fabricated by most metalworking processes, and they are easily welded. At room temperature, magnesium work hardens rapidly, reducing cold formability; thus, cold forming is limited to mild deformation or roll bending around large radii. Pure magnesium is usually alloyed with other elements to develop sufficient strength for structural purposes. Some alloys are heat treated to further improve properties.

Cast magnesium alloys are dimensionally stable to about 200°F. Some cast magnesium-aluminum-zinc alloys may undergo permanent growth if used above that temperature for long periods. Permanent-mold castings are as strong as sand castings, and they generally provide closer dimensional tolerances and better surface finish. Typical applications of magnesium gravity castings are aircraft engine components and wheels for race and sports cars.

Design of die-cast magnesium parts follows the same principles established for other die-casting metals. Maximum mechanical properties in a typical alloy are developed in wall thicknesses ranging from 0.078 to 0.150 in. Chain-saw and power-tool housings, archery-bow handles, and attache-case frames are typical die-cast applications.

Magnesium is easy to hot work, so fewer forging steps are usually required than for other metals. Bending, blocking, and finishing are usually the only operations needed. Typical magnesium forgings are missile fuselage connector rings.

Standard extruded shapes include round, square, rectangular, and hexagonal bars; angles, beams, and channels; and a variety of tubes. Luggage frames and support frames for military shelters are examples of magnesium extrusions.

Methods used for joining magnesium are gas tungsten-arc (TIG) and gas metal-arc (MIG) welding, spotwelding, riveting, and adhesive bonding. Mechanical fasteners can be used on magnesium, provided that stress concentrations are held to a safe minimum. Only ductile aluminum rivets should be used, preferably alloy 5056-H32, to minimize galvanic-corrosion failure at riveted joints.

Specification: Magnesium alloys are designated by a system established by the ASTM that covers both chemical compositions and tempers.


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The first two letters of the designation identifies the two alloying elements specified in the greatest amount. The letters are arranged in order of decreasing percentages or alphabetically if the elements are present in equal amounts. The letters are followed by respective percentages rounded off to whole numbers, followed by a final serial letter. The serial letter indicates some variation in composition of minor alloying constituents or impurities.

The letters that designate the more common magnesium alloying elements are

• A -- Aluminum • E -- Rare earths • H – Thorium • K – Zirconium • L – Lithium • M – Manganese • Q – Silver • S – Silicon • Z -- Zinc

For example, magnesium alloy AZ31B contains 3% aluminum (code letter A) and 1% zinc (code letter Z).

Resisting corrosion

A problem with magnesium has been its lack of sufficient corrosion resistance for many applications, particularly in the alloys used for die and sand casting. The problem has been solved by the two major supplies, Dow and AMAX; both have developed corrosion-resistant, high-purity AZ91 alloys for die casting, and both also offer a sand-casting grade.

The die-casting grade is now designated by ASTM as AZ91D and will, for all practical purposes, replace AZ91B. The sand-casting grade received the designation AZ91E from ASTM. The high-purity alloys are said to be as much as 100 times more corrosion resistant than standard magnesium alloys, and more resistant to saltwater than die-cast 380 aluminum alloy or cold-rolled steel, tested according to ASTM B117. Research in magnesium metallurgy has shown that the ability of magnesium to resist corrosion in a service environment of salt-laden air or spray depends heavily on keeping contaminants (iron, nickel, copper) below their maximum tolerance limits during all production operations.

The high-purity magnesium die-casting alloy has already replaced other metals as well as a number of plastics in a variety of U.S. passenger-car and lightweight-truck components. Examples include valve and timing-gear covers, brackets, clutch and transfer-case housings, grille panels, headlamp doors, windshield-wiper motor housings, and various interior trim parts.

2.6 NICKEL

Structural applications that require specific corrosion resistance or elevated temperature strength receive the necessary properties from nickel and its alloys. Some nickel alloys are among the toughest structural materials known. When compared to steel, other nickel alloys have ultrahigh strength, high proportional limits, and high moduli of elasticity. Commercially pure nickel has good electrical, magnetic, and magnetostrictive properties.

• Common nickel alloy families include

commercially pure nickel; binary systems, such as Ni-Cu, Ni-Si, and Ni-Mo; ternary systems, such as Ni-Cr-Fe and Ni-Cr-Mo; more complex systems, such as Ni-Cr-Fe-Mo-Cu (with other possible additions); and superalloys. Nickel content throughout the alloy families ranges from 32.5 to 99.5%.


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At cryogenic temperatures, nickel alloys are strong and ductile. Several nickel-base superalloys are specified for high-strength applications at temperatures to 2,000°F. High-carbon nickel-base casting alloys are commonly used at moderate stresses above 2,200°F.

• Alloy characteristics

Commercial nickel and nickel alloys are available in a wide range of wrought and cast grades; however, considerably fewer casting grades are available. Wrought alloys tend to be better known by tradenames such as Monel, Hastelloy, Inconel, Incoloy, etc. Casting alloys are identified by Alloy Casting Institute and ASTM designations. Wrought and cast nickel alloys are often used together in systems built up from wrought and cast components. The casting alloys contain additional elements, such as silicon and manganese, to improve castability and pressure tightness.

Commercially pure nickels and extrahigh nickel alloys: Primary wrought materials in this group are Nickel 200 and 201, both of which contain 99.5% Ni. The cast grade, designated CZ-100, is recommended for use at temperatures above 600°F because its lower carbon content prevents graphitization and attendant ductility loss. Both wrought grades are particularly resistant to caustics, high-temperature halogens and hydrogen halides, and salts other than oxidizing halides. These alloys are particularly well suited for food-contact applications.

Duranickel 301, a precipitation-hardened, 94% nickel alloy, has excellent spring properties to 600°F. During thermal treatment, Ni3AlTi particles precipitate throughout the matrix. This action enhances alloy strength. Corrosion resistance is similar to that of commercially pure wrought nickel.

• Binary nickel alloys

The primary wrought alloys in this category are the Ni-Cu grades known as Monel alloy 400 (Ni-31.SCu) and K-500 (Ni-29.SCu), which also contain small amounts of Al, Fe, and Ti. The Ni-Cu alloys differ from Nickel 200 and 201 because their strength and hardness can be increased by age hardening. Although the Ni-Cu alloys share many of the corrosion characteristics of commercially pure nickel, their resistance to sulfuric and hydrofluoric acids and brine is better. Handling of waters, including seawater and brackish water, is a major application. Monel alloys 400 and K-500 are immune to chloride-ion stress-corrosion cracking, which is often considered in their selection.

Other commercially important binary nickel compositions are Ni-Mo and Ni-Si. One binary type, Hastelloy alloy B-2 (Ni-28Mo), offers superior resistance to hydrochloric acid, aluminum-chloride catalysts, and other strongly reducing chemicals. It also has excellent high-temperature strength in inert atmospheres and vacuum.

Cast nickel-copper alloys comprise a low and high silicon grade. M-35-1 and QQ-N-288, Grades A and E (1.5% Si), are commonly used in conjunction with wrought nickel-copper in pumps, valves, and fittings. A higher silicon grade, QQ-N-288, Grade B (3.5% Si), is used for rotating parts and wear rings because it combines corrosion resistance with high strength and wear resistance. Grade D (4.0% Si) offers exceptional galling resistance.

Two other binary cast alloys are ACI N-12M-1 and N-12M-2. These Ni-Mo alloys are commonly used for handling hydrochloric acid in all concentrations at temperatures up to the boiling point. These alloys are produced commercially under the tradenames Hastelloy alloy B and Chlorimet 2.

• Ternary nickel alloys

Two primary wrought and cast compositions are Ni-Cr-Fe and Ni-Cr-Mo. Ni-Cr-Fe is known commercially as Haynes alloys 214 and 556, Inconel alloy 600, and Incoloy alloy 800. Haynes new alloy No. 214 (Ni-16Cr-2.5Fe-4.5Al-Y) has excellent resistance to oxidation to 2,200°F, and resists carburizing


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and chlorine-contaminated atmospheres. Haynes patented alloy No. 556 (Fe-20Ni-22Cr-18Co) combines effective resistance to sulfidizing, carburizing, and chlorine-bearing environments with good oxidation resistance, fabricability and high-temperature strength. Inconel alloy 600 (Ni-15.5Cr-8Fe) has good resistance to oxidizing and reducing environments. Intended for severely corrosive conditions at elevated temperatures, Incoloy 800 (Ni-46Fe-21Cr) has good resistance to oxidation and carburization at elevated temperatures, and it resists sulfur attack, internal oxidation, scaling, and corrosion in many atmospheres.

A cast Ni-Cr-Fe alloy CY-40, known as Inconel, has higher carbon, Mn, and Si contents than the corresponding wrought grade. In the as-cast condition, the alloy is insensitive to the type of intergranular attack encountered in as-cast or sensitized stainless steels.

Significant additions of molybdenum make Ni-Cr-Mo alloys highly resistant to pitting. They retain high strength and oxidation resistance at elevated temperatures, but they are used in the chemical industry primarily for their resistance to a wide variety of aqueous corrosives. In many applications, these alloys are considered the only materials capable of withstanding the severe corrosion conditions encountered.

In this group, the primary commercial materials are C-276, Hastelloy alloy C-22, and Inconel alloy 625. Hastelloy alloy C-22 (Ni-22Cr-13Mo-3W-3Fe) has better overall corrosion resistance and versatility than any other Ni-Cr-Mo alloy. Alloy C-276 (57Ni-15.5Cr-16Mo) has excellent resistance to strong oxidizing and reducing corrosives, acids, and chlorine-contaminated hydrocarbons. Alloy C-276 is also one of the few materials that withstands the corrosive effects of wet chlorine gas, hypochlorite, and chlorine dioxide. Hastelloy alloy C-22, the newest alloy in this group, has outstanding resistance to pitting, crevice corrosion, and stress-corrosion cracking. Present applications include the pulp and paper industry, various pickling acid processes, and production of pesticides and various agrichemicals.

Two grades of cast Ni-Cr-Mo alloys, ACI CW-12M-1 and CW-12M-2, are used in severe corrosion service, often involving combinations of acids at elevated temperatures. The two versions of CW-12M are also produced as Hastelloy C and Chlorimet.

• Complex alloys

Ni-Cr-Fe-Mo-Cu is the basic composition in this category of nickel alloys. They offer good resistance to pitting, intergranular corrosion, chloride-ion stress-corrosion cracking, and general corrosion in a wide range of oxidizing and reducing environments. These alloys are frequently used in applications involving sulfuric and phosphoric acids.

Important commercial grades include Hastelloy alloys G-30 and H, Haynes alloy No. 230, Inconel alloys 617, 625, and 718, and Incoloy alloy 825.

Haynes alloy No. 230 (Ni-22Cr-14W-2Mo) has excellent high-temperature strength, oxidation resistance, and thermal stability, making it suitable for various applications in the aerospace, airframe, nuclear, and chemical-process industries.

Hastelloy alloy G-30 (Ni-30Cr-6Mo-2.5W-15Fe) has many advantages over other metallic and nonmetallic materials in handling phosphoric acid, sulfuric acid, and oxidizing acid mixtures. Hastelloy alloy H (Ni-22Cr-9Mo-2W-18Fe) is a patented alloy with localized corrosion resistance equivalent or better to alloy 625. Alloy H also has good resistance to hot acids and excellent resistance to stress-corrosion cracking. It is often used in flue gas desulfurization equipment.

Inconel alloy 617 (Ni-22Cr-12.5Co-9Mo-1.5Fe-1.2Al) resists cyclic oxidation at 2,000°F, and has good stress-rupture properties above 1,800°F.

Inconel alloy 625 (Ni-21.5Cr-2.5Fe-9Mo-3.6Nb+Ta) has high strength and toughness from cryogenic temperatures to 1,800°F, good oxidation resistance, exceptional fatigue strength, and good resistance to


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many corrosives. Furnace mufflers, electronic parts, chemical and food-processing equipment, and heat-treating equipment are among a few of the many applications for alloy 615.

Inconel alloy 718 (Ni-18.5Fe-19Cr-3Mo-5Nb+Ta) has excellent strength from -423 to 1,300°F. The alloy is age hardenable, can be welded in the fully aged condition, and has excellent oxidation resistance up to 1,800°F.

Incoloy 825 (42Ni-30Fe-21.5Cr-3Mo-2.25Cu) offers excellent resistance to a wide variety of corrosives. It resists pitting and intergranular corrosion, reducing acids, and oxidizing chemicals. Applications include pickling-tank heaters and hooks, spent nuclear-fuel-element recovery, chemical-tank trailers, evaporators, food-processing equipment, sour-well tubing, hydrofluoric-acid production, pollution-control equipment, and radioactive-waste systems.

• Super alloys

One class of Ni-based superalloys is strengthened by intermetallic compound precipitation in a face-centered cubic matrix. The strengthening precipitate is gamma prime, typified by Waspaloy (Ni-19.5Cr-13.5Co-4.3Mo-3.0Ti-1.4Al-2.0Fe), Udimet 700 (Ni-15Cr-18.5Co-5Mo-3.4Ti-4.3Al-<1fe), and the modern but complex Rene 95 (Ni-14Cr-8Co-3.5Mo-3.5W-3.5Nb-2.5Ti-3.5Al).

Another type of Ni-based superalloy is represented by Hastelloy alloy X (Ni-22Fe-9Mo-22Cr-1.5Co). This alloy is essentially solid-solution strengthened, but probably also derives some strengthening from carbide precipitation through a working-plus-aging schedule.

A third class includes oxide-dispersion-strengthened (ODS) alloys such as IN MA-754 (Ni-20Cr-0.6yttria) and IN MA-6000 (Ni-15Cr-2Mo-4W-2.5Ti-4.5Al), which are strengthened by dispersions such as yttria coupled (in some cases) with gamma prime precipitation (MA-6000).

Nickel-based superalloys are used in cast and wrought forms, although special processing (powder metallurgy/isothermal forging) often is used to produce wrought versions of the more highly alloyed compositions (U-700, Astroloy, IN-100).

An additional dimension of Ni-based superalloys has been the introduction of grain-aspect ratio and orientation as a means of controlling properties. In some instances, grain boundaries have been removed. Wrought powder-metallurgy alloys of the ODS class and cast alloys such as MAR M-247 have demonstrated property improvements due to grain morphology control by directional crystallization or solidification. Virtually all uses of the cast and wrought nickel-base superalloys are for gas-turbine components.

• Fabrication

Most wrought-nickel alloys can be hot and cold worked, machined, and welded successfully. The casting alloys can be machined or ground, and many can be welded and brazed.

Nearly any shape that can be forged in steel can also be forged in nickel and nickel alloys. However, because nickel work hardens easily, severe cold-forming operations require frequent intermediate annealing to restore soft temper. Annealed cold-rolled sheet, not stretcher leveled, is best for spinning and other manual work. In general, cold-drawn rods machine much more cleanly and readily than hot-rolled or annealed material.

Nickel alloys can be joined by shielded metal-arc, gas tungsten-arc, gas metal-arc, plasma-arc, electron-beam, oxyacetylene, and resistance welding; silver and bronze brazing; and soft soldering. Resistance welding methods include spot, seam, projection, and flash welding.


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Special nickel alloys, including superalloys, are best worked at about 1,800 to 2,200°F. In the annealed condition, these alloys can be cold worked by all standard methods. Required forces and rate of work hardening are intermediate between those of mild steel and Type 304 stainless steel. These alloys work harden to a greater extent than the austenitic stainless steels, so they require more intermediate annealing steps.

Both cold-worked and hot-worked Ni-Cu require thermal treatment to develop optimum ductility and to minimize distortion during subsequent machining. Stress relieving before machining is recommended to minimize distortion after metal removal. Stress equalizing of cold-worked Cu-Ni increases yield strength without marked effects on other properties.

Many Hastelloy alloys can be upset forged if the length of the piece is no greater than twice its diameter. However, upsetting should never be attempted on a cast ingot. Cast ingots must be reduced at least 75% before hot upsetting.

Most wrought nickel-based alloys can be formed from sheet into complex shapes involving considerable plastic flow. These alloys are processed in the annealed condition.

2.7 REFRACTORY METALS

Refractory metals are characterized by their extremely high melting points, which range well above those of iron, cobalt, and nickel. They are used in demanding applications requiring high-temperature strength and corrosion resistance. The most extensively used of these metals are tungsten, tantalum, molybdenum, and columbium (niobium). They are mutually soluble and form solid-solution alloys with each other in any proportion. These four refractory metals and their alloys are available in mill forms as well as products such as screws, bolts, studs, and tubing.

Although the melting points of these metals are all well above 4,000°F, they oxidize at much lower temperatures. Accelerated oxidation in air occurs at 190°C for tungsten, 395°C for molybdenum, and 425°C for tantalum and columbium. Therefore, protective coatings must be applied to these metals if they are to be used at higher temperatures. Tensile and yield strengths of the refractory metals are substantially retained at high temperature.

• Columbium and tantalum

These metals are usually considered together because most of their working characteristics are similar. They can be fabricated by most conventional methods at room temperature. Heavy sections for forging can be heated, without protection, to approximately 425°C.

Out of several commercial-grade tantalum alloys, those containing tungsten, columbium, and molybdenum generally retain the corrosion resistance of tantalum and provide higher mechanical properties. Columbium is also available in alloys containing tantalum, tungsten, molybdenum, vanadium, hafnium, zirconium, or carbon. Alloys provide improved tensile, yield, and creep properties, particularly in the 1,100 to 1,650°C range.

Most sheet-metal fabrication of columbium and tantalum is done in the thickness range of 0.004 to 0.060 in. Columbium, like tantalum, can be welded to itself and to certain other metals by resistance welding, tungsten inert-gas (TIG) welding, and to itself by inert-gas arc welding. Electron-beam welding can also be used, particularly for joining to other metals. However, surfaces that are heated above 315°C during welding must be protected with an inert gas to prevent embrittlement.

Principal applications for tantalum are in capacitor anodes, filaments, gettering devices, chemical-process equipment, and high-temperature aerospace engine components. Columbium is used in superconducting


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materials, thin-film substrates, electrical contacts, heat sinks, and as an alloying addition in steels and superalloys.

• Molybdenum

Probably the most versatile of the refractory metals, molybdenum is also a natural resource of the United States. It is an excellent structural material for applications requiring high strength and rigidity at temperatures to 3,000°F where it can operate in vacuum or under inert or reducing atmospheres.

Unalloyed molybdenum and its principal alloy, TZM, are produced by powder-metallurgy methods and by vacuum-arc melting. Both are commercially available in ordinary mill product forms: forging billets, bars, rods, wire, seamless tubing, plate, strip, and thin foil. Compared to unalloyed molybdenum, the TZM alloy (Mo-0.5%Ti-0.1%Zr) develops higher strength at room temperature and much higher stress-rupture and creep properties at all elevated temperatures. At 1,800 to 2,000°F, TZM can sustain a 30,000-psi stress for over 100 hr, three times that for unalloyed molybdenum.

Molybdenum and TZM are readily machined with conventional tools. Sheet can be processed by punching, stamping, spinning, and deep drawing. Some parts can be forged to shape. Molybdenum wire and powder can be flame sprayed onto steel substrates to salvage worn parts or to produce long-wearing, low-friction surfaces for tools.

In nonoxidizing environments, the metal resists attack by hydrochloric, hydrofluoric, sulfuric, and phosphoric acids. Molybdenum oxidizes at high temperatures to produce volatile, nontoxic, molybdenum trioxide; however, parts such as gimbled nozzles have been used successfully in rocket and missile-guidance systems when exposure time to the very-high temperatures of ballistic gases was brief.

Molybdenum parts can be welded by inertia, resistance, and spot methods in air; by TIG and MIG welding under inert atmospheres; and by electron-beam welding in vacuum. The best welds are produced by inertia (friction) welding and electron-beam welding; welds produced by the other techniques are less ductile. Generally, arc-cast metal develops better welds than do powder-metallurgy products. Heavy sections of molybdenum should be preheated and postheated when they are welded to reduce thermal stresses.

Because molybdenum has a modulus of elasticity of 47 × 106 psi at room temperature, it is used for boring bars and the quills for high-speed internal grinders to avoid vibration and chatter. Its relatively high electrical conductivity makes unalloyed molybdenum useful for electrical and electronic applications. It is used in the manufacture of incandescent lamps, as substrates in solid-state electronic devices, as electrodes for EDM equipment and for melting glass, and as heating elements and reflectors or radiation shields for high-temperature vacuum furnaces.

Because it retains usable strength at elevated temperatures, has a low coefficient of thermal expansion, and resists erosion by molten metals, the TZM alloy is used for cores in die casting of aluminum, and for die cavities in casting of brass, bronze, and even stainless steel. Dies of the TZM alloy weighing several thousand pounds are used for isothermal forging of superalloy components for aircraft gas turbines, and die inserts made of TZM have been used for extruding steel shapes. Piercer points of TZM are used to produce stainless-steel seamless tubing.

• Tungsten

In many respects, tungsten is similar to molybdenum. The two metals have about the same electrical conductivity and resistivity, coefficient of thermal expansion, and about the same resistance to corrosion by mineral acids. Both have high strength at temperatures above 2,000°F, but because the melting point of tungsten is higher, it retains significant strength at higher temperatures than molybdenum does. The elastic


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modulus for tungsten is about 25% higher than that of molybdenum, and its density is almost twice that of molybdenum. All commercial unalloyed tungsten is produced by powder-metallurgy methods; it is available as rod, wire, plate, sheet, and some forged shapes. For some special applications, vacuum-arc-melted tungsten can be produced, but it is expensive and limited to relatively small sections.

Several tungsten alloys are produced by liquid-phase sintering of compacts of tungsten powder with binders of nickel-copper, iron-nickel, iron-copper, or nickel-cobalt-molybdenum combinations; tungsten usually comprises 85 to 95% of the alloy by weight. These alloys are often identified as heavy metals or machinable tungsten alloys. In compact forms, the alloys can be machined by turning, drilling, boring, milling, and shaping; they are not available in mill product forms because they are unable to be wrought at any temperature.

The heavy-metal alloys are especially useful for aircraft counterbalances and as weights in gyratory compasses. Heavy-metal inserts are used as the cores of high-mass military projectiles. Tungsten alloys are widely used for counterbalances in sports equipment such as golf clubs and tennis racquets. X-ray shielding is another important application of the tungsten alloys.

Filaments for incandescent lamps are usually coils of very fine unalloyed tungsten wire. Electronic tubes are often constructed with tungsten as the heaters; some advanced tubes use heaters made from a tungsten alloy containing 3% rhenium. A thermocouple rated to 4,350°F consists of one tungsten wire alloyed with 25% rhenium and another wire alloyed with 5% rhenium.

Nozzle throats of forged and machined unalloyed tungsten have been used in solid-fuel rocket engines; at one time, throats were machined from porous consolidations of tungsten powder that were infiltrated with silver for exposure to gases at temperatures near 3,500°C. Unalloyed tungsten is used for X-ray targets, for filaments in vacuum-metallizing furnaces, and for electrical contacts such as the distributor points in automotive ignition systems. Tungsten electrodes form the basis for TIG welding. Water-cooled tungsten tips are used for nonconsumable electrode vacuum-arc melting of alloys.

Cutting tools and parts that must resist severe abrasion are often made of tungsten carbide. Tungsten-carbide chips or inserts, with the cutting edges ground, are attached to the bodies of steel tools by brazing or by screws. The higher cutting speeds and longer tool life made feasible by the use of tungsten-carbide tools are such that the inserts are discarded after one use. Tungsten-carbide dies have been used for many years for drawing wire. Inserts of tungsten carbide are used in rotary bits for drilling oil and gas wells and in mining operations. Fused tungsten carbide is applied to the surfaces of mining machinery that is subjected to severe wear.

2.8 TITANIUM

Depending on the predominant phase or phases in their microstructure, titanium alloys are categorized as alpha, alpha-beta, and beta. This natural grouping not only reflects basic titanium production metallurgy, but it also indicates general properties peculiar to each type. The alpha phase in pure titanium is characterized by a hexagonal close-packed crystalline structure that remains stable from room temperature to approximately 1,620°F. The beta phase in pure titanium has a body-centered cubic structure, and is stable from approximately 1,620°F to the melting point of about 3,040°F.

Adding alloying elements to titanium provides a wide range of physical and mechanical properties. Certain alloying additions, notably aluminum, tend to stabilize the alpha phase; that is, they raise the temperature at which the alloy will be transformed completely to the beta phase. This temperature is known as the beta-transus temperature.

Alloying additions such as chromium, columbium, copper, iron, manganese, molybdenum, tantalum, and vanadium stabilize the beta phase by lowering the temperature of transformation from alpha to beta. Some


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elements, notably tin and zirconium, behave as neutral solutes in titanium and have little effect on the transformation temperature, acting instead as strengtheners of the alpha phase.

• Alpha alloys

The single-phase and near-single-phase alpha alloys of titanium have good weldability. The generally high aluminum content of this group of alloys ensures good strength characteristics and oxidation resistance at elevated temperatures (in the range of 600 to 1,100°F). Alpha alloys cannot be heat treated to develop higher mechanical properties because they are single-phase alloys.

• Alpha-beta alloys

The addition of controlled amounts of beta-stabilizing alloying elements causes the beta phase to persist below the beta transus temperature, down to room temperature, resulting in a two-phase system. These two-phase titanium alloys can be strengthened significantly by heat treatment consisting of a quench from some temperature in the alpha-beta range, followed by an aging cycle at a somewhat lower temperature. Beta-phase transformation, which would normally occur on slow cooling, is suppressed by the quenching. The aging cycle causes the precipitation of some fine alpha particles from the metastable beta, imparting a structure that is stronger than the annealed alpha-beta structure. Although heat-treated alpha-beta alloys are stronger than the alpha alloys, their ductility is proportionally lower.

• Beta alloys

The high percentage of beta-stabilizing elements in these alloys results in a microstructure that is substantially beta. The metastable beta can be strengthened considerably by heat treatment.

Titanium is used in corrosive environments or in applications that require light weight, high strength-to-weight ratio, and nonmagnetic properties. While commercially available in many alloys, most requirements can be met by a grade of commercially pure titanium, titanium-0.2% palladium alloy, or by the high-strength Ti-Al-V-Cr (beta type) alloys. These grades, which are available in most common wrought mill forms, are covered by ASTM-AMS specifications and, in most cases, by a similar ASME specification.

Beta-21S is a new beta alloy developed as an oxidation-resistant aerospace material and as a matrix for metal-matrix composites. Composition is Ti-15Mo-2.7Nb-3Al-0.2Si, with molybdenum and niobium working synergistically to raise corrosion resistance to very high levels. It also offers one of the lowest hydrogen uptake efficiency levels of any titanium alloy. The combination of high strength and high corrosion resistance make it an ideal candidate for orthopedic implants, deep sour oil wells, and geothermal brine wells.

Like stainless steel, titanium sheet and plate work harden significantly during forming. Minimum bend-radius rules are nearly the same for both, although springback is greater for titanium. Commercially pure grades of heavy plate are cold formed or, for more severe shapes, warm formed at temperatures to about 800°F. Alloy grades can be formed at temperatures as high as 1,400°F in inert-gas atmospheres. Tube can be cold bent to radii three times the tube OD, provided that both inside and outside surfaces of the bend are in tension at the point of bending. In some cases, tighter bends can be made.

Despite their high strength, some alloys of titanium have superplastic characteristics in the range of 1,500 to 1,700°F. The alloy used for most superplastically formed parts is the standard Ti-6Al-4V alloy. Several aircraft manufacturers are producing components formed by this method. Some applications involve assembly by diffusion bonding.

Titanium plates or sheets can be sheared, punched, or perforated on standard equipment. Titanium and Ti-Pd alloy plates can be sheared subject to equipment limitations similar to those for stainless steel. The


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harder alloys are more difficult to shear, so thickness limitations are generally about two-third those for stainless steel.

Titanium and its alloys can be machined and abrasive ground; however, sharp tools and continuous feed are required to prevent work hardening. Tapping is difficult because the metal galls. Coarse threads should be used where possible.

Titanium castings can be produced by investment or graphite-mold methods. Casting must be done in a vacuum furnace, however, because of the highly reactive nature of titanium in the presence of oxygen. Typical applications for titanium castings are surgical implants and hardware for marine and chemical equipment such as compressors and valve bodies.

Generally, titanium is welded by gas-tungsten arc (GTA) or plasma-arc techniques. Metal inert-gas processes can be used under special conditions. Thorough cleaning and shielding are essential because molten titanium reacts with nitrogen, oxygen, and hydrogen, and will dissolve large quantities of these gases, which embrittles the metal. In all other respects, GTA welding of titanium is similar to that of stainless steel. Normally, a sound weld appears bright silver with no discoloration on the surface or along the heat-affected zone.

2.9 ZIRCONIUM

In addition to resisting HCl at all concentrations and at temperatures above the boiling temperature, zirconium and its alloys also have excellent resistance in sulfuric acid at temperatures above boiling and concentrations to 70%. Corrosion rate in nitric acid is less than 1 mil/year at temperatures above boiling and concentrations to 90%. The metals also resist most organics such as acetic acid and acetic anhydride as well as citric, lactic, tartaric, oxalic, tannic, and chlorinated organic acids.

Relatively few metals besides zirconium can be used in chemical processes requiring alternate contact with strong acids and alkalis. However, zirconium has no resistance to hydrofluoric acid and is rapidly attacked, even at very low concentrations.

Zirconium alloys can be machined by conventional methods, but they have a tendency to gall and work harden during machining. Consequently, tools with higher than normal clearance angles are needed to penetrate previously work-hardened surfaces. Results can be satisfactory, however, with cemented carbide or high-speed steel tools. Carbide tools usually provide better finishes and higher productivity.

Mill products are available in four principal grades: 702, 704, 705, and 706. These metals can be formed, bent, and punched on standard shop equipment with a few modifications and special techniques. Grades 702 (unalloyed) and 704 (Zr-Sn-Cr-Fe alloy) sheet and strip can be bent on conventional press-brake or roll-forming equipment to a 5t bend radius at room temperature and to 3t at 200°C. Grades 705 and 706 (Zr-Cb alloys) can be bent to a 3t and 2.5t radius at room temperature and to about 1.5t at 200°C.

Zirconium has better weldability than some of the more common construction metals including some alloy steels and aluminum alloys. Low distortion during welding stems from a low coefficient of thermal expansion. Zirconium is most commonly welded by the gas-tungsten arc (GTAW) method, but other methods can also be used, including gas metal-arc (GMAW), plasma-arc, electron-beam, and resistance welding.

Welding zirconium requires proper shielding because of the metal's reactivity to gases at welding temperatures. Welding without proper shielding (argon or helium) causes absorption of oxygen, hydrogen, and nitrogen from the atmosphere, resulting in brittle welds. Although a clean, bright weld results from the use of a proper shielding system, discoloration of a weld is not necessarily an indication of its unacceptability. However, white deposits or a black color in the weld area are not acceptable. A bend test is usually the best way to determine acceptability of a zirconium weld.


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Major uses for zirconium and its alloys are as a construction material in the chemical-processing industry. Applications include heat exchangers (for producing hydrogen peroxide, rayon, etc.), drying columns, pipe and fittings, pump and valve housings, and reactor vessels.

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MECHANICAL PROPERTIES

CHAPTER

THREE MECHANICAL

PROPERTIES 3.1 INTODUCTION This section is devoted primarily to the terms used in describing various properties and characteristics of metals in general. Of primary concern in aircraft maintenance are such general properties of metals and their alloys as hardness, brittleness, malleability, ductility, elasticity, toughness, density, fusibility, conductivity, and contraction and expansion. You must know the definition of the terms included here because they form the basis for further discussion of aircraft metals. The mechanical properties of metals determine the range of usefulness of the metal and establish the service that can be expected. Mechanical properties are also used to help specify and identify metals. The most common properties considered are strength, hardness, ductility, and impact resistance. 3.2 HARDNESS Hardness refers to the ability of a metal to resist abrasion, penetration, cutting action, or permanent distortion. Hardness may be increased by working the metal and, in the case of steel and certain titanium and aluminum alloys, by heat treatment and cold-working (discussed later). Structural parts are often formed from metals in their soft state and then heat treated to harden them so that the finished shape will be retained. Hardness and strength are closely associated properties of all metals.

3.3 BRITTLENESS

Brittleness is the property of a metal that allows little bending or deformation without shattering. In other words, a brittle metal is apt to break or crack without change of shape. Because structural metals are often subjected to shock loads, brittleness is not a very desirable property. Cast iron, cast aluminum, and very hard steel are brittle metals.

3.4 MALLEABILITY

A metal that can be hammered, rolled, or pressed into various shapes without cracking or breaking or other detrimental effects is said to be malleable. This property is necessary in sheet metal that is to be worked into curved shapes such as cowlings, fairings, and wing tips. Copper is one example of a malleable metal.



3.5 DUCTILITY

Ductility is the property of a metal that permits it to be permanently drawn, bent, or twisted into various shapes without breaking. This property is essential for metals used in making wire and tubing. Ductile metals are greatly preferred for aircraft use because of their ease of forming and resistance to failure under shock loads. For this reason, aluminum alloys are used for cowl rings, fuselage and wing skin, and formed or extruded parts, such as ribs, spars, and bulkheads. Chrome-molybdenum steel is also easily formed into desired shapes. Ductility is similar to malleability.

3.6 ELASTICITY

Elasticity is that property that enables a metal to return to its original shape when the force that causes the change of shape is removed. This property is extremely valuable, because it would be highly undesirable to have a part permanently distorted after an applied load was removed. Each metal has a point known as the elastic limit, beyond which it cannot be loaded without causing permanent distortion. When metal is loaded beyond its elastic limit and permanent distortion does result, it is referred to as strained. In aircraft construction, members and parts are so designed that the maximum loads to which they are subjected will never stress them beyond their elastic limit.

NOTE: Stress is the internal resistance of any metal to distortion.

3.7 TOUGHNESS

A material that possesses toughness will withstand tearing or shearing and may be stretched or otherwise deformed without breaking. Toughness is a desirable property in aircraft metals.

3.8 DENSITY

Density is the weight of a unit volume of a material. In aircraft work, the actual weight of a material per cubic inch is preferred, since this figure can be used in determining the weight of a part before actual manufacture. Density is an important consideration when choosing a material to be used in the design of a part and still maintain the proper weight and balance of the aircraft.

3.9 STRENGTH

The strength of a metal is its ability to withstand the action of external forces without breaking. Tensile strength, also called ultimate strength, is the maximum strength developed in a metal in a tension test. The tension test is a method for determining the behavior of a metal under an actual stretch loading. This test provides the elastic limit, elongation, yield point, yield strength, tensile strength, and the reduction in area. Tensile tests are normally taken at standardized room temperatures but may also be made at elevated temperatures. Strength



Many tensile testing machines are equipped to plot a curve which shows the load or stress and the strain or movement that occurs during the test operation. In the testing operation the load is increased gradually and the specimen will stretch or elongate in proportion to the tensile load. 3.10 STIFFNESS This is a general term which may be applied to materials or structures. When a force is applied to a structure, there is a displacement in the direction of the force; stiffness is the ratio of the force divided by the displacement. High stiffness means that a large force produces a small displacement. When discussing the stiffness of a material, the concept is the same, except that stress substitutes for force, and strain substitutes for displacement; see modulus of elasticity. Measure of resistance of plastics to bending. It includes both plastic and elastic behavior, so it is an apparent value of elastic modulus rather than a true value.

3.11 FATIGUE

It has been recognized since 1830 that a metal subjected to a repetitive or fluctuating stress will fail at a stress much lower than that required to cause fracture on a single application of load. Failures occurring under conditions of dynamic loading are called fatigue failures, presumably because it is generally observed that these failures occur only after a considerable period of service. Fatigue has become progressively more prevalent as technology has developed a greater amount of equipment, such as automobiles, aircraft, compressors, pumps, turbines, etc., subject to repeated loading and vibration. Today it is often stated that fatigue accounts for al least 90 percent of all service failures due to mechanical causes.

A fatigue failure is particularly insidious because it occurs without any obvious warning. Fatigue results in a brittle-appearing fracture, with no gross deformation at the fracture. On a macroscopic scale the fracture surface is usually normal to the direction of the principal tensile stress. A fatigue failure can usually be recognized from the appearance of the fracture surface, which shows a smooth region, due to the rubbing action as the crack propagated through the section, and a rough region, where the member has failed in a ductile manner when the cross section was no longer able to carry the load. Frequently the progress of the fracture is indicated by a series of rings, or "beach marks", progressing inward from the point of initiation of the failure.

Three basic factors are necessary to cause fatigue failure. These are:

1. maximum tensile stress of sufficiently high value, 2. large enough variation or fluctuation in the applied stress, and 3. sufficiently large number of cycles of the applied stress.

In addition, there are a host of other variables, such as stress concentration, corrosion, temperature, overload, metallurgical structure, residual stresses, and combined stresses, which tend to alter the conditions for fatigue. Since we have not yet gained a complete understanding of what causes fatigue in metals, it will be necessary to discuss each of these factors from an essentially empirical standpoint. Because of the mass of data of this type, it will be possible to describe only the highlights of the relationship between these factors and fatigue.



Stress Cycles

At the outset it will be advantageous to define briefly the general types of fluctuating stresses which can cause fatigue. Figure 1 serves to illustrate typical fatigue stress cycles.

Figure 1a illustrates a completely reversed cycle of stress of sinusoidal form. For this type of stress cycle the maximum and minimum stresses are equal. Tensile stress is considered positive, and compressive stress is negative.

Figure 1b illustrates a repeated stress cycle in which the maximum stress σmax (Rmax) and minimum stress σmin (Rmin) are not equal. In this illustration they are both tension, but a repeated stress cycle could just as well contain maximum and minimum stresses of opposite signs or both in compression.

Figure 1c illustrates a complicated stress cycle which might be encountered in a part such as an aircraft wing which is subjected to periodic unpredictable overloads due to gusts.

Fig. 1 Typical fatigue stress cycles. (a) Reversed stress; (b) repeated stress; (c) irregular or random stress cycle.

A fluctuating stress cycle can be considered to be made up of two components, a mean, or steady, stress σm (Rm), and an alternating, or variable, stress σa. We must also consider the range of stress σr. As can be seen from Fig. 1b, the range of stress is the algebratic difference between the maximum and minimum stress in a cycle.



The S-N Curve

The basic method of presenting engineering fatigue data is by means of the S-N curve, a plot of stress S against the number of cycles to failure N. A log scale is almost always used for N. The value of stress that is plotted can be σa, σmax, or σmin. The stress values are usually nominal stresses, i.e., there is no adjustment for stress concentration. The S-N relationship is determined for a specified value of σm, R (R=σmin/σmax), or A (A=σa/σm). Most determinations of the fatigue properties of materials have been made in completed reversed bending, where the mean stress is zero.

It will be noted that this S-N curve is concerned chiefly with fatigue failure at high numbers of cycles (N > 105 cycles). Under these conditions the stress, on a gross scale, is elastic, but as we shall see shortly the metal deforms plastically in a highly localized way. At higher stresses the fatigue life is progressively decreased, but the gross plastic deformation makes interpretation difficult in terms of stress. For the low-cycle fatigue region (N < 104 or 105 cycles) tests are conducted with controlled cycles of elastic plus plastic strain instead of controlled load or stress cycles.

The usual procedure for determining an S-N curve is to test the first specimen at a high stress where failure is expected in a fairly short number of cycles, e.g., at about two-thirds the static tensile strength of the material. The test stress is decreased for each succeeding specimen until one or two specimens do not fail in the specified numbers of cycles, which is usually at least 107 cycles.

The highest stress at which a runout (non-failure) is obtained is taken as the fatigue limit. For materials without a fatigue limit the test is usually terminated for practical considerations at a low stress where the life is about 108 or 5x108 cycles. The S-N curve is usually determined with about 8 to 12 specimens.

Statistical Nature of Fatigue

A considerable amount of interest has been shown in the statistical analysis of fatigue data and in reasons for the variability in fatigue-test results. Since fatigue life and fatigue limit are statistical quantities, it must be realized that considerable deviation from an average curve determined with only a few specimens is to be expected.

It is necessary to think in terms of the probability of a specimen attaining a certain life at a given stress or the probability of failure at a given stress in the vicinity of the fatigue limit. To do this requires the testing of considerably more specimens than in the past so that the statistical parameters for estimating these probabilities can be determined.

The basic method for expressing fatigue data should then be a three-dimensional surface representing the relationship between stress, number of cycles to failure, and probability of failure.

In determining the fatigue limit of a material, it should be recognized that each specimen has its own fatigue limit, a stress above which it will fail but below which it will not fail, and that this critical stress varies from specimen to specimen for very obscure reasons. It is known that inclusions in steel have an important effect on the fatigue limit and its variability, but even vacuum-melted steel shows appreciable scatter in fatigue limit.

The statistical problem of accurately determining the fatigue limit is complicated by the fact that we cannot measure the individual value of the fatigue limit for any given specimen. We can only test a



specimen at a particular stress, and if the specimen fails, then the stress was somewhere above the fatigue limit of the specimen. The two statistical methods which are used for making a statistical estimate of the fatigue limit are called probit analysis and the staircase method. The procedures for applying these methods of analysis to the determination of the fatigue limit have been well established.

Effect of Mean Stress on Fatigue

Much of the fatigue data in the literature have been determined for conditions of completely reversed cycles of stress, σm = 0. However, conditions are frequently met in engineering practice where the stress situation consists of an alternating stress and a superimposed mean, or steady, stress. There are several possible methods of determining an S-N diagram for a situation where the mean stress is not equal to zero.

Cyclic Stress-Strain Curve

Cyclic strain controlled fatigue, as opposed to our previous discussion of cyclic stress controlled fatigue, occurs when the strain amplitude is held constant during cycling. Strain controlled cyclic loading is found in thermal cycling, where a component expands and contracts in response to fluctuations in the operating temperature. In a more general view, the localized plastic strains at a notch subjected to either cyclic stress or strain conditions result in strain controlled conditions near the root of the notch due to the constraint effect of the larger surrounding mass of essentially elastically deformed material.

Since plastic deformation is not completely reversible, modifications to the structure occur during cyclic straining and these can result in changes in the stress-strain response. Depending on the initial state a metal may undergo cyclic hardening, cyclic softening, or remain cyclically stable. It is not uncommon for all three behaviors to occur in a given material depending on the initial state of the material and the test conditions.

Generally the hysteresis loop stabilizes after about 100 cycles and the material arrives at an equilibrium condition for the imposed strain amplitude. The cyclically stabilized stress-strain curve may be quite different from the stress-strain curve obtained on monotonic static loading. The cyclic stress-strain curve is usually determined by connecting the tips of stable hysteresis loops from constant-strain-amplitude fatigue tests of specimens cycled at different strain amplitudes. Under conditions where saturation of the hysteresis loop is not obtained, the maximum stress amplitude for hardening or the minimum stress amplitude for softening is used. Sometimes the stress is taken at 50 percent of the life to failure. Several shortcut procedures have been developed.

Low-Cycle Fatigue

Although historically fatigue studies have been concerned with conditions of service in which failure occurred at more than 104 cycles of stress, there is growing recognition of engineering failures which occur at relatively high stress and low numbers of cycles to failure. This type of fatigue failure must be considered in the design of nuclear pressure vessels, steam turbines, and most other types of power machinery. Low-cycle fatigue conditions frequently are created where the repeated stresses are of thermal origin. Since thermal stresses arise from the thermal expansion of the material, it is easy to see that in this case fatigue results from cyclic strain rather than from cyclic stress.



3.12 CREEP

High temperature progressive deformation of a material at constant stress is called creep. High temperature is a relative term that is dependent on the materials being evaluated. A typical creep curve is shown below:

In a creep test a constant load is applied to a tensile specimen maintained at a constant temperature. Strain is then measured over a period of time. The slope of the curve, identified in the above figure, is the strain rate of the test during stage II or the creep rate of the material.

Primary creep, Stage I, is a period of decreasing creep rate. Primary creep is a period of primarily transient creep. During this period deformation takes place and the resistance to creep increases until stage II. Secondary creep, Stage II, is a period of roughly constant creep rate. Stage II is referred to as steady state creep. Tertiary creep, Stage III, occurs when there is a reduction in cross sectional area due to necking or effective reduction in area due to internal void formation.

Stress Rupture

Stress rupture testing is similar to creep testing except that the stresses used are higher than in a creep test. Stress rupture testing is always done until failure of the material. In creep testing the main goal is to determine the minimum creep rate in stage II. Once a designer knows the materials will creep and has accounted for this deformation a primary goal is to avoid failure of the component.



Stress rupture tests are used to determine the time to cause failure. Data is plotted log-log as in the chart above. A straight line is usually obtained at each temperature. This information can then be used to extrapolate time to failure for longer times. Changes in slope of the stress rupture line are due to structural changes in the material. It is significant to be aware of these changes in material behavior, because they could result in large errors when extrapolating the data.



Failure Analysis

High temperature failures is a significant problem. A failure analysis can identify the root cause of your failure to prevent reoccurrence. AMC can provide failure analysis of high temperature failures to identify the root cause of your component failure.

3.13 STRESS

Stress is defined as force per unit area. It has the same units as pressure, and in fact pressure is one special variety of stress. However, stress is a much more complex quantity than pressure because it varies both with direction and with the surface it acts on.

Stress = Load / cross-sectional area ( N / mm2)

• Compression:

Stress that acts to shorten an object.

• Tension:

Stress that acts to lengthen an object.

• Normal Stress:

Stress that acts perpendicular to a surface. Can be either compressional or tensional.

• Shear:

Stress that acts parallel to a surface. It can cause one object to slide over another. It also tends to deform originally rectangular objects into parallelograms. The most general definition is that shear acts to change the angles in an object.

• Hydrostatic:

Stress (usually compressional) that is uniform in all directions. A scuba diver experiences hydrostatic stress. Stress in the earth is nearly hydrostatic. The term for uniform stress in the earth is lithostatic.

• Directed Stress:

Stress that varies with direction. Stress under a stone slab is directed; there is a force in one direction but no counteracting forces perpendicular to it. This is why a person under a thick slab gets squashed but a scuba diver under the same pressure doesn't. The scuba diver feels the same force in all directions.



3.14 STRAIN

You will also be able to find the amount of stretch or elongation the specimen undergoes during tensile testing This can be expressed as an absolute measurement in the change in length or as a relative measurement called "strain". Strain itself can be expressed in two different ways, as "engineering strain" and "true strain". Engineering strain is probably the easiest and the most common expression of strain used. It is the ratio of the change in length to the original length,

. Whereas, the true strain is similar but based on the instantaneous length of the specimen as the test progresses,

, where Li is the instantaneous length and L0 the initial length.

• Longitudinal or Linear Strain

Strain that changes the length of a line without changing its direction. Can be either compressional or tensional.

• Compression

Longitudinal strain that shortens an object.

• Tension

Longitudinal strain that lengthens an object.

• Shear

Strain that changes the angles of an object. Shear causes lines to rotate.

• Infinitesimal Strain:

Strain that is tiny, a few percent or less. Allows a number of useful mathematical simplifications and approximations.

• Finite Strain:

Strain larger than a few percent. Requires a more complicated mathematical treatment than infinitesimal strain.

• Homogeneous Strain:

Uniform strain. Straight lines in the original object remain straight. Parallel lines remain parallel. Circles deform to ellipses. Note that this definition rules out folding, since an originally straight layer has to remain straight.



• Inhomogeneous Strain:

How real geology behaves. Deformation varies from place to place. Lines may bend and do not necessarily remain parallel.

3.15 Terms for behavior of materials

• Elastic:

Material deforms under stress but returns to its original size and shape when the stress is released. There is no permanent deformation. Some elastic strain, like in a rubber band, can be large, but in rocks it is usually small enough to be considered infinitesimal.

• Brittle:

Material deforms by fracturing. Glass is brittle. Rocks are typically brittle at low temperatures and pressures.

• Ductile:

Material deforms without breaking. Metals are ductile. Many materials show both types of behavior. They may deform in a ductile manner if deformed slowly, but fracture if deformed too quickly or too much. Rocks are typically ductile at high temperatures or pressures.

• Viscous:

Materials that deform steadily under stress. Purely viscous materials like liquids deform under even the smallest stress. Rocks may behave like viscous materials under high temperature and pressure.

• Plastic:

Material does not flow until a threshhold stress has been exceeded.

• Viscoelastic:

Combines elastic and viscous behavior. Models of glacio-isostasy frequently assume a viscoelastic earth: the crust flexes elastically and the underlying mantle flows viscously.

3.16 Strain-Stress Diagram A stress-strain curve is a graph derived from measuring load (stress - σ) versus extension (strain - ε) for a sample of a material. The nature of the curve varies from material to material. The following diagrams illustrate the stress-strain behaviour of typical materials in terms of the engineering stress and engineering strain where the stress and strain are calculated based on the original dimensions of the sample and not the instantaneous values. In each case the samples are loaded in tension although in many cases similar behaviour is observed in compression.



The stress value at the point P is called the limit of proportionality:

σ = F / Sp P 0

This behavior conforms to the Hook’s Law:

σ = E*δ

Where E is a constant, known as Young’s Modulus or Modulus of Elasticity.

The value of Young’s Modulus is determined mainly by the nature of the material and is nearly insensitive to the heat treatment and composition.

Modulus of elasticity determines stiffness - resistance of a body to elastic deformation caused by an applied force.

The line 0E in the strain-stress curve indicates the range of elastic deformation – removal of the load at any point of this part of the curve results in return of the specimen length to its original value.

The elastic behavior is characterized by the elasticity limit (stress value at the point E):

σ = F / Sel E 0

For the most materials the points P and E coincide and therefore σ =σel p.

A point where the stress causes sudden deformation without any increase in the force is called yield limit (yield stress, yield strength):



σ = F / Sy Y 0

The highest stress (point YU) , occurring before the sudden deformation is called upper yield limit .

The lower stress value, causing the sudden deformation (point YL) is called lower yield limit.

The commonly used parameter of yield limit is actually lower yield limit.

If the load reaches the yield point the specimen undergoes plastic deformation – it does not return to its original length after removal of the load.

Hard steels and non-ferrous metals do not have defined yield limit, therefore a stress, corresponding to a definite deformation (0.1% or 0.2%) is commonly used instead of yield limit. This stress is called proof stress or offset yield limit (offset yield strength):

σ = F / S0.2% 0.2% 0

The method of obtaining the proof stress is shown in the picture.

As the load increase, the specimen continues to undergo plastic deformation and at a certain stress value its cross-section decreases due to “necking” (point S in the strain-stress diagram). At this point the stress reaches the maximum value, which is called ultimate tensile strength (tensile strength):

σ = F / St S 0



Continuation of the deformation results in breaking the specimen - the point B in the diagram.

The actual strain-stress curve is obtained by taking into account the true specimen cross-section instead of the original value.

Other important characteristic of metals is ductility - ability of a material to deform under tension without rupture.

Two ductility parameters may be obtain from the tensile test:

Relative elongation - ratio between the increase of the specimen length before its rupture and its original length:

δ = (L – L ) / Lm 0 0

Where Lm– maximum specimen length.

Relative reduction of area - ratio between the decrease of the specimen cross-section area before its rupture and its original cross-section area:

ψ= (S – S ) / S0 min 0

Where Smin– minimum specimen cross-section area.

3.17 HOOKE,S LAW

For most tensile testing of materials, you will notice that in the initial portion of the test, the relationship between the applied force, or load, and the elongation the specimen exhibits is linear. In this linear region, the line obeys the relationship defined as "Hooke's Law" where the ratio of stress to strain is a constant, or

. E is the slope of the line in this region where stress (σ) is proportional to strain (ε) and is called the "Modulus of Elasticity" or "Young's Modulus".

Modulus of elasticity

The modulus of elasticity is a measure of the stiffness of the material, but it only applies in the linear region of the curve. If a specimen is loaded within this linear region, the material will return to its exact same condition if the load is removed. At the point that the curve is no longer linear and deviates from the



straight-line relationship, Hooke's Law no longer applies and some permanent deformation occurs in the specimen. This point is called the "elastic, or proportional, limit". From this point on in the tensile test, the material reacts plastically to any further increase in load or stress. It will not return to its original, unstressed condition if the load were removed.

Yield strength

A value called "yield strength" of a material is defined as the stress applied to the material at which plastic deformation starts to occur while the material is loaded.

________________________________________


APPLICATIONS TO MATERIALS TESTING

FOUR CHAPTER


4.1 INTRODUCTION

Major branches of engineering depend on the results of mechanical tests for design and/or quality control purposes. Test specimens are prepared for metallic and non-metallic materials in the evaluation of tensile, compression, impact, fracture toughness, fatigue and bend properties. Routine testing of fasteners, chain materials, weld coupons, wire rope, castings, sheet, plate, forgings and other components is done in an expedient manner providing an efficient, quality conscious service. Many fabricators, heat treaters and foundries rely on Bodycote’s definitive mechanical testing services to facilitate early production release or production start. Some materials require more in-depth testing, such as dynamic fracture test, or cryogenic and elevated temperature mechanical properties. Our customers can feel confident that not only routine but also the most diverse test requests will be handles by highly experienced engineers and technicians.

Capabilities include:

• Bend, Compression, Plastic Strain Ratio, Ring Flaring/Flattening • Tensile, Nick Break, Fillet Fracture, Hydrogen Embrittlement • Impact, Fracture Toughness • Hardness, Jominy Hardenability • Creep, Stress Rupture • Drop Weight, Dynamic Load • Fatigue • Component Testing • Vibration & Shock • Fasteners • Tribology/Wear

The test involve comparison of the behavior of test pieces under conditions which ore approximately similar to those conditions in which the metals are used. The tests take a lot of time, sometimes months and in some case several years to give conclusive results, which may be applied for the development of components of machinery, etc. The test for creep, fatigue, stress, corrosion, etc are also made at elevated temperature. Theses tests essentially require long time.

The material testing can be divided into two main groups:

1. Destructive 2. Non –Destructive




4.2 DESTRUCTIVE TEST

In destuctive testing, tests are carried out to the specimen’s failure. These tests are generally much easier to carry out, yield more information, and are easier to interpret than nondestructive testing.

Testing of an object is often done in view of future use, which would make destructive testing pointless. However, it can be useful if the result gives information about similar specimens which are not tested.

Some types of destructive testing are:

• Fracture Toughness Test • Spark Test • Bend Test • Hardness Test • Compression test • Fatigue Test • Flexure Test • Jominy End-Quench Test • Impact Test • Torsion test • Tensile Test • Creep Test • Charpy Test • Izod Test

4.3 FRACTURA TOUGHNESS TEST

Background

The resistance to fracture of a material is known as its fracture toughness. Fracture toughness generally depends on temperature, environment, loading rate, the composition of the material and its microstructure, together with geometric effects (constraint). [1] These factors are of particular importance for welded joints, where the metallurgical and geometric effects are complex [2,3]

Fracture toughness is a critical input parameter for fracture-mechanics based fitness-for-service assessments. Although fracture toughness can sometimes be obtained from the literature, or materials properties databases, it is preferable to determine this by experiment for the particular material and joint being assessed.

Various measures of 'toughness' exist, including the widely used but qualitative Charpy impact test. Although it is possible to correlate Charpy energy with fracture toughness, a large degree of uncertainty is associated with correlations because they are empirical. It is preferable to determine fracture toughness in a rigorous fashion, in terms of K (stress intensity factor), CTOD (crack tip opening displacement), or J (the J integral); see also What is a fracture toughness test? Standards exist for performing fracture mechanics tests, with the most common specimen configuration shown in Fig.1 (the single-edge notch bend SENB specimen). A sharp fatigue notch is inserted in the specimen, which is loaded to failure. The crack driving force is calculated for the failure condition, giving the fracture toughness.


Fig. Fracture mechanics testing

Various national Standards have been developed for fracture toughness testing:

• The British Standard BS 7448 [4] includes four parts, for testing of metallic materials, including parent materials, weldments, high strain rates (dynamic fracture toughness testing, to be published in 2005) and resistance curves (R-curves for ductile tearing). BS 7448: Part 2 is the first Standard worldwide to apply specifically to weldments.

• A series of American ASTM Standards cover K, CTOD, J testing (including R-curves), ASTM E1290 (CTOD testing) and ASTM E1820 (K, J & CTOD). None specifically address testing of welds. together with a summary of applicable terminology. [5-8]

• A series of international (ISO) standards are being developed. ISO 12135 covers all aspects of fracture testing (K, J & CTOD) of plain material. Standards are being prepared on testing of welds (ISO/CD 15653) and stable crack growth in low constraint specimens (ISO/CD 15653). The latter is mainly concerned with testing thin, sheet material.

• The European Structural Integrity Society (ESIS) has published procedures for R-curve and standard fracture toughness testing of metallic materials. [9-10] Currently, a draft unified testing procedure (ESIS P3-04), which includes weld testing, is being developed. (These are not standards in the usual sense, but rather testing protocols that have been agreed by experts).

Although different standards have historically been published for determining K, CTOD and J, the tests are very similar, and generally all three values can be established from one test. See Are there any differences between fracture toughness tests carried out to BS7448 and E1820?

Test specimens

The most widely used fracture toughness test configurations are the single edge notch bend (SENB or three-point bend), and the compact (CT) specimens, as shown in Fig.2. The compact specimen has the advantage that it requires less material, but is more expensive to machine and more complex to test compared with the SENB specimen. Also, special requirements are needed for temperature control (e.g. use of an environmental chamber). SENB specimens are typically immersed in a bath for low temperature tests. Although the compact specimen is loaded in tension, the crack tip conditions are predominantly bending (high constraint). If limited material is available, it is possible to fabricate SENB specimens by



welding extension pieces (for the loading arms) to the material sample. (Electron beam welding is typically used, because the weld is narrow and causes little distortion).

Fig. Examples of common fracture toughness test specimen types

Other specimen configurations include centre-cracked tension (CCT) panels, single edge notch tension (SENT) specimens, and shallow-crack tests. These specialised tests are associated with lower levels of constraint, and can be more structurally representative than standard SENB or CT specimens.

The position and orientation of the specimen is important. In particular, the location and orientation of the notch is critical, especially for welded joints. Typically, the notch (fatigue pre-crack) is positioned such that a chosen microstructure is sampled. The orientation of the notch is defined with respect to either the weld axis for welded joints, or the rolling direction or forging axis for other components.

In standard SENB & C T specimens (see Fig.1), the notch depth is within the range 45-70% of the specimen width, W, giving a lower-bound estimate of fracture toughness, because of the high level of crack tip constraint generated by the specimen design.

A notch is machined into the fracture toughness specimen, following which a fatigue crack is grown by applying cyclic loading to the specimen. Specialised high frequency resonance or servo-hydraulic machines are often used for this process.

The fracture mechanics test standards include many checks to ensure that results are credible. These include restrictions on the fatigue crack size, position and shape, together with limitations on the maximum allowable fatigue force (this is to ensure that the crack-tip plastic zone produced during fatigue pre-cracking is small in comparison with the plastic zone produced during testing). Many of these checks can only be performed after testing.

Instrumentation and loading

During fracture toughness testing, the force applied to the specimen and specimen displacements and loading rate (using load cells and displacement transducers), together with the temperature are recorded.

One of the displacements is the crack-mouth opening. This is measured using a clip gauge either attached to knife edges mounted at the crack mouth (see Fig.1) or integral knife edges machined into the notch. These gauges comprise two cantilevered beams on which are positioned four strain gauges. By measuring the elastic strains and calibration it is possible to infer the crack-mouth opening.

Fracture toughness tests are performed in universal hydraulic test machines, generally using displacement control.



Fracture toughness parameters

The following are the fracture toughness parameters commonly obtained from testing:

• K (stress intensity factor) can be considered as a stress-based estimate of fracture toughness. It is derived from a function which depends on the applied force at failure. K depends on geometry (the flaw depth, together with a geometric function, which is given in test standards for each test specimen geometry).

• CTOD or (crack-tip opening displacement) can be considered as a strain-based estimate of fracture toughness. However, it can be separated into elastic and plastic components. The elastic part of CTOD is derived from the stress intensity factor, K. In some standards, the plastic component of CTOD is obtained by assuming that the specimen rotates about a plastic hinge. The plastic component is derived from the crack mouth opening displacement (measured using a clip gauge). The position of the plastic hinge (defined by r p ) is given in test standards for each specimen type. Alternative methods exist for estimating CTOD, which make no assumption regarding the position of the plastic hinge. These require the determination of J from which CTOD is derived. [6,7] CTOD values determined from formulations assuming a plastic hinge [4] may differ from those determined from J. [6,7]

• J (the J-integral) is an energy-based estimate of fracture toughness. It can be separated into elastic and plastic components. As with CTOD, the elastic component is based on K, while the plastic component is derived from the plastic area under the force-displacement curve.

4.4 SPARK TEST

Spark testing metals is done by noting the type of sparks that issue from a piece of steel that has been put to a grinding wheel, From this one can deduce with some accuracy the type of alloy present (for instance; percentage carbon, vanadium, chromium).

Spark characteristics to note are:

1. Colour 2. Length 3. Branching

One of the simplest tests is to note how the sparks branch. Higher carbon steels will produce shorter streams of sparks with a large amount of branching, in contrast low carbon steel will produce a longer stream with less branching. Additionally wrought iron will produce practically no branching, and cast iron extremely short stream with excessive branching


Fig. Spark test


4.5 BENDING TEST

The bend test is a simple and inexpensive qualitative test that can be used to evaluate both the ductility and soundness of a material. It is often used as a quality control test for butt-welded joints, having the advantage of simplicity of both test piece and equipment.

No expensive test equipment is needed, test specimens are easily prepared and the test can, if required, be carried out on the shop floor as a quality control test to ensure consistency in production.

The bend test uses a coupon that is bent in three point bending to a specified angle.

The outside of the bend is extensively plastically deformed so that any defects in, or embrittlement of, the material will be revealed by the premature failure of the coupon.

The bend test may be free formed or guided.

The guided bend test is where the coupon is wrapped around a former of a specified diameter and is the type of test specified in the welding procedure and welder qualification specifications. For example, it is a requirement in ASME IX, the EN 287 and EN 288 series of specifications and ISO 15614 Part 1.

As the guided bend test is the only form of bend test specified in welding qualification specifications it is the only one that will be dealt with in this article.

Typical bend test jigs are illustrated in Fig.1(a) and 1(b).

Fig.1(a) shows a guided bend test jig that uses a male and a female former, the commonest form of equipment

Fig.1(b) shows a wrap-around guided bend test machine that works on the same principles as a plumber's pipe bender



The strain applied to the specimen depends on the diameter of the former around which the coupon is bent and this is related to the thickness of the coupon 't', normally expressed as a multiple of 't' eg 3t, 4t etc.

The former diameter is specified in the test standard and varies with the strength and ductility of the material - the bend former diameter for a low ductility material such as a fully hard aluminium alloy may be as large as 8t. An annealed low carbon steel on the other hand may require a former diameter of only 3t. The angle of bend may be 90°, 120° or 180° depending on the specification requirements.

On completion of the test the coupon is examined for defects that may have opened up on the tension face. Most specifications regard a defect over 3mm in length as being cause for rejection.

For butt weld procedure and welder qualification testing the bend coupons may be oriented transverse or parallel to the welding direction.

Below approximately 12mm material thickness transverse specimens are usually tested with the root or face of the weld in tension. Material over 12mm thick is normally tested using the side bend test that tests the full section thickness, Fig.2.

Fig.2

Where the material thickness is too great to permit the full section to be bent the specifications allow a number of narrower specimens to be taken provided that the full material thickness is tested. Conventionally,

most welding specifications require two root and two face bend coupons or four side bends to be taken from each butt welded test piece.

The transverse face bend specimen will reveal any defects on the face such as excessive undercut or lack of sidewall fusion close to the cap. The transverse root bend is also excellent at revealing lack of root fusion or penetration. The transverse side bend tests the full weld thickness and is particularly good at revealing lack of side-wall fusion and lack of root fusion in double-V butt joints. This specimen orientation is also useful for testing weld cladding where any brittle regions close to the fusion line are readily revealed.

Longitudinal bend specimens are machined to include the full weld width, both HAZs and a portion of each parent metal. They may be bent with the face, root or side in tension and are used where there is a difference in mechanical strength between the two parent metals or the parent metal and the weld. The test will readily reveal any transverse defects but it is less good at revealing longitudinally oriented defects such as lack of fusion or penetration.

Whilst the bend test is simple and straightforward to perform there are some features that may result in the test being invalid.

In cutting the coupon from the test weld the effects of the cutting must not be allowed to affect the result. Thus it is necessary to remove any HAZ from flame cutting or work hardened metal if the sample is sheared.

It is normal to machine or grind flat the face and root of a weld bend test coupon to reduce the stress raising effect that these would have. Sharp corners can cause premature failure and should be rounded off to a maximum radius of 3mm.



The edges of transverse bend coupons from small diameter tubes will experience very high tensile stresses when the ID is in tension and this can result in tearing at the specimen edges.

Weld joints with non-uniform properties such as dissimilar metal joints or where the weld and parent metal strengths are substantially different can result in 'peaking' of the bend coupon. This is when most of the deformation takes place in the weaker of the two materials which therefore experiences excessive localised deformation that may result in premature failure.


A dissimilar metal joint where one of the parent metals is very high strength is a good example of where this may occur and similar peaking can be seen in fully hard welded aluminium alloy joints.

In these instances the roller bend test illustrated in Fig.1(b) is the best method of performing a bend test as each component of the coupon is strained by a similar amount and peaking is to a great extent eliminated.

4.6 HARDNESS TEST

Simply stated, hardness is the resistance of a material to permanent indentation. It is important to recognize that hardness is an empirical test and therefore hardness is not a material property. This is because there are several different hardness tests that will each determine a different hardness value for the same piece of material. Therefore, hardness is test method dependent and every test result has to have a label identifying the test method used. Hardness is, however, used extensively to characterize materials and to determine if they are suitable for their intended use. All of the hardness tests described in this section involve the use of a specifically shaped indenter, significantly harder than the test sample, that is pressed into the surface of the sample using a specific force. Either the depth or size of the indent is measured to determine a hardness value.

Why Use a Hardness Test?

• Easy to perform • Quick - 1 to 30 seconds • Relatively inexpensive • Non-destructive • Finished parts can be tested - but not ruined



• Virtually any size and shape can be tested • Practical QC device - incoming, outgoing

The most common uses for hardness tests is to verify the heat treatment of a part and to determine if a material has the properties necessary for its intended use. Establishing a correlation between the hardness result and the desired material property allows this, making hardness tests very useful in industrial and R&D applications.

Hardness Scales There are five major hardness scales:

• Brinell - HB • Knoop - HK • Rockwell - HR • Shore - HS • Vickers - HV

Each of these scales involve the use of a specifically shaped diamond, carbide or hardened steel indenter pressed into the material with a known force using a defined test procedure. The hardness values are determined by measuring either the depth of indenter penetration or the size of the resultant indent. All of the scales are arranged so that the hardness values determined increase as the material gets harder. The hardness values are reported using the proper symbol, HR, HV, HK, etc. indicating the test scale performed.

Five Determining Factors

The following five factors can be used to determine the correct hardness test for your application.

1. Material - grain size, metal, rubber, etc. 2. Approximate Hardness - hardened steel, rubber, etc. 3. Shape - thickness, size, etc. 4. Heat Treatment – through or casehardened, annealed, etc. 5. Production Requirements - sample or 100%

The application guide is designed to help determine the hardness tests that can be used on some typical materials.

• Brinell Hardness Test

Dr. J. A. Brinell invented the Brinell test in Sweden in 1900. The oldest of the hardness test methods in common use today, the Brinell test is frequently used to determine the hardness of forgings and castings that have a grain structure too course for Rockwell or Vickers testing. Therefore, Brinell tests are frequently done on large parts. By varying the test force and ball size, nearly all metals can be tested using a Brinell test. Brinell values are considered test force independent as long as the ball size/test force relationship is the same.

In the USA, Brinell testing is typically done on iron and steel castings using a 3000Kg test force and a 10mm diameter carbide ball. Aluminum and other softer alloys are frequently tested using a 500Kg test force and a 10 or 5mm carbide ball. Therefore the typical range of Brinell testing in this country is 500 to 3000kg with 5 or 10mm carbide balls. In Europe Brinell testing is done using a much wider range of forces


and ball sizes. It's common in Europe to perform Brinell tests on small parts using a 1mm carbide ball and a test force as low as 1kg. These low load tests are commonly referred to as baby Brinell tests.

Standards

Brinell Test methods are defined in the following standards:

• ASTM E10 • ISO 6506

Fig. Brinell hardness tester. Fig. Microscopic view of impression.

Brinell Test Method

All Brinell tests use a carbide ball indenter. The test procedure is as follows:




• The indenter is pressed into the sample by an accurately controlled test force. • The force is maintained for a specific dwell time, normally 10 - 15 seconds. • After the dwell time is complete, the indenter is removed leaving a round indent in the sample. • The size of the indent is determined optically by measuring two diagonals of the round indent

using either a portable microscope or one that is integrated with the load application device. • The Brinell hardness number is a function of the test force divided by the curved surface area of

the indent. The indentation is considered to be spherical with a radius equal to half the diameter of the ball. The average of the two diagonals is used in the following formula to calculate the Brinell hardness.

The Brinell number, which normally ranges from HB 50 to HB 750 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple. A typical Brinell hardness is specified as follows:

356HBW

Where 356 is the calculated hardness and the W indicates that a carbide ball was used. Note- Previous standards allowed a steel ball and had an S designation. Steel balls are no longer allowed.

Applications

Because of the wide test force range the Brinell test can be used on almost any metallic material. See the application guide. The part size is only limited by the testing instrument's capacity.

Strengths

1. One scale covers the entire hardness range, although comparable results can only be obtained if the ball size and test force relationship is the same.

2. A wide range of test forces and ball sizes to suit every application. 3. Nondestructive, sample can normally be reused.

Weaknesses

1. The main drawback of the Brinell test is the need to optically measure the indent size. This requires that the test point be finished well enough to make an accurate measurement.

2. Slow. Testing can take 30 seconds not counting the sample preparation time.

• Knoop Test

Knoop (HK) hardness was developed by at the National Bureau of Standards (now NIST) in 1939. The indenter used is a rhombic-based pyramidal diamond that produces an elongated diamond shaped indent. Knoop tests are mainly done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size. Because of this, Knoop tests have mainly been known as microhardness tests. The newer standards more accurately use the term microindentation tests. The magnifications required to measure Knoop indents dictate a highly polished test surface. To achieve this surface, the samples are normally mounted and metallurgically polished, therefore Knoop is almost always a destructive test.

Standards

Knoop test methods are defined in ASTM E384.


Knoop Test Method

Knoop testing is done with a rhombic-based pyramidal diamond indenter that forms an elongated diamond shaped indent.

• The indenter is pressed into the sample by an accurately controlled test force. • The force is maintained for a specific dwell time, normally 10 - 15 seconds. • After the dwell time is complete, the indenter is removed leaving an elongated diamond shaped

indent in the sample. • The size of the indent is determined optically by measuring the longest diagonal of the diamond

shaped indent. • The Knoop hardness number is a function of the test force divided by the projected area of the

indent. The diagonal is used in the following formula to calculate the Knoop hardness.

HK = Constant x test force / indent diagonal squared

The constant is a function of the indenter geometry and the units of force and diagonal. The Knoop number, which normally ranges from HK 60 to HK1000 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple, while all digital test instruments do it automatically. A typical Knoop hardness is specified as follows:

450HV0.5

Where 450 is the calculated hardness and 0.5 is the test force in kg.

Applications

Because of the wide test force range, the Knoop test can be used on almost any metallic material. See the application guide. The part size is only limited by the testing instrument's capacity.

Strengths

• The elongated diamond indenter and low test forces allows testing very small parts or material features not capable if being tested any other way.

• One scale covers the entire hardness range. • Test results a mainly test force independent over 100g. • A wide range of test forces to suit every application.



Weaknesses

• The main drawback of the Knoop test is the need to optically measure the indent size. This requires that the test point be highly polished to be able to see the indent well enough to make an accurate measurement.

• Slow. Testing can take 30 seconds not counting the sample preparation time.

• Rockwell Test

Stanley P. Rockwell invented the Rockwell hardness test. He was a metallurgist for a large ball bearing company and he wanted a fast non-destructive way to determine if the heat treatment process they were doing on the bearing races was successful. The only hardness tests he had available at time were Vickers, Brinell and Scleroscope. The Vickers test was too time consuming, Brinell indents were too big for his parts and the Scleroscope was difficult to use, especially on his small parts.

To satisfy his needs he invented the Rockwell test method. This simple sequence of test force application proved to be a major advance in the world of hardness testing. It enabled the user to perform an accurate hardness test on a variety of sized parts in just a few seconds.

Rockwell test methods are defined in the following standards:

• ASTM E18 Metals • ISO 6508 Metals • ASTM D785 Plastics

Types of the Rockwell Test

There are two types of Rockwell tests:

1. Rockwell: the minor load is 10 kgf, the major load is 60, 100, or 150 kgf. 2. Superficial Rockwell: the minor load is 3 kgf and major loads are 15, 30, or 45 kgf.

In both tests, the indenter may be either a diamond cone or steel ball, depending upon the characteristics of the material being tested.

Fig. Rockwell hardness tester. PRESTON UNIVERSITY, LAHORE CAMPUS 57


Rockwell Scales

Rockwell hardness values are expressed as a combination of a hardness number and a scale symbol representing the indenter and the minor and major loads. The hardness number is expressed by the symbol HR and the scale designation.

There are 30 different scales. The majority of applications are covered by the Rockwell C and B scales for testing steel, brass, and other metals. However, the increasing use of materials other than steel and brass as well as thin materials necessitates a basic knowledge of the factors that must be considered in choosing the correct scale to ensure an accurate Rockwell test. The choice is not only between the regular hardness test and superficial hardness test, with three different major loads for each, but also between the diamond indenter and the 1/16, 1/8, 1/4 and 1/2 in. diameter steel ball indenters.

If no specification exists or there is doubt about the suitability of the specified scale, an analysis should be made of the following factors that control scale selection:

• Type of material • Specimen thickness • Test location • Scale limitations

Principal of the Rockwell Test

• Select image to enlarge The indenter moves down into position on the part surface • A minor load is applied and a zero reference position is established • The major load is applied for a specified time period (dwell time) beyond zero • The major load is released leaving the minor load applied

The resulting Rockwell number represents the difference in depth from the zero reference position as a result of the application of the major load.

• Shore Test

The Shore test has been used since 1907 to determine the hardness of a wide variety of rubber and soft plastics. Originally there were only 4 different scales for rubbers. However, now there are 12 scales to allow testing an even wider range of materials from small rubber O rings to very soft foam products. The testers that perform Shore tests have been commonly referred to as Durometers and the results frequently called Durometer hardness. With the exception of the M scale testers, all Durometers can be used either as a portable unit or in an operating stand. This flexibility adds greatly to the usefulness of the Shore scale.




Standards

Shore test methods are defined in the following standards:

• ASTM D-2240 • DIN 53 505 • ISO 7619 Part 1 • JIS K 6301* • ASKER C-SRIS-0101

NOTE: The JIS standard is very similar to the ASTM 2240 standard. However, there are small but important differences.

Shore Test Method

The Shore test uses a hardened indenter, an accurately calibrated spring, a depth indicator, and a flat presser foot. The indenter is mounted in the middle of the presser foot and extends 2.5mm from the surface of the foot. In the fully extended position the indicator displays zero. When the indenter is depressed flat even with the presser foot's surface, the indicator displays 100. Therefore, every Shore point is equal to 0.0025mm penetration (M scale is 0.00125mm).

In use the unit is placed on the sample so that the presser foot is held firmly against the test surface. The spring pushes the indenter into the sample and the indicator indicates the depth of penetration. The deeper the indentation the softer the material and the lower the indicator reading.

The different Shore scales, A, B, C, D, DO, E, M, O, OO, OOO, OOO-S and R are created by using 7 different indenter shapes, 5 different springs, 2 different indenter extensions an 2 different presser foot specifications. The A and D scales are by far the most commonly used. The M scale uses a very low force spring and was developed to allow testing very small parts like O rings that can not be tested in the normal A scale. Because different materials respond to the test scales in different ways, there is no correlation between the different scales.

Applications

All Durometers except for the M scale units can be used as a portable device. Test stands are recommended for best accuracy and are required for M scale testing because of its increased sensitivity. Some stands have extra weights to make sure that the force on the presser foot is constant from test to test. Normally multiple tests are done on each sample and the average result is used.

Strengths

• Fast, easy to use • Inexpensive • Wide range of materials can be tested • Non destructive, part can normally be used after testing

Weaknesses

• Dwell time variables can cause poor readings. • Inconsistent force on the presser foot will cause errors. • Difficulties keeping the indenter perpendicular to the test surface will cause errors.


• The test surface must be large enough to support the presser foot.

• Vickers Test

The Vickers (HV) test was developed in England is 1925 and was formally known as the Diamond Pyramid Hardness (DPH) test. The Vickers test has two distinct force ranges, micro (10g to 1000g) and macro (1kg to 100kg), to cover all testing requirements. The indenter is the same for both ranges therefore Vickers hardness values are continuous over the total range of hardness for metals (typically HV100 to HV1000). With the exception of test forces below 200g, Vickers values are generally considered test force independent. In other words, if the material tested is uniform, the Vickers values will be the same if tested using a 500g force or a 50kg force. Below 200g, caution must be used when trying to compare results.

Standards

Vickers test methods are defined in the following standards:

• ASTM E384 – micro force ranges – 10g to 1kg • ASTM E92 – macro force ranges - 1kg to 100kg • ISO 6507-1,2,3 – micro and macro ranges

Vickers Test Method

All Vickers ranges use a 136° pyramidal diamond indenter that forms a square indent.

• The indenter is pressed into the sample by an accurately controlled test force. • The force is maintained for a specific dwell time, normally 10 – 15 seconds. • After the dwell time is complete, the indenter is removed leaving an indent in the sample that

appears square shaped on the surface. • The size of the indent is determined optically by measuring the two diagonals of the square indent. • The Vickers hardness number is a function of the test force divided by the surface area of the

indent. The average of the two diagonals is used in the following formula to calculate the Vickers hardness.

HV = Constant x test force / indent diagonal squared

The constant is a function of the indenter geometry and the units of force and diagonal. The Vickers number, which normally ranges from HV 100 to HV1000 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple, while all digital test instruments do it automatically. A typical Vickers hardness is specified as follows:



356HV0.5

Where 356 is the calculated hardness and 0.5 is the test force in kg.

Applications

Because of the wide test force range, the Vickers test can be used on almost any metallic material. See the application guide. The part size is only limited by the testing instrument's capacity.

Strengths

• One scale covers the entire hardness range. • A wide range of test forces to suit every application. • Nondestructive, sample can normally be used.

Weaknesses

• The main drawback of the Vickers test is the need to optically measure the indent size. This requires that the test point be highly finished to be able to see the indent well enough to make an accurate measurement.

• Slow. Testing can take 30 seconds not counting the sample preparation time.

4.7 COMPRESSION TEST


A compression test determines behavior of materials under crushing loads. The specimen is compressed and deformation at various loads is recorded. Compressive stress and strain are calculated and plotted as a stress-strain diagram which is used to determine elastic limit, proportional limit, yield point, yield strength and, for some materials, compressive strength.

Why Perform a Compression Test?

The ASM Handbook®, Volume 8, Mechanical Testing and Evaluation states: "Axial compression testing is a useful procedure for measuring the plastic flow behavior and ductile fracture limits of a material. Measuring the plastic flow behavior requires frictionless (homogenous compression) test conditions, while measuring ductile fracture limits takes advantage of the barrel formation and controlled stress and strain conditions at the equator of the barreled surface when compression is carried out with friction. Axial


compression testing is also useful for measurement of elastic and compressive fracture properties of brittle materials or low-ductility materials. In any case, the use of specimens having large L/D ratios should be avoided to prevent buckling and shearing modes of deformation1."

The image at right shows variation of the strains during a compression test without friction (homogenous compression) and with progressively higher levels of friction and decreasing aspect ratio L/D (shown as h/d)1.

Fig. Modes of Deformation in Compression Testing

The figure illustrates the modes of deformation in compression testing. (a) Buckling, when L/D > 5. (b) Shearing, when L/D > 2.5. (c) Double barreling, when L/D > 2.0 and friction is present at the contact surfaces. (d) Barreling, when L/D < 2.0 and friction is present at the contact surfaces. (e) Homogenous compression, when L/D < 2.0 and no friction is present at the contact surfaces. (f) Compressive instability due to work-softening material1.

Typical Materials

The following materials are typically subjected to a compression test.

• Concrete • Metals • Plastics • Ceramics • Composites • Corrugated Cardboard

4.8 FATIGUE TEST

The definition of fatigue testing can be thought of as simply applying cyclic loading to your test specimen to understand how it will perform under similar conditions in actual use. The load application can either be a repeated application of a fixed load or simulation of in-service loads. The load application may be repeated



millions of times and up to several hundred times per second.

Why Do a Fatigue Test? In many applications, materials are subjected to vibrating or oscillating forces. The behavior of materials under such load conditions differs from the behavior under a static load. Because the material is subjected to repeated load cycles (fatigue) in actual use, designers are faced with predicting fatigue life, which is defined as the total number of cycles to failure under specified loading conditions. Fatigue testing gives much better data to predict the in-service life of materials.

4.9 FLEXURE TEST

The flexure test method measures behavior of materials subjected to simple beam loading. It is also called a transverse beam test with some materials. Maximum fiber stress and maximum strain are calculated for increments of load. Results are plotted in a stress-strain diagram. Flexural strength is defined as the maximum stress in the outermost fiber. This is calculated at the surface of the specimen on the convex or tension side. Flexural modulus is calculated from the slope of the stress vs. deflection curve. If the curve has no linear region, a secant line is fitted to the curve to determine slope.

Why Perform a Flexure Test?

A flexure test produces tensile stress in the convex side of the specimen and compression stress in the concave side. This creates an area of shear stress along the midline. To ensure the primary failure comes from tensile or compression stress the shear stress must be minimized. This is done by controlling the span to depth ratio; the length of the outer span divided by the height (depth) of the specimen. For most materials S/d=16 is acceptable. Some materials require S/d=32 to 64 to keep the shear stress low enough.

Types of Flexure Tests

Flexure testing is often done on relatively flexible materials such as polymers, wood and composites. There are two test types; 3-point flex and 4-point flex. In a 3-point test the area of uniform stress is quite small and concentrated under the center loading point. In a 4-point test, the area of uniform stress exists between the inner span loading points (typically half the outer span length).



Typical Materials

Polymers

The 3-point flexure test is the most common for polymers. Specimen deflection is usually measured by the crosshead position. Test results include flexural strength and flexural modulus.

Wood and Composites

The 4-point flexure test is common for wood and composites. The 4-point test requires a deflectometer to accurately measure specimen deflection at the center of the support span. Test results include flexural strength and flexural modulus.

Brittle Materials When a 3-point flexure test is done on a brittle material like ceramic or concrete it is often called modulus of rupture (MOR). This test provides flex strength data only, not stiffness (modulus). The 4-point test can also be used on brittle materials. Alignment of the support and loading anvils is critical with brittle materials. The test fixture for these materials usually has self-aligning anvils.

4.10 JOMINY END-QUENCH TEST

One standard procedure that is widely used to measure hardenability of steel is the Jominy end-quench test. In this test water is sprayed on one end of a bar of steel while it is hot. This leads to a one dimensional heat transfer cooling. Except near the surface of the bar the temperature is controlled by that flow along the length of the bar. Moving axially inward from the quenched end of the bar, the temperature and the rate of change of temperature are changing. The temperature is higher and the rate is slower away from the quenched end. If hardness is measured as a function of distance from the end, a hardness profile can be obtained which applies to any part made from the same steel.

Experimental Procedure

You will be given two steels (1045) and an alloy steel (4130). Before heating the specimens practice mounting the specimens in the rack and at the proper water flow to spray the ends of the specimen. Mark each specimen noting the hardness on the Rockwell C scale. Check to make sure the collar of the Jominy is secure and put the specimen in the furnace at 1600 degrees F for 45 minutes. While you are waiting to heat the specimens examine the microstructure of the allow steel and carbon steel specimens provided by the instructor. At the end of the austenizing treatment remove one specimen and carefully but rapidly place the specimen in the hold with the water turned on.


Method of Test

Fig. 4. Schematic of Jominy end-

quench test specimen (a) mounted during quenching and (b) after hardness testing.


The standard method for the Jominy test is ASTM-A255. The specimen consists of a cylindrical bar with a 1-in diameter and 4-in length and with a 1/16 in flange at one end. The test consists of austenitizing at 5°F above the solvus line on the Fe-C phase which separates γ from γ + α iron. Thereafter the specimen is removed from the furnace and is placed in the hardenability fixture as in Figure 4a. The time spent transferring the specimen from the furnace to the fixture should not be more than 5 sec. The fixture is constructed so that the specimen is held 1/2 inch above the water opening so that a column of water is directed only at the bottom of the bar. The water opening is 1/2 inch in diameter and the flow is previously adjusted to cause the column to rise 2-1/2 inches without the specimen in place. The test piece is held 10 minutes in the fixture under the action of cooling before quenching in cold water. After cooling, shallow flats 0.015 in. deep are ground along the specimen length (Figure 4b). Hardness (Rockwell C scale) measurements are taken for the first 2 ½ in. along each flat; for the first ½ in., hardness readings are taken at 1/16 in. intervals, for the remaining 2 in., hardness readings are taken every 1/8 in. Figure 5 shows the correlation between the hardness and the distance from the quenched end is due to the variation in the cooling rates that result to different microstructures at different distances from the quenched end.

Fig. 5. Correlation of hardenability and continuous cooling information for eutectoid steel.

Using the observed hardness values at different distances from the quenched end, hardenability curves can be plotted. Figure 6 shows typical hardenability curves for commonly used steel alloys.


Fig. 6 Hardenability curves for five different steel alloys


4.11 IMPACT TEST

In this test the pendulum is swing up to its starting position (height H ) and then it is allowed to strike the notched specimen, fixed in a vice. The pendulum fractures the specimen, spending a part of its energy. After the fracture the pendulum swings up to a height H.

The impact toughness of the specimen is calculated by the formula:

a = A/ S

Where

a-impact toughness,

A – the work, required for breaking the specimen ( A = M*g*H0–M*g*H),

M - the pendulum mass,

S - cross-section area of the specimen at the notch.

One of the most popular impact tests is the Charpy Test, schematically presented in the figure below:

Why is Impact Testing Important?


Impact resistance is one of the most important properties for a part designer to consider, and without question, the most difficult to quantify. The impact resistance of a part is, in many applications, a critical measure of service life. More importantly these days, it involves the perplexing problem of product safety and liability.

One must determine:

• the impact energies the part can be expected to see in its lifetime, • the type of impact that will deliver that energy, and then


• select a material that will resist such assaults over the projected life span.

Molded-in stresses, polymer orientation, weak spots (e.g. weld lines or gate areas), and part geometry will affect impact performance. Impact properties also change when additives, e.g. coloring agents, are added to plastics.

Further complication is offered by the choice of failure modes: ductile or brittle. Brittle materials take little energy to start a crack, little more to propagate it to a shattering climax. Other materials possess ductility to varying degrees. Highly ductile materials fail by puncture in drop weight testing and require a high energy load to initiate and propagate the crack.

Many materials are capable of either ductile or brittle failure, depending upon the type of test and rate and temperature conditions. They possess a ductile/brittle transition that actually shifts according to these variables.

4.12 TORSION TEST

As in the macroscopic world, several kinds of tests have to be performed to be able to establish reliable failure criteria. An important experiment in this context is the torsional test, as it allows to validate or expand the criteria gained from tensile tests. The experimental setup is a major challenge. The actuator part has to be able to apply pure torsion by minimizing errors due to bending. The sensor part is able to measure the resulting angle and torque, with a resolution of 0.03o and 0.3 mNm, respectively. A balance records the tensile forces occurring in the specimens during the experiments. Experiments have been performed on both silicon and metallic specimens. The resulting curves are shown in Figure 1 .

Fig. 1 Torque/rotation diagrams for silicon and Ni specimens

A numerical simulation (see Figure 2 ), using finite element techniques, of the experiments in combination with an analytical analysis allows the determination of the governing elastic moduli, which in this case consist of two shear-moduli, as the material behaviour is considered to be transversely isotropic.

Fig. 2 FE-model of LIGA specimen



In addition to the determination of elastic moduli, it is important to know the relevant failure criterion in order to have engineering design rules. For Ni specimens, which show an isotropic behaviour, it is possible to show that the von Mises yield criterion is in good agreement with the experiments. The anisotropic Ni-Fe alloys do not show the same behaviour. Their yielding point in torsion lies beyond the point predicted by von Mises, using the data from the tensile test and assuming an isotropic yield criterion.

Why Perform a Torsion Test?

Many products and components are subjected to torsional forces during their operation. Products such as biomedical catheter tubing, switches, fasteners, and automotive steering columns are just a few devices subject to such torsional stresses. By testing these products in torsion, manufacturers are able to simulate real life service conditions, check product quality, verify designs, and ensure proper manufacturing techniques.

Types of Torsion Tests

Torsion tests can be performed by applying only a rotational motion or by applying both axial (tension or compression) and torsional forces. Types of torsion testing vary from product to product but can usually be classified as failure, proof, or product operation testing.

• Torsion Only: Applying only torsional loads to the test specimen. • Axial-Torsion: Applying both axial (tension or compression) and torsional forces to the test

specimen. • Failure Testing: Twisting the product, component, or specimen until failure. Failure can be

classified as either a physical break or a kink/defect in the specimen. • Proof Testing: Applying a torsional load and holding this torque load for a fixed amount of time. • Operational Testing: Testing complete assemblies or products such as bottle caps, switches, dial

pens, or steering columns to verify that the product performs as expected under torsion loads.

4.13 TENSILE TESTING

• As mentioned earlier the tensile test is used to provide information that will be used in design calculations or to demonstrate that a material complies with the requirements of the appropriate specification - it may therefore be either a quantitative OR a qualitative test.

• The test is made by gripping the ends of a suitably prepared standardised test piece in a tensile test machine and then applying a continually increasing uni-axial load until such time as failure occurs. Test pieces are standardised in order that results are reproducible and comparable as shown in Fig 2.


Fig.2. Standard shape tensile specimens


Specimens are said to be proportional when the gauge length, L 0 , is related to the original cross sectional area, A 0 , expressed as L 0 =k A 0 . The constant k is 5.65 in EN specifications and 5 in the ASME codes. These give gauge lengths of approximately 5x specimen diameter and 4x specimen diameter respectively - whilst this difference may not be technically significant it is important when claiming compliance with specifications.

Fig.3. Stress/strain curve

• Both the load (stress) and the test piece extension (strain) are measured and from this data an engineering stress/strain curve is constructed, Fig.3. From this curve we can determine:

• a) the tensile strength, also known as the ultimate tensile strength, the load at failure divided by the original cross sectional area where the ultimate tensile strength (U.T.S.), max = P max /A 0 , where P max = maximum load, A 0 = original cross sectional area. In EN specifications this parameter is also identified as 'R m ';

• b) the yield point (YP), the stress at which deformation changes from elastic to plastic behaviour ie below the yield point unloading the specimen means that it returns to its original length, above the yield point permanent plastic deformation has occurred, YP or y = P yp /A 0 where P yp = load at the yield point. In EN specifications this parameter is also identified as 'R e ';

• c) By reassembling the broken specimen we can also measure the percentage elongation, El% how much the test piece had stretched at failure where El% = (L f - L 0 /L o ) x100 where Lf = gauge length at fracture and L0 = original gauge length. In EN specifications this parameter is also identified as 'A' ( Fig.4a).

• d) the percentage reduction of area, how much the specimen has necked or reduced in diameter at the point of failure where R of A% =(A 0 - A f /A 0 ) x 100 where A f = cross sectional area at site of the fracture. In EN specifications this parameter is also identified as 'Z', ( Fig.4b).



Fig.4 a) Calculation of percentage elongation b) Calculation of percentage reduction of area

• Fig (a) and (b) are measures of the strength of the material, (c) and (d) indicate the ductility or ability of the material to deform without fracture.

• The slope of the elastic portion of the curve, essentially a straight line, will give Young's Modulus of Elasticity, a measure of how much a structure will elastically deform when loaded.

• A low modulus means that a structure will be flexible, a high modulus a structure that will be stiff and inflexible.

• To produce the most accurate stress/strain curve an extensometer should be attached to the specimen to measure the elongation of the gauge length. A less accurate method is to measure the movement of the cross-head of the tensile machine.

• The stress strain curve in Fig.3 shows a material that has a well pronounced yield point but only annealed carbon steel exhibits this sort of behaviour. Metals that are strengthened by alloying, by heat treatment or by cold working do not have a pronounced yield and some other method must be found to determine the 'yield point'.

• This is done by measuring the proof stress ( offset yield strength in American terminology), the stress required to produce a small specified amount of plastic deformation in the test piece.

• The proof stress is measured by drawing a line parallel to the elastic portion of the stress/strain curve at a specified strain, this strain being a percentage of the original gauge length, hence 0.2% proof, 1% proof (see Fig.5).

• For example, 0.2% proof strength would be measured using 0.2mm of permanent deformation in a specimen with a gauge length of 100mm. Proof strength is therefore not a fixed material characteristic, such as the yield point, but will depend upon how much plastic deformation is specified. It is essential therefore when considering proof strengths that the percentage figure is always quoted. Most steel specifications use 0.2% deformation, R P0.2 in the EN specifications.


• Some materials such as annealed copper, grey iron and plastics do not have a straight line elastic portion on the stress/strain curve. In this case the usual practice, analogous to the method of


determining proof strength, is to define the 'yield strength' as the stress to produce a specified amount of permanent deformation.

Fig.5. Determination of proof (offset yield) strength

Results generally required from a commercial tensile test:

The result generally required from a commercial tensile test are as follow:

• Tensile strength • Elongation % • Reduction in area • Stress at yield point (when this is present)

Tensile strength

This is found by dividing the maximum load by the original cross sectional area of the specimen

(Note that for ductile materials the maximum load may be greater than the breaking load).

Tensile Strength = ________________________ Maximum load

Original cross-sectional area

Elongation %

This is given by expressing the stretch as a percentage of the of the original test length.

Elongation % = ______________ x 100 Stretch


Original length


Good elongation combined with a reasonable strength indicates a ductile material.

Besides these two main properties a specification may stipulate other requirement which may be obtained from the tensile test :

Reduction in area %

This is given by:

Reduction in area % = ___________________________ x 100 Reduction in area at fracture

Original area

= ____________________________ x 100 Original area - Area at fracture Original area

A high reduction of area indication that the material will lend itself more readily to cold working

Stress at yield point (when this is present)

Stress at yield point = ___________________ Load at yield pointCross-sectional area

Note that as a stress will be expressed as a load per unit area . for example if a load in Newton’s is divided by an area in square millimeter. the unit stress will be Newton per square millimeter, the units of stress will be Newton per square millimeter.

Example :

Determine the results of a test on a BS test piece, 14mm test diameter , 70mm gauge length which gave the following results when tested:

• Load at yield point 51 KN • Maximum load 82 KN • Length between gauge point with broken end put together 90 mm • Diameter at fracture 11.5 mm

Solution: Tensile strength = __________________ Maximum Load

Cross-sectional area


14 = _____________ = 533 N / mm282 x 103

3.14 / 4 x 2


Stress at yield point = _________________ Load at yield point

Cross-sectional area = ______________ = 331 N / mm251 x 103

3.14 / 4 x 142

Elongation % = ______________ x 100 Stretch

Original length

= _________ x 100 = 28.6 % 90 - 70 70

Reduction in area % = ____________________________ x 100 Original area - Area at fractureOriginal area

= _________________________________ x 100 (3.14 / 4 x 14 ) - (3.14 / 4 x 11.5 ) 2 2

3.14 / 4 x 142

= 32.4 %

4.14 CREEP TEST

Creep is a phenomenon of slow plastic deformation (elongation) of a metal at high temperature under a constant load.

The creep mechanism:

At low stresses the creep is controlled by the diffusion of atoms through the grain boundaries. At higher stresses the creep strain proceeds due to the dislocations movement.

The rate of creep is a function of the material, the applied stress value, the temperature, and the time exposure.

Considerable creep deformation, causing damage of machines and structures occur at high temperatures (about a half of the melting point measured in the absolute temperature scale). Therefore this phenomenon is taken into account in design and operation of heat exchangers, steam boilers and pipes, jet engines and other loaded equipment, working at high temperatures.

Soft metals (lead, tin) may experience creep at room temperature.

A typical creep behavior is presented in the diagram:



Fig. Creep behavior

The initial strain is not time dependent and it is caused mainly by elastic deformation.

The first stage creep is characterized by relatively fast plastic deformation occurring at decreasing rate. During this stage resistance creep increases causing decrease the deformation rate.

The second stage creep occurs at a constant and relatively low deformation rate. This rate is used in the engineering design.

The rate of creep at the second stage depends on both the load (stress) and the temperature.

The third stage creep is associated with accelerated strain rate caused by decrease of the cross sectional area of the specimen (necking). This stage is finalized by the specimen fracture.

At room temperature creep is negligible at any stress below the yield point.

The quantity, which is used in precise design of machines and structures working at elevated temperatures, is creep strength.

Creep strength is a stress which causes a definite creep strain after a specified period of time at a given temperature.

Creep strength of a material is much lower, than its tensile strength.

If a large amount of deformation is tolerated rupture strength is used in design.

Rupture strength is a stress which causes a fracture of a metal after a specified period of time at a given temperature.

Creep strength and rupture strength are determined in stress-rupture tests conducted in [Tensile test and Strain-Stress Diagram|tensile test]] machines equipped with a furnace providing uniform heating of the tested specimens.

This machine records amount of strain at every moment after the test has started and until the specimen failure.

How to Perform a Creep Test?


To determine creep properties, a material is subjected to prolonged constant tension or compression loading at constant elevated temperature. Deformation is recorded at specified time intervals and a creep vs. time diagram is plotted. Slope of curve at any point is creep rate. If failure occurs, it terminates the test


and the time for rupture is recorded. If specimen does not fracture within the test period, creep recovery may be measured.

How to Determine Stress-Relaxation?

To determine stress-relaxation of a material, the specimen is deformed a given amount and decrease in stress is recorded over prolonged period of exposure at constant elevated temperature. The stress-relaxation rate is the slope of the curve at any point.

Typical Applications

• Metal Working • Springs • Soldered Joints • High-Temperature Materials

4.15 CHARPY TEST

While most commonly used on metals, it is also used on polymers, ceramics and composites. The Charpy test is most commonly used to evaluate the relative toughness or impact toughness of materials and as such is often used in quality control applications where it is a fast and economical test. It is used more as a comparative test rather than a definitive test.

Fig. The hammer striking energy in the Charpy test is 220 ft*lbf (300 J).

Charpy Test Specimens

Charpy test specimens normally measure 55x10x10mm and have a notch machined across one of the larger faces. The notches may be:

• V-notch – A V-shaped notch, 2mm deep, with 45° angle and 0.25mm radius along the base • U-notch or keyhole notch – A 5mm deep notch with 1mm radius at the base of the notch.



What Does the Charpy Test Involve?

The Charpy test involves striking a suitable test piece with a striker, mounted at the end of a pendulum. The test piece is fixed in place at both ends and the striker impacts the test piece immediately behind a machined notch.

Fig. 1 Schematic of the Charpy impact test.

4.16 IZOD TEST

The Izod test is has become the standard testing procedure for comparing the impact resistances of plastics. While being the standard for plastics it is also used on other materials.

The Izod test is most commonly used to evaluate the relative toughness or impact toughness of materials and as such is often used in quality control applications where it is a fast and economical test. It is used more as a comparative test rather than a definitive test. This is also in part due to the fact that the values do not relate accurately to the impact strength of moulded parts or actual components under actual operational conditions.

Izod Test Specimens

Izod test specimens vary depending on what material is being tested. Metallic samples tend to be square in cross section, while polymeric test specimens are often rectangular, being struck parallel to the long axis of the rectangle.

Izod test sample usually have a V-notch cut into them, although specimens with no notch as also used on occasion.

What Does the Izod Test Involve?

The Izod test involves striking a suitable test piece with a striker, mounted at the end of a pendulum. The test piece is clamped vertically with the notch facing the striker. The striker swings downwards impacting the test piece at the bottom of its swing.



Some Izod impact testers are equipped to be able to utilise different sized strikers, which impart different amounts of energy. Often a series of stri8kers may be used to determine the impact energy, starting with small strikers and working up until failure occurs.

Fig. 1 Schematic of the Izod impact test.

Izod Tests at Different Temperatures

Tests are often performed at different temperatures to more closely simulate the actual service conditions. In the case of low temperature tests, specimens may are kept in a freezer until their temperature has equilibrated. They are then immediately removed and tested within seconds of removal from the freezer.

4.17 NON-DESTRUCTIVE TEST

Nondestructive testing (NDT), also called nondestructive evaluation (NDE) and nondestructive inspection (NDI), is testing that does not destroy the test object. NDE is vital for constructing and maintaining all types of components and structures. To detect different defects such as cracking and corrosion, there are different methods of testing available, such as X-ray (where cracks show up on the film) and ultrasound (where cracks show up as an echo blip on the screen). This article is aimed mainly at industrial NDT, but many of the methods described here can be used to test the human body. In fact methods from the medical field have often been adapted for industrial use, as was the case with Phased array ultrasonics and Computed radiography.

While destructive testing usually provides a more reliable assessment of the state of the test object, destruction of the test object usually makes this type of test more costly to the test object's owner than nondestructive testing. Destructive testing is also inappropriate in many circumstances, such as forensic investigation. That there is a tradeoff between the cost of the test and its reliability favors a strategy in which most test objects are inspected nondestructively; destructive testing is performed on a sampling of test objects that is drawn randomly for the purpose of characterizing the testing reliability of the nondestructive test.



The need of NDT

It is very difficult to weld or mold a solid object that has no risk of breaking in service, so testing at manufacture and during use is often essential. During the process of molding a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture.

During their service lives, many industrial components need regular nondestructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example:

Aircraft skins need regular checking to detect cracks;

• Underground pipelines are subjected to corrosion and stress corrosion cracking; • Pipes in industrial plants may be subject to erosion and corrosion from the products they carry; • Concrete structures may be weakend if the inner reinforcing steel is corroded; • Pressure vessels may develop crcacks in welds; • The wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing

for broken wires and other damage is important

NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT.

Some types of non-destructive tests are:

• Ultrasonic testing • Liquid penetrant testing • Radiographic testing • Magnetic particle testing • Magnetic flux leackage test

4.18 ULTRASONIC TESTING

In ultrasonic testing, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterize materials. It is also commonly used to determine the thickness of the test object - monitoring pipework corrosion being a good example.

Ultrasonic Inspection is often performed on steel and other metals and alloys, though it can be used on concrete and other materials such as composites. It is a form of non-destructive testing used in many industries including aerospace, automotive and other transportation sectors.



An example of Ultrasonic Testing (UT) on blade roots of a V2500 IAE aircraft engine. Step 1: The UT probe is placed on the root of the blades to be inspected with the help of a special borescope tool(videoprobe). Step2:Instrumentsettingsareinput. Step 3: The probe is scanned over the blade root. In this case, an indication (peak in the data) through the red line (or gate) indicates a good blade; an indication to the left of that range indicates a crack.

Many structures and components are being successfully inspected with Zetec products that use ultrasound technologies, such as:

• Pressure retaining welds in nuclear and fossil power plants: piping welds, reactor pressure vessel

welds, and nozzle welds are just a few examples

• Turbine blade roots and attachments

• Turbine bores

• Feeder tubes in CANDU type nuclear power plants

How it works?

In ultrasonic testing, a transducer connected to a diagnostic machine is passed over the object being inspected. In reflection (or pulse-echo) mode, the transducer sends pulsed waves through a couplant (such as water or oil) on the surface of the object, and receives the "sound" reflected back to the device. Reflected ultrasound comes from an interface - such as the back wall of the object or from an imperfection. The screen on the calibrated diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance taken for the reflection to return to the transducer. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after travelling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted thus indicating their presence.

Advantages

1. Superior penetrating power, which allows the detection of flaws deep in the part. 2. High sensitivity, permitting the detection of extremely small flaws. 3. Only one surface need to be accessible.



4. Greater accuracy than other nondestructive methods in determining the depth of internal flaws and the thickness of parts with parallel surfaces.

5. Some capability of estimating the size, orientation, shape and nature of defects. 6. Nonhazardous to operations or to nearby personnel and has no effect on equipment and materials

in the vicinity. 7. Capable of portable or highly automated operation.

Disadvantages

1. Manual operation requires careful attention by experienced technicians. 2. Extensive technical knowledge is required for the development of inspection procedures. 3. Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to

inspect. 4. Surface must be prepared by cleaning and removing loose scale, paint, etc. (UT can often be used

successfully through paint that is properly bonded to a surface) 5. Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers

and parts being inspected unless a non-contact technique is used. Non-contact techniques include Laser and Electro Magnetic Acoustic Transducers (EMAT).

6. Inspected items must be water resistant, when using water based couplants that do not contain rust inhibitors.

4.19 LIQUID PENETRANT TESTING

1. Section of material with a surface-breaking crack that is not visible to the naked eye. 2. Penetrant is applied to the surface. 3. Excess penetrant is removed. 4. Developer is applied, rendering the crack visible.

Liquid penetrant inspection is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). Penetrant may be applied to all non-ferrous materials, but for inspection of ferrous components magnetic particle inspection is preferred




for its subsurface detection capability. LPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components.

Principles

LPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, and a developer is applied. The developer helps to draw penetrant out of the flaw where a visible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending upon the type of dye used - fluorescent or nonfluorescent (visible).

Mtaerials

Penetrants are classified into sensitivity levels. Visible penetrants are typically red in color, and represent the lowest sensitivity. Fluorescent penetrants contain two or more dyes that fluoresce when excited by ultraviolet (UV-A) radiation (also known as black light). Since FPI is performed in a darkened environment, and the excited dyes emit brilliant yellow-green light that contrasts strongly against the dark background, this material is more sensitive to small defects.

When selecting a sensitivity level one must consider many factors, including the environment under which the test will be performed, the surface finish of the specimen, and the size of defects sought. One must also assure that the test chemicals are compatible with the sample so that the examination will not cause permanent staining, or degradation. This technique can be quite portable, because in its simplest form the inspection requires only 3 aerosol spray cans, some paper towels, and adequate visible light. Stationary systems with dedicated application, wash, and development stations, are more costly and complicated, but result in better sensitivity and higher sample through-put.

Inspection steps

Below are the main steps of Liquid Penetrant Inspection:

1. Precleaning

The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep penetrant out of a defect, or cause irrelevant or false indications. Cleaning methods may include solvents, alkaline cleaning steps, vapor degreasing, or media blasting. The end goal of this step is a clean surface where any defects present are open to the surface, dry, and free of contamination.

2. Application of penetrant

The penetrant is then applied to the surface of the item being tested. The penetrant is allowed time to soak into any flaws (generally 10 to 30 minutes). The soak time mainly depends upon the material being testing and the size of flaws sought. As expected, smaller flaws require a longer penetration time. Due to their incompatible nature one must be careful not to apply visible red dye penetrant to a sample that may later be inspected with fluorescent penetrant.

3. Excess penetrant removal

The excess penetrant is then removed from the surface. Removal method is controlled by the type of penetrant used. Water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable are the common choices. Emulsifiers represent the highest sensitivity level, and chemically interact with the oily penetrant to make it removable with a water spray. When using solvent remover and lint-free cloth it is important to not spray the solvent on the test surface directly, because this can the remove the penetrant from the flaws. This process must be performed under controlled conditions so that



all penetrant on the surface is removed (background noise), but penetrant trapped in real defects remains in place.

4. Application of developer

After excess penetrant has been removed a white developer is applied to the sample. Several developer types are available, including: non-aqueous wet developer, dry powder, water suspendible, and water soluble. Choice of developer is governed by penetrant compatibility (one can't use water-soluble or suspedible developer with water-washable penetrant), and by inspection conditions. When using non-aqueous wet developer (NAWD) or dry powder the sample must be dried prior to application, while soluble and suspendible developers are applied with the part still wet from the previous step. NAWD is commercially available in aerosol spray cans, and may employ acetone, isopropyl alcohol, or a propellant that is a combination of the two. Developer should form a thin, even coating on the surface.

The developer draws penetrant from defects out onto the surface to form a visible indication, a process similar to the action of blotting paper. Any colored stains indicate the positions and types of defects on the surface under inspection.

5. Inspection

The inspector will use visible light with adequate intensity (100 foot-candles is typical) for visible dye penetrant. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations. Inspection of the test surface should take place after a 10 minute development time. This time delay allows the blotting action to occur. The inspector may observe the sample for indication formation when using visible dye, but this should not be done when using fluorescent penetrant. Also of concern, if one waits too long after development the indications may "bleed out" such that interpretation is hindered.

6. Post cleaning

The test surface is often cleaned after inspection and recording of defects (if found), especially if post-inspection coating processes are scheduled.

Features

The flaws are more visible, because;

• The defect indication has a high visual contrast (e.g. red dye against a white developer background, or a bright fluorescent indication against a dark background).

• The developer draws the penetrant out of the flaw over a wider area than the real flaw, so it looks wider.

• Limited training is required for the operator — although experience is quite valuable. • Low testing costs. • Proper cleaning is necessary to assure that surface contaminants have been removed and any

defects present are clean and dry. Some cleaning methods have been shown to be detrimental to test sensitivity, so acid etching to remove metal smearing and re-open the defect may be necessary.

• Penetrant dyes stain cloth, skin and other porous surfaces brought into contact. One should verify compatibility on the test material, especially when considering the testing of plastic components.

4.20 RADIOGRAPHIC TESTING

Radiographic Testing (RT), or industrial radiography, is a nondestructive testing (NDT) method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materialS.



Either an X-ray machine or a radioactive source (Ir-192, Co-60, or in rare cases Cs-137) can be used as a source of photons. Neutron radiographic testing (NR) is a variant of radiographic testing which uses neutrons instead of photons to penetrate materials. This can see very different things from X-rays, because neutrons can pass with ease through lead and steel but are stopped by plastics, water and oils.

Since the amount of radiation emerging from the opposite side of the material can be detected and measured, variations in this amount (or intensity) of radiation are used to determine thickness or composition of material. Penetrating radiations are those restricted to that part of the electromagnetic spectrum of wavelength less than about 10 nanometres.

Inspection of welds

The beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point, except in special techniques where known defects are best revealed by a different alignment of the beam. The length of weld under examination for each exposure shall be such that the thickness of the material at the diagnostic extremities, measured in the direction of the incident beam, does not exceed the actual thickness at that point by more than 6%. The specimen to be inspected is placed between the source of radiation and the detecting device, usually the film in a light tight holder or cassette, and the radiation is allowed to penetrate the part for the required length of time to be adequately recorded.

The result is a two-dimensional projection of the part onto the film, producing a latent image of varying densities according to the amount of radiation reaching each area. It is known as a radiograph, as distinct from a photograph produced by light. Because film is cumulative in its response (the exposure increasing as it absorbs more radiation), relatively weak radiation can be detected by prolonging the exposure until the film can record an image that will be visible after development. The radiograph is examined as a negative, without printing as a positive as in photography. This is because, in printing, some of the detail is always lost and no useful purpose is served.

Before commencing a radiographic examination, it is always advisable to examine the component with one's own eyes, to eliminate any possible external defects. If the surface of a weld is too irregular, it may be desirable to grind it to obtain a smooth finish, but this is likely to be limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult.

After this visual examination, the operator will have a clear idea of the possibilities of access to the two faces of the weld, which is important both for the setting up of the equipment and for the choice of the most appropriate technique.

Defects such as delaminations and planar cracks are difficult to detect using radiography, which is why penetrants are often used to enhance the contrast in the detection of such defects. Penetrants used include silver nitrate, zinc iodide, chloroform and diiodomethane. Choice of the penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed. Diiodomethane has the advantages of high opacity, ease of penetration, and ease of removal because it evaporates relatively quickly. However, it can cause skin burns.

Safety

Industrial radiography appears to have one of the worst safety profiles of the radiation professions, possibly because there are many operators using strong gamma sources (> 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within hospitals.

4.21 MEGNATIC PARTICLE TESTING

Magnetic particle inspection processes are non-destructive methods for the detection of defects in ferrous materials. They make use of an externally applied magnetic field or DC current through the material, and



the principle that the magnetic susceptibility of a defect is markedly poorer (the magnetic resistance is greater) than that of the surrounding material.

The presence of a surface or near surface flaw (void) in the material causes distortion in the magnetic flux through it, which in turn causes leakage of the magnetic fields at the flaw. This deformation of the magnetic field is not limited to the immediate locality of the defect but extends for a considerable distance; even through the surface and into the air if the magnetism is intense enough. Thus the size of the distortion is much larger than that of the defect and is made visible at the surface of the part by means of the tiny particles that are attracted to the leakage fields.

The most common method of magnetic particle inspection uses finely divided iron or magnetic iron oxide particles, held in suspension in a suitable liquid (often kerosene). This fluid is referred to as carrier. The particles are often colored and usually coated with fluorescent dyes that are made visible with a hand-held ultraviolet (UV) light. The suspension is sprayed or painted over the magnetized specimen during magnetization with a direct current or with an electromagnet, to localize areas where the magnetic field has protruded from the surface. The magnetic particles are attracted by the surface field in the area of the defect and hold on to the edges of the defect to reveal it as a build up of particles.

This inspection can be applied to raw material in a steel mill (billets or slabs), in the early stages of manufacturing (forgings, castings), or most commonly to machined parts before they are put into service. It is also very commonly used for inspecting structural parts (e.g. landing gear) that have been in-service for some time to find fatigue cracks.

4.22 MAGNETIC FLUX LEACKAGE TEST

Magnetic flux leakage (MFL) is a magnetic method of nondestructive testing that is used to detect corrosion and pitting in steel structures, most commonly pipelines and storage tanks. The basic principle is that a powerful magnet is used to magnetize the steel. At areas where there is corrosion or missing metal, the magnetic field "leaks" from the steel. In an MFL tool, a magnetic detector is placed between the poles of the magnet to detect the leakage field. Analysts interpret the chart recording of the leakage field to identify damaged areas and hopefully to estimate the depth of metal loss. This article currently focuses mainly on the pipeline application of MFL, but links to tank floor examination are provided at the end.

________________________________________

MATERIAL TESTING EQUIPEMENT

CHAPTER

FIVE MATERIAL TESTING

EQUIPEMENT 5.1 INTRODUCTION Mechanical testing equipment covers devices used for adhesion, compression, drop (shock), tensile, vibration and fatigue testing. The growing importance of quality control and assurance in production has contributed to an increasing demand for mechanical testing equipment with quality-control procedures existing on all production levels of many industrial markets. Mechanical testing for quality control serves two major purposes: product-endurance analysis and product-safety assurance. Understanding the strength and endurance of the product is beneficial to the end-user and to the supplier. Mechanical testing contributes to quality enhancement of a product because it enables manufacturers to test material characteristics before and after the final assembly stage. Because of the diverse nature of mechanical testing equipment, materials and structures of all sizes can be quality tested. Mechanical test method, user interface options, display options, additional output options, and environmental parameters. User interface options for mechanical testing equipment include local interfaces that are analog or digital, computer interfaces, serial or parallel communications, and application software. Display options for mechanical testing equipment include analog meters, digital readouts, and video displays. Additional output options include analog voltage, pulse signal, analog current, and switch or relay. Important environmental parameters to consider for mechanical testing equipment include operating temperature and operating humidity. 5.2 BRINELL TESTER

The Brinell hardness tester, shown in figure 1-25, uses a hardened spherical ball, which is forced into the surface of the metal. The ball is 10 millimeters (0.3937

inch) in diameter. A pressure of 3,000 kilograms (6,600 pounds) is used for ferrous metals and 500 kilograms for nonferrous metals. Normally, the load should be applied for 30 seconds. In order to produce equilibrium, this period may be increased to 1 minute for extremely hard steels. The load is applied by means of hydraulic pressure. The hydraulic pressure is built up by a hand pump or an electric motor, depending on the model of tester. A pressure gauge indicates the amount of pressure. There is a release mechanism for relieving the pressure after the test has been made, and a calibrated microscope is provided for measuring the diameter of the impression in millimeters. The machine has various shaped anvils for supporting the specimen and an elevating screw for bringing the specimen in contact with the ball penetrator. There are attachments for special tests.

To determine the Brinell hardness number for a metal, the diameter of the impression is first measured, using the calibrated microscope furnished with the tester.

85 PRESTON UNIVERSITY, LAHORE CAMPUS


Table 1-3.-Portion of Conversion Table Furnished with Brinell Tester

the microscope. After measuring the diameter of the impression, the measurement is converted into the Brinell hardness number on the conversion table furnished with the tester. A portion of the conversion table is shown in table 1-3.

Fig. 1-25.-Brinell hardness tester.

5.3 ROCKWELL TESTER

The Rockwell hardness tester, shown in figure 1-27, measures the resistance to penetration as does the Brinell tester, but instead of measuring the diameter of the impression, the Rockwell tester measures the depth, and the hardness is indicated directly on a dial attached to the machine. The more shallow the penetration, the higher the hardness number.

Two types of penetrators are used with the Rockwell tester–a diamond cone and a hardened steel ball. The load that forces the penetrator into the metal is called the "major load," and is measured in kilograms. The



results of each penetrator and load combination are reported on separate scales, designated by letters. The penetrator, the major load, and the scale vary with the kind of metal being tested.

For hardened steels, the diamond penetrator is used, the major load is 150 kilograms, and the hardness is read on the C scale. When this reading is recorded, the letter C must precede the number indicated by the pointer. The C-scale setup is used for testing metals ranging in hardness from C-20 to the hardest steel (usually about C-70). If the metal is softer than C-20, the B-scale setup is used. With this setup, the 1/16-inch ball is used as a penetrator, the major load is 100 kilograms, and the hardness is read on the B scale.

In addition to the C and B scales, there are other setups for special testing. The scales, penetrators, major loads, and dial numbers are listed in table 1-4. The dial numbers in the outer circle are black, and the inner numbers are red.

Fig. 1-27.-Rockwell hardness tester.

The Rockwell tester is equipped with a weight pan, and two weights are supplied with the machine. One weight is marked in red. The other weight is marked in black. With no weight in the weight pan, the machine applies a major load of 60 kilograms. If the scale setup calls for a 100-kilogram load, the red weight is placed in the pan. For a 150-kilogram load, the black weight is added to the red weight. The black weight is always used in conjunction with the red weight; it is never used alone. Practically all testing is done with either the B-scale setup or the C-scale setup. For these scales, the colors may be used as a guide in selecting the weight (or weights) and in reading the dial. For the B-scale test, use the red weight and read the red numbers. For a C-scale test, add the black weight to the red weight and read the black numbers.



In setting up the Rockwell machine, use the diamond penetrator for testing materials that are known to be hard. If in doubt, try the diamond, since the steel ball may be deformed if used for testing hard materials. If the metal tests below C-22, then change to the steel ball.

Use the steel ball for all soft materials-those testing less than B-100. Should an overlap occur at the top of the B scale and the bottom of the C scale, use the C-scale setup.

Before the major load is applied, the test specimen must be securely locked in place to prevent slipping and to properly seat the anvil and penetrator. To do this, a load of 10 kilograms is applied before the lever is tripped. This preliminary load is called the "minor load." The minor load is 10 kilograms regardless of the scale setup. When the machine is set up properly, it auto-matically applies the 10-kilogram load.

The metal to be tested in the Rockwell tester must be ground smooth on two opposite sides and be free of scratches and foreign matter. The surface should be perpendicular to the axis of penetration, and the two opposite ground surfaces should be parallel. If the specimen is tapered, the amount of error will depend on the taper. A curved surface will also cause a slight error in the hardness test. The amount of error depends on the curvature–the smaller the radius of curvature, the greater the error. To eliminate such error, a small flat should be ground on the curved surface if possible.

5.4 RIEHLE TESTER

The Riehle hardness tester is a portable unit that is designed for making Rockwell tests comparable to the bench-type machine. The instrument is quite universal in its application, being readily adjustable to a wide range of sizes and shapes that would be difficult, or impossible, to test on a bench-type tester.

Fig. 1-28.-Riehle portable hardness tester.

Figure 1-28 shows the tester and its proper use. It may be noted that the adjusting screws and the penetration indicator are set back some distance from the penetrator end of the clamps. This makes it practicable to use the tester on either the outside or inside surface of tubing, as well as on many other applications where the clearance above the penetrator or below the anvil is limited. The indicator brackets are arranged so that it is possible to turn the indicators to any angle for greater convenience in a specific application, or to facilitate its use by a left-handed operator. Adjustment of the lower clamp is made by the small knurled knob below the clamp. The larger diameter knob, extending through the slot in the side of the clamp, is used for actual clamping.

Each Riehle tester is supplied with a diamond pene-trator and a 1/16-inch ball penetrator. The ball penetrator should not be used on materials harder than B-100 nor on a load heavier than 100 kilograms. This is to avoid the danger of flattening the ball.



The diamond penetrator, when used with a 150-kilogram load, may be used on materials from the hardest down to those giving a reading of C-20. When the expected hardness of a material is completely unknown to the operator, it is advisable to take a preliminary reading on the A scale as a guide in selecting the proper scale to be used.

Testing Procedure

The basic procedures for making a test with the Riehle tester are as follows:

1. Apply a minor load of 10 kilograms. 2. Set the penetration indicator to zero. 3. Apply a major load of 60, 100, or 150 kilograms (depending on the scale), and then reduce the

load back to the initial 10-kilogram load. 4. Read the hardness directly on the penetration indicator.

The hardness reading is based on the measurement of the additional increment of penetration produced by applying a major load after an initial penetration has been produced by the minor load. In reporting a hardness number, the number must be prefixed by the letter indicating the scale on which the reading was obtained.

Removal and Replacement of a Penetrator

The penetrator is retained in the tester by means of a small knurled clamp screw extending from the top of

Fig. 1-29.-Barcol portable hardness tester.

5.5 BARCOL TESTER

The Barcol hardness tester, the upper clamp. To remove a penetrator, there should beat least 2 or 3 inches of space between the upper and lower clamps so that one hand can be placed underneath the upper clamp to catch the penetrator when it is released. Two or three turns of the clamp screw will release the penetrator. The two contact pins that extend through the penetrator on either side of the point are retained in the tester when the penetrator is removed.

To replace a penetrator, it must be turned so that the flat side faces the clamp screw, and the locating pin on the penetrator is in line with the slot provided to take the pin. The contact pins should be guided into their respective holes through the penetrator. With the penetrator in place, it should then be clamped securely by turning the clamp screw. Before you make an actual test, one or two preliminary tests should be made to properly seat the penetrator.

shown in figure 1-29, is a portable unit designed for testing aluminum alloys, copper, brass, and other relatively soft materials. Approximate range of the tester is 25 to 100 Brinell. The unit can be used in any



position and in any space that will allow for the operator’s hand. The hardness is indicated on a dial conveniently divided in 100 graduations.

Fig. 1-30-Cutaway of Barcol tester.

Figure 1-30 is a cutaway drawing of the tester, showing the internal parts and their general arrangement within the case.

The lower plunger guide and point are accurately ground so that attention need be given only to the proper position of the lower plunger guide within the frame to obtain accurate operation when a point is replaced. The frame, into which the lower plunger guide and spring-tensioned plunger are screwed, holds the point in the proper position. Adjustment of the plunger upper guide nut, which regulates the spring tension, is made when the instrument is calibrated at the factory.

CAUTION

The position of this nut should not be changed. Any adjustment made to the plunger upper guide nut will void the calibrated settings made at the factory.

The leg is set for testing surfaces that permit the lower plunger guide and the leg plate to be on the same plane. For testing rivets or other raised objects, a block may be placed under the leg plate to raise it to the same plane. For permanent testing of this type, the leg maybe removed and washers inserted, as shown in the drawing. The point should always be perpendicular to the surface being tested.

The design of the Barcol tester is such that operating experience is not necessary. It is only necessary to exert a light pressure against the instrument to drive the spring-loaded indenter into the material to be tested.



Table 1-5.-Typical Barcol Readings for Aluminum Alloys

Alloy and temper Barcol number1100-0

3003-0

3003 -1/2H

2024-0

5052-0

5052-1/2H

6061-T

2024-T

35

42

56

60

62

75

78

85

The hardness reading is instantly indicated on the dial. Several typical reading for aluminum alloys are listed in table 1-5. The harder the material, the higher the Barcol number.

To prevent damage to the point, avoid sliding or scraping when it is in contact with the material being tested. If the point should become damaged, it must be replaced with a new one. No attempt should be made to grind the point.

Each tester is supplied with a test disc for checking the condition of the point. To check the condition of the point, press the instrument down on the test disc. When the downward pressure brings the end of the lower plunger guide against the surface of the disc, the indicator reading should be within the range shown on the test disc.

To replace the point, remove the two screws that hold the halves of the case together. Lift out the frame, remove the spring sleeve, loosen the locknut, and unscrew the lower plunger guide, holding the point upward so that the spring and plunger will not fall out of place. Insert the new point and replace the lower plunger guide, screwing it back into the frame, Adjust the lower plunger guide with the wrench that is furnished until the indicator reading and the test disc average number are identical. After the lower plunger guide is properly set, tighten the locknut to keep the lower plunger guide in place, This adjustment should be made only after installing anew point; any readjustment on a worn or damaged point give erroneous readings.

Fig. 1-31.-Ernst portable hardness tester.



THE ERNST PORTABLE HARDNESS TESTER HAS A DIAMOND-TIPPED PENETRATOR AND READS IN ROCKWELL OR BRINELL SCALES.

NOTE : MATERIAL MUST BE SOLIDLY SUPPORTED FROM BEHIND. PRESS DOWN WITH A STEADY, EVEN FORCE.

5.6 ERNST TESTER

The Ernst tester is a small versatile tool that requires access to only one side of the material being tested. There are two models of the tester-one for testing hardened steels and hard alloys and one for testing unhardened steels and most nonferrous metals. It has a diamond point penetrator, and it is read directly from the Rockwell A or B or the Brinell scales, depending on the model used. Figure 1-31 shows the Ernst portable hardness tester and its proper use.

The correct procedures for using the Ernst tester are as follows:

1. Solidly support the metal being tested by placing a bucking bar behind the metal. This will minimize flexing of the metal and yield a more accurate reading of hardness.

2. The handgrip must be pressed down with a steady, even force to ensure accurate readings. 3. Press down until the fluid column has stopped moving. The hardness value is given at the point

where the fluid column has stopped moving on the scale.

As with other portable testers of similar type, the material must be smooth and backed up so there will be no tendency to sag under the load applied on the tester. The test block supplied with each tester should be used frequently to check its performance.

5.7 UNIVERSAL HARDNESS TESTER

For customers who require the flexibility of three hardness testing machines in one then Indentec's range of Universal Hardness Testers are an ideal choice.

The units are capable of Rockwell, Vickers and low load Brinell hardness tests, with the option of a hand held portable microscope or built in optical system for Vickers and Brinell indentation measurement.

If you need to eliminate uncertainty from Vickers and Brinell testing, you should check out our CAMS system. This computer-aided innovation replaces operator judgement with a CCD camera for distinguishing impressions. Through mouse-driven software, the indentation is electronically projected onto a PC monitor and measured automatically eliminating operator influence and reducing gang R+ R values. Available on all our Universal machines, CAMS makes Vickers and Brinell testing very easy and trouble free.

The multi-test facility makes the machine ideal for educational purposes in helping to demonstrate the three classical hardness tests. The unit is supplied with UKAS accredited test blocks and indentors for each of the three test methods.



5.8 MICRO VICKERS HARDNESS TESTER

Our advanced line of QV-1000 Series Vickers hardness testers are low-cost and precise testing systems suitable for hardness analysis of metallic specimens in metallography laboratories or production environments.

Features

• Motorized turret • High quality microscope with digital reading (QV-1000DAT Model) • Fully automatic load control • Easy operating system • Two optical paths • Built-in high speed thermal printer • XY stage with minimum reading of 0.01mm • QV-MONITOR or QV-CCD system (optional)

Micro Vickers Hardness Tester - Analogue with Auto Turret (motorized) Standard configuration includes:

• Main unit • Diamond indenter Vickers • Objectives 10x, 40x • Eyepiece 15x • XY-stage with micrometers • 3 adjustable feet • 3 clamping devices • Extension tube for CCD-camera • Digital eyepiece incl. protection cover



• Spirit level • Micro-Vickers test plates 2x • Spare light bulb 12V-30W • Spare fuses 2x • Installation & user manual • Portable tester

• If you can't bring the specimen to the tester, then you can take the test to the remote or large specimen with one of our handheld portable hardness testers.

• Producing on-the-spot readings and print-outs of any popular hardness scale, these pocket-sized instruments can be applied to any surface, from any direction, including upside down.

• Top models store up to 200 test results and display hardness, scale, time, material tested, number of tests, running average hardness, test direction and ultimate tensile strength.

• The units are totally portable, requiring no external power source cables and connect to a printer via an infra-red port.

• Instruments are supplied with an Indentec calibration certificate and a UKAS traceable test block.

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Material Testing Techniques

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