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HARDNESS & HARDNESS TESTING

PART-1

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Introduction

• Hardness measurements widely used in laboratory and as industrial tests (widely used on castings and forgings) as a tool characterizing the mechanical properties of materials.

• It can be used to classify the materials or to identify a material in a group.

• Hardness tests are employed at a very large scaleto assess the useful properties of materials.

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DEFINITION

• Hardness is defined as the resistance of a material to permanent deformation such as:– Indentation

– Wear

– Abrassion

– Scratch

• Hardness is a mechanical property that is related to:– Wear resistance of a material

– Its ability to abrade or indent another material or

– Its resistance to permanent or plastic deformation

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• There are many hardness tests currently in use.

• The neccessity for all these different hardness tests is due to the need for categorizing the great range

of hardness from soft rubber to hard ceramics.

• Selecting of the appropirate hardnees test is dependent upon the;

– Relative hardness of the material being tested and

– Amount of damage that can be tolerated on the surface of the specimen (or material)

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TYPES OF HARDNESS TESTS

• The types of hardness testers in use of product are based on a number of “arbitrary definitions” of hardness.

Some of these definitions are:• 1- Resistance to permanent indentation (indentation

test ) • 2-Resistance to scratching (scratch test)• 3-Energy absorption under impact loads (dynamic

hardness test)• 4-Rebound of a falling weight (rebound test)

3rd and 4th : Same principle collision & absorbed energy of one hard metal by the other soft metal

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• Of these methods, indentation test is employed the most. For this type of test, hardness is defined as the resistance of a material to permanent indentation.

• The general principle of the indentation test:

- A hard indenter (a small sphere, pyramid or cone) is pressed into the surface of the material to be tested under a specific load for a definite intervall of time,

- The size or the depth of the indentation is measured.

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Brinell-indenters

Rockcell Ball indenters

Rockcell Diamond indenters

2.5 mm carbide

ball indenter

5 mm carbide ball

indenter

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• Hardness test is one of the prominent tests in determining the properties of materials.

• Because, a reasonable relationship exists between the hardness and other physical properties of a material.

• For example, hardness and the tensile test both measure the resistance of a material to plastic deformation.Therefore a correlation exists between the hardness and tensile strength of the steels :

St = 0.33 x BHN

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• However, hardness is not a fundamental property of a material.

• It depends upon the particular treatment to which the material has been subjected.

• Hardness values are actually arbitrary and there are no absolute standards of hardness.

• The hardness tests are also popular in the sense that they are easy, simple and relatively non-destructive tests (compared to tensile, bending and torsion test)

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• The indentation type hardness testers are classified into two main categories based on the level of the destruction of the surface of the specimen.

The Most Commonly Used Indentation Hardness Tests Are :

1-Macro Hardness Testers Loads > 1 kg

2-Micro Hardness TestersLoads < 1 kg

BRINELL H.T.

ROCKWELL H.T.

VICKERS H.T

VICKERS DIAMONDPYRAMID H.T.

KNOOP DIAMOND H.T.

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Micro hardness test

• The size of indentation produced is so small that a powerfull microscope is required to

measure the size of indentation.

• This method is used for testing;

- very thin materials (down to 0.01 mm),

- extremely small parts,

- thin superficially hardened parts,

- plated surfaced and,

- In research applications.

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• In indentation type of testers, the shape and size of the indenter and the magnitude of the load applied are selected in accordance with the :

– Purpose of the test

– The structural properties of the material being tested

– The state of its surface and

– The size of the part or specimen

• All of which shall be dealt with in appropriate places

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• The ADVANTAGES of The Hardness Measurements can be summarized as :

1) The hardness of a Ductile Material:

Determined by the indentation test, is related with its other mechanical properties (in particular with the ultimate tensile strength).

The Sut of most steels (except for those having austenitic or mantensitic structure) and many non-ferrous alloys can be assessed quickly by hardness measurements.

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Though the Brittle Materials undergo plastic deformation in indentation tests, a correlation between hardness and Su

is not observed for brittle materials.

A limited exception is the gray iron for which a qualitative relationship exists. A high indentation hardness is usually an indication of high ultimate compressive strength (Suc) for this material (gray iron).

– Hardness, as determined by indentation tests, is also shown to be correlated with the „fatigue strength‟

of some metals such as annealed steels, copper andduraluminum.

– These correlations, both for Su and fatigue strengths, apply only over a range of materials on which tests have previously been made. Therefore, users should refrain from making over generalization.

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2-Hardness tests are substantially simplier than other mechanical tests.

Specially prepared specimens are not needed. The machine parts can usually be tested directly. Though depends on the size of the part and type of test, the parts tested are not damaged. Furthermore, hardness tests can be made quickly, taking about in the order of seconds to few minutes

3-Similar materials may be graded according to hardness, thus making specifications for a certain type of application clear-cut. Likewise, the quality level of materials or products may be controlled effectively by hardness tests.

4- Hardness can be measured on parts of small thickness and in very thin layers, or over microscopic sections.

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TYPES OF HARDNESS TESTS

INDENTATION TESTS

MACRO

MICRO

7-SHORE SCLERSCOPE

6-SCRATCH 9-ULTRASONIC

10-HOT HARDNESS

5-KNOOP DIAMOND

4-VICKERS DIAMOND

3-VICKERS

2-ROCKWELL

1-BRINELL

8-DUROMETER

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THE BRINELL HARDNESS TEST• The Brinell methods consists of indenting the metal (usually)

with a 10 mm diameter steel ball subjected to a load of 3000 kg.

• For soft materials the load is reduced to 1500 or 500 kg, as may be required to avoid too deep an indentation

• In Brinell Test, the BHN (Brinell Hardness Number) of nearly all materials is influenced by the

– The magnitude of the indenting load,

– Diameter of the ball indenter and

– The elastic characteristics of the indenter ball.

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Different ball diameter and load values can be used for the Brinell Test as long as the ratio is a constant value where;

F = indenting load

D = diameter of the ball

For example :

2 2 2

3000 kg 187.5 kg 750 kg30

10 mm 2.5 mm 5 mm

31 2

2 2 2

1 2 3

FF F= = = constant

D D D

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• In general, a ball of 10 mm diameter is used with the standard loads of 3000, 1500 and 500 kg. However, there are also commercial Brinell hardness testers which employ other test loads and use smaller ball indenters.

• The Brinell Hardness Number is calculated by:

F : applied load, kg

D : Diameter of the ball indenter, mm

d : mean diameter of impression, mm

2 2

FBHN=

DD D d

2

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• For a reliable Brinell hardness test result the following must be observed during testing and the evolution of the results.

1-It is essential that the load is not applied too rapidly for this will add an extra load to the nominal load, resulting from the inertia of the load application system.

An additional error would also result from allowing insufficient (not enough) time for plastic flow to take place. The rate of loading should be uniform and should not exceeed 500 kg/sec. Many of the testers are equipped with dash pots to ensure the load is applied slowly.

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2-Measurement of the impression should be accurate enough. The direct measurement of the impression diameter has to be done down to 0.1 mm, and the estimation of diameter to 0.02 mm

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3-Thickness: of the test piece must be such that no bulge or other marks should appear on the other side. The thickness (t) must be at least ten times the depth (h) of indentation

4-Spacing of indentation: The distance of the center of indentation from the edge of the specimen or edge of another indentation should be at least 2.5 times the diameter of indentation.

distance >2.5 d

t>10 OR t >10h

h

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5-Radius of curvature : When indentations are produced on a curved surface, the minimum radius of curvature of the surface should not be less than 25 mm for a 10 mm ball. The diameter of indentation shall be taken as the mean of the two principal diameters.

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5-Radius of curvature : When indentations are produced on a curved surface, the minimum radius of curvature of the surface should not be less than 25 mm for a 10 mm ball. The diameter of indentation shall be taken as the mean of the two principal diameters.

6-Selection of test load : The load in the standard Brinell test is 3000, 1500, or 500 kg. The load should be seleceted to keep the ratio of the indentation to the diameter of the ball (d/D), greater than 0.25 and less than 0.60.

The lower limit is dictated by the diameter of the indentation. If the diameter is very small, the edges of the impression are not defined well enough. For a ratio greater than 0.60, the test becomes supersensitive.

d0.25 < < 0.60

D

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• To meet these requirements, the following load ranges are suggested for materials of different hardness for 10 mm ball:

– 3000 kg BHN 96 to 600 ( = 30)

– 1500 kg BHN 48 to 300 ( = 15)

– 500 kg BHN 16 to 100 ( = 5)

Approximate HB F/D^2 ratio Represantative Material

1 Lead, tin and tin alloys

30Above 100

10200 to 300

100 to 15

4 -- 6

Alluminium5

TABLE 5.1 RECOMMENDED F/D^2 RATIO

Cu & Cu alloys and

alluminium alloys

Steels and Cast iron

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STANDARDS

• In order to be able to standardize the results of such tests, standart methods of test has been issued by various institutions, sucs as TS139 of TSE and E10-73 of ASTM (American Society for Testing and Materials).

• According to these standarts, the deviation in the diameter of the steel ball of 10 mm is limited to 0.005 mm

(10 mm – 0.005 mm OR 10 mm + 0.005 mm)

In any diameter, the use of steel ball for testing is limited to materials which have a maximum hardness of 450 HB.

For harder materials it is essential that a carbide ball be used. However, under all circumstances, the Brinell method of hardness testing is limited up to about 630 HB.

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• If the Brinell hardness number symbol HB is used without any suffix numbers.

• HB denotes a hardness number obtained using a 10 mm ball and a test load of 3000 kg, and with the duration of loading 10-15 seconds. e.g. 350 HB.

• On the other hand;

75 HB/10 500/30: indicates a Brinell hardness of 75 measured with a ball of 10 mm diameter and a load of 500 kg applied for 30 seconds.

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Portable Combined Hardness Tester

MET-UD measurements of hardness

of metals and alloys : Rockwell

(HRC), Brinell (HB), Vickers (HV) and

Shore (HSD).

THE BRINELL HARDNESS TESTERS

THE ROCKWELL TEST• It is different from Brinell Hardness testing method.• The Rockwell test consists of measuring the

additional depth to which a steel ball or “Brale” diamond penetrator is forced by a heavy (major) load beyond the depth of previously applied light (minor) load.

•The two superimposed impressions is a special design feature of the tester, aimed at elimination of errors in measurement due to surface imperfections or distortions around the periphery of the indentation.

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• As the result of application of the minor load, an initial identation of depth is made on the specimen. The depth of this impression serves also as the datum line, (for the dial gauge is “SET” to zero) before the major load is applied.

• Since the major load is applied without removing the minor load the penetrator is forced beyond the depth of previously applied load by the depth

δm

δM

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B 1/16'' Bilya 100 kg Kırmızı

C Elmas Koni 150 kg Siyah

A Elmas Koni 60 kg Siyah

D Elmas Koni 100 kg Siyah

E 1/8'' bilya 100 kg Kırmızı

F 1/16'' bilya 60 kg Kırmızı

G 1/16'' bilya 150 kg Kırmızı

H 1/8'' bilya 60 kg Kırmızı

K 1/8'' bilya 150 kg Kırmızı

L 1/4'' bilya 60 kg Kırmızı

M 1/4'' bilya 100 kg Kırmızı

P 1/4'' bilya 150 kg Kırmızı

R 1/2'' bilya 60 kg Kırmızı

STANDARD ROCKWELL CİHAZI ÖLÇME TABLOSU

SEMBOL DALICI AĞIRLIK (KG) RENK

SEMBOL DALICI UÇ AĞIRLIK (KG)

15 N Elmas Koni 15 Kg

30 N Elmas Koni 30 Kg

45 N Elmas Koni 45 Kg

15 T 1/16'' Bilya 15 Kg

30 T 1/16'' Bilya 30 Kg

45 T 1/16'' Bilya 45 Kg

15 W 1/8'' Bilya 15 Kg

30 W 1/8'' Bilya 30 Kg

45 W 1/8'' Bilya 45 Kg

15 W 1/4'' Bilya 15 Kg

30 X 1/4'' Bilya 30 Kg

45 X 1/4'' Bilya 45 Kg

15 Y 1/2'' Bilya 15 Kg

30 Y 1/2'' Bilya 30 Kg

45 Y 1/2'' Bilya 45 Kg

YÜZEYSEL ROCKWELL CİHAZI ÖLÇME TABLOSU

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• After the major load is applied and removed, reading is taken while the minor load is still in position. The scale letter and Rockwell number indicated by the large pointer is the hardness of the specimen.

• Removal of the additional load allows a partial recovery, so reducing the depth of penetration. The permanent increase in depth of penetration, e, resulting from application and removal of the major load is used to deduce the Rockwell hardness number by means of equations.

Where

HR :Rockwell Hardness NumberK : a constant depending on the hardness scale

*** K is 100 units for full A,C,D (brale penetration) scales130 units for full steel ball penetrator scales

HR = K e

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The Penetrators for the Rockwell Test1- A diamond sphero-conical penetrator (Brale Penetrator), having an

included angle of 120 degree, with a spherical tip having a radius of 0.2 mm.

The diamond penetrator is used for hardened steels and cemented carbides.

2- and a steel ball penetrators

of different diameter:

1/16 , 1/8 , 1/4 and 1/2

The ball penetrators are used

when testing materials:

such as steel, copper alloys, aluminium, plastics and likewise.

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THE TYPES OF ROCKWELL TEST

• The Rockwell testing falls into two categories:

– Normal (regular) Rockwell Testing ( e.g. B and C scales)

– Rockwell Superficial Testing (e.g. 30N and 30T scales)

• In normal testing the minor load is always 10 kg. The major load, however can be either 60, 100 or 150 kg and we use 1/16‟‟, 1/8‟‟,1/4‟‟ and 1/2‟‟ ball penetrators.

• The superficial testing is used when the depth of penetrator has to be small. This is achieved by using a smaller minor load, smaller major load and a more sensitive depth measuring. In superficial testing, the minor load is always 3 kg. The major load, however, can be either 15, 30, or 45 kg.

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• In choosing the right scale (A,B,C,etc) there are four basic consideration.

1- Type of material (See Table 5.2)

2- Thickness of specimen (Table5.3)

3- Spacing of indentations

4- Scale limitations

Rockwell hardness numbers

(HR) will be lower on the

convex surfaces than on a flat

surface for the same material.

For a concave surface the

opposite is true.

•If D is larger than 25 mm the difference between actual and

measured values are negligible but for diameter of 25 mm or

smaller a correction factor,supplied by the manufacturer,must

be applied

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TABLE 5.2 TYPICAL APPLICATION OF SCALES

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TABLE 5.3 THICKNESS OF SPECIMENS

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THE ROCKWELL HARDNESS TESTERS

Rockwell hardness is never shown by numbers alone but by letters showing

the scale. For example:

•C60 or 60RC (TSE) means a Rockwell hardness value of 60 with scale C

and a major load of 150 kg forcing a brale diamond penetrator

•30N80 indicates a superficial hardness value of 80 with scale 30N obtained

under a load of 30kg forcing the brale penetrator

Vickers hardness test

The Vickers hardness test was developed in the early 1920s as an alternative method to measure the hardness of materials.

Vickers tests has two versions:

Macro Vickers (load over 1kg) and

Micro Vickers (load less than 1kg)

The Vickers macrotest is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and the indenter can be used for all materials irrespective of hardness.

However, Vickers macrohardness test is not as widely used as the Brinell or Rockwell test.

A diamond, in the form of a right pyramid wıth a square base and an angle of 1360 between opposıte faces is forced into the material under a load F.

The two diagonals, d1 and d2, of the indentation left in the surface of the materıal after removing the load are measured, and their arithhmetic mean d is calculated.

VHN=1.8544*F/d²

F=load ,kg

d=mean of diagonal impression, mm

θ=face angle of diamond=136o

d=(d1+d2) /2

VHN

Diagonal measuremet in Vickers Test:

Diagonals of the indentation produced are approximately seven times larger than the depth of indentation, especially with high hardness metals it provides better accuracy than that could be obtained with the Brinell or Rockwell.

This is because of the fact that, by virtue of its indentation, higher measurement accuracy can be obtained even if the indentation depth is small. So this makes the Vickers test especially suitable for hardness measurements in thin layers and very hard alloys.

Vickers hardness number is nearly independent of the test loads for homogeneous materials due to the fact that the ratio between the diagonals of the impression remains constant under different test loads above 5kg. At lower loads it may be load dependent.

Standart designation of Vickers hardness is usually given with load and duration values:

450VHN/30/20 means a 450 Vickers hardness value measured under a load of 30kg for a duration of 20 sec (similar to the Brinell)

Vickers hardness tests and Brinell test are similar to each other both in principle and also the hardness values.

Both calculate the hardness as the load/area of impression

While Brinell use a ball as indenter Vickers use a diamond with angle of 136degrees resembling a ball as seen in fig.5.9

Therefore numerical values of HB and VHN of the same test piece are so close to each other up to HB400

Advantages of Vickers Hardness Test Method

1. Soft, as well as hard metals may be tested.

2. Tests may be conducted in low-load and micro range as well

3. The vickers hardness –apart from the micro and low-load range is independent of the test-load applied (above 5kg).

4. The pyramidal impression damages the work-pieces only slightly.

The microhardness test is generally employed for testing;

small precision parts

surface layers

thin materials

small diameter wires

for investigation of small areas

hardness of constituents

hardness near edges such as in cutting tools,etc.

MICROHARDNESS TESTs

The term microhardness refers to static indentatin tests made with load up to 1 kg.

In such tests the load must be applied with great care for order of magnitude of the applied load is usually from 1 gram to 500 grams.

The indenter is;

either a 1360 diamond pyramid [vickers test] or

the Knoop diamond indenter.

In microhardness tests, it is very important that the surface being tested be lapped flat and be free from scratches.

Knoop indenter produces (nearly 3 times)larger diagonal than what Vickers indenter produces. Similarly depth of impression is nearly half that of the Vickers.

Therefore Vickers test is less sensitive to surface conditions than Knoop for the same test load, but more sensitive to errors in measuring the impression size

Microhardness test values are usually dependent on the test load since loads are rather low.

Knoop hardness value is also calculated on the principle of force divided by the area of impression similar to Brinell and Vickers tests.

In Vickers it was;

VHN=1.8544xF/d²

Now in Knoop it is

KHN=P/A=P/CL² or

KHN=14.230xP/L²

SHORE SCLEROSCOPE TEST

This test consists of dropping a diamond tipped hammer under its own weight, from a fixed height on to test speciment, and reading the rebound height.

The rebound of the hammer varies in proportion to the hardness of the metal: the higher the rebound, the harder the metal.

The Schleroscope hardness numbers are indicated by the following symbols (ASTM A448):

HSc XX or HSd XX where c and d refer to two different types of instruments used for the test.

For scale (model) C height and mass of the hammer are 251.2mm and 23 gr

For scale (model) D height and mass of the hammer are 17.9mm and 36 gr

Lots of different size and shapes of specimens can be tested with Scleroscope test method.

Small parts including thin sheets and strips and large parts like steel rolling rolls can be tested with Scleroscope test method.

Scleroscope is a portable test equipment and can be taken to the field or out of lab to test the part in place like heavy and large rolls of steel rolling.

For thin parts, minimum thickness of sheets in various categories are given on page 265

For a reliable Scleroscope test result, the degree of surface finish is important since small loads (masses) are used.

An excessively coarse finish will yield low and erratic readings. Harder is the test surface, better has to be the surface finish for a reliable and reproducible results.

Test should not be done more than once in the same spot since impact of the hammer cold works that spot and causes subsequent redings thereon to be high.

The indentations has to be spaced at least 0.5mm apart

DUROMETER

The durometer is a popular instrument for measuring the indentation hardness of the elastrometers and polymers.

This is one of the standardised ASTM procedure for testing rubber and plastic (ASTM D2240-68)

The operation is similar to rockwell test method. Durometer hardness is a measure of the depth of indentation.

The indenter is pressed against the material being tested by a calibrated spring and the indentation depth is measured by an indicating device

1) Model A for testing soft materials

2) Model D for hard materials

Both durometers are quite similar, differing primarily in the sharpness of indenters and the magnitude of the load applied to the indenter by the calibrated spring.

The load is applied by foot and the reading is taken within seconds of contact of load with the material.

HA=0-100 or HD= 0-100 are usual designations

The shore durometer has two models:

ULTRASONIC HARDNESS TEST

A magnetostrictive diamond-tipped rod vibrating at ultrasonic frequencies is brought into contact with the metal at a load of 3.3 kg.

The change in freqency of the tip due to hardness of the metal is read on a meter scale in terms of either Rockwell or Vickers numbers.

The shallowness of the indentation produced by the tester (less than 0.013 mm) makes it particularly suitable for testing.

The equipment is very portable but

Its use is resricted primarily to steel.»

HOT HARDNESS TESTING

At low temperatures, a correlation has been identified between hardness and tensile strength of many metals.

It is, however, not clear if this correlation exists at high temp’s too.

Therefore it is a wonder how the hardness of metal changes at elevated/high temperatures.

Hot hardness gives a good indication of the potential usefulness of a metal or alloy for high- temperature applications.

Some degree of the success has been obtained in correlating the hot-hardness with strength at elavated temperatures.

The points that have to be watched for hot-hardness testing are:

1) Hardness tester and the specimen both have to be at the same temperature.

2) A cold indenter will cause local cooling and thus produce an effect similar to quenching (of the specimen)

3) Tests have to be carried out in either vacuum or an inert gas otherwise the surface of the specimen will be oxidised.

While some of these effects will cause totally useless results some will cause appriceable errors in results.

Very special Rockwell hardness testers (models 6JR, 6JS & 6JT) are available for testing at temp’s up to 980 C.

SCRATCH HARDNESS TEST

The basic principle of the test is that a diamond is drawn under a vertical load steadily over a horizontal surface.

A special diamond whose shape can be adapted to the particular requirements is used. It may be conical as in Rockwell test or pyramidal as in Vickers test.

Indentation hardeness was closely related to the tensile strength of most metals

Likewise, the resistance of a material to wear is closely related to its peformance in a scratch test as above.

Duo to this relation the scratch test is of the technological interest.

This property of resistance to wear is measured by means of various scratch hardness tests.

Types of Scratch Test

The MOH’s scale catogorizes the materials on the basis of relative hardness.

Each new material is used to scratch the old materials and placed in a proper place on the scale.

1) A

2) B B scratch A

3) C C “ B

4) D D “ C etc

.

10)

A new material N is used as the diamond to scratch materals starting with the hardest material in the scale, D in this case

Note : This test is not objective test since the hardness of the two material can be so close that just by scratching them and observing the surface they can not be classified or scaled.

During the early 1800s, a German mineralogist named Friedrich Mohs devised a scale that tested mineral hardness, which means the resistance of a mineral to being scratched. This scale, which ran from 1 to 10, was named after Mohs, and is known as the Mohs Hardness Test.

Keep in mind that the Mohs scale does not indicate exact hardness, which depends upon the purity of the mineral sample and its degree of crystallinity.

While a reference set of the above minerals may be used to test the hardness of an unknown mineral, such a set is rarely used in actual practice; particularly because diamond is rather expensive to be used as an abrasive. Instead, common objects are used as tools to perform the hardness test:

The Mohs Hardness Test

Bierbaum Scratch Hardness is defined as the ratio of load to the square of measured width of the

scratch.

Bier.Sc.H = F / W²

in (kg/mm²)

Marten’s Scratch Hardness

is the value of the load (in kg)

required to produce a scratch of w=0.01mm.

Relative Scratch Hardness Number of materials can be

determined by comparing the scratches produced on the specimen materials against a reference scratch. Another non-objective test

CHOICE OF TESTING METHOD

The points or the considerations that play a role in the choice of the suitable hardness testing method are:

-The scope of the test (whether it is done to do a direct hardness measurement or to do an indirect strength or wear resistance measurement of the material)

-The degree of accuracy required (Vickers, Knoop or Rockwell)

-The rapidity with which the tests must be conducted (time).

-The specimen or part size (macro test or micro test or Schleroscope etc).

Shore Scleroscope is the fastest by all standartd means, to be followed by the Rockwell tester.

The Brinell or Vickers method of testing are comperatively slower even if these tester are equipped with micro-projectors.

The structure of the material effect the selection of the size of the indenter (hence type of the test)

For Heteregeneous structures;

with large inclusions of individual structural components (e.g. gray iron or nonferrous) hardness tests should be done with a large size indenter;

because the large size of indentation averages out local heterogeneties.

Homogeneous structure :requires to be done at smaller

areas with smaller indenters since test done at this point will be sufficiently representative of the bulk of the metal.

Ferrous Materials :Practically all ferrous meterials of suitable

thickness can be tested by the standart Brinell, Rockwell, Vickers and Shore schleroscope method.

However, the Brinell test gives better results with coarse grained materials because of the averaging provided by large 10 mm ball.

For hardened and tempered steel:Rockwell and Vickers

testing produce more accurate results. When material is thin Rockwell A scale or superfacial testing can be used.

Case Hardened Steels: the penetrator should not produce an

impression which distrubs this thin hardened layer.

The Rockwell tester is more advantageous in this case. The scale to be used depends upon the thickness of the case.

Likewise, Vickers tester can also be used effectively. Variation of hardness across the depth can only be achieved by microhardness test.

Steel Forging:Brinell test with 10mm ball and 3000kg load gives

better results. A good relationship exists between BHN and tensile strenght of the forgings.

Cast Iron: Cast iron is generaly tested by the standard Brinell test

because of the nonunıformity of the structure of CI.

Plastics: There are three important ASTM standarded procedures for

testing plastic.

1)DUROMETER

2)BARCOL IMPRESSOR

3)ROCKWELL TEST(R,L,M,E,K)

The relationship between their hardness and strength is not so determinate as with metals.

Elastomers: Rubber and rubber like materials are tested on

durometer.

Very Hard Materials are tested with diamond indenter or

penetrators and very high contact pressure are required to produce on impression of measurable size.The hardness of ceramics is usually determined according to Knoop scale.

Rockwell test is relatively insensitive to surface finish.

Brinell test can also be said not to be highly influenced by the surface scratches and roughness as long as the diameter of the impression could be defined accurately.

The Vickers and microhardness tester require reasonably better surface finish.

A General Rule is that lighter the test load, the higher the degree of surface finish required.

Hardness Conversion Chart