Unit 4

Virtually any metal or alloy that can be melted can be cast. The most common ferrous metals

include grey iron, ductile iron, malleable iron and steel. Alloys of iron and steel are used for high

performance applications, such as temperature, wear and corrosion resistance. The most common

non-ferrous metals include aluminum, copper, zinc and magnesium based alloys. The production

and application of ductile iron and aluminum castings are steadily increasing. Aluminum has

overtaken steel in terms of production by weight. The consumption of magnesium alloys is

rapidly increasing in automobile and other sectors, owing its high strength to weight ratio.

Important and emerging metal titanium is stronger than steel, but has found limited applications

owing to the difficulty in casting and machining. Table 1 lists the major metals in use today (by

weight) along with their main characteristics and typical applications.

Table 1: Major cast metals

Cast irons

Though ferrous alloys with more than 2.14 wt.% C are designated as cast irons, commercially

cast irons contain about 3.0-4.5% C along with some alloying additions. Alloys with this carbon

content melt at lower temperatures than steels i.e. they are responsive to casting. Hence casting is

the most used fabrication technique for these alloys. Hard and brittle constituent presented in

these alloys, cementite is a meta-stable phase, and can readily decompose to form α-ferrite and

graphite. In this way disadvantages of brittle phase can easily be overcome. Tendency of cast

irons to form graphite is usually controlled by their composition and cooling rate. Based on the

form of carbon present, cast irons are categorized as gray, white, nodular and malleable cast

irons.

Gray cast iron: These alloys consists carbon in form graphite flakes, which are surrounded by

either ferrite or pearlite. Because of presence of graphite, fractured surface of these alloys look

grayish, and so is the name for them. Alloying addition of Si (1-3wt.%) is responsible for

decomposition of cementite, and also high fluidity. Thus castings of intricate shapes can be

easily made. Due to graphite flakes, gray cast irons are weak and brittle. However they possess

good damping properties, and thus typical applications include: base structures, bed for heavy

machines, etc. they also show high resistance to wear.

White cast iron: When Si content is low (< 1%) in combination with faster cooling rates, there

is no time left for cementite to get decomposed, thus most of the brittle cementite retains.

Because of presence of cementite, fractured surface appear white, hence the name. They are very

brittle and extremely difficult to machine. Hence their use is limited to wear resistant

applications such as rollers in rolling mills. Usually white cast iron is heat treated to produce

malleable iron.

Nodular (or ductile) cast iron: Alloying additions are of prime importance in producing these

materials. Small additions of Mg / Ce to the gray cast iron melt before casting can result in

graphite to form nodules or sphere-like particles. Matrix surrounding these particles can be either

ferrite or pearlite depending on the heat treatment. These are stronger and ductile than gray cast

irons. Typical applications include: pump bodies, crank shafts, automotive components, etc.

Malleable cast iron: These formed after heat treating white cast iron. Heat treatments involve

heating the material up to 800-900ْC, and keep it for long hours, before cooling it to room

temperature. High temperature incubation causes cementite to decompose and form ferrite and

graphite. Thus these materials are stronger with appreciable amount of ductility. Typical

applications include: railroad, connecting rods, marine and other heavy-duty services.

Melting Practices

Melting is an equally important parameter for obtaining a quality castings. A number of furnaces

can be used for melting the metal, to be used, to make a metal casting. The choice of furnace

depends on the type of metal to be melted. Some of the furnaces used in metal casting are as

following:

Crucible furnaces

Cupola

Induction furnace

Reverberatory furnace

Crucible Furnace.

Crucible furnaces are small capacity typically used for small melting applications. Crucible

furnace is suitable for the batch type foundries where the metal requirement is intermittent. The

metal is placed in a crucible which is made of clay and graphite. The energy is applied indirectly

to the metal by heating the crucible by coke, oil or gas.The heating of crucible is done by coke,

oil or gas. .

Coke-Fired Furnace (Figure 1).

Primarily used for non-ferrous metals

Furnace is of a cylindrical shape

Also known as pit furnace

Preparation involves: first to make a deep bed of coke in the furnace

Burn the coke till it attains the state of maximum combustion

Insert the crucible in the coke bed

Remove the crucible when the melt reaches to desired temperature

Figure 1: Coke Fired Crucible Furnace

Oil-Fired Furnace.

Primarily used for non-ferrous metals

Furnace is of a cylindrical shape

Advantages include: no wastage of fuel

Less contamination of the metal

Absorption of water vapor is least as the metal melts inside the closed metallic furnace

Cupola

Cupola furnaces are tall, cylindrical furnaces used to melt iron and ferrous alloys in foundry

operations. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the

furnace from the top. A schematic diagram of a cupola is shown in Figure 2. This diagram of a

cupola illustrates the furnace's cylindrical shaft lined with refractory and the alternating layers of

coke and metal scrap. The molten metal flows out of a spout at the bottom of the cupola. .

Description of Cupola

The cupola consists of a vertical cylindrical steel sheet and lined inside with acid

refractory bricks. The lining is generally thicker in the lower portion of the cupola as

the temperature are higher than in upper portion

There is a charging door through which coke, pig iron, steel scrap and flux is charged

The blast is blown through the tuyeres

These tuyeres are arranged in one or more row around the periphery of cupola

Hot gases which ascends from the bottom (combustion zone) preheats the iron in the

preheating zone

Cupolas are provided with a drop bottom door through which debris, consisting of coke,

slag etc. can be discharged at the end of the melt

A slag hole is provided to remove the slag from the melt

Through the tap hole molten metal is poured into the ladle

At the top conical cap called the spark arrest is provided to prevent the spark emerging to

outside

Operation of Cupola

The cupola is charged with wood at the bottom. On the top of the wood a bed of coke is built.

Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from

the top. The purpose of adding flux is to eliminate the impurities and to protect the metal from

oxidation. Air blast is opened for the complete combustion of coke. When sufficient metal has

been melted that slag hole is first opened to remove the slag. Tap hole is then opened to collect

the metal in the ladle.

Figure 2: Schematic of a Cupola

Reverberatory furnace

A furnace or kiln in which the material under treatment is heated indirectly by means of a flame

deflected downward from the roof. Reverberatory furnaces are used in opper, tin, and nickel

production, in the production of certain concretes and cements, and in aluminum. Reverberatory

furnaces heat the metal to melting temperatures with direct fired wall-mounted burners. The

primary mode of heat transfer is through radiation from the refractory brick walls to the metal,

but convective heat transfer also provides additional heating from the burner to the metal. The

advantages provided by reverberatory melters is the high volume processing rate, and low

operating and maintenance costs. The disadvantages of the reverberatory melters are the high

metal oxidation rates, low efficiencies, and large floor space requirements. A schematic of

Reverberatory furnace is shown in Figure 3

Figure 3: Schematic of a Reverberatory Furnace

Induction furnace

Induction heating is a heating method. The heating by the induction method occurs when an

electrically conductive material is placed in a varying magnetic field. Induction heating is a rapid

form of heating in which a current is induced directly into the part being heated. Induction

heating is a non-contact form of heating.

The heating system in an induction furnace includes:

1. Induction heating power supply,

2. Induction heating coil,

3. Water-cooling source, which cools the coil and several internal components inside the

power supply.

The induction heating power supply sends alternating current through the induction coil, which

generates a magnetic field. Induction furnaces work on the principle of a transformer. An

alternative electromagnetic field induces eddy currents in the metal which converts the electric

energy to heat without any physical contact between the induction coil and the work piece. A

schematic diagram of induction furnace is shown in Figure 4. The furnace contains a crucible

surrounded by a water cooled copper coil. The coil is called primary coil to which a high

frequency current is supplied. By induction secondary currents, called eddy currents are

produced in the crucible. High temperature can be obtained by this method. Induction furnaces

are of two types: cored furnace and coreless furnace. Cored furnaces are used almost exclusively

as holding furnaces. In cored furnace the electromagnetic field heats the metal between two coils.

Coreless furnaces heat the metal via an external primary coil.

Figure 4: Schematic of a Induction Furnace

Advantages of Induction Furnace

Induction heating is a clean form of heating

High rate of melting or high melting efficiency

Alloyed steels can be melted without any loss of alloying elements

Controllable and localized heating

Disadvantages of Induction Furnace

High capital cost of the equipment

High operating cost

Unit 5

Casting Defects:

Some defects are common to any and all process. These defects are illustrated in Fig 1 and

briefly described in the following: There are numerous opportunities in the casting operation for

different defects to appear in the cast product. Some of them are common to all casting

processes:

Misruns: Casting solidifies before completely fill the mold. Reasons are low pouring

temperature, slow pouring or thin cross section of casting.

Cold shut: Two portions flow together but without fusion between them. Causes are similar to

those of a misrun.

Cold shots: When splattering occurs during pouring, solid globules of metal are entrapped in the

casting. Proper gating system designs could avoid this defect.

Shrinkage cavity: Voids resulting from shrinkage. The problem can often be solved by proper

riser design but may require some changes in the part design as well.

Microporosity: Network of small voids distributed throughout the casting. The defect occurs

more often in alloys, because of the manner they solidify.

Hot tearing: Cracks caused by low mold collapsibility.They occur when the material is

restrained from contraction during solidification. A proper mold design can solve the problem.

Some defects are typical only for some particular casting processes, for instance, many defects

occur in sand casting as a result of interaction between the sand mold and the molten metal.

Defect found primarily in sand casting are gas cavities, rough surface areas, shift of the two

halves of the mold, or shift of the core, etc.

a) Misruns: A Misruns is a casting that has solidified before completely filling the mold cavity.

Typical causes include

1) Fluidity of the molten metal is insufficient,

2) Pouring Temperature is too low,

3) Pouring is done too slowly and/or

4) Cross section of the mold cavity is too thin.

b) Cold Shut: A cold shut occurs when two portion of the metal flow together, but there is lack

of fusion between them due to premature freezing, Its causes are similar to those of a Misruns.

Fig. 1 Some common defects in castings

c) Cold Shots: When splattering occurs during pouring, solid globules of the metal are formed

that become entrapped in the casting. Poring procedures and gating system designs that avoid

splattering can prevent these defects.

d) Shrinkage Cavity: These defects area depression in the surface or an internal void in the

casting caused by solidification shrinkage that restricts the amount of the molten metal available

in the last region to freeze. It often occurs near the top of the casting in which case it is referred

to as a pipe. The problem can often be solved by proper riser design.

e) Microporosity: This refers to a network of a small voids distributed throughout the casting

caused by localized solidification shrinkage of the final molten metal in the dendritic structure.

The defect is usually associated with alloys, because of the protracted manner in which freezing

occurs in these metals.

f) Hot Tearing: This defect, also called hot cracking, occurs when the casting is restrained or

early stages of cooling after solidification. The defect is manifested as a separation of the metal

(hence the terms tearing or cracking) at a point of high tensile stress caused by metal’s inability

to shrink naturally. In sand casting and other expandable mold processes, compounding the mold

to be collapsible prevents it. In permanent mold processes, removing the part from the mold

immediately after freezing reduces hot tearing.

Some defects are related to the use of sand molds and therefore they occur only in sand castings.

To a lesser degree, other expandable mold processes are also susceptible to these problems.

Defects found primarily in sand castings are shown in Fig 2 and describe here:

a) Sand Blow: This defect consists of a balloon-shaped gas cavity caused by release of mold

gases during pouring. It occurs at or below the casting surface near the top of the casting. Low

permeability, poor venting and high moisture content of the sand mold are the usual causes.

Fig. 2 Other defects found primarily in sand castings

b) Pinholes: A defect similar to a sand blow involves the formation of many small gas cavities at

or slightly below the surface of the casting.

c) Sand Wash: A wash is an irregularity in the surface of the casting that results from erosion of

the sand mold during pouring. The contour of the erosion is imprinted into surface of the final

cast part.

d) Scabs: This is a rough area of the casting due to encrustations of sand and metal. It is caused

by portions of the mold surface flaking off during solidification and becoming embedded in the

casting surface.

e) Penetration: When the fluidity of the liquid metal is high, it may penetrate into the sand mold

or sand core after freezing, the surface of the casting consists of a mixture of sand grins and

metal. Harder packing of the sand molds helps to alleviate this condition.

f) Mold Shift: This is manifested as a step in the cast product at the parting line caused by

sidewise displacement of the cope with respect to the drag.

g) Core Shift: A similar movement can happen with the core but the displacement is usually

vertical. Core shift and mold shift are caused by buoyancy of the molten metal.

h) Mold Crack: If mold strength is insufficient a crack may develop in to which liquid metal can

seep to form a fin on the final casting.

Issues in Casting

• Shrinkage

• Porosity

• Piping

• Microstructure

Shrinkage

•Can amount to 5-10% by volume

•Gray cast iron expands upon solidification due to phase changes

•Need to design part and mold to take this amount into consideration

Porosity

•Types

–due to gases –smooth bubbles

–due to shrinkage –rough voids

Porosity due to Gases

•Smooth bubbles

–result from entrapped gases

–solubility in liquid is high, in solid is low, so gas is rejected during cooling

Remedies for Gas Bubbles

•Control atmosphere

–vacuum

–gases with less solubility

•Proper venting to let gases out

•Proper design of runners and gates to avoid turbulence

Porosity due to Shrinkage

•Rough bubbles -voids

•Stages

–cooling liquid

–rejects latent heat at melting point

•alloys become slushy -liquid and solid co-exist

–cooling solid

Differential Cooling •Transition between thicker and thinner sections can lead to porosity

Porosity / Shrinkage Solutions

•Risers allow molten metal to flow into mold to make up for shrinkage

•Design flow so no part freezes early –large channels

•“Flexible” molds

–allow metal to shrink, not hold metal

Heating or cooling certain areas to maintain uniform cooling (thermit or chills)

•Uniform part thickness

–leads to uniform cooling, less residual stress

Pipe Defect

•Due to shrinkage giving rise to a funnel-like cavity

•Solutions

–insulate top (glass wool)

–heat top (exothermic mixture -thermit)

Microstructure

•Post-treatment may be necessary to get desired properties -grain structure

–annealing

–tempering

–cold working

Inspections of Casting

Foundry inspection procedures include:

a. Visual Inspection to detect obvious defects, such as Misruns, cold shut and severe surface

flaws;

b. Dimensional measurements to ensure that tolerances have been met;

c. Metallurgical, chemical, physical and other tests concerned with the inherent quality of the

cast metal. Tests in category 3 include

1) Pressure testing to locate leaks in the casting

2) Radiographic methods, magnetic particle tests, the use of fluorescent penetrants and

supersonic testing to detect either surface or internal defects in the casting;

3) Mechanical testing to determine properties such as tensile strength and hardness. If defects are

discovered but are not too serious, it is often possible to save the casting by welding, grinding or

other salvage methods to which the customer has agreed.

Visual inspection

Visible defects that can be detected provide a means for discovering errors in the pattern

equipment or in the molding and casting process. Visual inspection may prove inadequate only

in the detection of sub surface or internal defects.

Dimensional inspection

Dimensional inspection is one of the important inspectionsfor casting. When precision casting is

required, we make some samples for inspection the tolerance, shape size and also measure the

profile of the cast. This dimensional inspection of casting may be conducted by various methods:

• Standard measuring instruments to check the size of the cast.

• Contour gauges for the checking of profile, curves and shapes

• Coordinate measuring and Marking Machine

• Special fixtures

X-Ray Radiography

In all the foundries the flaw detection test are performed in the casting where the defects are not

visible. This flaw detection test is usually performed for internal defects, surface defects etc.

These tests are valuable not only in detecting but even in locating the casting defects present

inthe interior of the casting. Radiography is one of the important flaw detection test for casting.

The radiation used in radiography testing is a higher energy (shorter wavelength) version of the

electromagnetic waves that we see as visible light. The radiation can come from an X-ray

generator or a radioactive source.

Magnetic particle inspection

This test is used to reveal the location of cracks that extend to the surface of iron or steel

castings, which are magnetic nature. The casting is first magnetized and then iron particles are

sprinkled all over the path of the magnetic field. The particles align themselves in the direction

of the lines of force. A discontinuity in the casting causes the lines of the force to bypass the

discontinuity and to concentrate around the extremities of the defect.

Fluorescent dye-penetration test

This method is very simple and applied for all cast metals. It entails applying a thin penetration

oil-base dye to the surface of the casting and allowing it to stand for some time so that the oil

passes into the cracks by means of capillary action. The oil is then thoroughly wiped and cleaned

from the surface. To detect the defects, the casting is pained with a coat of whitewash or

powdered with tale and then viewed under ultraviolet light. The oil being fluorescent in nature,

can be easily detect under this light, and thus the defects are easily revealed.

Ultrasonic Testing

Ultrasonic testing used for detecting internal voids in casting is based on the principle of

reflection of high frequency sound waves. If the surface under test contains some defect, the high

frequency sound waves when emitted through the section of the casting, will be reflected from

the surface of defect and return in a shorter period of time.

The advantage this method of testing over other methods is that the defect, even if in the interior,

is not only detected and located accurately, but its dimension can also be quickly measured

without in any damaging or destroying the casting.

Fracture test

Fracture test is done by examining a fracture surface of the casting. it is possible to observe

coarse graphite or chilled portion and also shrinkage cavity, pin hole etc. The apparent soundness

of the casting can thus be judged by seeing the fracture.

Macro-etching test (macroscopic examination)

The macroscopic inspection is widely used as a routine control test in steel production because it

affords a convenient and effective means of determining internal defects in the metal. Macro-

etching may reveal one of the following conditions:

• Crystalline heterogeneity, depending on solidification

• Chemical heterogeneity, depending on the impurities present or localized segregation and

• Mechanical heterogeneity, depending on strain introduced on the metal, if any.

Sulphur Print test

Sulphur may exist in iron or steel in one of two forms; either as iron sulphide or manganese

sulphide. The distribution of sulphur inclusions can easily examined by this test.

Microscopic Examination

Microscopic examination can enable the study of the microstructure of the metal alloy,

elucidating its composition, the type and nature of any treatment given to it, and its mechanical

properties. In the case of cast metals, particularly steels, cast iron, malleable iron, and SG iron,

microstructure examination is essential for assessing metallurgical structure and composition.

Composition analysis can also be done using microscopic inspection. Distribution of phase can

be observed by metallographic sample preparation of cast product. Grain size and distribution,

grain boundary area can be observed by this procedure. Distribution of nonmetallic inclusion can

also be found from this process of inspection.

Chill Test

Chill test offers a convenient means for an approximate evaluation of the graphitizing tendency

of the iron produced and forms an important and quick shop floor test for ascertaining whether

this iron will be of the class desired. In chill test, accelerated cooling rate is introduced to induce

the formation of a chilled specimen of appropriate dimension. It is then broken by striking with a

hammer in such a manner that the fracture is straight and midway of its length. The depth of chill

obtained on the test piece is affected by the carbon and silicon present and it can therefore be

related to the carbon equivalent, whose value in turn determines the grade of iron.

Application of Robots in Foundry

Like many other industries, foundries are constantly on the lookout for new ways to boost their

productivity, cut costs and increase quality. But once the decision for ABB’s leading high-

performance robot technology has been made, there is no need to look any further: lower

production costs and scrap rates, increased up-time and consistent, superior quality are the

compelling benefits with ABB robots.

Following the automotive trend with the massive shift from iron to aluminium and other light

alloys –for both ecological and economic reasons –foundries are investing heavily in new

machinery. With the aluminium content in vehicles rising by 5.5 % each year, some 12 million

tonnes of aluminium will be cast in 2010. To handle this workload, around 70 new foundries will

have to be built annually. At ABB we are moving right along with this trend, providing the new

businesses with proven robot-based solutions including progressive production cell technology.

Experienced solutions for downstream aluminium our commitment to foundry automation is

based on almost 40 years of experience and covers every aspect of the production process –all

the way from smelter to the finished automotive part. ABB’s robots are always there to gain

efficiency along the entire value chain. This synergised-system concept based on specific robots

designed for the need of each process offers many advantages like enormous flexibility, high

levels of reliability, and consistent capacity utilisation all along the foundry line.

Even for a robot, a foundry is not a workplace like any other. The exceptionally tough work

environment demands appropriate protection –the more comprehensive the better. ABB offers an

extensive range of foundry-adapted robots with payloads up to 650 kg, by specialized high

function controllers and a wide range of software products.

IRC5: the modularised way to success ABB’s innovative IRC5 robot control system sets new

standards with its modularised concept, a human-engineered Flex Pendant programming unit

with special foundry applications interface and fully synchronous, simultaneous control of up to

four robots using Multi Move. The patented True Move and Quick Move functions assure

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Robot Studio: for genuine offline programming Cost-efficient offline programming is the best

way to maximise return on investment in robotics. ABB’s simulation and offline programming

software, Robot Studio, allows robot programming to be carried out in the office without

shutting down production. It also enables robot programs to be prepared in advance, increasing

overall productivity.

Teach Saver: more than a time-saver for a long time, the elaborate programming required was

the biggest impediment to using robots to clean cast components. ABB’s Teach Saver software

package reduces this process by up to 90%. More: using a virtual offline robot cell also ensures

significantly greater accuracy than with classical teaching.

Completely sealed, equipped with a two-component high-resistance enamel surface and IP67

certified, ABB’s Foundry Plus range of fully foundry adapted industrial robots can take more

than just the heat. These robots are ready to meet the challenges of spits, sands and lubricants of

modern high-performance foundries on a daily basis.

Fig. 3 Typical Robots used in Foundry

Foundries are a very complex environment to work in. The automation of specialized tasks such

as investment casting, ingot handling or forging requires detailed process know-how and the

right hardware to handle castings and cores with power and precision. This is where ABB’s

robots enter the arena.

Dipping wax trees in water based slurry to continuously build the ceramic shell with special

sand, is a process in investment casting that is often robotised. With a reach of up to 3.5 m and a

handling capacity of 150 kg, ABB’s IRB 7600 is the perfect alternative to get the job done.

Furthermore, robots are frequently used for post processing applications such as grinding and

polishing.

Ingot handling is an application found in casting shops where aluminium ingots are produced.

When it comes to handling, the IRB 660 four-axis robot is the perfect tool for the task: it comes

equipped with a special purpose pneumatic gripper for handling the solidified aluminium ingots

and features a payload of up to 250 kg. For even heavier handling, the IRB 7600 is the perfect

choice with a capacity of up to650 kg.

Tending a zinc die-casting machine requires total reliability and efficiency in a harsh

environment. The IRB 140 Foundry Plus robot featuring full IP67-classification is ready to take

this challenge. Thanks to its compact dimensions, it is even suited for portable robot cells that

can be moved away for tool changes and servicing of the die-casting machine.

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