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Course No: Experiment No:

Name of the experiment:

STRUCTURAL STUDY OF MILD STEEL AFTER HEAT TREATMENT

Date of Performance: Name:

Reg. No:

Date of Submission: Session:

Group:

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NAME OF THE EXPERIMENT:

STRUCTURAL STUDY OF MILD STEEL

AFTER HEAT TREATMENT

OBJECTS OF STUDY

Getting introduced to HEAT TREATMENT.

To study structures of mild steel after heat treatment.

Learning different microstructures that are produced after heat treatment.

To produce a flat, scratch-free, mirror like surface.

INTRODUCTION

The definition of heat treatment of steel given in the handbook of metal is as

A combination of heating and cooling operations timed and applied to a metal or

alloy in the solid state in a way that will produce desired properties.

All heat treatment process involves the transformation or decomposition of austenite.

1.

Metallography or microscopy consists of the microscopic study of the structuralcharacteristics of a metal or an alloy. The microscope is by far the most important tool of

the metallurgist from both the scientific and technical standpoints. It is possible to

determine grain size and the size, shape, and distribution of various phases and inclusionswhich have a great effect on the mechanical properties of the metal. The microstructure

will reveal the mechanical and thermal treatment of the metal, and it may be possible to

predict its expected behavior under a given set of conditions.

The procedures and steps of heat treatment will be discussed briefly in the report:

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PROCEDURE

HEAT TREATMENT PROCESS:

Fig. Exploded view of electric furnace used for heat-treating.

Electric Furnaces

Electric furnaces with a controlled atmosphere are frequently used for heat-treating on

repair ships and tenders. Quite often two such units are used on the same ship. One is arelatively low-temperature furnace used for preheating or tempering, and the other is ahigher temperature furnace used for hardening. Both types are equipped with control

devices for regulating temperature. The high-temperature furnace may also be equipped

with rheostats used to increase the rate of heating. An exploded view of a slightly

The outer casing of the furnace is usually made of sheet steel. Just inside the casing is alayer of insulating material, such as mica, spun glass, or asbestos. Inside this insulating

material is a lining of refractory material, such as firebrick and insulating brick. The

refractory lining insulates the furnace, helps maintain the required high temperatures, andsupports the heating elements and the hearth plate.

Hearth plates are placed on the bottom of the heating chamber to support the pieces being

heated. Hearth plates must withstand high temperatures without sagging or scaling. Theyare often made of a special nickel-chromium, heat-resistant alloy. If the furnace isdesigned for the heat treatment of high-speed steels, the hearth plate may be made of a

carbon and silicon. Grids, usually made of iron-chromium-nickel alloy, keep heavy or

long sections of material off the hearth plate. The use of grids ensures more uniformheating of the material and tends to prevent warping.

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COOLING EQUIPMENT

The rate of cooling is controlled by selecting an appropriate cooling medium and cooling

procedure. The equipment required for cooling includes the substances used for cooling,

a tank or other container to hold the cooling medium, and various kinds of tongs, baskets,

and other devices for handling and holding the work. The rate at which a metal coolsdepends upon a number of factors. The size, shape, temperature, and composition of the

material and the temperature and composition of the cooling medium are the major

Factors involved. The rate at which a cooling medium can absorb heat is also greatlyinfluenced by circulation. When the cooling medium is agitated, the rate of cooling is

much faster than when the cooling medium is not in motion. The volume of the cooling

medium is also important. As the metal cools, the cooling medium absorbs heat. If thevolume is insufficient, the cooling medium will become too hot to cool the work at the

Required rate. In regular heat-treating shops where the cooling mediums must be used

continuously, mechanical cooling systems are used to maintain the cooling medium at the

correct temperature. Liquids, gases, and solids are all used as cooling mediums for heat-

treating operations. Solid materials such as lime, sand, ashes, and cast-iron chips aresometimes used when the rate of cooling must be slower than that produced by

liquids or gases. We have used water as our quenching medium in the hardening

process.

Figure. Portable quenching tank for use in heat-treating.

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TRANSFORMATION TEMPERATURES

The first arrest at 2,800F marks the temperature at which the iron freezes. The otherarrests (known as transformation temperatures or critical points) mark

temperatures at which certain internal changes take place in the solid iron. Some of these

temperatures are very important in the heat treatment of steel. As was mentioned before,

the atoms in all solid metals are arranged in a definite geometric pattern. The atoms iniron immediately after freezing are arranged in the body-centered cubic structure. In this

crystal structure the unit cell consists of a cube with an iron atom at each of the eight

comers and another in the center. Each of the many individual grains (crystals) of whichthe solid metal is composed is built up of a very large number of these unit cells, all

oriented alike in the same grain. This high-temperature iron is known as delta iron. At

2,550F, iron undergoes an allotropic transformation; that is, the arrangement of theatoms in the crystal changes. The new crystal structure is face-centered cubic, and the

unit cell again consists of a cube with an iron atom at each of the

eight corners, but with an iron atom in the center of each of the six faces instead on one in

the center of the cube. This form is known as gamma iron. At 1,670F, iron undergoes

another allotropic transformation and reverts to the body-centered cubic system. Thisstructure, which is basically the same as the structure of delta iron, is stable at all

temperatures below the A3 point and is known as alpha iron. The arrest at 1,420F is notcaused by an allotropic change. It marks the temperature at which iron becomes

ferromagnetic and is, therefore, termed the magnetic transition. Above this temperature

iron is nonmagnetic.

Figure. Iron-carbon phase diagram.

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ANNNEALING: (EXPERIMENT PERFORMED BY OUR GROUP)

ANNEALING is a term used to describe any heat treatment process that is used for the

primary purpose of softening the metal. Two types of annealing processes are commonly

used. FULL ANNEALING is done to soften the metal and make it more ductile, and

to relieve internal stresses caused by previous treatment such as casting, cold working, orhot working. The operation known as PROCESS ANNEALING or

STRESS RELIEF ANNEALING is done to soften the metal somewhat, although not asmuch as by full annealing, and to relieve internal stresses. In general, full annealing

requires higher temperatures, longer soaking time, and slower cooling than process

annealing. In the full annealing of steels, the steel is heated to a temperature that is 25 to50F above the upper transformation point. In the process annealing of steels, lower

temperatures are generally used. The rate of cooling used for annealing varies

greatly, depending upon the metal being annealed and the degree of softening required.

Physical properties:

Annealed metals are relatively soft and can be cut and shaped more easily. They bend easily

when pressure is applied. As a rule they are heated and allowed to cool slowly.

The animation above shows that an annealed metal is usually softer and can be deformed

more easily than metals that are not annealed.

NORMALIZING:

It is held at this temperature to fully convert the structure into Austenite, and then removed

from the furnace and cooled at room temperature under natural convection. This results in a

grain structure of fine Pearlite with excess of Ferrite or Cementite. The resulting material is

soft; the degree of softness depends on the actual ambient conditions of cooling. This process

is considerably cheaper than full annealing since there is not the added cost of controlledfurnace cooling.

The main difference between full annealing and normalizing is that fully annealed parts are

uniform in softness (and machinablilty) throughout the entire part; since the entire part is

exposed to the controlled furnace cooling. In the case of the normalized part, depending on

the part geometry, the cooling is non-uniform resulting in non-uniform material properties

across the part. This may not be desirable if further machining is desired, since it makes themachining job somewhat unpredictable. In such a case it is better to do full annealing.

HARDENING:

Martensitic stainless steels are hardened by austenitising, quenching and tempering much like

low alloy steels. Austenitising temperatures normally are 980 to 1010C, well above the

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critical temperature. As-quenched hardness increases with austenitising temperature to about

980C and then decreases due to retention of austenite. For some grades the optimumaustenitising temperature may depend on the subsequent tempering temperature.

Preheating before austenitising is recommended to prevent cracking in high-carbon types and

in intricate sections of low-carbon types. Preheating at 790C and then heating to the

austenitising temperature is the most common practice.

COOLING AND QUENCHING:

Martensitic stainless steels have high hardenability because of their high alloy content. Air

cooling from the austenitising temperature is usually adequate to produce full hardness, but

oil quenching is sometimes used, particularly for larger sections. Parts should be tempered as

soon as they have cooled to room temperature, particularly if oil quenching has been used, to

avoid delayed cracking. Parts sometimes are frozen to approximately -75C before tempering

to transform retained austenite, particularly where dimensional stability is important, such as

in gauge blocks made of grade 440C. Tempering at temperatures above 510C should be

followed by relatively rapid cooling to below 400C to avoid "475C" embrittlement.

Some precipitation-hardening stainless steels require more complicated heat treatments than

standard martensitic types. For instance, a semi-austenitic precipitation-hardening type may

require annealing, trigger annealing (to condition austenite for transformation on cooling to

room temperature), sub-zero cooling (to complete the transformation of austenite) and aging

(to fully harden the alloy). On the other hand, martensitic precipitation-hardening types (such

as Grade 630) often require nothing more than a simple aging treatment.

TEMPERING

After hardening, most alloys are tempered to reduce brittleness and to relieve some of thehigh internal stresses developed during hardening. Tempering always follows, rather thanprecedes, the hardening process. Tempering is occasionally done after materials have

been normalized, but its major use is after hardening. In some alloy steels tempering may

increase hardness when tempered to certain temperature ranges. In most other materials,however, tempering causes an unavoidable loss of some hardness. The amount of

hardness removed by tempering depends upon the tempering temperature; the higher the

temperature, the softer the material will be. Tempering is always done at temperatures

below the lower transformation point. In this respect, tempering differs from hardening,annealing, and normalizing, which all involve heating the material to temperatures above

the upper transformation point. The temperatures used for tempering are selected on the

basis of the properties required in the final product. For example, permanent magnets aretempered at 121F because they must retain considerable strength and hardness. Case-hardened objects are also tempered at relatively low temperatures (212 to 400F)

because the surface of such objects must remain hard. Many cutting tools are tempered at

430F or below so they will retain hardness. Battering tools must have great impactresistance and must be able to cut or penetrate metal; therefore, battering tools are

tempered between 450 and 600F even though the higher temperatures mean some

sacrifice of hardness to produce impact resistance. Springs are tempered between 600

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Fig. Filing operation

and 900F because the property of elasticity is more important in a spring than the

property of hardness. Tools made of high-speed steels are tempered at 1,050 to 1,100F.Note, however, that with high-speed tools the high tempering temperature increases,

rather than decreases, hardness. This increase in hardness occurs because high-speed

steels retain austenite during quenching; when the hardened steel is tempered, the

austenite changes to martensite. Since tempering uses temperatures below the lowertransformation point, the rate of cooling generally has no effect upon the structure of the

material. However, some nickel-chromium steels and a few other special steels become

brittle if they are heated to the tempering temperature and then allowed to cool slowly.These steels, which are often called temper brittle or blue brittle steels, must be quenched

rapidly from the tempering temperature to prevent brittleness. In general, however, steels

are cooled

SAMPLING

The choice of a sample for microscopic study may be very important. If a failure is

to be investigated, the sample should be chosen as close as possible to the area of

failure and should be compared with one taken from the normal section.

If the material is soft, such as nonferrous metals or alloys and non-heat treated steels,

the section may be obtained by manual hack sawing and ifthe material is hard, thesection may be obtained by use of an abrasive cutoff wheel. This wheel is a thin disk

of suitable cutting abrasive rotating at high speed. The specimen should be kept cool

during the cutting operation

ROUGH GRINDING OR FILING.

Whenever possible, the specimen

should be of a size that is

convenient to handle. A soft sample

may be made flat by slowly moving

it up and back across the surface of

a flat smooth file. The soft or hard

specimen may be rough-ground on

a belt sander, with the specimen

kept cool by frequent dropping in

water during the grinding operation

In all grinding and polishing

operations the specimen should be moved perpendicular

to the existing scratches This will facilitate recognition of

the stage when the deeper scratches have been replaced by shallower onescharacteristic of the finer abrasive, The rough grinding is continued until the surface

is flat and free of nicks, burrs, etc. and all scratches due to the hacksaw or cutoff

wheel are no longer visible.

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MOUNTING

Specimens that are small or awkwardly shaped should be mounted to facilitate

intermediate and final polishing. Wires, small rods, sheet metal specimens, thin

sections. etc., must be appropriately mounted in a suitable material or rigidlyclamped in a mechanical mount.

Synthetic plastic materials applied in a special mounting press will yield mounts of a

uniform convenient size (usually 1 in, 1.25 in., or 1.5 in. in diameter) for handling in

subsequent polishing operations. These mounts when properly made, are very

resistant to attack by the etching reagents ordinarily used. The most commonthermosetting resin for mounting is Bakelite, Bakelite molding powders are available

in a variety of colors, which simplifies the identification .of mounted specimens The

specimen and the correct amount of Bakelite powder, or a Bakelite perform, are

placed in the cylinder of the mounting press. The temperature is gradually raised to

150oC, and a molding pressure of about 4,000 psi is applied simultaneously. SinceBakelite is set and cured when this temperature is reached, the specimen mount maybe ejected from the molding die while it is still hot.

Lucite is the most common thermoplastic resin for mounting. Lucite is completelytransparent when properly molded. This transparency is useful when it is

necessary to observe the exact section that is being polished or when it is desirable

for any other reason to see the entire specimen in the mount. Unlike thethermosetting plastics, the thermoplastic resins do not undergo curing at the

molding temperature; rather they set on cooling. The specimen and a proper

amount of Lucite powder are placed in the mounting press and are subjected to the

same temperature and pressure as for Bakelite (150oC 4,000 psi) after this tem-

perature has been reached. The heating coil is removed, and cooling fins areplaced around the cylinder to cool the mount below 75

oC in about 7 min while the

molding pressure is maintained. Then the mount may be ejected from the mold.Ejecting the mount while still hot or allowing it to cool slowly in the molding

cylinder to ordinary temperature before ejection will cause the mount to be

opaque.

Small specimens may be conveniently mounted for metallographic preparation in

a laboratory-made clamping device. Thin sheet specimens, when mounted in suchas clamping device are usually alternated with metal "filler (sheets which haveapproximately the same hardness as the specimens. The use of filler sheets will

preserve surface irregularities of the specimen and will prevent, to some extent,the edges of the specimen from becoming rounded during polishing.

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INTERMEDIATE POLISHING

After mounting, the specimen is polished on a series of emery papers containing

successively finer abrasives. The first paper is usually No. 3(36), 2.5(46), 2(60),

1.5(80), 1(100), 0(120). The intermediate polishing operations using emery paper

are usually done dry; however, in certain cases such as the preparation ofsoftmaterials, silicon carbide abrasive may be used. As compared to emery paper,

silicon carbide has a greater removal rate and. as it is resin-bonded, can be usedwith a lubricant. Using a lubricant prevents overheating the sample, minimizes

smearing of soft metals, and also provides a rinsing action to flush away surface

removal products so the paper will not become clogged.

FINE POLISHING

The time consumed and the successes of fine polishing depend largely upon the

care that was exercised during the previous polishing steps. The final

approximation to a fiat scratch-free surface is obtained by use of a wet rotatingwheel covered with a special cloth that is charged with carefully sized abrasiveparticles. A wide range of abrasives is available for final polishing. While many

will do a satisfactory job, there appears to be a preference for the gamma form of

aluminum oxide for ferrous and copper-based materials, and cerium oxide foraluminum, magnesium, and their alloys. Other final polishing abrasives often used

are diamond paste, chromium oxide, and magnesium oxide.

The choice of a proper polishing cloth depends upon the particular materialbeing polished and the purpose of the metallographic study. Many cloths areavailable of varying nap or pile, from those having no pile, such as silk, to

those of intermediate pile, such as broadcloth, billiard Cloth, and canvas duck,and finally to a deep pile, such as velvet. Synthetic polishing cloths are alsoavailable forgeneral polishing purposes, of which two, under the trade namesof Gamal and Microcloth, are most widely used. A properly polished samplewill show only the nonmetallic inclusions and will be scratch free.

Fig. Scratch-free surface after final polishing.

Magnification 5OX. Black spots are oxide

impurities.

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ETCHING

The purpose of etching is to make visible the many structural characteristics of the

metal or alloy. The process must be such that the various parts of the

microstructure may be clearly differentiated. This is accomplished by use of an

appropriate reagent which subjects the polished surface to chemical action.Inalloys composed of two or more phases, the components are revealed during etching bya preferential attack of one or more of these constituents by the reagent, because of

difference in chemical composition of the phases. In uniform single-phase alloys or pure

metals, contrast is obtained and grain boundaries are made visible because of

differences in the rate at which various grains are attacked by the reagent .This

difference in the rate of attack is mainly associate with the angle of the different grain

sections to the plane of the polished surface Because of chemical attack by the etching

reagent, the grain boundaries will appear as valleys in the polished surface. Light from

the microscope hitting the side of these valleys will be reflected out of the microscope,

making the grain boundaries appear as dark lines.

TABLEEtching Reagents for Microscopic Examination

ETCHING REAGENT COMPOSITION

Nitric acid(nital)

White nitric acid 1-5 ml

Ethyl or methyl alcohol(95% or absolute)(also amyl alcohol)

100 ml

Picric acid(picral)

Picric acid 4 g

Ethyl or methyl alcohol(95% or absolute)

100 ml

Ferric chloride 5 g

Hydrochloric acid 50 mlWater 100 mlAmmonium hydroxide 5 partsWater S:partsHydrogen peroxide 2-5 parts

Ammonium per sulfate Ammonium persulfate 10 gWater 90 ml

Palmerton reagent Chromic oxide 200 gSodium sulfate 15 gWater .1,000 ml

Ammonium molybdate

Molybdic acid (85%) 100 g

Ammonium hydroxide(sp gr 0.9)

140 ml

Water 240 mlFilter and add to nitricacid (sp gr 1.32)

60 ml

Hydrofluoric acidHydrofluoric acid(conc.)

0.5 ml

H2O 99.5 ml

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Fig. Photograph obtained from electron microscope after annealing.

WORKPIECE SPECIFICATION

Material: Mild Steel

Type: Round Shaped

Diameter: 25 mm

Height: 10 cm

Furnace temperature: 703o C

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DISCUSSION

From this experiment we are taught to learn the process of examining the microstructure

of mild steel after heat treatment. The process was time consuming and a lot of care wasnecessary. The heating was done with care and accuracy. Filing was necessary to be done

in only one way to avoid unwanted scratches. Nital was quite dangerous and it had tohandle very carefully.

CONCLUSION

Success in microscopic study depends largely upon the care taken in the preparation of

the specimen. The most expensive microscope will not reveal the structure of a specimenthat has been poorly prepared. The procedure to be followed in the preparation of a

specimen is comparatively simple and involves a technique which is developed only after

constant practice. The ultimate objective is to produce a flat, scratch-free, mirror likesurface.