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EXPERIMENT 4

THE EFFECTS OF HEAT TREATMENT ON THE MICROSTRUCTURE OF STEEL

OBJECTIVES

I. To study on preparing of metallographic sample for microstructures observation.

II. To study the effects of heat treatment on the microstructure of steel.

III. To discover the microstructure of ferrite, pearlite, cementite, austenite and martensite under

microscopic view.

APPARATUS

I. Five different sample of heat treated steel

Sample number Type ! "ea# #rea#men#

1 Without heat treatment2 Water quenched

!il quenched

" #ormali$ing

% &nnealed

&brasive cutter machine ' ()*& T+ -&W )tching machine


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/ompression (ounting 0ress and polishing machine

*rinding+polisher machine ' #! 2T Fume hood


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!ptical microscope

SUMMAR$

The properties of steels can be changed or altered by several techniques such as alloying and heat

treatment. eat treatment process is a process is a process of ability to change the properties by applying

heat. -uch treatment modifies microstructures, producing a variety of mechanical properties that are

important in manufacturing, such as improve formability and machinability.

eat Treatment 0rocess

I. &nnealing

&nnealing is the process to maes metal as soft as possible. The material is e3posed to anelevated temperature for an e3tended time period and then slowly cooled to allow phase

changes.

There are three stages of annealing

eating to the desired temperature4 The material is austeniti$ed by heating to 1 / to5

" / above the & or & lines under equilibrium is achieved which is the alloy

changes to austenite.

-oaing or holding time 4 the material is held for an hour at the annealing

temperature for the every inch of thicness

/ool slowly, usually to room temperature. /ooling method is slowly cooled in air or

turn off the furnace.

The purposes of annealing are

I. 5elieve internal stresses. Internal stresses can build up in metal such as a result of

processing. -uch as welding, cold woring, casting, forging or machining. If internal

stresses are allowed to remain in a metal, the part may eventually distort or crac. Thus,

&nnealing helps relieve internal stresses and reduce the chances of distortion and

cracing.


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II. Increasing softness, machinability and formability. & softer and more ductile material is

easier to machine in the machine shop. &n annealed part will respond better to forming

operations.

III. 5efinement of grain structures. &fter some types of metalworing, the crystal structures

are elongated. &nnealing can change the shape of the grains bac to the desired form.

II. #ormali$ing

The name of normali$ing comes from the original intended purpose of the process+ to return steel

to 6 normal7 condition it was in before it was altered by cold woring or order processing.

eating the alloy to %% / to 8% / above the & and holding for sufficient time so that the alloy

completely transforms to austenite , followed by air cooling.

The process of normali$ing

eat up to critical temperature, at which point the structure is all austenite.

/ool slowly in air

The purpose of normali$ing

I. To refine the grains and produce a more uniform and desirable si$e distributions for steels

that has been plastically deformed.

II. & normali$ed part will usually be a little stronger, harder and more brittle than a full+

annealed part.III. To improve machinability

I9. (ae the steels higher in strengthens and harder.

III. :uenching

:uenching is the act of rapid cooling of alloy from an elevated temperature to harden the steel in

any quenching media. 9arious quenching mediums can be used during the process. For e3ample

water, oil, air and brine which will give different result on the structures and materials properties.


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The process of quenching

eated to / to % / above the upper critical temperature and then quenched.

The effect of cooling rate on the material can be summari$ed as in table below4

Cl%n& ra#e 'uen("%n& Sl)

0roperties ard -oft

-trong Wea

;rittle ectives of sample preparation for metallographic observations with a mirror lie surface. 0roper

preparation of metallographic specimens to determine microstructure and content requires that a rigid

step+by+step process be followed. In sequence, the steps include sectioning, mounting, course grinding,

fine grinding, polishing, etching and microscopic e3amination. -pecimens must be ept clean and

preparation procedure carefully followed in order to reveal accurate microstructures.

(etallography consists of the study of the constitution and structure of metals and alloys. (uch can be

learned through specimen e3amination with the naed eye, but more refined techniques require

magnification and preparation of the material?s surface. !ptical microscopy is sufficient for general

purpose e3amination@ advanced e3amination and research laboratories often contain electron microscopes

'-)( and T)(, 3+ray and electron diffract meters and possibly other scanning devices. Incorrect

techniques in preparing a sample may result in altering the true microstructure and will most liely lead to

erroneous conclusions. It necessarily follows that the microstructure should not be altered. ot or cold

woring can occur during the specimen preparation process if the metallurgist is not careful. )3pertise at

the methods employed to produce high quality metallographic samples requires training and practice. The

basic techniques can be learned through patient persistence in a matter of hours.

In order to obtain the smooth and flat surface, several techniques are required. These include cutting,

molding, grinding, polishing and etching.

I. /utting

When cutting a specimen from a larger piece of material, care must be taen to ensure that it is

representative of the features found in the larger sample, or that it contains all the information required to

investigate a feature of interest.


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!ne problem is that preparation of the specimen may change the microstructure of the material, for

e3ample through heating, chemical attac, or mechanical damage. The amount of damage depends on the

method by which the specimen is cut and the material itself.

/utting with abrasives may cause a high amount of damage, while the use of a low+speed diamond saw

can lessen the problems. There are many different cutting methods, although some are used only for

specific specimen types. (TI provides the -AB+1% low speed diamond saw for cutting !( 'optical

microscope, -)( 'scanning electron microscope, and even T)( 'transmission electron microscope

specimen.

0roper sectioning is required to minimi$e damage, which may alter the microstructure and produce false

metallographic characteri$ation. 0roper cutting requires the correct selection of abrasive type, bonding,

and si$e@ as well as proper cutting speed, load and coolant. Table I list the most common type of abrasive

blades used for metallographic sectioning and Table II lists cutting parameters for diamond wafer cutting

Table 1

Ma#er%al* + ally, Cla**%!%(a#%n Abra*%-e.bn/

&luminum, brass, $inc, etc. -oft non+ferrous -i/C 5olled rubber

eat treated alloys ard non+ferrous &luminaC 5ubber resin

D5c "% steel -oft ferrous &luminaC 5ubber resin

E5c "% steel ard ferrous &luminaC 5ubber resin

-uper alloys igh #i+/r alloys -i/C 5olled rubber

Table 2

Ma#er%al C"ara(#er%*#%(* Spee/+rpm, La/+&ram*, Bla/e+&r%#.(n(0,

-ilicon substrate -oftCbrittle D D1 FineClow

*allium arsenide -oftCbrittle D2 D1 FineClow

;oron composites 9ery brittle % 2% FineClow

/eramic fibre

composites

9ery brittle 1 % FineClow

*lasses ;rittle 1 % FineClow

(inerals FriableCbrittle E1% E% FineClow

&lumina ceramic ardCtough E1% E% (ediumClow

irconia ardCtough E% E8 (ediumClow

-ilicon nitride ardCtough E% E8 (ediumClow

(etal matri3

composites

+ E% E% (ediumChigh

*eneral purpose + 9ariable 9ariable (ediumChigh

II. (ounting


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(ounting of specimens is usually necessary to allow them to be handled easily. It also minimi$es the

amount of damage liely to be caused to the specimen itself.

The mounting material used should not influence the specimen as a result of chemical reaction or

mechanical stresses. It should adhere well to the specimen, and if the specimen is to be electropolished

later in the preparation then the mounting material should also be electrically conducting.

-pecimens can be hot mounted 'about 1% G/ using a mounting press either in a thermosetting plastic,

e.g.phenolic resin, or a thermo softening plastic e.g.acrylic resin. If hot mounting will alter the structure

of the specimen a cold+setting resin can be used, e.g. epo3y, acrylic or polyester resin. 0orous materials

must be impregnated by resin before mounting or polishing, to prevent grit, polishing media or etchant

being trapped in the pores, and to preserve the open structure of the material.

/astable mounting resins are commonly used for electronic and ceramic materials. /astable mounting

resins are recommended for brittle and porous materials. These mounting compounds are typically two

component systems '1+ resin and 1+hardener. Typical curing times range from minutes to hours with the

faster curing resins producing higher e3othermic temperature which causes the mounting material to

shrin away from the edge during curing. For e3ample, the &crylic /old (ounting 5esins cure in less

than 1 minutes and )po3y /astable 5esins cure in appro3imately "+H hours. #ote that adding an e3ternal

energy source such as heat or microwave energy can enhance the )po3y /astable 5esin curing cycle. It is

recommended that the room temperature be less than 8%G F to avoid overheating and uncontrollable

curing of the mounting compound. Table I9 lists the basic properties of castable

& mounted specimen usually has a thicness of about half its diameter, to prevent rocing during grinding

and polishing. The edges of the mounted specimen should also be rounded to minimi$e the damage to

grinding and polishing discs.

III. *rinding

-urface layers damaged by cutting must be removed by grinding. (ounted specimens are ground

with rotating discs of abrasive paper, for e3ample wet silicon carbide paper. The coarseness of the

paper is indicated by a number4 the number of grains of silicon carbide per square inch. -o, for

e3ample, 18 grit paper is coarser than 12 grit.


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The grinding procedure involves several stages, using a finer paper 'higher number each time. )ach

grinding stage removes the scratches from the previous coarser paper. This can be easily achieved by

orienting the specimen perpendicular to the previous scratches. ;etween each grade the specimen is

washed thoroughly with soapy water to prevent contamination from coarser grit present on the

specimen surface. Typically, the finest grade of paper used is the 12, and once the only scratches

left on the specimen are from this grade.

The series of photos below shows the progression of the specimen when ground with progressively

finer paper.

/opper specimen ground with 18 grit paper /opper specimen ground with " grit paper

/opper specimen ground with 8 grit paper /opper specimen ground with 12 grit

paper

*rinding


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&utomated 0reparation + The ey to successful automated preparation is to thoroughly clean the

specimens between each abrasive grit si$e that is used. The tracing of the specimens should also

uniformly brea down the -i/ paper, otherwise non+uniform grinding will occur 'especially for hard

specimens in soft mounts. In other words, the specimen should trac across the entire diameter of the

-i/ paper.

&brasive -election + The most common abrasive used for planar grinding metal and polymer

metallographic specimens is silicon carbide '-i/. -ilicon carbide is an e3cellent abrasive for planar

grinding because it is very hard and maintains a sharp cutting edge as it breas down during cutting.

-i/ abrasives are typically listed by the grit si$e. Table 9I shows the two most common grit si$e+

numbering systems, along with their respective median diameter particle si$e and Table below

provides a list of recommendation for planar grinding specific materials.

Planar 1r%n/%n& Re(mmen/a#%n*

(etallic

-pecimens

For metallic specimen grinding, sequential grinding with silicon carbide '-i/

abrasive paper is the most efficient and economical rough grinding process.

&lthough e3tremely coarse grit abrasive papers can be found, it is recommendedthat a properly cut specimen not be rough ground with an abrasive greater than

12 grit -i/ paper. & typical abrasive grinding procedure would consist of 12

or 2" grit -i/ paper followed by decreasing the si$e of the -i/ paper '2,

", and H grit. Finer papers are also available for continued abrasive paper

grinding '8 and 12 grit. In addition to the correct sequence and abrasive

si$e selection, the grinding parameters such as grinding direction, load and

speed can affect the specimen flatness and the depth of damage. The basic idea

is to remove all of the previous specimen damage before continuing to the ne3t

step while maintaining planar specimens

)lectronic-pecimens

*rinding electronic components is very dependent upon both the substrate'silicon, alumina, barium titanate, plastic 0/;?s, etc and the metallic materials

used. In general, when grinding plated or coated materials, it is recommended

that the coating be prepared in compression to prevent the coating from

separating from the substrate. -ilicon specimens should be have been sectioned

with a fine grit diamond blade and cut as near as possible to the area of interest.

For rough grinding, fine abrasives such as " or H grit -i/ or a 1%+micron

diamond lapping film is recommended to avoid producing more damage than

created during sectioning. ard ceramic substrates 'especially porous materials

should be rough polished with diamond lapping films to minimi$e edge

rounding and relief between the widely varying materials hardness?

0lasma -pray

/omponents

-imilar to the preparation of electronic components, plasma spray coatings

should be ept in compression to minimi$e the possibility of delamination at the

coatingC substrate interface. For ceramic plasma spray coatings, diamond+

lapping films are recommended to minimi$e edge rounding or relief and to

maintain the integrity of any inclusions within the coating.

/eramics and

/eramics

5ough grinding ceramics and ceramic matri3 composites should be performed

with 1% or micron diamond on a metal mesh cloth in order to obtain adequate


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/omposites stoc removal and to minimi$e surface and subsurface damage.

0lastics and

0lastics

/omposites

0lastics are generally very soft and therefore can be planar ground with

sequentially decreasing -i/ abrasive paper grit si$es. When plastics are used in

con>unction with hard ceramics, planar grinding can be very tricy. For these

composite materials cutting must minimi$e damage as much as possible because

almost all grinding and polishing will cause relief between the soft plastic andthe hard ceramic. Following proper cutting, grinding with as small as possible a

diamond 'H+micron diamond on a metal steel mesh cloth or the use of lapping

films is suggested.

I9. 0olishing

0olishing discs are covered with soft cloth impregnated with abrasive diamond particles and an oily

lubricant or water lubricant. 0articles of two different grades are used 4 a coarser polish + typically

with diamond particles H microns in diameter which should remove the scratches produced from the

finest grinding stage, and a finer polish J typically with diamond particles 1 micron in diameter, to

produce a smooth surface. ;efore using a finer polishing wheel the specimen should be washed

thoroughly with warm soapy water followed by alcohol to prevent contamination of the disc. The

drying can be made quicer using a hot air drier.

5ough 0olishing + The purpose of the rough polishing step is to remove the damage produced during

cutting and planar grinding. 0roper rough polishing will maintain specimen flatness and retain all

inclusions or secondary phases. ;y eliminating the previous damage and maintaining the

microstructural integrity of the specimen at this step, a minimal amount of time should be required to

remove the cosmetic damage at the final polishing step. 5ough polishing is accomplished primarily

with diamond abrasives ranging from micron down to 1+micron diamond. 0olycrystalline diamond

because of its multiple and small cutting edges, produces high cut rates with minimal surface damage,

therefore it is the recommended diamond abrasive for metallographic rough polishing on low napped

polishing cloths. Table below show the basic rough polishing guidelines for a number of materials.

(etals 'ferrous, nonferrous, toolsteels, ys, etc.

5ough polishing typically requires two polishing steps, suchas a H+micron diamond followed by a 1+micron diamond on

low napped polishing cloths.

/eramics and ceramic matri3

composites '/(/

=ow nap+polishing pads using polycrystalline diamond

alternating with colloidal silica. This provides a chemical

mechanical polishing '/(0 effect, which results in a

damage free surface finish.

0olymer matri3 composites '0(/


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;iomaterials =ow napped polishing pads with polycrystalline diamond

alternating with colloidal silica. &lternatively, diamond

lapping films may wor well

(icroelectronic specimen


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/opper specimen polished to H micron level /opper specimen polished to 1 micron level

9. )tching

The purpose of etching is to optically enhance microstructural features such as grain si$e and phase

features. )tching selectively alters these microstructural features based on composition, stress, or

crystal structure. The most common technique for etching is selective chemical etching and numerous

formulations have been used over the years. !ther techniques such as molten salt, electrolytic,

thermal and plasma etching have also found speciali$ed applications. /hemical etching selectively

attacs specific microstructural features. It generally consists of a mi3ture of acids or bases with

o3idi$ing or reducing agents. For more technical information on selective chemical etching consult

corrosion boos which discuss the relationship between p and )h 'o3idationCreduction potentials,

often nown as )h+p diagrams or 0ourbai3 diagrams. !ver the years numerous chemical etchants

have been formulated. -ome of the more common chemical etchants are listed in the following table.

Table below lists some of the more common etchants.

)tchant /omposition &pplications /onditions

Kellers )tch 1 ml


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H ml #itric acid

2 ml ydrofluoric acid

#ital 1 ml )thanol

1+1 ml #itric acid

/arbon steels, tin and

nicel alloys

-econds to minutes

Kallings 5eagent " ml


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advantage of the higher internal energy associated at a materials grain boundaries. &t the elevated

temperature of molten salts, the higher energy at the grain boundaries is relieved, producing a

rounded grain boundary edge, which can be observed by optical, or electron microscope techniques.

Thermal etching is a useful etching technique for ceramic materials. Thermal etching is technique that

relieves the higher energy associated at the grain boundaries of a material. ;y heating the ceramic

material to temperatures below the sintering temperature of the material and holding for a period of

minutes to hours the grain boundaries will see a level of lower energy. The result is that the grain

boundary edge will become rounded so that it can be evaluated by optical or electron microscope

techniques.


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EXPERIMENT PROCE2URES

1. We were provided with five different samples of heat+ treated steel.

2. The microstructure of each sample were observed and setched.

. We identified and labeled the microstructures.

RESULTS

&ustenite


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Ferrite

0earlite

(artensite

Lpper bainite


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=ower bainite

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