Metallography Lab

Mechanical laboratory II

Metallography LabMetallography LabMetallography LabMetallography Lab

Metallography of steel and cast Metallography of steel and cast Metallography of steel and cast Metallography of steel and cast

ironironironiron

Vishal Yadav 0901ME22

C.Kausik

B.Reddy

Mechanical laboratory II-ME310 (2011)

Metallography of steel and cast Metallography of steel and cast Metallography of steel and cast Metallography of steel and cast

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INTRODUCTION :

Metallography –

the study of the microstructure of metals using various techniques—has been an invaluable tool for

the advancement of science and industry for over one hundred years. Metallography is used to reveal

the microstructure of metals, which is affected by alloy composition and processing conditions;

including cold working, heat treatment and welding. A finished part's environment can also affect its

microstructure and cause problems such as corrosion and decarburization.

Analysis of a material's metallographic microstructure aids in determining if the material

has been processed correctly and is therefore a critical step for determining product reliability and/or

for determining why a material failed.

A. Metallographic Procedure:

REQUISITES:

The key to obtaining an accurate interpretation of a microstructure is a properly prepared specimen

which is truly representative of the material being examined. The definition of a properly prepared

metallographic surface states that the section must meet the following criteria.

• Be flat and free from scratches, stains, and

other imperfections which tend to mar

the surface.

• Contain all non-metallic inclusions intact.

• Show no chipping or galling of hard and

brittle intermetallic compounds.

• Be free from all traces of disturbed metal

To ensure achievement of such true surfaces, preparation must be carried out not only with

accuracy, but also with a clear understanding of what must be accomplished during each specific

stage..

1.Stages of Preparation (Definitions):

The most straight-forward approach is to divide the entire process into a logical series of stages

involved and the purpose of same.

Stage 1-Sectioning:

The removal of a representative sample from the parent piece.

This was done using a hacksaw which is made of secondary-hardened tool steel. Although

the blade is significantly flexible, it is very hard and can fracture violently if the direction of

the stroke deviates much from the plane of the cut.

Stage 2-Mounting :

Small samples are generally mounted in plastic for convenience in handling and to protect the edges

of the specimen being prepared. Compression-type molding is commonly applied to encase specimens

in 1 to 1.5 inch diameter plugs of a hard polymer. Compression molding materials are classified as

either thermosetting or thermoplastic:

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Mounted Sample

Stage 3-Coarse Grinding :

The purpose of the coarse grinding stage is to generate the initial flat surface necessary for the

subsequent grinding and polishing steps. As a result of sectioning and grinding, the material may get

cold worked to a considerable depth with a resultant transition zone of deformed material between the

surface and the undistorted metal. Course grinding can be accomplished either wet or dry using 80 to

180 grit electrically powered disks or belts, but care must be taken to avoid significant heating of the

sample. The final objective is to obtain a flat surface free from all previous tool marks and cold

working due to specimen cutting.

An important factor throughout the Coarse Grinding and Fine Grinding Stages is that the scratches be

uniform in size and parallel to each other in any one grinding stage. Proper grinding involves rotation

of the sample by 90o between stages while the grinding angle must be held constant during the

grinding at any one stage.

Grinding machine.

Stage 4-Medium and Fine Grinding

Medium and Fine Grinding of metallurgical samples are closely allied with the Coarse Grinding

which precedes them. Each stage of metallographic sample preparation must be carefully performed;

the entire process is designed to produce a scratch free surface by employing a series of successively

finer abrasives. Failure to be careful in any stage will result in an unsatisfactory sample.Movement

from one stage to the next should only proceed when all of the scratches from the preceding stage are

completely removed. Manual Fine Grinding is performed by drawing the specimens in one direction

across the surface of the water lubricated abrasive paper. (Back to front is recommended) Use of

backward and forward motion is less desirable because there is a tendency to rock the sample,

producing a curved rather than a flat surface.To monitor progress, each fine grinding step should be

performed in a direction off-angle with respect to the previous step.

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Abrasive paper.

Stage 5-Mechanical Polishing

Polishing involves the use of abrasives, suspended in a water solution, on a cloth-covered electrically

powered wheel. Diamond abrasives provide the best, and most expensive, compounds utilized in

polishing;standard sized aluminum oxide powders are applied for general use purposes. Following the

final 600 grit fine-grinding stage, the sample MUST be washed and carefully dried before proceeding

to the first polishing stage! At the polishing stages, even hard dust particles in the air which settles on

the polishing cloth can cause unwanted scratching of the specimen! Careful washing of the specimen

and the operator's hands must be carried out prior to each stage of polishing.

The polishers consist of rotating discs covered with soft cloth

impregnated with diamond particles (3 and 1/3 micron size) and an oily lubricant. Begin

with the 3 micron grade and continue polishing until the grinding scratches have been removed. It

is of vital importance that the sample is thoroughly cleaned using soapy water, followed by alcohol,

and dried before moving onto the final 1/3 micron stage. Any contamination of the 1/3 micron

polishing disc will make it impossible to achieve a satisfactory polish.

Stage 6-Etching:

The purpose of etching is two-fold. < Grinding and polishing operations produce a highly deformed, thin layer on the surface which is

removed chemically during etching.

< Secondly, the etchant attacks the surface with preference for those sites with the highest energy,

leading to surface relief which allows different crystal orientations, grain boundaries, precipitates,

phases and defects to be distinguished in reflected light microscopy. For iron & steel the etchant used

composed of 1-5 parts of Nitric Acid & 100 parts of Alcohol.

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2.Heat Treatement Process:

Quenching – It is the rapid cooling of a workpiece to obtain certain material properties. It prevents low-temperature processes, such as phase transformations, from occurring by only

providing a narrow window of time in which the reaction is both thermodynamically

favorable and kinetically accessible. For instance, it can reduce crystallinity and thereby

increase toughness of both alloys and plastics (produced through polymerization).

Annealing – It is a heat treatment wherein a material is altered, causing changes in its

properties such as strength and hardness. It is a process that produces conditions by heating

to above the recrystallization temperature, maintaining a suitable temperature, and then

cooling. Annealing is used to induce ductility, soften material, relieve internal stresses,

refine the structure by making it homogeneous, and improve cold working properties.

Normalising – Normalization is an annealing process in which a metal is cooled in air after

heating in order to relieve stress. It can also be referred to as: Heating a ferrous alloy to a

suitable temperature above the transformation temperature range and cooling in air to a

temperature substantially below the transformation range. This process is typically confined

to hardenable steel. It is used to refine grains which have been deformed through cold

work, and can improve ductility and toughness of the steel. It involves heating the steel to

just above its upper critical point. It is soaked for a short period then allowed to cool in air.

Small grains are formed which give a much harder and tougher metal with normal tensile

strength and not the maximum ductility achieved by annealing. It eliminates columnar

grains and dendritic segregation that sometimes occurs during casting. Normalizing

improves machinability of a component and provides dimensional stability if subjected to

further heat treatment processes.

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B.Iron Carbon Phase Diagram :-

When carbon in small quantities is added to iron, ‘Steel’ is obtained. Since the influence of

carbon on mechanical properties of iron is much larger than other alloying elements. The

atomic diameter of carbon is less than the interstices between iron atoms and the carbon

goes into solid solution of iron. As carbon dissolves in the interstices, it distorts the original

crystal lattice of iron.

This mechanical distortion of crystal lattice interferes with the external applied strain to

the crystal lattice, by mechanically blocking the dislocation of the crystal lattices. In

other words, they provide mechanical strength. Obviously adding more and more carbon to

iron (upto solubility of iron) results in more and more distortion of the crystal lattices and

hence provides increased mechanical strength. However, solubility of more carbon

influences negatively with another important property of iron called the ‘ductility’ (ability of

iron to undergo large plastic deformation). The a-iron or ferrite is very soft and it flows

plastically. Hence we see that when more carbon is added, enhanced mechanical strength is

obtained, but ductility is reduced. Increase in carbon content is not the only way, and

certainly not the desirable way to get increased strength of steels. More amount of carbon

causes problems during the welding process. We will see later, how both mechanical

strength and ductility of steel could be improved even with low carbon content. The iron-

carbon equilibrium diagram is a plot of transformation of iron with respect to carbon

content and temperature. This diagram is also called Fe-Fe carbon phase diagram (Fig.1).

The important metallurgical terms, used in the diagram, are presented below.

Ferrite (α): Virtually pure iron with body centered cubic crystal structure (bcc). It is stable at all temperatures

upto 9100C. The carbon solubility in ferrite depends upon the temperature; the maximum being

0.02% at 723C.

Cementite: Iron carbide (Fe3C), a compound iron and carbon containing 6.67% carbon by weight.

Pearlite:

A fine mixture of ferrite and cementite arranged in lamellar form. It is stable at all

temperatures below 723C.

Austenite (γ):

Austenite is a face centred cubic structure (fcc). It is stable at temperatures

above 723oC depending upon carbon content. It can dissolve upto 2% carbon. The

maximum solubility of carbon in the form of Fe3C in iron is 6.67%. Addition of carbon to iron

beyond this percentage would result in formation of free carbon or graphite in iron. At

6.67% of carbon, iron transforms completely into cementite or Fe3C (Iron Carbide).

Generally carbon content in structural steels is in the range of 0.12-0.25%. Upto 2% carbon,

we get a structure of ferrite + pearlite or pearlite + cementite depending upon whether

carbon content is less than 0.8% or beyond 0.8%. Beyond 2% carbon in iron, brittle cast iron

is formed.

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Fig1. Iron Carbon phase diagram

The iron-iron carbide portion of the phase diagram that is of interest to structural engineers

is shown in Fig.1. The phase diagram is divided into two parts called “hypoeutectoid steels”

(steels with carbon content to the left of eutectoid point *0.8% carbon+) and “hyper

eutectoid steels” which have carbon content to the right of the eutectoid point. It is seen

Metallography of steel and cast iron from the figure that iron containing very low percentage of

carbon (0.002%) called very low carbon steels will have 100% ferrite microstructure (grains or

crystals of ferrite with irregular boundaries) as shown in Fig 1. Ferrite is soft and ductile with very

low mechanical strength. This microstructure at ambient temperature has a mixture of what is known

as ‘pearlite and ferrite’ as can be seen in Fig. 1. Hence we see that ordinary structural steels

have a pearlite + ferrite microstructure. However, it is important to note that steel of 0.20%

carbon ends up in pearlite + ferrite microstructure, only when it is cooled very slowly from

higher temperature during manufacture. When the rate of cooling is faster, the normal

pearlite+ ferrite microstructure may not form, instead some other microstructure called

bainite or martensite may result.

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C. Microstructure Analysis:

1. Microstructure analysis of Mild steel:

When carbon content in iron is <

common form of steel because its price is

acceptable for many applications. Mild steel contains 0.16

brittle nor ductile. Mild steel has a relatively low tensile strength, but it i

surface hardness can be increased through carburizing. The steel has two major constituents, which

are ferrite and pearlite

The properties of the steel depends upon the microstructure.

decreasing the amount of pearlite improves the

inclusions can also affect the toughness.

For example, they can encourage ductile fracture.

can be machined and worked into co

2. Microstructure analysis of Cast iron

When carbon content in iron is > 2% , then we generally consider as cast iron. A typical chemical

composition to obtain a graphitic microstructure is 2.5 to 4.0% carbon and 1 to 3% silicon. Silicon is

important to making grey iron as opposed

element in cast iron, which means it helps the alloy produce graphite instead of iron carbides. Another

factor affecting graphitization is the solidification rate; the slower the rate, the greater the tendency for

graphite to form. A moderate cooling rate forms a more pearlitic matrix, while a slow cooling rate

forms a more ferritic matrix. To achieve a fully ferritic matrix the alloy must be annealed. Rapid

cooling partly or completely suppresses graphitization and lead

called white iron. The graphite takes on the shape of a three dimensional flake.

a polished surface will appear under a microscope, the graphite flakes

graphite has no appreciable strength, so they can be treated as

existing notches and so it is brittle.

C. Microstructure Analysis:

Microstructure analysis of Mild steel:

on is < 2% , then we generally consider it as steel. Mild steel is the most

common form of steel because its price is relatively low while it provides material propertie

applications. Mild steel contains 0.16–0.29% carbon; theref

ductile. Mild steel has a relatively low tensile strength, but it is cheap and malleable;

hardness can be increased through carburizing. The steel has two major constituents, which

pends upon the microstructure. Decreasing the size of the grains and

mount of pearlite improves the strength, ductility and the toughness of the steel. The

toughness.

can encourage ductile fracture. Mild steel is a very versatile and useful material. It

can be machined and worked into complex shapes, has low cost and good mechanical properties.

Mild Steel

Microstructure analysis of Cast iron:

content in iron is > 2% , then we generally consider as cast iron. A typical chemical


important to making grey iron as opposed to white cast iron, because silicon is a graphite stabilizing



ite to form. A moderate cooling rate forms a more pearlitic matrix, while a slow cooling rate


cooling partly or completely suppresses graphitization and leads to formation of cementite, which is

The graphite takes on the shape of a three dimensional flake. In two dimensions, as

a polished surface will appear under a microscope, the graphite flakes appear as fine lines. The

appreciable strength, so they can be treated as voids.The tips of the flakes act as pre

existing notches and so it is brittle.

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consider it as steel. Mild steel is the most

relatively low while it provides material properties that are

0.29% carbon; therefore, it is neither

s cheap and malleable;

hardness can be increased through carburizing. The steel has two major constituents, which

Decreasing the size of the grains and

strength, ductility and the toughness of the steel. The

and useful material. It

good mechanical properties.

content in iron is > 2% , then we generally consider as cast iron. A typical chemical


se silicon is a graphite stabilizing



ite to form. A moderate cooling rate forms a more pearlitic matrix, while a slow cooling rate


s to formation of cementite, which is

In two dimensions, as

appear as fine lines. The

The tips of the flakes act as pre

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3.Comparison between ash-polished and etched specimen:

The microstructure between etched mild steel and

only to make the microstructure visible.

Ash polished Mild steel

Grain boundaries are not visible in case of ash polished specimen.

Grain boundaries are clearly visible in case of etched

2.Cast Iron

polished and etched specimen:

between etched mild steel and unetched mild steel does not change. Etching is

only to make the microstructure visible.

Grain boundaries are not visible in case of ash polished specimen.

Grain boundaries are clearly visible in case of etched specimen. Etched Mild steel

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unetched mild steel does not change. Etching is

`

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4.Comparison between etched Mild steel and Cast iron:

The light coloured region of the microstructure is

grains can be seen quite clearly.The dark

ferrite and iron carbide, which can be seen as a "wormy" texture.You can also see small spots w

the ferrite grains. These are inclusions or impurities such as oxides and sulphides.Etchant used is 2%

Nital.

Grey cast iron, containing graphite flakes in a matrix

pearlite can be resolved, appearing to consist

speckled white regions represent a

5.Comparison of quenched mild steel and cast iron:

normalizing temperatures and then rapidly cooled (quenched) in water to the critical

temperature. The critical temperature is dependent on the carbon content, but as a general

rule is lower as the carbon content increases. This results in a martensitic s

Comparison between etched Mild steel and Cast iron:

Mild steel

The light coloured region of the microstructure is the ferrite. The grain boundaries between the ferrite

grains can be seen quite clearly.The dark regions are the pearlite. It is made up from a fine mixture of

can be seen as a "wormy" texture.You can also see small spots w

are inclusions or impurities such as oxides and sulphides.Etchant used is 2%

Cast Iron

ng graphite flakes in a matrix which is pearlitic. The lamellar structure of the

resolved, appearing to consist of alternating layers of cementite and ferrite. The

kled white regions represent a phosphide eutectic. Etchant used is 2% Nital.

Comparison of quenched mild steel and cast iron:

Mild steel normalizing temperatures and then rapidly cooled (quenched) in water to the critical


rule is lower as the carbon content increases. This results in a martensitic structure. Thus

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the ferrite. The grain boundaries between the ferrite

regions are the pearlite. It is made up from a fine mixture of

can be seen as a "wormy" texture.You can also see small spots within

are inclusions or impurities such as oxides and sulphides.Etchant used is 2%

which is pearlitic. The lamellar structure of the

of alternating layers of cementite and ferrite. The

normalizing temperatures and then rapidly cooled (quenched) in water to the critical


tructure. Thus

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quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These

internal stresses cause stress cracks on the surface. Quenched steel is approximately three

to four-fold harder than normalized steel.

Cast iron except tempered at 950°C after

quenching results in the formation of graphite flakes in a matrix of tempered martensite.

6.Comparison of normalised mild steel and cast iron:

Mild steel normalized by austenitizing at 950 °C and air cooling.

We obtain a pearlite which looks like a coarse grain structure in a ferrite matrix.

The temperature range for normalizing gray iron is

approximately 885 to 925°C .Normaliz

combines good wear resistance with reasonable machinability and an excellent response to

induction or flame hardening, provided that the cooling rate is fast enough and the



fold harder than normalized steel.

cast iron Cast iron except tempered at 950°C after


Comparison of normalised mild steel and cast iron:

Mild steel.

Mild steel normalized by austenitizing at 950 °C and air cooling.

We obtain a pearlite which looks like a coarse grain structure in a ferrite matrix.

Cast iron

The temperature range for normalizing gray iron is

approximately 885 to 925°C .Normalizing generally produces affine pearlitic matrix that



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ing generally produces affine pearlitic matrix that



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hardenability is sufficient for the section thickness.

7.Comparison of annealed mild steel and cast iron:

Mild steel annealed by austenitizing at 950°C and cooling slowly in a furnace. The structure consists

of coarse lamellar pearlite (dark) in a matrix of ferrite (light).

If the microstructure of gray iron contains massive carbide particles, higher annealing temperatures

are necessary. Annealing may simply serve to convert massive carbide to pearlite and graphite. To

break down massive carbide with reasonable speed, temperatures of at least 870°C are required. With

each additional 55 °C increment in holding temperature, the rate of carbide decomposition doubles.

Consequently, it is general practice to employ holding temperatures of 900 to 955°C.

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Ref: ASM Metal hand book volume 09: Metallography and Microstructure.

www.nptel.iitm.ac.in

www.keytomeatls.com

www.ehow.com

www.wikipedia.org

www.msm.cam.ac.uk

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