SYNOPSIS

Problem Definition:

The chemical composition of a material gives approximate analysis of properties. However it

is not possible to predict the expected behaviour of the material based only on the chemical

composition. Therefore microstructure and micro hardness study is essential to verify different

phases and based on these phases the expected behaviour of a material.

Objectives:

To understand the process of preparation of Grey cast iron, aluminium and brass specimens

for microstructure studies.

The main objective of this project is to study the various micro-constituents in the above said

materials.

Methodology:

Grey cast iron, aluminium and brass have been selected to observe the various micro-

constituent present.

The raw materials are cut to get the required dimensions using abrasive cutting machine.

The materials are subjected to polishing and etching with the help of belt grinder, polishing

machine/electro polisher.

Using optical microscope the microstructure photographs are generated.

Correlation between various micro-constituents of the microstructure with mechanical

properties will be studied.

Tools and Techniques to be used:

Pneumatic Mounting Press

Metallography Abrasive Cutting Machine

General Purpose Belt Grinder

Metallography Polishing Machine

Micro hardness Tester (10gms to 2000gms)

Optical Microscope (2000X)

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Introduction

The examination of microstructure is one of the principal means of evaluating alloys and products to

determine the effects of various fabrication and thermal treatments and to analyse the cause of

failure. Main microstructural changes occur during freezing, homogenisation, hot or cold working,

annealing, etc. Good interpretation of the structure relies on having a complete history of the

specimen.

In general, the metallography of metals and metallic alloys is a hard job in the meaning that

materials represent a great variety of chemical compositions and thus a wide range of hardness and

different mechanical properties. Therefore the techniques required for metallographic examination

may vary considerably between soft and hard alloys. Moreover, one specific alloy can contain

several microstructural features, like matrix, second phases, dispersoids, grains, sub grains and

thus grain boundaries or sub boundaries according to the type of the alloy and its thermal or thermo

mechanical history. However, some methods of sample preparation and observation are quite

general and apply to all such materials.

As a general rule, examination should start at normal eye vision level and proceed to higher

magnification. Simplicity and cost make optical examination (macro and micro) the most useful.

When the magnification and the depth of focus become too low, the electron microscopes are

required.

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1. Microstructure

Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by

a microscope above 25× magnification. The microstructure of a material (which can be broadly

classified into metallic, polymeric, ceramic and composite) can strongly influence physical properties

such as strength, toughness, ductility, hardness, corrosion resistance, high/low temperature

behaviour, wear resistance, and so on, which in turn govern the application of these materials in

industrial practice.

1.1 What is microstructure?

When describing the structure of a material, we make a clear distinction between its crystal

structure and its microstructure. The term ‘crystal structure’ is used to describe the average

positions of atoms within the unit cell, and is completely specified by the lattice type and the

fractional coordinates of the atoms (as determined, for example, by X-ray diffraction). In other

words, the crystal structure describes the appearance of the material on an atomic (or Å) length

scale. The term ‘microstructure’ is used to describe the appearance of the material on the nm-cm

length scale. A reasonable working definition of microstructure is:

Microstructure can be observed using a range of microscopy techniques. The microstructural

features of a given material may vary greatly when observed at different length scales. For this

reason, it is crucial to consider the length scale of the observations you are making when describing

the microstructure of a material.

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“The arrangement of phases and defects within a material.”

Fig.1.1 Microstructure of Cartridge Brass

1.2 Why is the microstructure of a material important?

The most important aspect of any engineering material is its structure. The structure of a material is

related to its composition, properties, processing history and performance. And therefore, studying

the microstructure of a material provides information linking its composition and processing to its

properties and performance. Interpretation of microstructures requires an understanding of the

processes by which various structures are formed. Physical Metallurgy is the science which

provides meaningful explanations of the microstructures, through understanding what is happening

is inside a metal during the various processing steps. Metallography is the science of preparing

specimens, examining the structures with a microscope and interpreting the microstructures.

The structural features present in a material are a function of the composition and form of the

starting material, and any subsequent heat treatments and or processing treatments the material

receives. Microstructural analysis is used to gain information on how the material was produced and

the quality of the resulting material. Microstructural features, such as grain size, inclusions,

impurities, second phases, porosity, segregation or surface effects, are a function of the starting

material and subsequent processing treatments. The microstructural features of metals are well

defined and documented, and understood to be the result of specific treatments. These

microstructural features affect the properties of a material, and certain microstructural features are

associated with superior properties.

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1.3 What is Microstructural Analysis used for?

Microstructural and microstructural examination techniques are employed in areas such as routine

quality control, failure analysis and research studies. In quality control, microstructural analysis is

used to determine if the structural parameters are within certain specifications. It is used as a

criterion for acceptance or rejection. The microstructural features sometimes considered are grain

size, amount of impurities, second phases, porosity, segregation or defects present. The amount or

size of these features can be measured and quantified, and compared to the acceptance criterion.

Various techniques for quantifying microstructural features, such as grain size, particle or pore size,

volume fraction of a constituent, and inclusion rating, are available for comparative analysis.

Microstructural analysis is used in failure analysis to determine the cause of failure. Failures can

occur due to improper material selection and poor quality control. Microstructural examination of a

failed component is used to identify the material and the condition of the material of the component.

Through microstructural examination one can determine if the component was made from specified

material and if the material received the proper processing treatments. Failure analysis, examining

the fracture surface of the failed component, provides information about the cause of failure. Failure

surfaces have been well documented over the years and certain features are associated with

certain types of failures. Using failure analysis it is possible to determine the type of stress that

caused the component to fail and often times determine the origin of the fracture.

Microstructural analysis is used in research studies to determine the microstructural changes that

occur as a result of varying parameters such as composition, heat treatment or processing steps.

Typical research studies include microstructural analysis and materials property testing. Through

these research programs the processing - structure - property relationships are developed.

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2. Metallography

Metallography is the science and art of preparing a metal surface for analysis by grinding and

polishing, and etching to reveal the structure of the specimen. Ceramic, sintered carbide or any

other solid material may also be prepared using metallographic techniques, hence the collective

term, materialography.

Metallographic and materialographic specimen preparation seeks to find the true structure of the

material. Mechanical preparation is the most common method of preparing the specimens for

examination. Abrasive particles are used in successively finer steps to remove material from the

specimen surface until the needed metallographic surface quality is achieved. A large number of

materialographic preparation machines for grinding and polishing are available, meeting different

demands on preparation quality, capacity, and reproducibility.

A systematic preparation method is the easiest way to achieve the true materialographic structure.

When the work routinely involves examining the same material, in the same condition, the

metallographer wants to achieve the same result each time. This means that the preparation result

must be reproducible. Different materials with similar properties (hardness and ductility) will respond

alike and thus require the same consumables during preparation. Specimen preparation must

therefore pursue rules which are suitable for most materials.

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Fig. 2.1 Henry Clifton Sorby (1826–1908), founder of metallography

2.1 Sample Preparation

A properly prepared metallographic sample can be aesthetically pleasing as well as revealing from a

scientific point of view. The purpose of this is to understand how to prepare and interpret

metallographic samples systematically.

2.1.1 Cutting Metallic Samples

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.

To use the hacksaw, the sample must be secured in a vice; obviously, the plane of the cut must

contain the direction of the gripping force.

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Fig. 2.2 Specimen to be cut

2.1.2 Sample Mounting

Small samples were difficult to hold safely during grinding and polishing operations, and their shape

was not suitable for observation on a flat surface. They were therefore mounted inside a polymer

block.

For mounting, the sample is surrounded by an organic polymeric powder which melts under the

influence of heat (about 200 oC). Pressure was also applied by a piston, ensuring a high quality

mould, free of porosity and with intimate contact between the sample and the polymer.

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Fig. 2.3 Prepared Mould

Fig. 2.4 Operating Conditions of Mould Preparation

2.1.3 Grinding and Polishingix

Fig. 2.5 Phenolic Powder for Mould

Fig. 2.6 Mould Making Machine

Phenolic Powder and Mould Release

Agent:

Phenolic powder was used as the mould

under 7 Bar of pressure at approximately

around 160 oC. The finished mount, for

better results was ejected after it was

cooled down under pressure to below 30 oC

from the press.

Mould release agent was sprayed prior to

compression mounting to make sure that

the prepared mould does not stick to the

surface of mounting press.

Mould Making Machine

Mould making machine or a Mounting press

was used to obtain the mould for specimen

to be used for further operations on mould

like grinding or polishing.

Regardless of the resin used to compression

mount specimens, the best results are

obtained when:

The specimen are clean and dry

The cured mounts are cooled under

full pressure below 30 oC before

ejection from the press.

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Grinding was done using rotating discs covered with silicon carbide paper and water. There are a

number of grades of paper, with 180, 240, 400, 800, 1200, 1500, 2000 grains of silicon carbide per

square inch. 180 grade therefore represents the coarsest particles and this is the grade to begin the

grinding operation.

Always use light pressure applied at the

centre of the sample. Continue grinding until all the blemishes have been removed, the sample

surface is flat, and all the scratches are in a single orientation. Wash the sample in water and move

to the next grade, orienting the scratches from the previous grade normal to the rotation direction.

This makes it easy to see when the coarser scratches have all been removed. After the final

grinding operation on 2000 paper, wash the sample in water followed by alcohol and dry it before

moving to the polishers.

The polishers consist of rotating discs covered with soft cloth impregnated with diamond particles (6

and 1 micron size) and an oily lubricant. Begin with the 6 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 micron

stage. Any contamination of the 1 micron polishing disc will make it impossible to achieve a

satisfactory polish.

2.1.4 Etching

The purpose of etching is two-fold. x

Fig. 2.7 Grinding Machine Fig. 2.8 Polishing Machine

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.

Materials Composition Application procedure

Brass 1 Part of Ammonium Hydroxide

1 Part 3% Hydrogen Peroxide 1 Part Water

Swab

Iron & Steel 1-5 Parts Nitric Acid 100 Parts Alcohol (nital) Immerse/Swab

Aluminum 10 g Sodium Hydroxide, 100 ml Water Immerse

3. Microstructure Analysis of Aluminium

3.1 Introduction

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Table 2.1 Etchants

Aluminium encompasses a wide range of chemical compositions and product forms that can be

manufactured by all available metalworking techniques and standard casting processes.

Manufactured forms of aluminum and aluminum alloys include standard mill products (e.g., sheet,

plate, foil, rod, bar, wire, tube, pipe, and structural forms) and engineered forms for specific

applications produced by extrusion, forging, stamping, powder metallurgy, semisolid processing,

and machining. Aluminum products also include metal-matrix composites with either particulate or

fiber reinforcement.

3.2 Composition and Phases

Aluminum alloys encompass more than three hundred commonly recognized alloy compositions

and many additional variations developed in supplier/consumer relationships. All commercial

aluminum alloys contain some iron and silicon as well as two or more elements intentionally added

to enhance properties.

Aluminium used to study microstructure is 12% Silicon- Aluminium.

3.3 Standard microstructures of Aluminium

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Fig. 3.1 The principal alloying elements of aluminum alloys.

3.3.1 Aluminium Silicon phase diagram

Element Temperature Liquid solubility Solid solubility

°C °F wt% at. % wt% at.%

Silicon 580 1080 12.6 12.16 1.65 1.59

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Fig. 3.2 Aluminum-silicon phase diagram and cast microstructures of hypoeutectic compositions (<12% Si), hypereutectic compositions (>12% Si), and one close to the eutectic composition of 12% Si

T 3.1 Solubility limits

Using the aluminum-silicon phase diagram, as shown above, the basic process of solidification and

morphology formation is as follows.

When the temperature goes below the liquidus line, the solid-solution phase (α) solidifies first, while

most of the copper remains in liquid form. As the temperature approaches the solidus, the α solid

phase becomes more enriched with Silicon. When the temperature falls below the solidus

temperature in alloys containing less than the maximum solubility (5.65 wt% Si), solidification is

complete to the solid-solution phase condition (α). In alloys containing more than 5.65 wt% Si, some

liquid remains when the eutectic temperature (548 °C, or 1018 °F) is reached. In this case, two

terminal solid-solution phases (α and θ) separate out simultaneously from the molten liquid. On

cooling below the eutectic temperature, a network of eutectic forms in the residual liquid

surrounding the dendrites or grains of primary α.

3.4 Experimental study on Aluminium

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Fig. 3.3 Microstructure of Aluminium (12% Silicon), 400X

(Explanation is made with reference to ASM Handbook, vol. 9)

Various types of (1) as-cast morphologies are obtained, depending on whether the alloy content is

above, below, or near the eutectic composition. In the case of eutectic compositions, the as-cast

structure may have mixed morphologies of both network like and dispersed second phases. In

these cases, a dispersed second phase may occur as a primary product during solidification above

the eutectic temperature. These types of mixed as-cast morphologies are shown below.

4. Microstructure Analysis of Gray Cast Iron

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Fig. 3.4 Microstructure of Aluminium (12% Silicon), 100X

Fig. 3.5 Dispersed phase and network like morphology of second-phase structure in two eutectic alloys. (a) As-cast 413 alloy at 750×. (b) As-cast aluminum-copper alloy at 400×.

4.1 Introduction

Grey iron is a cast iron alloy that has a graphitic microstructure. It’s named after the gray color of

the fracture it forms, which is due to the presence of graphite.

Grey cast irons are softer with a microstructure of graphite in transformed-austenite and cementite

matrix. The graphite flakes, which are rosettes in three dimensions, have a low density and hence

compensate for the freezing contraction, thus giving good castings free from porosity.

The flakes of graphite have good damping characteristics and good machinability. In applications

involving wear, the graphite is beneficial because it helps retain lubricants.

Sulphur in cast irons is known to favour the formation of graphite flakes. The graphite can be

induced to precipitate in a spheroidal shape by removing the sulphur from the melt using a small

quantity of calcium carbide. This is followed by a minute addition of magnesium or cerium, which

poisons the preferred growth directions and hence leads to isotropic growth resulting in spheroids of

graphite. The calcium treatment is necessary before the addition of magnesium since the latter also

has an affinity for both sulphur and oxygen, whereas its spheroidising ability depends on its

presence in solution in the liquid iron. The magnesium is frequently added as an alloy with iron and

silicon (Fe-Si-Mg) rather than as pure magnesium.

However, magnesium tends to encourage the precipitation of cementite, so silicon is also added (in

the form of ferro-silicon) to ensure the precipitation of carbon as graphite. The ferro-silicon is known

as an inoculant.

Spheroidal graphite cast iron has excellent toughness and is used widely, for example in

crankshafts.

4.2 Graphitization

A solid-state transformation of thermodynamically unstable non-graphitic carbon into graphite by xvi

means of heat treatment.

Properties of Gray Cast Iron

ASTM Number

Tensile Strength

(Kpsi)

Compressive Strength 

(Kpsi)

Shear Modulus

of Rupture (Kpsi)

Modulus of Elasticity (Mpsi) Endurance Limit (Kpsi)

Brinell Hardness

H_bTension Torsion

20 22 83 26 9.6-14 3.9-5.6 10 156

25 26 97 32 11.5-14.8 4.6-6.0 11.5 174

30 31 109 40 13.0-16.4 5.6-6.6 14 201

35 36.5 124 48.5 14.5-17.2 5.8-6.9 16 212

40 42.5 140 57 16.0-20 6.4-7.8 18.5 235

50 52.5 164 73 18.8-22.8 7.2-8.0 21.5 262

60 62.5 187.5 88.5 20.4-23.5 7.8-8.5 24.5 302

4.3 TYPICAL USES

Cast iron is used in a wide variety of structural and decorative applications, because it is relatively

inexpensive, durable and easily cast into a variety of shapes. Most of the typical uses include:

- Historic markers and plaques

- Hardware: hinges, latches

- Columns, balusters

- Stairs

- Structural connectors in buildings and monuments

- Decorative features

- Fences

- Tools and utensils

- Stoves and firebacks

- piping.

4.4 Standard Microstructures of Gray Cast Iron

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Table 4.1 Properties of Gray Cast Iron

4.5 IRON-CARBON (Fe-C) DIAGRAM

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As-cast gray iron (Fe-2.8%C-0.8%Si-0.4%Mn-

0.1%S-0.35%P-0.3%Cr). Pearlite Etched with

4% nital. Arrows show the white areas with

weakly etched or non-etched pearlite, 500X

As-cast gray iron, (Fe-3.24%C-2.32%Si-

0.54%Mn-0.71%P-0.1%S). E, phosphorous

ternary eutectic. Etched with 4% nital, 100X

Spheroidal graphite cast iron, Fe-3.2C-2.5Si-

0.05Mg wt%, contains graphite nodules in a

matrix which is pearlitic. One of the nodules is

surrounded by ferrite, simply because the

region around the nodule is decarburized as

carbon deposits on to the graphite. Etchant:

Nital 2%.

The best way to understand the metallurgy is to examine the iron-carbon binary phase diagram.

From the figure above we can make out the phases present in the material taken for analysis which

is cementite, pearlite and transformed leduberite at 3% carbon.

4.6 Experimental Study on Gray Cast Ironxix

Fig. 4.1 Iron Carbon Phase Diagram

4.6.1 Composition

Constituents G-25 G-30

Carbon 2.8% - 3.2% 2.8% - 3.2%

Silicon 1.6% - 2.0% 1.6% - 2.0%

Manganese 0.6% - 1.0% 0.6% - 1.0%

Chromium 0.2% max. 0.35% - 0.5%

Phosphorous 0.2% max. 0.2% max.

Sulphur 0.2% max. 0.2% max.

4.6.2 Microstructure Obtained

(Explanation is made with reference to ASM Handbook, vol. 9)

Figure above shows the graphite flakes surrounded by ferrite immersed in pearlitic matrix.

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Graphite Flakes

Ferrite

Pearlitic matrix

Inclusions

Fig. 4.2 Microstructure obtained (G 25), 100X

Figure shows the closer view of

graphite flakes and the ferrite around it.

Table. 4.2 Composition of Grades of Gray Cast Iron Used

4.7 Inference

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Fig. 4.3 Microstructure obtained (G 25), 400X

Fig. 4.4 Microstructure obtained (G 30), 100X

Figure shows the microstructure of

grey cast iron with finer graphite flakes

which show that the percentage of

chromium is higher than the previous

image.

Figure shows a closer view of the

previous image which shows the finer

graphite flakes and ferrite around it.

Fig. 4.5 Microstructure obtained (G 30), 400X

As it is seen from the various microstructures the graphite is present in the form of flakes which has

precipitates from the austenitic phase. Also it is surrounded by the ferritic phase. All this is present

in a matrix of pearlite as seen in the microstructure.

Presence of certain inclusions can also be seen. From the experimental analysis it was seen that G-

30 has finer flakes than G-25. This is due to the variation of chromium% in the two materials. Hence

variation in percentage of chromium makes the graphite flakes finer.

5. Microstructure Analysis of Brass

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5.1 Introduction

Brass is a metal composed primarily of copper and zinc. Copper is the main component, and brass

is usually classified as a copper alloy. The colour of brass varies from a dark reddish brown to a light

silvery yellow depending on the amount of zinc present; the more zinc, the lighter the colour. Brass

is stronger and harder than copper, but not as strong or hard as steel. It is easy to form into various

shapes, a good conductor of heat, and generally resistant to corrosion from salt water. Because of

these properties, brass is used to make pipes and tubes, weather-stripping and other architectural

trim pieces, screws, radiators, musical instruments, and cartridge casings for firearms.

5.2 Properties

Brass has higher malleability than copper or zinc. The relatively low melting point of brass

(900 to 940°C, depending on composition) and its flow characteristics make it a relatively

easy material to cast.

Today almost 90% of all brass alloys are recycled. Because brass is not ferromagnetic, it can

be separated from ferrous scrap by passing the scrap near a powerful magnet.

Aluminium makes brass stronger and more corrosion resistant. Aluminium also causes a

highly beneficial hard layer of aluminium oxide (Al2O3) to be formed on the surface that is

thin, transparent and self healing.

5.3 Standard Microstructures of Brass

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5.4 Experimental Study on Brass

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Fig. 5.1 Standard Microstructure of Brass (C26000)

Alloy C26000 (cartridge brass), processed

to obtain specific grain size. Preliminarily

hot rolled, annealed, cold rolled, annealed

to a grain size of 25 μm, cold rolled to 70%

reduction. Final anneal at 330 °C (625 °F)

for 5 μm grain size.

Fig. 5.2 Copper-Zinc diagram(Figure shows variation in phases with respect to change in composition and temperature)

Material Used Cartridge Brass (C26000)

Constituents Copper (70%)

Zinc (30%)

5.5 Microstructures of Brass

CONCLUSION

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Table 5.1 Constituents of Brass

Fig. 5.3 Obtained Microstructure of Brass, 100X

Figure shows the eutectic precipitate of

copper and zinc (dark region) α- copper

(white region).

Fig. 5.4 Enlarged View at 400X

This figure is same as the previous figure

at 400X. Shows α-copper grains clearly

and eutectic precipitates at the grain

boundaries as black phase.

We learnt the procedure to prepare the specimen for microstructure analysis.

Extreme literature survey of various microstructure helped us to understand the

microstructure of our materials.

With the literature survey we can explain the respective microstructure.

SCOPE

To observe the microstructure under high magnification microscopes.

To observe the microstructure under different heat treatment condition.

It is interesting to observe the microstructures under SEM (Scanning electron microscope) &

perform ERAX at various places.

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