a project report on microstructure analysis of gray cast iron, aluminium and brass using optical micrographs

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 microconstituent 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 i

•

Micro hardness Tester (10gms to 2000gms)

•

Optical Microscope (2000X)

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

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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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.

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

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. vi

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.

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.

Fig. 2.2 Specimen to be cut vii

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.

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.

Fig. 2.3 Prepared Mould

Fig. 2.4 Operating Conditions of Mould Preparation viii

Phenolic Powder and Mould Release 8

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.

Fig. 2.5 Phenolic Powder for Mould

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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Fig. 2.6 Mould Making Machine

2.1.3 Grinding and Polishing 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.

Fig. 2.7 Grinding Machine

Always use light pressure applied at the Fig. 2.8 Polishing Machine

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. x

2.1.4 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.

Materials

Composition

Application procedure

Brass

1 Part of Ammonium Hydroxide

Swab

1 Part 3% Hydrogen Peroxide 1 Part Water Iron & Steel

Aluminum

1-5 Parts Nitric Acid 100 Parts Alcohol (nital)

Immerse/Swab

10 g Sodium Hydroxide, 100 ml Water

Immerse

Table 2.1 Etchants

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3. Microstructure Analysis of Aluminium 3.1 Introduction 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.

Fig. 3.1 The principal alloying elements of aluminum alloys.

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3.3 Standard microstructures of Aluminium

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

Element Temperature Liquid solubility

Silicon

Solid solubility

°C

°F

wt%

at. %

wt%

at.%

580

1080

12.6

12.16

1.65

1.59

T 3.1 Solubility limits

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3.3.1 Aluminium Silicon phase diagram 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.

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

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3.4 Experimental study on Aluminium

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

(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.

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×.

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4. Microstructure Analysis of Gray Cast Iron 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.

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4.2 Graphitization A solid-state transformation of thermodynamically unstable non-graphitic carbon into graphite by means of heat treatment.

Properties of Gray Cast Iron Shear Tensile Compressive Modulus Modulus of Elasticity (Mpsi) Endurance Brinell ASTM Strength Strength of Limit Hardness Number (Kpsi) (Kpsi) Rupture (Kpsi) H_b Tension Torsion (Kpsi) 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

Table 4.1 Properties of Gray Cast Iron

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.

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4.4 Standard Microstructures of Gray Cast Iron As-cast gray iron (Fe-2.8%C-0.8%Si-0.4%Mn0.1%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,

0.54%Mn-0.71%P-0.1%S).

(Fe-3.24%C-2.32%SiE,

phosphorous

ternary eutectic. Etched with 4% nital, 100X

Spheroidal graphite cast iron, Fe-3.2C-2.5Si0.05Mg 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%.

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4.5 IRON-CARBON (Fe-C) DIAGRAM

Fig. 4.1 Iron Carbon Phase Diagram

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.

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4.6 Experimental Study on Gray Cast Iron 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.

Table. 4.2 Composition of Grades of Gray Cast Iron Used

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

Graphite Flakes

Ferrite

Pearlitic matrix

Inclusion s

Figure shows the closer view of graphite Fig. 4.2 Microstructure obtained (G 25), 100X

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

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

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.

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

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

4.7 Inference xxi

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 G30 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 xxii

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 xxiii

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. 5.1 Standard Microstructure of Brass (C26000)

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

5.4 Experimental Study on Brass xxiv

Material Used

Cartridge Brass (C26000)

Constituents

Copper (70%) Zinc (30%)

Table 5.1 Constituents of Brass

5.5 Microstructures of Brass

Figure shows the eutectic precipitate of copper and zinc (dark region) α- copper (white region).

Fig. 5.3 Obtained Microstructure of Brass, 100X

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.

CONCLUSION Fig. 5.4 Enlarged View at 400X

•

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. xxv

•

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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