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MATERIAL SCIENCE

TERM PAPER

ON

IMPORTANCE OF MICROSTRUCTURES ANDMICROSTRUCTURES OF CAST IRON

Submitted By:

Pranav Dimri

Sec:M2R11

Roll No:A56

Reg. No:11013579

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CONTENTS

Introduction

Methods

o Microscopy

Optical

X-Ray microtomographic Electron microscopy

Types of Microstructures

Conclusions

http://en.wikipedia.org/wiki/Microstructure#Methodshttp://en.wikipedia.org/wiki/Microstructure#Microscopyhttp://en.wikipedia.org/wiki/Microstructure#Opticalhttp://en.wikipedia.org/wiki/Microstructure#Opticalhttp://en.wikipedia.org/wiki/Microstructure#X-Ray_microtomographichttp://en.wikipedia.org/wiki/Microstructure#Electron_microscopyhttp://en.wikipedia.org/wiki/Microstructure#Microscopyhttp://en.wikipedia.org/wiki/Microstructure#Opticalhttp://en.wikipedia.org/wiki/Microstructure#X-Ray_microtomographichttp://en.wikipedia.org/wiki/Microstructure#Electron_microscopyhttp://en.wikipedia.org/wiki/Microstructure#Methods

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INTRODUCTION

Microstructure is defined as the structure of a prepared surface orthin foil of material as revealed by a microscope above 25magnification. The microstructure of a material (which can bebroadly classified into metallic, polymeric, ceramic and composite)can strongly influence physical properties such as strength,toughness, ductility, hardness, corrosion resistance, high/low

temperature behavior, wear resistance, and so on, which in turngovern the application of these materials in industrial practice.

http://en.wikipedia.org/wiki/Metallographyhttp://en.wikipedia.org/wiki/Polymerichttp://en.wikipedia.org/wiki/Ceramographyhttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Metallographyhttp://en.wikipedia.org/wiki/Polymerichttp://en.wikipedia.org/wiki/Ceramographyhttp://en.wikipedia.org/wiki/Composite_material

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METHODS

The concept of microstructure is perhaps more accessible to thecasual observer through macrostructural features in commonplaceobjects. If one ever comes across a piece of galvanized steel, suchas the casing of a lamp post or road divider, one observes that thesurface is not uniformly colored, but is covered with a patchwork ofinterlocking polygons of different shades of grey or silver. Eachpolygon (the most frequently occurring would be hexagons) is a

single crystal ofzinc adhering to the surface of the steel beneath.Zinc and lead are two common metals which form large crystalsvisible to the naked eye. The metallic atoms in each crystal arewell-organized into one of seven crystal lattice systems possible formetals (cubic, tetrahedral, hexagonal, monoclinic, triclinic,rhombohedral, orthorhombic); these systems dictate that the atomsare all lined up like points in a 3-D matrix. However, the direction ofalignment of the matrices differ from crystal to adjacent crystal,leading to variance in the reflectivity of each presented face of theinterlocked crystals on the galvanized surface. Symmetricalcrystals are generally unstressed, unworked. They grow in alldirections equally and were not subjected to deforming stresseseither during or after. For large crystals, the ratio of crystal bulk tointer-crystal boundary (more properly, intergranullar boundary) ishigh. This indicates high ductility but correspondingly, lowerstrength (see Hall-Petch Strengthening), but a true study wouldtake into quantitative account the relative strengths of the crystal

and that of inter-crystal bonding.

http://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Crystal_latticehttp://en.wikipedia.org/wiki/Hall-Petch_Strengtheninghttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Crystal_latticehttp://en.wikipedia.org/wiki/Hall-Petch_Strengthening

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MICROSCOPY

1.Optical

When a polished flat sample reveals traces of its microstructure, itis normal to capture the image using macrophotography. Moresophisticated microstructure examination involves higher poweredinstruments: optical microscopy, electron microscopy, X-raydiffraction and so on, some involving preparation of the material

sample (cutting, microtomy, polishing, etching, vapor-depositionetc.). The methods are known collectively as metallography asapplied to metals and alloys, and can be used in modified form forany other material, such as ceramics, glasses, composites, andpolymers.

Two kinds of optical microscope are generally used to examine flat,polished and etched specimens: a reflection microscope and an

inverted microscope. Recording the image is achieved using adigital camera working through the eyepiece.

http://en.wikipedia.org/wiki/Macrophotographyhttp://en.wikipedia.org/wiki/Optical_microscopyhttp://en.wikipedia.org/wiki/Electron_microscopyhttp://en.wikipedia.org/wiki/X-ray_diffractionhttp://en.wikipedia.org/wiki/X-ray_diffractionhttp://en.wikipedia.org/wiki/Microtomyhttp://en.wikipedia.org/wiki/Etchinghttp://en.wikipedia.org/wiki/Metallographyhttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/Glasshttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Inverted_microscopehttp://en.wikipedia.org/wiki/Macrophotographyhttp://en.wikipedia.org/wiki/Optical_microscopyhttp://en.wikipedia.org/wiki/Electron_microscopyhttp://en.wikipedia.org/wiki/X-ray_diffractionhttp://en.wikipedia.org/wiki/X-ray_diffractionhttp://en.wikipedia.org/wiki/Microtomyhttp://en.wikipedia.org/wiki/Etchinghttp://en.wikipedia.org/wiki/Metallographyhttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/Glasshttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Inverted_microscope

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2.X-Ray microtomographic

Nondestructive testing of microstructure for biological materials is achallenge and computermicrotomography is the current solution. Infact, CMT can be used for the evaluation of microstructure of manyother materials also. CMT can be very expensive though, and forresearch purposes, it is a necessity to generate a three-dimensional microstructure from two-dimensional cross-sectionalimages of the material. This is an area of active research andpursued by many scientists.

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3.Electron microscopy

Aluminium copper (4 at% Cu) alloy showing copper precipitationwithin the aluminium matrix. The image is a projection through themetal where the dark regions are plate like copper precipitates

For high-resolution information on metallurgical microstructures,electron microscopic methods can be employed. This can allow fordirect observation of atomic-scale features such as very fineprecipitation reactions, dislocations or grain-boundary interfaces.Such methods may be critical in determining parameters such assolid state diffusivities.

http://en.wikipedia.org/wiki/Dislocationhttp://en.wikipedia.org/wiki/File:Al-Cu-BrightField.jpghttp://en.wikipedia.org/wiki/Dislocation

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TYPES OF MICROSTRUCTURES

This is the structure the soil has under the microscope. It is relatedwith the size, shape and arrangement of the primary particles andthe voids in aggregate and non-aggregate materials and the size,shape and arrangement of any aggregate present.

Microstructures of sand grains

Single grains. Almost entirely formed of sand grains, without, orwith very little, fine material between the grains. Loose grains, orslightly touching.

Bridged grains. Sand grains joined by bridges of fine material.

Pellicular grains. Sand grains covered with a pellicle of fine material.

Intergrain micro-aggregates. There are fine material micro-aggregatesbetween the sand grains.

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Intergrain vesicles. With vesicles between the sand grains.

Intergrain channels. With channels between the sand grains.

Compact grains. With sand grains very close together, leading to avery compact structure.

From nickel alloys such as RA330, alloy 600 and others that arecommercially available. Chemistries for some common alloys are shown inprovides a guide for what impact these alloy additions have on high-temperature properties.

In the search for the lowest cost, some important details may beoverlooked. Although the chemistries may be the same, alloys fromdifferent sources may perform much differently in service.

Critical alloy property requirements for most furnace fixturesinclude strength, resistance to thermal fatigue, oxidation andarburization resistance. These properties are interrelated inimpacting the life of alloy fixtures. Strength is normally consideredto be defined by the creep or rupture behavior and is improved by

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coarser grain structures. Thermal-fatigue resistance is also stronglyrelated to grain size, but finer grains are desirable in this case.Finer-grained alloys typically ASTM 4 or finer provide superiorlife in thermal fatigue but with some sacrifice in creep-rupture

strength.

In practice, quenching fixtures are not subject to dead loads butrather to a combination of mechanical loads and cyclic thermalstrains. Thermal-fatigue damage in a coarse-grained bar can leadto more significant distortion and damage in service than the use ofa finer-grained alloy with lesser creep-rupture strength.

Cast and Wrought Structures:-

Case in point is that cast alloys, which typically have a very coarsegrain structure, offer superior structural strength at red heat ascompared to a finer-grained wrought product. They do, however,commonly suffer from cracking when subjected to cyclic duty dueto their coarse grain structure.

Figures 1-4 compare cast and wrought grids used at a wireprocessor that provides spheroidized, normalized and annealingservices. Traditional cast HT alloy grids were used in theirfurnaces. Their process involves the heating of steel-rod coils in anitrogen atmosphere to approximately 1500F followed by acontrolled cooling.Cast grids typically provided a lifetime of 18-24 months until majorrepairs were required, with cracking becoming visible after about a

year of service. At that point, cracks in the castings led tosegments breaking away from the trays (Fig. 3). A leading heat-resistant alloy fabricator noted the cracking problems associatedwith the cast trays and proposed a new fabricated design usingRA330 alloy.

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In contrast to cast HT, this wrought alloy has a carbon content of0.05% and is also manufactured to control its grain size, whichgreatly enhances resistance to cracking from thermal cycling.RA330 is also immune to sigma-phase formation. As a result, even

after long-term exposure at 1500F, RA330 alloy retains itsductility. This allows for life extension through restraighteningand/or weld repair. After six years in service, the fabricated gridsare still performing well. Good ductility has allowed for occasionalrestraightening as required.

Grain Size Variation in Wrought Alloys:-

Over the years, alloy suppliers have looked for ways to reduce theproduction costs of these heat-resistant alloys by streamlining theproduction process. While the chemistry remains consistent,altering the mill production process can yield a rod coil product witha wide variety of microstructures. Some of these are suitable forhigh-temperature quenching, and others are not. Figures 5 and 6show acceptable and unacceptable microstructures seenthroughout the years. Experience shows that both the chemistryand the microstructure are critical to the performance of a wroughtfabricated tray as well.

Case Study Alloy-600 Bar Baskets:-

A study done on alloy-600 bar baskets fabricated using acombination of coarse-grained (ASTM 4) and fine-grained (ASTM

8-9) rods also showed the benefit of fine grains in quenchingservice. In this study, the baskets were evaluated after one year ofservice in a combination of 50% straight hardening at 1650F, 30%carburizing at 1650F and carbonitriding at 1500-1600F. All cyclesincluded an oil quench. It was found that carburization of the barbaskets was 0.022-0.043 inches deep in the fine-grained bars

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versus 0.050-0.060 inches in coarse-grained samples. Thermal-fatigue cracks were twice as deep in the coarse bars as the fine-grained bars on average.

Case Study Corrugated Boxes:-

J.C. Kelly likewise indicates in his experience that grain sizes finerthan ASTM 4 are desirable for thermal-fatigue resistance inquenching fixtures. A test was conducted with corrugated boxeswhere half the box was constructed of material with a grain size of

ASTM 4 and the other half was ASTM 00. After 18 months of

service in a carbonitriding operation at 1650F followed by an oilquench, the corners of the box cracked in the coarse-grainedproduct while no cracking was evident in the finer-grained product.

Similar support for the benefit of fine-grained material in quenchingservice can be derived by considering the markets for two nickelalloys. RA330 at 35% nickel and 19% chromium is fairly similar inchemistry to alloy 800H/AT (32% Ni, 20% Cr). RA330 is common tothe heat-treating industry, whereas 800H/AT is commonly used inthe petrochemical industry.

The difference between the two involves their processing at the milllevel. RA330 is mill annealed at a more moderate temperature typically around 1900F whereas 800H/AT is required to besolution annealed at 2100F minimum. Higher annealingtemperatures yield a coarse grain structure (commonly ASTM 4 orcoarser) and maximize creep-rupture strength in the 800H/AT

product. RA330 has a typical grain size finer than ASTM 4. As aresult, 800H/AT is most suited to structural components ofpetrochemical furnaces where operation is continuous andtemperature cycling is minimal. RA330, with its finer grain structure,is more suitable for the rigors of thermal shock encountered byfixtures in the heat-treat industry.

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CONCLUSIONS

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While chemistry plays an integral part in the performance of a heat-resistant alloy, the microstructure of a material, as shown here,

impacts resistance to thermal fatigue. Understanding both willensure the proper material and method of manufacture areselected for your application. Before substituting alloys of similarchemistry, it is suggested that you check with your alloy supplier toensure that the products are designed for similar operatingconditions.

It would be an unusual heat-treat operation if it has notexperienced dramatically rising costs in recent years. The cost ofenergy and materials has been on the rise. Heat-resistant alloypricing has risen whether cast or wrought. A significant factor in thehigher costs for these alloys is the recent rise of nickel prices.Many heat-treat fixtures are used in applications other thancarburizing, nitriding or carbonitriding. In services like annealing,vacuum heat treating, etc., alloys high in nickel (35-76%) such asRA330, HR-120 and alloy 600 are used because nickel alloys are

typically stronger than stainless steels, but not necessarily forenvironmental resistance. RA 253 MA (UNS S30815) is an 11%nickel alloy with added nitrogen. It has been utilized for high-temperature service for more than two decades. Its most significantuse has been in steel mills and power-plant applications. It is,however, gaining in popularity in the industrial heat-treat industrybecause of its low initial cost and high creep strength. The nitrogenprovides high strength without the need for a coarse grain structure

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Importance of microstructure in determining environmentalsusceptibility of stainless steels

Weight loss and stress corrosion studies on ASTM A304 stainlesssteels subjected to different degrees of cold working and exposed

to magnesium chloride solutions at different pH levels revealed that

environmental resistance is a sensitive function of microstructure.

Slip bands are preferential regions of attack in cold worked

structures, while sensitization alters the crack path to follow the

grain boundaries. The similarities in the relative susceptibility of the

various microstructural features under conditions of generalcorrosion, stress corrosion, and hydrogen embrittlement suggests

that any mechanism that is developed must be able to

comprehensively explain the observed phenomena under all three

forms of environmental degradation.