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EXPERIMENT 1

HEAT TREATMENT OF METALS

Objectives:

- To understand the basic principle of hardness measurement- To be able to perform the hardness testing.- To be able to analyze the results obtained.- To recognize and identify each micro constituent present in the microstructure of metals- To use phase diagram or TTT diagram to relate the microstructure developed with the effects

of composition, impurities and heat treatment- To deduce the materials properties based on microscopic examination.

Introduction for Hardness Measurement:

Hardness is a measure of a materials resistance to localized plastic deformation (e.g., a smalldent or a scratch). Hardness tests are performed more frequently than any other mechanical testfor several reasons:1. They are simple and inexpensive ordinarily no special specimen need be prepared, and the

testing apparatus is relatively inexpensive.2. The test is nondestructive the specimen is neither fractured nor excessively deformed; a small

indentation is the only deformation.3. Other mechanical properties often may be estimated from hardness data, such as tensile

strength.

There are the most common hardness test methods used in today`s technology:1. Rockwell hardness test2. Brinell hardness

3. Vickers hardness4. Knoop hardness5. Shore hardness.

Steels are based on the Fe-C system and exhibit both diffusional and diffusionless transformation.Martensite is the name given to the diffusionless shear-type transformation that occurs when thehigh-temperature austenitic phase is rapidly cooled or quenched at such a rate as to avoiddecomposition by a diffusional mechanism. The rapid cooling retains carbon in super-saturatedsolid solution to produce a body-centered tetragonal crystal structure.As-quenched martensite is very hard, brittle and in a state of stress. In order to change it to amore useful form it needs stress relieving and tempering to restore ductility and toughness.

Tempering involves reheating it to a temperature below the eutectoid temperature. Duringtempering, the carbon trapped in solution is able to diffuse, relaxing the BCT to a BCC structure,and iron carbides precipitates begin to form; the precise nature of the carbide phase depends onthe composition of the steel and tempering temperature and duration. Generally, the iron carbideswill not be detected by light microscopy at any but the highest (~650 0C) tempering temperatures;they are finely dispersed at lower tempering temperatures and indeed can contribute to strength.Thus, tempered martensite can deliver excellent combinations of strength and toughness superiorto diffusionally formed microstructures. Consequently, it is the preferred structure for a number of engineering applications. This experiment is designed to compare the heat treatment response of aplain carbon steel BS 970/080M40 (En8) with that of a low alloy steel BS 970/817M40 (En24)containing similar concentrations of carbon.

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Introduction for Microstructures:

Microstructure is subject to direct microscopic observation, using optical or electron microscopes.In metal alloys, microstructure is characterized by the number of phases present, their proportions,and the manner in which they are distributed or arranged. The microstructure of an alloy dependson such variables as the alloying elements present, their concentrations, and the heat treatment of

the alloy (i.e., the temperature, the heating time at temperature, and the rate of cooling to roomtemperature).The term phase equilibrium, often used in the context of this discussion, refers to equilibrium asit applies to systems in which more than one phase may exist. Phase equilibrium is reflected byconstancy with time in the phase characteristics of a system. Equilibrium phase diagramsrepresent the relationships between temperature and the compositions and the quantities of phasesat equilibrium.For a binary system of known composition and temperature that is at equilibrium, at least threekinds of information are available: (1) the phases that are present, (2) the compositions of thesephases, and (3) the percentages or fractions of the phases. The procedures for making thesedeterminations will be presented in this experiment.

Procedure:Samples:

Specimen No.:

1. Normalized - EN8/EN242. Water quench with agitation - EN8/EN243. Oil quench with mild agitation - EN8/EN244. Copper5. Copper after annealing

The Hardness Measurements:1. Remove any oxidation scale from 2 opposite faces by grinding on SiC paper and measure the

Rockwell Hardness on C scale (only for EN8/EN24 samples) and Rockwell Hardness on Fscale (for copper samples) or any appropriate scale . 3 consistent readings should beobtained from each sample. Spurious readings, usually associated with quench-cracks, shouldbe discarded. Submit your raw data in table format at the end of the experiment.

2. Remove the oxidation scale completely and polish smoothly for the micro-hardness test onvickers scale (HV). Perform the micro-hardness test using THREE (3) different COUNTERLOADS , 100gm, 200gm, and 300gm. Take and record TEN (10) readings on each counterload. Analyze the results obtained statistically and write your comments.

The microstructure determination:

Examine the specimens using optical microscope with appropriate magnification and field of view. The microscope is linked to a computer which could display and store the microstructureimages. In the computer, launch the microstructure software and capture the photomicrographs tosave in JPEG file format.

Using table below, hand-sketch all the photomicrographs of the specimens using the mostappropriate magnification (usually >100x). Indicate the field of view and carefully annotate yourdrawings with the corresponding micro-constituents identity. Record all the relevant microscopicobservation in your results.

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Discussion Write-up:

Draw a line graph showing the hardness response for all specimen no.1 to no.5. Comment onthe differences. Also, by using line graph, show the effects of (a) tempering time and (b)tempering temperature on the hardness response of tempered martensite; results for the twotypes steels must be shown on the same graphs.For micro-hardness testing (i.e microvickers) comment on the possible errors encounterduring the testing. Propose the best counter load used in order to give accurate and consistentresults. Justify your reasons. Explain what are the typical products or components that aresuitable for this kind of hardness measurement. In your formal report, include the properly hand drawn microstructures alongside with thephotomicrograph. The figures should complete with annotation and brief explanation notes.Recognise the phases present for each specimen and include in your discussion in each casehow the microstructures developed in terms of heat treatment, cooling rate, carbon content,impurities content etc. Iron-iron carbide phase diagram in Fig.E8.1should be used extensivelyin order to determine the phases present in each specimen. Also, identify the distinguishablematerials properties based on its microstructure and name some of the applications for each

specimen.

Note : Etching is used in order to make the specimen visible under the inspection of microscope.Etching involves chemical reaction which reacts with binary/multi-phase alloys differently andproduces different texture for each phase.

References:

1. William D. Callister, Jr., Materials Science and Engineering, An Introduction , 2008 , JohnWiley.

2. http://www.calce.umd.edu.

3.

J. F. Shackelford, Introduction to Materials Science for Engineers ,2007, Practice Hall.4. R.W.K. Honeycombe & H.K.D.H Bhadeshia, Steels: Microstructure and Properties ,1995 ,Edward Arnold.