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Title:

Microstructure examination of steel

Objective:

To observe the constituents and structure of metals and their alloys by means of an optical microscope.

Introduction:

The properties of metals highly depend on their structures. The internal structures determine how mate

perform under a given application. The effects of most industrial processes applied to metals to control

properties can be explained by studying their micro structures. Metallography is the study of metal

optical electron microscope. Structures which are coarse enough to be discernible by the naked eye or u

low magnifications are termed macro structures.

Those which require high magnification to be visible are termed micro structures. Microscopes are requ

for the examination of the metals. Optical microscopes are used for resolutions down to roughly

wavelength of light and electron microscopes are used for detail below this level, down to atomic resolut

The most commonly used microscope is the conventional light microscope. In principle, optical microsc

may be used to look through specimens as well as at them. Many materials, however, do not transmit

and so we are restricted to looking at the surface of the specimens with an optical microscope.

Theory:

The most common method used to examine the structures of materials is optical technique. A mirror poli

produced on one face of the specimen by grinding on successively fine emery (sand) papers and polishin

revolving cloth wheels with fine abrasives such as diamond and alumina powder. The micro structu

revealed by a surface treatment using an appropriate chemical reagent in a procedure termed etching.

etching reagents depend on the material used and after etching the specimen must be washed with alc

and ether to remove the grease. The atoms at the grain boundaries are chemically more active,

consequently dissolve more readily than those within the grains forming small grooves. These gro

become discernible when viewed under a microscope because they reflect light angle different from th

the grains themselves.

To reveal the structural details such as grain boundaries, phases and inclusions this polished surface is et

with chemical solutions. The etchant attacks various parts of the specimen at different rates and reveal

structure. A metallographic microscope is used to examine the microstructure.

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Materials and Apparatus:

Light microscope.

Steel specimen.

Abrasive papers and powders.

Alcohol

Procedure:

1. Observed the microstructure and features of the steel specimens under the optical microscope.

2. Sketched the observed microstructure and their feature.

3. Determined the average grain size.

Grain size determination

1. Straight lines, all of the same length, are drawn through several photomicrographs that sho

the grain structure.

2. The grain intersected by each line segment are counted.

3. The line length is then divided by an average of the number of grains intersected, taken ov

the line segments.

Calculations:

Total amount of grains that cut the lines

Average number of grain per line = = 42/8

Number of lines that you drawn

= 5.25

Length of a line

Average grain size = = 102.04 micrometer/5.25

Average number of grain per line

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Results:

Average number of grain per line = 5.25

Length of a line = (50 micrometer/4.9 cm) x 10 cm = 102.04 micrometer

Average grain size = 19.43 micrometer

Conclusion:

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

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

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

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propert ie s and per fo rmance. In te rp re ta tion of mi crost ruc tur es requi re s an unders tandi ng

of thepr ces s es by whi c h va r i ous s t r uc t u r es a r e f o r med . Phys i ca l Met a l l u r gy i s t he

science which provides meaningful explanations of the microstructures, through understanding what

happening is inside a metal during the v arious processing steps.

Metallography is the science of preparing specimens, examining the structures with a microscope a

interpreting the microstructures. Th e st ru ct ur al fe at ur es pr es en t in a ma te ri al ar e a fu nc t

of the composition and form of the starting material, and any subsequent heat treatmen

and or processing treatments the material receives. Microstructural analysis is used to gain informa

on how the material was produced and the quality of the resulting material.

Mic ros t ruc tur al feat ure s , s uch as g rain s ize , i nclu s ion s , impuri ties , second phases,

po rosi ty, segr egation or su rface effects , are a function of the st art ing material and subsequ

process ing tr ea tmen ts. Th e mi cros tructural features of me ta ls are

welld e f i n e d a n d d o c u m e n t e d , a n d u n d e r s t o o d t o b e t h e r e s u l t o f s p e c i f i c

a t m e n t s . T h e s e microstructural features affect the properties of a material, and certain microstructu

features are associated with superior properties.

References:

Van Vlack, L. H., Elements of Materials Science and Engineering, Addison-Wesley Pub. Co., (Mass:19

Guy, A. G., Elements of Physical Metallurgy, Addison-Wesley Pub. Co., (Mass:1959)

Kehl, G. L., Principles of Metallographic Laboratory Practice, McGraw Hill Book Co. , (New York:194

Adapted from ASM Metals Handbook, Ninth Edition, v. 9, "Metallography and Microstructures", Ameri

Society for Metals, Metals Park, OH, 1985, p. 12

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