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Etching

Microscopy analysis of a properly polished, unetched specimen will reveal only a few structural features such

as inclusions and cracks or other physical imperfections. Etching is used to highlight, and sometimes identify,

micro structural features or phases present. Even in a carefully prepared sample, a surface layer of disturbedmetal, resulting from the final polishing stage, is always present and must be removed. Etchants are usually

dilute acid or dilute alkalis in water, alcohol or some other solvent. Etching occurs when the acid or base is

 placed on the specimen surface because of the difference in rate of attack of the various phases present and

their orientation. The etching process is usually accomplished by merely applying the appropriate solution to

the specimen surface for several seconds to several minutes. Nital, a Nitric acid – Alcohol miture !e.g. "#$%, is

the etchant commonly utili&ed with common irons and steels. The specimen is immersed into Nital solution.

'ive seconds is usually sufficient for proper etching depending on sample and nital concentration. The sample

is immediately washed under running water and dried in an air blast. (o not touch, wipe or swab the specimen

following etching and then move on to the microscopic eamination stage)

Microscopy analysis

". *nitial microscopic viewing should be done utili&ing a stereomicroscope, which reveals a three+

dimensional scanning of the specimen surface. The specimen is placed on the stage of the microscope so that

its surface is perpendicular to the optical ais.

. (etailed viewing is done with a Metallurgical Microscope. A metallurgical microscope has a system

of lenses !ob-ectives and eyepiece% so that different magnifications !"/, 0/ and "/% can be achieved.

The important characteristics of the microscope are# !"% magnification, !% resolution and !1% flatness of field.

1. Take a photograph of each specimen at the optimal magnification. !if the camera is not working then

sketch the microstructure of each specimen%.

0. Make sure that the specimen that you prepared is clearly labelled and the specimen surface is in its

final condition2 it must be submitted with your lab write+up to the laboratory instructor.

The resultant magnification is the product of the magnifying power of the ob-ective and that of the ocular.

3canning Electron Microscopes !3EMs% are capable of magnifications up to ,/ and Transmission

Electron Microscopes !TEMs% are utili&ed to view at magnifications up to ",/ for highly detailed micro

structural study.

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4.Why are metallographic samples sometimes mounted

in plastic?

Metallographic samples need to be well levelled and polished. Polishing is best done on a

machine with a mechanical holder (to apply uniform motion and a steady down force)

 which can only hold samples of standard cylindrical diameters. But one very often

cannot cut the sample material into that shape or the sample is just too small. So it is

easier to cast the sample inside plastic. Also that way the bac! side of the sample can be

easily levelled. "inally the stages of metallographic microscopes are usually set to

accommodate such samples.

#he plastic is not chosen at random$ it is an epo%y resin which is easy to prepare (bymi%ing the two precursors into a solvent) and cast with your samples into standard si&ed

cylindrical moulds. 't has also the advantage of not smearing on your metallic surface

and not clogging your abrasive papers.

5.What is the difference between hypo-eutectoid steel

and hyper-eutectoid steel?

ypoeutectoid steel has less than *+, of - in its composition.

ypereutectoid steel has between *.+, and , of -.

/utectoid would be with a percentage of *.+ every percentage is of weight.

 A ypo/utectoid phase is a phase in which the concentration of the component on the%a%is of the phase diagram is less than the concentration of the same component at theeutectoid point. Similarly A hypereutectoid phase is the a phase in which theconcentration of this component is more than the /utectoid point.

'n case of steel the hypereutectoid phase will have lesser carbon content than the

eutectoid or hypereutectoid steel. More specifically less than *.00, (which is the carboncontent at the eutectoid point. ypereutectoid steel will contain *.00, carbon.

1ow coming to properties they differ due to the presence of different micro structures. At temperatures above eutectoid temperature the hyper eutectoid phase contains 2ferrite 3 cementite ("e4-) whereas hypereutectoid steel contains 5ferrite and 2ferrite.6n cooling below the eutectoid temperature the resulting microstructures are alsodifferent. According to the 7i!ipedia page for carbon steels8

-ooling a hypoeutectoid steel (less than *.00 wt, -) results in a lamellarpearlitic

structure of iron carbide layers with 5ferrite (pure iron) between. 'f it is hypereutectoidsteel (more than *.00 wt, -) then the structure is full pearlite with small grains (larger

http://en.wikipedia.org/wiki/Ferrite_(iron)http://en.wikipedia.org/wiki/Ferrite_(iron)

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than the pearlite lamella) of  cementitescattered throughout. #he relative amounts of

constituents are found using the lever rule.

6.

i)What are the phases of eutectoid steel at roomtemperature?

The lowest temperature at which austenite transforms into ferrite and cementite. 3teel with

.44 percent carbon transforms at this temperature.

Conclusion

As conclusion, the ob-ectives of the eperiment were successfully achieved. The method of 

 preparation of metallographic sample was understood. There are $ process need to be through

 by the specimen which are cutting, molding, grinding, polishing and finally etching. The

other aim which is to understand the concept of etching and the purpose of etching also

successfully achieved. Etching process was carried out to selectively attack the surface of 

metals to reveal grains boundaries, phases, precipitates, inclusions and variations in

composition.

5ther than that, the basic function of optical microscope and it operation also understood.

The structure of the material can be observed with the aid of optical microscope. 'inally, the

ob-ective of the used and performed analysis from the microstructure6s observed also

successfully achieved. The microstructure of the sample obtain was a miture of ferrite and

 pearlite which is a hypoeutectoid steel.

http://en.wikipedia.org/wiki/Cementitehttp://en.wikipedia.org/wiki/Cementitehttp://en.wikipedia.org/wiki/Lever_rulehttp://en.wikipedia.org/wiki/Cementitehttp://en.wikipedia.org/wiki/Lever_rule