Work of an Optical Microscope

Dear Student.

The obvious answer would be so that you could see the difference in structure between the gas nitrided layer and the non-nitrided core. You will not, however, be taking "images of case hardness," you will be taking images of the structure. It is possible that you will be attempting a Knoop microhardness test, in which case you will be measuring the dimensions of an indenter in the nitrided case to determine hardness ( resistance to penetration).

for student work the following wikipedia link should suffice:

http://en.wikipedia.org/wiki/Knoop_hardness_test

The more important (and even more obvious) question is "Why on earth would anyone Gas nitride a low carbon austenitic stainless steel?"

If anyone can give me an answer to THAT question I'll be obliged!

milo

Good question from Milo. Perhaps the stainless is needed for corosion resistance but needs local hardening for some reason? I know that 316LN is used in the vacuum industry for its low outgassing properties, perhaps that's a clue.

I'd answer with another quite obvious fact but nit mentioned by our friend: The optical microscope IS the instrument to observe the different microstructure in the material, and also to evaluate the grain size and inclusions presence if you observe the specimem with and without etching, due to its magnifying range. Of course, a more powerfull instrument would not help you in this task.

Great Point bh.

I stuck to the specifics of the question- IE whats the microscope got to do with hardness.

You correctly establish other ways that optical microscopy can contribute to the "characterization" of the material being studied.

Thanks for contributing.

milo

i personally thank everyone 4 helping me out wit that question.the specimen was gas nitrided for corrosion resisting some of d components of d nuclear industry that require a hard case and a less hardened core.

Careful here student.

I'd ask the Nuclear engineers to rethink their design. The nitriding will actually subject the material to greater threat of corrosion by galvanic cells within the nitrided layer between the layer itself and contained nitride. The second reference below also mentions an issue with 'expended austenite.'

First reference: Moreover, the corrosion. resistance of nitrided stainless steel is additionally reduced due to the presence in nitrided layers of galvanic cells.

Article: Corrosion resistance of stainless steel after high-temperature nitriding

Journal Materials Science Publisher Springer New York ISSN 1068-820X (Print) 1573-885X (Online) Subject Chemistry and Materials Science and Russian Library of Science Issue Volume 6, Number 1 / January, 1972

Second Reference:The results revealed that corrosion resistance for layers containing "expended austenite" is better even than that of austenite. However, the presence of surface defects enables austenitic matrix to come into contact with aggressive environment and corrosion proceeds beneath the layer. The presence of nitrides in the nitrided layers reduces their protective properties. For layers mainly composed of nitrides the corrosion proceeds in the form of gradual decomposition of the layers. -

Surface and Coatings Technology
Volume 200, Issues 22-23 , 20 June 2006, Pages 6623-6628
EMRS 2005 Symposium K - Protective Coatings and Thin Films

milo

thank u very much sir but it is just a research tats goin on safe till its implemented