ASNT Level III- Visual & Optical Testing
My Level III Self Study Notes Reading 4 Section 4B
My Level III Self Study Notes Reading 4 Section 4B
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Microstructural replication is done in two steps: surface preparation followed<br />
by the replication procedure. Surface preparation involves progressive<br />
grinding and polishing until the test surface is relatively free of scratches<br />
(metallurgical quality). Depending on the material type and hardness, this<br />
can be obtained by using a I to 0.05 p.m (0.04 to 0.002 mil) polishing<br />
compound as the final step. Electrolytic polishing can increase efficiency if<br />
many areas are being tested. Surfaces can be electropolished with a 320-400<br />
grit finish. The disadvantages of electropolishing are that (1) the equipment is<br />
costly, (2) with most systems only a small area can he polished at one time<br />
and (3) pitting has been known to occur with some alloy systems containing<br />
large amounts of carbides.<br />
Next, the polished surface is etched to provide microstructural topographic<br />
contrast which may be necessary for evaluation. Etchants vary with material<br />
type and can be applied electrolytically, by swabbing or spraying the etchant<br />
onto the surface. With some materials, a combination of etch-polish etch<br />
intervals yields the most favorable results.<br />
Charlie Chong/ Fion Zhang