Posted by PB on July 08, 2008 at 00:20:00:
I have just performed a grain size evaluation of a Q&T 4140 and 4340 material. I used the oxidation procedure described in ASTM E112, and found a grain size of No. 8 and No. 10, finer than the requirements stipulated in ASTM A434.
The procedure involves placing the specimen in an oxidising atmosphere at 855degC for 1 hour, which causes preferential oxidation of the prior austenite grain boundaries. The sample is then quenched, the scale removed, lightly repolished and etched. I used Villelas.
Can anyone shed some light on the actual process, and what i'm looking at under the microscope? It looks like a perfect austenitic structure! Is it actually ferrite? have i decarbed it? I also have some concerns about the fineness of the grain size. Is the temperature high enough and the time long enough to affect the grain size?
Any information you can provide would be extremely helpful!
Thanks!