Posted by MLH on July 21, 2010 at 18:52:14:
In Reply to: Determining Grain Size of Tempered Martensite posted by Meng Tzee Luai on July 19, 2010 at 09:32:43:
You must be referring to the oxidation method per E112. It decorates surface grain boundaries by intergranular oxidation. Keep in mind this procedure will recrystallize your starting microstructure upon re-austenitization. It is a capability test more than anything else. So you are right, the oxidation may modify the grain size and perhaps the microstructure of the steel.
If you want to know your sample's current grain size before it is changed with an furnace cycle (McQuaid-Ehn or oxidation method), you should try using the saturated picric acid etchant method described in E112, Appendix A3.3.1.5. This should work for a 4130 steel.