Geno,
You and I are looking for the same holy grail, the exact science behind the art...unfortunately the diagram is gone from your posting but I know the one you are referring to. 727C is 1341F. So if you are around .8% carbon in the case, 1350F should create a hard martensitic surface when quenched.
Miller's point regarding the tempering effect on the case of stopping at 1250F, ie below the decalescence point is a good one. That's probably why Brownells says to recolor at 1400F. I got the 1250F recipe from a metallurgist who is also a gunsmith so I will ask him about tempering.
I think what Oscar was doing was going to around 1650 to get max carbon absorption, then coming back to maybe 1400 (or even lower as the recalescence point is much lower than the decalescence point when you come down in temp) before quenching to still get the case hard but to retain the colors (which disappear if you quench from too high temp...and you also risk cracking and/or warping if you quench from too high temp). In industry, this is normally done in two steps: first step is the caburizing at 1650F, then the part is allowed to cool. I suppose any working/polishing is then done. Second step is hardening of the case through quenching in water or brine from 1450F.
Anyone know if Oscar used a layer of oil on top of his quench tank?
