Heat Treat Results Today and a Question


I heat treated 8 blades today.

1st 3 were D2. Preheat 1200° hold 15 minutes. Ramp to 1850° soak for 30 minutes. After one 2-hour temper at 350° blades are 61, 61 and 62.

2nd 3 were 440C. Preheat 1425° hold 15 minutes. Ramp to 1875° soak for 30 minutes. After one 2-hour temper at 300° blades are 61, 60 and 61.5.

3rd 2 were AEB-L. Preheat 1500° hold 15 minutes. Ramp to 1950° soak for 15 minutes. After one 2-hour temper at 350° blades are 60 and 62.

All blades were plate quenched in aluminum in their foil pouches then removed and put in the freezer for an hour. It's all I had.

The Question - I am now curious if I need the second 2-hour cycle in the temper oven.
Nope. Won't change it at all.

During tempering, some retained austenite will convert to martensite. That fresh martensite is untempered and brittle. And while it is distributed in real small doses throughout, it does cause problems n some steels worse than others.
I'm new at the heat treat process. I am keeping accurate records now and I plan to make small changes to see what happens.

I am very satisfied with the results on all the blades yesterday!
even at low temperature tempering there is martensite becoming tempered martensite, but also RA converting to martensite and also tiny bits converting to carbides. especially in a slow-ish quench... that leaves more RA... i am going to guess the second blades were the softer ones, that increased in hardness after second temper. maybe chuck plates in the freezer or spray with cold water between quenches