Thoughts on retempering.

Matt Rochester

Matt Rochester

Joined
Nov 28, 2014
Messages
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I recently heat treated some Aebl oyster knives as follows. 1940 for 10 mins, plate quenched with forced air, into deep freezer while oven cooled and tempered twice for 2 hours at 375. That should've given me 59RC according to the chart I used. It's been a week or two since that and I just checked them and they were 60.5-61RC. My question is how effective is another temper at higher temps now? I have tried it before by going up another 25-50 on some cpm154 without a change in hardness. Should I try another temper at 450 or so? Thanks.
 
You already did that as the end of the quench. Cryo is not needed in tempering with normal knife steels.

Quench
Cryo/sub-zero
temper #1
cool to room temp
Temper #2
cool to room temp
Finish knife
 
Stacy E. Apelt - Bladesmith said:
You already did that as the end of the quench. Cryo is not needed in tempering with normal knife steels.

Quench
Cryo/sub-zero
temper #1
cool to room temp
Temper #2
cool to room temp
Finish knife
Click to expand...

I did not do a subzero I only placed them in my deep freezer while my oven cooled. So either my aust temp was higher than 1940 which means my oven is a little hotter or my temper temp wasn't quite high enough which means it's a little cool on the bottom side. I've heat treated a lot of Cpm154 and my results have been right on the money per my charts. Only thing I can figure is maybe the couple hour soak in the deep freezer did more than I thought. I seem to remember Hoss saying to use the freezer if you didn't have access to a subzero setup. I was not doing a subzero on purpose as I figure the RA wouldn't hurt an oyster knife. Anyway I'll bump up my temper temp and try again.
 
I wouldn't worry about small differences in hardness. There could be many causes from a slightly different batch of steel, a temperature variation. quench difference, and the test could have been off a tad.

I regularly answer the freezer question by saying that putting the blades in the freezer won't hurt them, but it isn't going to help much either.
The Mf is at -100F, so -10F isn't significantly close to it to do much more than room temp does. There will be some improvement over ambient, but it won't be the huge gain a certain cowboy knifemaker claims.

AEB-L has a very tight HT regime. Longer/shorter soaks and any variance in temperature will change the results. If you really need exactly RC59 on this batch of blades, test the hardness again to make sure it is high, and re-temper as needed to get the drop in hardness needed. Try 400F for 30 minutes.
 
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