Higher Tempering Temps needed after Cryo??

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PugKnifeCompany

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Apr 25, 2017
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I have recently been playing with stainless. I bought some 440c from admiral steel. Heated to 1850 and plate quenched, then into LN for 24 hrs. Hardness after cryo was 62-63! Then tempered at 400 and was expecting 58HRC, 400 degrees didn’t lower it AT ALL! I checked my oven temps, checked my hardness tester...then decided Admiral must have sent me the wrong steel....600F brought me down to 59HRC, so I decided it must be 154 CM steel. Next blade I made I just treated it like 154-cm and I worked great.

Now I’m trying to temper A2 after cryo treat, prior to temper the HRC is 64-65, put in temper oven at 425, again the HRC didn’t move, still 64....what’s going on? Does cryo treating require higher Temper temps? Am I doing something wrong?
 
Yes and No. Cryo will raise the AQ a point or two. If you need a specific Rc in use,say Rc 58-59, then you would raise the temper about 25°F to attain that. However, the cryo allows the blade to be used at a higher Rc than would be allowed without it. So instead of a stable blade at Rc 58-59, you now can use the blade at Rc 60-61. This would be attained with the same temper that was used with the in-cryoed blade regime.
 
I’m having to raise my temper temps more then just 25degrees though...for the 440c (or what was suppose to be 440c) it took 600 degrees to get it down to 59HRC?! I tried tempering the A2 at 425, and it was 64HRC after an hr in the oven?!
 
My 25°F was the first step. Temper at 350° and test, if needed raise the temper in 25° steps until you hit the desired hardness.

440C should hit Rc 62 as quenched, and Rc 63-64 with cryo.

PugKnifeCo - I would check out your oven and the readout, then calibrate your hardness tester. 440C has an as-quenched max hardness of Rc 62, or Rc 64 with cryo. It drops to Rc 60/61 at a 350° temper, and drops to Rc 58/59 at 375°F.
600° would be way too high for a knife blade. If all the stars aligned, you might get one more point over the above numbers, but there is no way a 600° temper would barely reach Rc 59. I don't think I have ever tempered 440C above 400°F.
 
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i Have checked my Rockwell tester with test blocks, it’s dead on. I will recheck my oven temps. I’m not sure what’s going on with the A2 I’m trying to temper Now, but when working with the alleged 440c, I tried it in my toaster oven at 450 degrees, (measured with an independent thermometer, as well as in my cook stove at 500 F. I am going to conclude that Admiral sent me the wrong steel. Anyone else get the wrong steel before?
I will recheck my tester, and my oven, and try the A2 again! Thanks for all the help!!
 
I can’t think of anything off the top of my head that they could have sent you which would have stayed at 59 after a 600 temper. Even CPM154 should have lost a lot more hardness than that at those kinds of temperatures. I would almost look at getting a second thermometer to verify the one you have. My own toaster oven that I tried tempering with before ended up being 75 degrees colder than the dial said it should. It showed 425 max and the two thermometers I tested with floated between 350 and 360. This was with a 1/4x3x9 plate in the bottom for thermal mass and after stabilizing for about an hour. The only thing I can think is that the tempering oven temps must be a lot lower than they are showing for some reason. My understanding is that 600 is not high enough to be getting into the secondary curve.
 
I was referencing a graph by alpha knife supply, and some info by crucible, for cryo treated blade, 58-59 looks about right for 600 degrees? Of course I have no experience here, just going by what I read. This is not hot enough to be getting into the secondary curve.
 
Some of the air hardening steels have a flat tempering curve between 250f to 600f. In those steels, if using cryo for full conversion of retained austenitite, you control final Rc#through austenitizing temp.

I’ve never used 440c, so I have no idea if this is one of those steels.

Warren
 
I'll have to pull up some tempering charts on 440C and double check. Perhaps I am not remembering the data right, but it still seems way to high.
Hopefully Larrin will check in with his comments.
 
I agree for 440c it would be way high..thinking 600F for 59 rockwell with 154 cm after cryo makes more sense. Going to check my tester again today with several different hardness blocks, as well as test my oven with a different second thermometer lol!
 
Bohler says the AQ is around Rc 61 and to temper between 212F and 400F. Their chart shows Rc60 at 212F, Rc 59 at 300F, and Rc 57 at 400F. From there it stabilizes at Rc55-56 up to 900F. (sounds like they included cryo inthe test)

Crucibles data sheet says it is as-quenched at Rc59, drops to Rc 56 at 400F, and to Rc 54 at 600F.

Carpenter says 300F = Rc60, 400F = Rc 59, 500F = Rc 57, 600F = Rc 56

I also checked my ASM last night and it had the numbers I gave originally.

I usually add 2 points for cryo, so according to all that data you should have hit around Rc 56 at 600F.
 
I temper my Cpm154 at 550 and get 61rc with subzero treatment. If it’s 440c and tempered to 600 then you should be way down around 55-56.
 
Stacy E. Apelt - Bladesmith said:
Bohler says the AQ is around Rc 61 and to temper between 212F and 400F. Their chart shows Rc60 at 212F, Rc 59 at 300F, and Rc 57 at 400F. From there it stabilizes at Rc55-56 up to 900F. (sounds like they included cryo inthe test)
Click to expand...

So there is no way the steel could have been 440c right. I mean my kitchen stove was set at 450 and it didn’t budge!
 
I tried to upload a picture, not sure how. The piece of A2 that I tempered @ 425 degrees is straw colored after temper, which leads me to believe the temp was indeed around 425.

I checked my Hardness tester with 5 separate test blocks, through a range of hardnesses...it's dead on.

My A2 after cryo and temper reads 64HRC every time.

I changed thermo-couples on my Heat Treat oven, and ordered another high heat thermometer to double check it.

Is it possible that Austentizing temps that are TOO HIGH can effect the numbers? I am 100% sure now, that my toaster oven, and my kitchen oven are within 25 degrees accurate. The heat treat oven MAY be off though... and i may be getting the steel too hot?
 
One thing you need to do is take a breath and decide what steel you are talking about You switch from 440C to A2 in the same thread. That is confusing.
 
Stacy E. Apelt - Bladesmith said:
One thing you need to do is take a breath and decide what steel you are talking about You switch from 440C to A2 in the same thread. That is confusing.
Click to expand...

Update
I heat treated a new batch of same “440c.” First I replaced the thermocouple in my oven, and also made sure Austenitizing temp was actually 1850.
After Cryo I was at 61
After temper at 350 I’m at 59rc

I can only surmise that my austenitizing temps were a bit to high...everything else was kept the same.
 
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