Cryogenic Processing of Steel Part 1 – Maximizing Hardness

Great article. It takes some of the voodoo out of the cryo quench. Although........there is that guy with the $20,000 swords out of mild steel with 20 cryo quenches......o_O
 
I look forward to parts 2 and 3 but I'm curious if there's a way to predict when a steel will respond to cryo with increased hardness, and if that's at the same aus temperatures or only with higher ones.

For example A2, I austenitize at 1750F, and generally get 65 RC as quenched. I've not been able to check RC after cryo, but when I do after tempering, I get exactly the RC I predict I'd get tempering A2 at xF, like 60-61 @400.

From your chart showing A2 quench rates and retained austenite, I'd assume there's enough RA to result in a bump in hardness after cryo but that's apparently not the case.

This extra hardness only seems to happen with steels that necessitate high temperature tempering (like 4V, Aus 1875 at room temp is 62 RC, after cryo and 400f temper, 64 RC.)
 
kuraki said:
I look forward to parts 2 and 3 but I'm curious if there's a way to predict when a steel will respond to cryo with increased hardness, and if that's at the same aus temperatures or only with higher ones.

For example A2, I austenitize at 1750F, and generally get 65 RC as quenched. I've not been able to check RC after cryo, but when I do after tempering, I get exactly the RC I predict I'd get tempering A2 at xF, like 60-61 @400.

From your chart showing A2 quench rates and retained austenite, I'd assume there's enough RA to result in a bump in hardness after cryo but that's apparently not the case.

This extra hardness only seems to happen with steels that necessitate high temperature tempering (like 4V, Aus 1875 at room temp is 62 RC, after cryo and 400f temper, 64 RC.)
Click to expand...
I can predict it but the method can't be used without thermodynamic software so teaching it isn't particularly useful.

You can see the hardness vs austenitizing chart along with retained austenite for A2 here: https://www.flamehardening.com.au/wp-content/uploads/2016/05/A2_data_sheet_09032013.pdf

Based on the typical 15% retained austenite with 1750°F I would think you can get another 1-2 Rc as-quenched with A2.
 
Thank you Larrin for the work. There are a couple of points that really clicked with me.

1. For the T1 Steel listed the difference between Dry Ice and LN are very small, only a couple of % points. Can we "assume" similar results for most SS steels? Especially like AEB-L?

2. The time from Quench to cold treatment is important, better to go direct into Dry Ice or LN - just as we always thought. Again I'm "assuming" this is valid for all SS steels? Especially like AEB-L?

Thank you so much for your work.

Ken >H
 
Interesting. I guess I need to bring my dewar to work (where the RC tester is) and get some data before tempering.

FWIW A2 is the only thing I've cryo'd that I've had come out of the cryo cracked. (so I was in part fishing for a reason to skip cryo with A2.)
 
Ken H> said:
1. For the T1 Steel listed the difference between Dry Ice and LN are very small, only a couple of % points. Can we "assume" similar results for most SS steels? Especially like AEB-L?
Click to expand...
It will be similar for steels and heat treatments that lead to 20% or less retained austenite without cryo. There may be more separation between dry ice and liquid nitrogen with much more retained austenite, though typically those austenitizing temperatures are not recommended anyway as it would lead to low hardness and poor properties without cryo.
2. The time from Quench to cold treatment is important, better to go direct into Dry Ice or LN - just as we always thought. Again I'm "assuming" this is valid for all SS steels? Especially like AEB-L?
Click to expand...
I would expect it to be similar, yes.
 
This is a great article. I’m really interested in the different structures that result from the low temper protocol, and the positive effect on fine edge stability.

One area I’m REALLY interested in right now is playing with pre-quenches, and the effects of lower austenitizing temps paired with lower tempering temps. With pre-quenches, is there an ideal temp such as between ac2 and ac3, or is it steel specific to be teased out by experimentation? Are there performance differences in as for example z-wear heat treated at 1975f, and tempered at 400f, giving Rc63, and austenitizing at 1925f, and tempered at 300f, hypothetically giving Rc63 as well? (Haven’t run the experiment as I’m quite low on z-wear, and my new diamond indented is still in the mail somewhere.) At what point do we go too low on temper, erasing the benefit of lower austenitizing temperatures?
 
There are too many questions to answer for one lifetime. But we will enjoy answering a few of them anyway.
 
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