How long to sub zero/cryo D2?

[SamaraVolk]

Hey all, I'm in the process of sub zeroing some D2 and A2 blades. I have not found the minimum time to keep them in the dry ice slurry. I see AEB-L only needs an hour or so but nothing definite for tool steels. I'm limited by the time it takes the dry ice to sublimate (8 hours or so, sometimes longer), but if I can get away with a shorter time that would be great. Any thoughts on the minimum time? Thanks

 

[Larrin]

Only the time required for the entire blade to reach the temperature is required to transform retained austenite (30 minutes). Some studies show that there is a further increase in wear resistance at the expense of some toughness with extended times. Some of those studies say that the extended time only works at liquid nitrogen temperatures but extended times in dry ice haven’t been studied much. If it were me I would do 30-60 minutes.

 

[SamaraVolk]

Only the time required for the entire blade to reach the temperature is required to transform retained austenite (30 minutes). Some studies show that there is a further increase in wear resistance at the expense of some toughness with extended times. Some of those studies say that the extended time only works at liquid nitrogen temperatures but extended times in dry ice haven’t been studied much. If it were me I would do 30-60 minutes.

Thank you

 

[Stacy E. Apelt - Bladesmith]

At sub-zero ... -100°F, you only need enough time to make sure all the steel is at that temperature. 10-15 minutes is plenty. This is to finish the martensitc conversion of any remaining austenite.

At cryo ... -400°F, you need to soak for 3-4 hours. 6 hours to overnight are done by some folks. This is for the formation of eta carbides in the subsequent temper.

 

[SamaraVolk]

At sub-zero ... -100°F, you only need enough time to make sure all the steel is at that temperature. 10-15 minutes is plenty. This is to finish the martensitc conversion of any remaining austenite.

At cryo ... -400°F, you need to soak for 3-4 hours. 6 hours to overnight are done by some folks. This is for the formation of eta carbides in the subsequent temper.

Thank you.

 

[samuraistuart]

Great advise being given here. Just to add a little tech know how, the conversion of retained austenite to “untempered martensite” happens at the speed of sound, literally. So what Larrin and Stacy have said is on the dot. Once the temp has been reached, the conversion happens. Larrin mentions 30 minutes. I will actullly leave my sub zero blades in overnight, although it isn’t needed that long.


When heavier alloyed steels get full on “cryo” treated (LN2), then extended soak times have been shown to add “eta carbides” once tempered.

But for sub zero/ dry ice treatments to ensure austenite conversion (which is all sub zero does anyway) the transition happens at the speed of sound once the temp has been reached.

 

[SamaraVolk]

Great advise being given here. Just to add a little tech know how, the conversion of retained austenite to “untempered martensite” happens at the speed of sound, literally. So what Larrin and Stacy have said is on the dot. Once the temp has been reached, the conversion happens. Larrin mentions 30 minutes. I will actullly leave my sub zero blades in overnight, although it isn’t needed that long.


When heavier alloyed steels get full on “cryo” treated (LN2), then extended soak times have been shown to add “eta carbides” once tempered.

But for sub zero/ dry ice treatments to ensure austenite conversion (which is all sub zero does anyway) the transition happens at the speed of sound once the temp has been reached.

Thanks. You mention overnight for sub zero. I've also read that the blades must go from sub zero to temper after reaching room temperature with no delay. How do you control this time if your ice sublimates in the early hours and the blades sit for 5 or6 hours? Is it even an issue?

 

[kuraki]

Thanks. You mention overnight for sub zero. I've also read that the blades must go from sub zero to temper after reaching room temperature with no delay. How do you control this time if your ice sublimates in the early hours and the blades sit for 5 or6 hours? Is it even an issue?

It's not an issue that they sit 5 or 6 hours at room temp after hitting the sub-zero temperatures and finishing RA conversion - outside the small risk of cracking due to untempered stress.

 

[SamaraVolk]

It's not an issue that they sit 5 or 6 hours at room temp after hitting the sub-zero temperatures and finishing RA conversion - outside the small risk of cracking due to untempered stress.

Thanks. The stress is what I was concerned about

 

[kuraki]

Thanks. The stress is what I was concerned about

That's hard to answer definitively. It depends on the steel alloy, how it was heat treated, even the shape of the blades. Knowing there's risk you can figure out through trial and error if it works for you.

 

[Willie71]

Thanks. You mention overnight for sub zero. I've also read that the blades must go from sub zero to temper after reaching room temperature with no delay. How do you control this time if your ice sublimates in the early hours and the blades sit for 5 or6 hours? Is it even an issue?

There is no point of long soaks at sub zero, as the temp is too warm for eta carbides. As Stuart noted, the conversion is quick.

 

[JG Custom Metal Works]

So as far as doing LN cryo after hardening/before tempering. Is there any negative effects from keeping them in the liquid nitrogen for 12-16hrs? Kuraki mentioned “cracking due to untempered stress.” Should I be letting them soak for no longer than 12hrs and then getting them into the oven for tempering ASAP? I usually put them in overnight and then get to them when I can in the morning. What is the absolute best(in your opinions) length of time to cryo in LN for higher alloyed steels(cpm-3v, cpm-s35, cpm-110, etc)?

 

[Stacy E. Apelt - Bladesmith]

Within reason, a long soak in LN does no harm. Temper immediately after reaching room temp.

 

[Willie71]

So as far as doing LN cryo after hardening/before tempering. Is there any negative effects from keeping them in the liquid nitrogen for 12-16hrs? Kuraki mentioned “cracking due to untempered stress.” Should I be letting them soak for no longer than 12hrs and then getting them into the oven for tempering ASAP? I usually put them in overnight and then get to them when I can in the morning. What is the absolute best(in your opinions) length of time to cryo in LN for higher alloyed steels(cpm-3v, cpm-s35, cpm-110, etc)?

I keep mine in overnight, and heat the oven for tempering as I let the blades come up to room temp.

 

[kuraki]

So as far as doing LN cryo after hardening/before tempering. Is there any negative effects from keeping them in the liquid nitrogen for 12-16hrs? Kuraki mentioned “cracking due to untempered stress.” Should I be letting them soak for no longer than 12hrs and then getting them into the oven for tempering ASAP? I usually put them in overnight and then get to them when I can in the morning. What is the absolute best(in your opinions) length of time to cryo in LN for higher alloyed steels(cpm-3v, cpm-s35, cpm-110, etc)?

I've had 1 blade crack in the LN. It was A2. There were other identical A2 blades that did not. I don't even know for sure if sitting around untempered was why it cracked, only that it's the only thing I can think of and so I avoid leaving them in overnight. Maybe it was a freak thing.