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Old 09-16-2017, 11:41 AM
samuraistuart samuraistuart is offline
Steel Addict
 
Join Date: Dec 2012
Location: San Antonio Texas
Posts: 163
Whether to cryo (LN2) or sub zero (dry ice) is really not complicated, but I understand how confused I was at first.

What are we trying to accomplish with these 2 things? It is the transformation of retained austenite into untempered martensite (that gets tempered later). Which one you pick will be simply determined by the steel itself, due to it's alloying. The more alloyed the steel, the more you NEED to go below -100F to convert that RA. Only the most heavily alloyed steels NEED, or I should say "should", get an LN2 soak. Think along the lines of S110V and the like. Steels like A2 and AEB-L, and honestly...MOST, of the steels we normally use do NOT NEED LN2. The dry ice slurry will be cold enough to convert most of the RA that needs to be converted. A2 falls easily into this category. However, more RA CAN be converted with LN2 over sub zero, but again, if it's a mid alloy steel like A2,D2, AEBl, etc, then sub zero does well enough.

However there is an added benefit when using LN2 over sub zero. Not only do you get a more complete RA conversion on all steels, but the LN2 WITH soak will allow the precipitation of extremely small "eta" carbides upon tempering. This BOTH increases toughness AND wear resistance. So with A2 and LN2 for example, you will get a theoretical 100% conversion of RA PLUS the extra eta carbides within the matrix after tempering. I say theoretical 100%, because even with XRAY diffraction they can only detect down to around 2% RA remaining.

Buying an LN2 dewar, one the right size and right mouth size, can be expensive. Plus finding a place to fill it. Dry ice is cheap, and the denatured alcohol is re-used over and over and over.
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