Eta carbides

[jdm61]

So the general consensus among us semi-serious steel nerds is that with higher alloy steels, your best bet is to do a sub-zero treatment right out of the quench in order to combat excess RA. But how and more importantly WHEN would you do a long soak at -300F in order to facilitate the formation of those elusive little eta carbides? Would you just leave the blades in the LN2 overnight before tempering or would you put them back in after the first temper cycle? For purposes of context, I am thinking about CPM 3V. Enquiring minds want to know.

 

[Nathan the Machinist]

The applications and timing of cryo depends on the steel. D2 reacts different than 3V for example. I was surprised when "rules of thumb" don't always universally apply. And there are some variables that you may not have considered that can effect the outcome.

IMO, the best way to find out is to try it and test it for yourself with experiments and controlled cut testing. I know it takes some time, and you have to control and minimize variables which can take some up-front setup. But real testing is a very cool process. I guarantee you'll come away from it learning things you didn't expect. One of the cool things about being a knife maker is you get to play with some of the coolest materials in human history with tools our predecessors could only dream of.

You're asking a very simple question. The answer to your question can be determined with a few hours work over a few days. And what you find will lead to more questions. :thumbup:

 

[jdm61]

Actually, I can't do that testing because I don't have LN or, IMO, the gear to HT high alloy steels. I would have to pay someone to do that just like i needed outside help grinding a very shallow hollow grind and cooling my platen. Hence all of the questions on here.

The applications and timing of cryo depends on the steel. D2 reacts different than 3V for example. I was surprised when "rules of thumb" don't always universally apply. And there are some variables that you may not have considered that can effect the outcome.

IMO, the best way to find out is to try it and test it for yourself with experiments and controlled cut testing. I know it takes some time, and you have to control and minimize variables which can take some up-front setup. But real testing is a very cool process. I guarantee you'll come away from it learning things you didn't expect. One of the cool things about being a knife maker is you get to play with some of the coolest materials in human history with tools our predecessors could only dream of.

You're asking a very simple question. The answer to your question can be determined with a few hours work over a few days. And what you find will lead to more questions. :thumbup:

 

[Larrin]

Eta carbides precipitate during tempering regardless of liquid nitrogen. There are one or two other carbide types as well which are all referred to as "transition carbides" which are very small and precipitate out during tempering: they are where the carbon goes as martensite is tempered. There is a growing body of literature pointing to several possible effects of increased cryo time: increased precipitation of transition carbides, more homogeneous distribution of said carbides, or a change in the alloy content of those carbides. I haven't done sufficient analysis of the literature to offer an opinion on whether these effects are likely or what type of cryo processing is required to achieve them. It still seems weird to me that you must reach a point where diffusion is extremely slow to later accelerate a diffusional process, and the proposed mechanisms still look like conjecture to me. But maybe the analysis is more solid than I give it credit.

 

[Larrin]

I should add that studies that have found an increase in wear resistance with extended cryo time have also found a corresponding decrease in toughness (when they looked at toughness at all, that is). Therefore even if wear resistance is increased by longer cryo time that may or may not be desirable, depending on the application.