The Two Commandments of Heat Treating

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Two general rules that come up time and time again in industrial heat treating discussions, are:

#1. The end use application of a part or component dictates it's heat treatment.

#2. Use the slowest possible quenching medium necessary to achieve the desired structures and/or properties.

Here’s a good article that talks about the problems associated with heat treating,… quench cracking, thermal gradients, stress risers etc. It even talks about differential or selective hardening to minimize or suppress distortion and cracking. It also talks about choosing appropriate quenching mediums etc.

http://www.getottenassociates.com/pdf_files/6072_0192_224_web.pdf
 
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... One of the main reasons I put this up is because there seems to be a trend in knifemaking, that,… “You should use the fastest possible quenching medium as long as you don’t see any visible cracking“. The problem with this is, “Most quench cracks aren’t readily visible to the naked eye,… and may not become apparent until after it’s too late“.

I know I’ve been painting a much different picture of steel, than a lot of what’s been going around… all I can say is, “Sit down, try to relax and take a few deep breaths“... too much stress isn’t good for steel or for people.
 
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I cant say I see it as a trend, but its worth discussing. The quench needs to be just fast enough to miss the undesirable phases, in our case typically pearlite and bainite.
 
True, but in the case of thermal gradients, push and pull, crack prone components etc., "knives specifically“,… we need to give special attention to the edge. In other words, a few grains of pearlite along the spine, wouldn’t be nearly as much of a concern as excessive micro-cracking along the edge etc.

Crystallography isn’t the only concern.
 
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I think that I would have to agree with Tai, at least to a certain extent. I think that there are those out there who do think that water or brine or even a fast oil is something special for super fast quenching without knowing the reason behind why they're doing it or the risks involved in using a fast quench. I think that they are getting confused and not seeing that what applies to a low manganese simple steel does not apply to a moderated chromium high carbon steel.

Doug
 
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Right. There are some who use quenchants that are faster than necessary, and certainly a little pearlit in the spine isnt a real concern. However there are those that usea quenchant that is too slow for the sake of not overstressing the steel, and that causes its own problems, and excessive brittleness at the edge is still one of them, potentially.
 
A crack is different than a crystal. “Crystallography” specifically and exclusively deals with crystals. A crystal can crack, but there are other types of cracks.

Oh brother! LOL
 
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I edited it, but I'm not sure it makes any better sense.

… just pointing out the difference between crystallography and microstructures or metallurgy in general.
 
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Good enough. :)

Crystallographic concerns are an important aspect of heat treating and metallurgy,… but not the only ones. A understanding of basic crystallography as it applies to steel, is very important,… but so are a number of other factors… “crackographics and “crackology”, not being the least of them. ;)
 
I dont think I'd be exagerating to say that if you use high carbon steel, its a safe bet you have a few quench cracks, microscopic though they may be.
 
True, all fully quenched and hardened hypereutectoids have micro fractures. The idea is to keep them to a minimum,... smaller and fewer.

Over-quenched and/or under-tempered blades may exhibit “high performance” qualities in the short run,… but fail in the long run.
 
Laurence, that cracked me up! LOL

... At any rate, the article I posted the link to talks about the possible problems and solutions. Well worth the read.

Whether anyone wants to be conservative or liberal with their heat treating is their choice, but it’s always good to have some basic information, guidelines and theory to work with. I don't think the basics can be overstated. They give a good solid working foundation and help separate the wheat from the chaff.
 
This "over quenching" is why I don't like the idea of the triple quench with hypereuticoid steels, especially once you get up close to 1% carbon. As Tai said, hypereuticoid steel like 52100, 1095, or high carbon W series steels will form ~100% plate martensite on quenching. Where these plates come together you will have some cracking and a decrease of toughness. These cracks are microscopic and by microscopic I mean that you may need to have a scanning electron microscope to see them. My concern is that triple quenching might produce triple the amount of these cracks. Without being able to afford to test this hypothesis I would rather err on the side of caution and quench such steels once.

It is also possible with these steels to dissolve too much carbon into the austinite by doing triple quenching. If the total time spend austinizing the steel puts too much carbon into the austinite then when you quench you can have an increase in retained austinite. To me it's much better to austinize long enough to dissolve enough carbon to form enough martensite on quenching but not so much as to increase retained austinite and then quench one time.

Doug
 
Good points Doug. I'd worry about the same types of things with multiple quenching. However, there may be a few ways to minimize and compensate for the stresses etc., to some degree,… slow quench oil, extreme grain refinement, using a consistent high quality steel/alloy, minimizing thermal gradients etc. Still a bit too risky and complicated for my liking though.

I tend to play it safe, keep it simple and lean more towards the conservative side,.. but to each his own. Long term performance is very important to me. I want my knives to withstand the test of time and keep on keeping on performing the way they were intended to and then some… Just plain old, good, sound, consistent heat treating is pretty hard to beat in the long run. This may sound boring to some makers, but I think it’s a good idea to at least be able to accomplish this, before trying to “hot rod” or “pimp” the steel.
 
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