Air Quenching

A

AfterTFD

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Oct 28, 2007
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I haven't been able to find any clear information on exactly what "air quenching" is.

Let's say we're heat treating some A2. After leaving it at 1750 degrees for a half hour then what? Does it just get pulled out and left to sit until it's cool enough to handle?
 
Yes, that's about all there is too it. Let it air cool inside the stainless steel envelope and when it gets cool enough to handle with gloves, pull it out and straighten it if needed.

You might try some A6 if you are forging. It moves easier under the hammer than A2 and has a relatively lower hardening temperature... It's great for real thin springy blades.
 
If you want to speed up the quench a little, just put it in front of a fan. :)

I've never had one crack that way... :D
 
The other consideration is plate quenching for smaller blades, take out at temp and clamp between two aluminum plates until can be handled. This will keep the blade straight.
 
My recipe for A2 closely follows the Crucible data sheet and goes like this : first I enclose my blade in a foil envelope. Then its into a cold kiln. Ramp full to 1200*F , equalize (5min) Ramp full to 1400*F , equalize (5min) Ramp full to 1775* F (austinitizing temp) Soak 40min.

Out of the kiln and directly in between 2 aluminum pieces of barstock that I use specifically for plate quenching (I should add that before I put my envelope in my kiln, I get a vice grip welders clamp set to a snug snap for the width of the 2 alum bars + my enveloped blade) Like I was saying, I take my envelope out of my kiln and clamp it between my alum plates. Immediately following that I snip the blade end of my envelope off with tin snips and blow air into the envelope with a vortec cold air gun. All of this is done on top of my biggest anvil. I have a clamp set-up for the cold air gun so that I can direct the air flow into the envelope hands free. As that is happening I snip the back of the envelope off so that the air free flows through. I have some nice thick alum plates and in combination with the vortec they are very efficient at removing the heat from my blades.

Temper @ ~ 475* for 2 hrs x 2 for a final HRC of ~ 59-60

My as quenched hardness is always right in the ball park (@~64HRC) with the data sheet.

Crucible Data Sheet defines A2 quench as "air or positive pressure quench (2 bar minimum) to below 150*F (65*C)"

All of this being said, I've seen other knifemakers get good results by simply holding the envelope in front of a fan or regular type air gun.

When I was working in the shop I saw our heat treat guys pull die punchs out of the kiln and place them on an expanded metal table to cool all on their own. No forced air whatsoever. A couple of temper cycles and then on to punching out tens of thousands of parts.

I try to cool my air quench blades as fast as possible, using both quench plates and forced, cold air. So far I've had very good results .

Everyone seems to do it a little bit different though.



:thumbup:
 
I discovered years ago that air quenched tool steels will quench just fine in oil which cuts down on oxidation during quench. I since have gone to plate quenching. I can't tell any difference between plate and oil quenched A2. The plate quenched blades come out cleaner though, and require less post heat treatment sanding.

When plate quenching, I remove the foil prior to getting down to Mf temperatures and check for any warpage then back between the plates to get to room temperature. Above 400°F they are easy to straighten.
 
David Wesner said:
I try to cool my air quench blades as fast as possible, using both quench plates and forced, cold air. So far I've had very good results .


:thumbup:
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I've read that some air hardening steels respond harder to an oil quench than an air quench. I'm not sure about A2. I've often wondered how plate quenching compares to oil quenching on an air quench steel.

Did that last sentence make any sense?
 
Thanks for the replies. :) I used A2 as an example because that's the steel I'm most interested in working with.
 
Nathan, yes you make sense. Plate quench is quite fast, comparable to oil.
When heat treating in foil it doesn't make sense to remove the blade from the foil. Just go directly to the plates .Any warpage problem will be taken care of by the plates.Very convenient !!
 
Nathan the Machinist said:
I've read that some air hardening steels respond harder to an oil quench than an air quench. I'm not sure about A2. I've often wondered how plate quenching compares to oil quenching on an air quench steel.

Did that last sentence make any sense?
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When I first started making knives I was surprised to find out that most ATS34 specs called for an oil quench. Many of the threads that I read on different forums led me to believe that it was an air quench steel. To this day I know of very few guys that quench their ATS34 in oil.

Good point Nathan & Steve, I've often wondered some of the same things. To this day, I haven't yet tried any of my air quench steels in oil :confused: But I am curious :)
 
David Wesner said:
Good point Nathan & Steve, I've often wondered some of the same things. To this day, I haven't yet tried any of my air quench steels in oil :confused: But I am curious :)
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I've oil quenched D2 blades. I don't recommend it if you can use plates because it is messy and hardness testing didn't show any benefit.

There is a school of thought that says the rate of quench can effect the corrosion resistance in stainless steels. D2 is so borderline that I think it is important to take advantage of this. The theory is that air quenching gives the carbon time to combine with more chromium on the way down to form carbides. A faster quench leaves more of it in martensite, freeing up chrome. When the carbon comes back out of the martensite during tempers if forms fine carbides with whatever it can find, including iron. This leaves more free chromium. I'm sure there is a down side, but I don't know what it would be...

BTW, thermal conductivity in metal is orders of magnitude greater than air (I looked into this once). By my reasoning steel quench plates would work just as well as aluminum and be easier for some people to obtain. The thermal conductivity of aluminum is several times that of steel (though the heat mass of steel is several times that of aluminum meaning it can absorb more heat) but compared to air I suspect it is moot. Anyway, despite the fact I have tons of aluminum plate laying around, I plate quench D2 in steel because I put it in a press and aluminum would get buggered up. I really don't think it matters.
 
Plate quenching is the way to go. I also blow compressed air inbetween the plates while the blade is sandwiched in there.
Scott
 
I should add that the data for HT from the makers are typically for at least 1" thick sections .The "air quench " is sometimes listed as moving air . But plate quenching takes care of things - clean , efficient .Don't make it more complex.
 
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