A-2 Heat Treating Revisited

G

gshamr

Joined
Jan 18, 2002
Messages
178
I did my first heat treat in my new knife kiln today. It didn't warp and appeared hard enough before I tempered it. I'm going to do a machete-size blade of A-2 tomorrow and want to double-check my recipe because I got bits and pieces of it from several makers.

1) Put the wrapped blade in the cold kiln and bring it up to 1500 degrees for stress relief. Hold it there for 10 min.

2) Bring it up to 1750-1775 degrees for 20 min to harden.

3) Plate quench or air quench and then temper 2x for 2 hours each at 500 degrees.
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Are my temps and times appropriate for 60 rc?

The oven heats so slowly that with the stress relief and then the climb to hardening temp, the blade is in there for over an hour. As I understand it, excess temperature can increase grain size but this time in the oven should be of no concern. Is that correct?

Thanks for the replies.

Gerry Hamrick
Everett, WA
 
As long as you don't exceed 1750 degyou should not get any grain growth.
Stan
 
You could try 1500' for 30 minutes to 1 hour and then to 1725-1800' F for 30 minutes. 20 minutes seems kinda short for A2.

I think to actually "stress relieve" A2 it would be more like to 1600'F for 2 hours and then cool 50'F per hour to around 1425'F and then freely in air to actually do any stress relief.

You are pre-heating with what you're saying. :D

500'F is around 60 RC

here is my take on plate quenching especially small blades. If your grinding is not the same on both sides and you have not stress relieved the blade in the correct manner, your blade may WARP from spine to edge instead of from left to right or right to left. This is much harder to correct then the normal warping. this is mainly for blades that are 1/8" or less in thickness. (ask me how i know :D)

The blade will get plenty hard just being hung up to air cool all by itself or even with a breeze. No real need to plate quench unless you are in a rush (which is already bad to begin with !). Most makers will say i'm on crack, but just trying to give you a little less headache incase it happens to you !

The time in the kiln (is of no concern) so long as it falls within the ballpark of the norm. I can't imagine if you left a blade in the kiln for a week that it wouldn't cause some undesired effect. :D All kilns are not going to RAMP up at the same rate and yet the recipes are not for any specific kiln exactly.
 
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I have always thought that the oven should be preheated to your temp be for you put the blade in. I have had blades over shoot the set temp because the oven is set to full on and the blade picks up radiated heat and comes up to temp faster then the entire oven. i only notice this problem when i temper so maybe you will be fine with how your doing it. just my 2 cents
 
Working with tool and die parts A2 (always thicker than a knife blade) I run the oven up to about 1200, let my part sit for about an hour per inch thickness at that. then I run the oven up to 1750. Heat treat 1 hour per inch thickness (sometimes longer if the part is really thick) and then pull it out and put a fan on it.

It usually tests at about 62-65 Rc, and it takes some heat to get the hardness down.

Now, regarding knife blades and anything that is considerably thinner, the steel will get up to heat pretty much as quick as the oven climbs because it is so thin. When I have something thin I just run the oven up to 1750, and put my piece in as soon as I start the temp up. That way it is in there while it is running up. Once it gets there, I may let it sit for about 30 minutes, but that's it for a knife blade 1/8 thick or less. After this, I pull it out and let it air dry.

Sometimes it takes up to 900 egrees to pull 65 Rc out of a part, but don't leave it in too long. After you pull it out and it tests at what you want, put it back in at about 300-400 for about 30 minutes a piece (2 times) to stress relieve it.
 
The A2 that I have treated have come in right at 60RC using close to the recipe you gave. The only difference I can see is that I pre-heat my A2 blades at 1375 for 1 hr. Then take it up to 1750- 1775 for a 45 minute soak. When they come out of the oven I place the blade on a thin wire rack in front of a fan and allow it to cool in the foil wrap until I can hold it in my bare hand.
Tempering is two, 2 hour cycles at 500 degrees allowing the blade to cool to room temp between cycles. Every A2 blade I have done in this manner has tested between 59-61 RC. I have not encountered any warping but my blades are not near as long as what you are talking about either.
 
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I'm glad I double-checked because it illustrates the different info on the subject.

I have info from Uddeholm that says to austenize preferably at 1720-1760 and to stress relief at 1200 for 2 hours. It does recommend holding for 30 min at 7400 to austenize, so I will try 30 min next time.

I have material from Evenheat that says to harden at 1800 for 10 or 15 min.

The info Matthew provided says to austenize at 1700-1800 for 20 min per inch of thickness and to stress relief at 12-1250 an hour per inch of thickness.

I've gotten different responses from knifemakers this time and previously.

Luckily, the stress relief I used was either not needed or was enough to keep a 10" and 14" (blade only, not handle) from warping. I only have two more long ones to do and then on to more normal sizes. I don't have a hardness tester but they seemed to pass the file test.

From the response by Leu Custom, I think I'm getting stress relief and preheat recommendations mixed up.
 
I'm going to beat this horse to death because I'm a little dense and I still have questions that will also apply to the D-2 and 154CM blades I am grinding.
One of the reasons I bought an oven is that Paul Bos charged something like a $75 minimum per batch of one specific steel. Unless this changed, budding knifemakers should know that if they grind 2 knives out of a particular steel, it may cost a great deal to have them heat treated by certain vendors. This wouldn't apply to Texas Knifemakers Supply, or some others. It's also why I have so many A-2 blades sitting around.
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The following is what I gathered prior to the responses to this thread. Please correct me where I am wrong.........

Generally speaking, normalization involves heating at a controlled temp or a period of time and then cooling at a controlled rate. This is mainly done with forged blades to relieve stress and occurs separate from the hardening heat.

Stress relief would apply to stock removal knives in an oven and involves heating to a controlled temp for a period of time, before ramping up to the austenizing temperature. This would be recommended in long, thin, or deeply hollow ground blades to avoid warpage. It might not be needed in simple flat ground knives that are ground evenly and are 3/16" or thinner.

Preheating would help avoid shocking the blade, and possibly warping it, by popping it into the oven at austenizing temp. Stress relief is a little different apparently involves longer times and possibly a different temperature. Can someone explain this to me?

Hardening A-2 closer to 1800 degrees for a shorter period of time would result in a hardness closer to 64 or 65. Hardening A-2 closer to 1700 would require a longer period of time and result in a hardness closer to 61 or 63. If those statements are correct, would the tempering of 500 x2 x2 hours result in the same final hardness in both cases?
I'm going to stick with 1750 and increase the soak time to 30 min.

Thanks for helping me with this.
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Leu Custom-I appreciate your advice on the plates and potential for warpage up and down. I will heed that when I do some thinner blades because up and down warpage would be a pain to correct.
 
gshamr said:
I'm going to beat this horse to death because I'm a little dense and I still have questions that will also apply to the D-2 and 154CM blades I am grinding.
One of the reasons I bought an oven is that Paul Bos charged something like a $75 minimum per batch of one specific steel. Unless this changed, budding knifemakers should know that if they grind 2 knives out of a particular steel, it may cost a great deal to have them heat treated by certain vendors. This wouldn't apply to Texas Knifemakers Supply, or some others. It's also why I have so many A-2 blades sitting around.
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The following is what I gathered prior to the responses to this thread. Please correct me where I am wrong.........

Generally speaking, normalization involves heating at a controlled temp or a period of time and then cooling at a controlled rate. This is mainly done with forged blades to relieve stress and occurs separate from the hardening heat.

Stress relief would apply to stock removal knives in an oven and involves heating to a controlled temp for a period of time, before ramping up to the austenizing temperature. This would be recommended in long, thin, or deeply hollow ground blades to avoid warpage. It might not be needed in simple flat ground knives that are ground evenly and are 3/16" or thinner.

Preheating would help avoid shocking the blade, and possibly warping it, by popping it into the oven at austenizing temp. Stress relief is a little different apparently involves longer times and possibly a different temperature. Can someone explain this to me?

Hardening A-2 closer to 1800 degrees for a shorter period of time would result in a hardness closer to 64 or 65. Hardening A-2 closer to 1700 would require a longer period of time and result in a hardness closer to 61 or 63. If those statements are correct, would the tempering of 500 x2 x2 hours result in the same final hardness in both cases?
I'm going to stick with 1750 and increase the soak time to 30 min.

Thanks for helping me with this.
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Leu Custom-I appreciate your advice on the plates and potential for warpage up and down. I will heed that when I do some thinner blades because up and down warpage would be a pain to correct.
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I have personally never heard of the term normalizing when speaking of A2. Then again, I could be wrong.

Stress Relieving would entail something almost like normalizing to relieve stress causing by grinding, EDM, etc etc, so the steel atoms can be all evenly distributed and there would be less chance of some crazy warping going on when the blade was quenched especially a blade that is ground chisel grind for instance only being ground on one side. With A2, i don't think it's that bad because you can cool in still air.

Stress Relieving I have heard does not yield a stronger cutting edge.

I would imagine if a blade is austenized at 1700 VS 1800'F the tempering temperature to achieve RC 60 would be either higher or lower slightly.

I have read to austenize at the low side for thinner sections (such as knife blades) and the high side of 1800'F for thicker sections. The higher austenizing temperature will yield a higher as quenched hardness which later on will be drawn down to the desired hardness. As far as whether one will yield a stronger cutting edge at one temperature vs another temperature and then tempered is something I have yet to try out myself.

Pre-heating is just that, you are pre-heating the steel as not to SHOCK the hell out of it. Putting the blades in a room temperature kiln and then ramping up to preheat temperature and then to austenizing temperature is the way to do it.

Many will say the a particular steel can be heat treated one way and you get the same results as others, i think this is false and how long your edge lasts all depends on your knowledge of how to heat treat the particular steel.
 
Leu, in my post I used the term normalizing in error, I pre-heat my A2 steel at 1375 to lesson the shock it goes through during the process just as you state. Must have had a senoir moment .
 
denim said:
Leu, in my post I used the term normalizing in error, I pre-heat my A2 steel at 1375 to lesson the shock it goes through during the process just as you state. Must have had a senoir moment .
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I was just replying to what gshamr said ! :) I don't think I even read your post that carefully so I was not commenting on what you might have said ! :thumbup:
 
I think in the end, there is no one definite recipe to heat treat A2 or any steels for that matter. One must use what works for them and that is simply by trial and error ! A lot of this heat treating data wasn't even developed for a knife but more a part of some kind that's a hell of a lot thicker than a knife blade ! :)

There is no right and wrong answer (unless you really go out of the ball park and preheat at like 100'F lol and try austenizing at 1400'F or something off the wall like that.

One blademaker (who is very known for making blades out of A2 and only A2) actually told me along time ago, that you can actually take A2 higher than 1800'F without a significant grain growth and that they made 1800'F a safety temperature (just in case you really messed up !) One of these days i'll try it out !
 
Leu, That's a good point. We have some parts at the shop I work in that we do frequently. We call them templates. They are usualy 4-6" long 3 -4" wide x .125" thick A-2 they don't get machined but ground square and I wire EDM differant profiles per print after heat treat. We always had problems with warpage in quench when we did it the traditional way taking them out of the wrap put them in a rack and blow a fan horizontal. We fixed this by raising the heat to 1800 take them out stab a hole in the corner of the wrap hang them up and blow the fan vertical from underneath the parts.The extra 50 degrees was to compensate for time before they hit the fan. Since then no warpage and parts come out of the quench 64-65 Rc.
 
JBS TOOLMAKER said:
Leu, That's a good point. We have some parts at the shop I work in that we do frequently. We call them templates. They are usualy 4-6" long 3 -4" wide x .125" thick A-2 they don't get machined but ground square and I wire EDM differant profiles per print after heat treat. We always had problems with warpage in quench when we did it the traditional way taking them out of the wrap put them in a rack and blow a fan horizontal. We fixed this by raising the heat to 1800 take them out stab a hole in the corner of the wrap hang them up and blow the fan vertical from underneath the parts.The extra 50 degrees was to compensate for time before they hit the fan. Since then no warpage and parts come out of the quench 64-65 Rc.
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I have even cracked the garage door a bit (where my blades were hanging for air cooling) and when I took them out of the oven they were straight ! And a few seconds later, they warped ! Due to the slight breeze coming in from the garage door being opened. Since then I do not air quench with the garage door opened any more.
 
I hang all of my blades on a rack over a fan point down. It promotes an even flow of air around the entire blade.I think A-2 is effected by this more than D-2. The garage door thing is true, when we figured this thing out about the templates I proved the point by taking a 1/8" piece out of the oven and imediately blowing one side with an air gun. We literaly watched it bend.
 
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