Help identifying cause of A2 warp

Isley

Isley

Joined
Feb 26, 2016
Messages
65
I heat treated four N690 blades and six AEB-L blades, all of which came out straight as an arrow. Then the problem, all four of my A2 blades came out with rather severe warps, only one of which I was able to bring back within tollerance, by clamping during temper. I've been seeing this a lot with A2 and from different material providers so I'm certain there is no issue there. All the blanks were profiled, drilled, and put through a two hour stress relief. no other prior grinding was performed, stock thickness was .150. My kiln is an even heat KO, 22.5" deep. 1200-equalize, 1400-equalize, 1750 hold for 20min, steady to the plate vice. Every blade warped in the same direction. I could stack them on top of each other like a little U. Blades on one side didn't seem to warp worse than others so I'm not sure if it's a kiln problem, and if so, why wouldn't it effect the other knives? Hoping for a little help before I lose my mind.
 
It sounds like a steel problem. Do you have certs or something that gives us more info about the steel? Is it precision-ground?

Warp on a coil product (strip or sheet metal, which describes most of the metal sold by the popular knife suppliers) is to be expected, but generally the domestic tool steels are not coil products. I had some serious warp on precision-ground S7 plate last year. The final thickness was 0.235", but the vendor started with a 0.500" plate. I learned that the plate was first face-milled, then blanchard ground, then surface ground. I believe the warp was caused by the face-milling.
 
This was hot rolled steel from AKS (not pointing fingers), as was the n690. I often taper the tangs, and the blades of course get their grind, after heat treat so I'm trying to avoid any prior surface prep. I've done limited A2, and that which I did before was precision ground. I thought the two hour stress relief would null any internal stress in the steel though. As far as the four that warped in this cycle, I always placed my template on the side opposite the sticker that's on the steel, so I still can't say for sure whether my kiln is to blame. As I thought about this I did notice that there is about a 3 inch gap in the coils where they enter the kiln from the control unit, could it be that this is creating a minor cold spot? If so I can remedy by pushing everything toward the rear of the kiln. I think I'll test this out shortly any way.
 
Warping comes from internal stesses in the steel. All knife steels grow after quenching, unless there is a lot of retained austenite like D2. During grinding, forging, heat treating there are stresses formed or released causing blades to warp. All hot rolled bar or sheet has a layer of decarb. Some decarb may be uneven from side to side. Decarb does not grow with the center of the bar because it lacks carbon to form martensite. In extreme cases I’ve seen bars that separate from the strain.

Heating is probably not the problem here. You may adjust the quench speed also, slower might be better.

Hoss
 
Isley said:
This was hot rolled steel from AKS (not pointing fingers), as was the n690. I often taper the tangs, and the blades of course get their grind, after heat treat so I'm trying to avoid any prior surface prep. I've done limited A2, and that which I did before was precision ground. I thought the two hour stress relief would null any internal stress in the steel though. As far as the four that warped in this cycle, I always placed my template on the side opposite the sticker that's on the steel, so I still can't say for sure whether my kiln is to blame. As I thought about this I did notice that there is about a 3 inch gap in the coils where they enter the kiln from the control unit, could it be that this is creating a minor cold spot? If so I can remedy by pushing everything toward the rear of the kiln. I think I'll test this out shortly any way.
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I'm having the same exact issues and from the same supplier.. I've wasted a couple sticks of A2 trying to figure it out. I've tried stress reliving, grinding and not grinding bevels, I've tried plate quench and still air also.. all blanks warp the same direction and for a 9" length piece I get a banana shape with about a .08" gap under the center when laid on glass, flip it over and its like a rocking horse. I guess I'm not the only one with an issue and I will have to contact AKS.

I was able to straighten a couple after HT and hardness tested at 60 with a 400 degree temper, 1775 Austenite temp.
 
At least I know I'm not alone. Please let me know what you learn from AKS, should you contact them. Seems like a 400 degree temper should yield a higher rockwell too, so long as the steel and heat treat are within specs.
 
Isley said:
At least I know I'm not alone. Please let me know what you learn from AKS, should you contact them. Seems like a 400 degree temper should yield a higher rockwell too, so long as the steel and heat treat are within specs.
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The reply I got was to “straighten it during the quench before it gets too cool. There is a time when this can be done before the conversion happens where the steel is pliable like plastic”.

I already knew about this technique, just didn’t realize it was “the” process for A2.. seems odd that I’ve wasted 4 feet of A2 due to very bad warps when I’ve yet to have any other steels like that. There was no answer regarding a known issue or anything though.. I am to try fixing during quench and report back to chuck on how that works for me.
 
Thank you for your response AKS. I'm familiar with this method of straightening, I generally expect A2 to be dimensionally stable assuming correct heat treat procedures, and historically it has been in my experience. If I deduce anything definitive with future makes, I'll be sure to post back. Thanks again.
 
Alpha Knife Supply said:
Straighten the blade immediately after quenching. I apologize if my response was confusing.

Chuck
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You had it right and I blurred the interpretation! Thanks. One of these nights I’ll give a few of them a try and let you know. Just stinks this would have to be the method for this steel when I’ve had so much good luck with your S35VN and AEBL.
 
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