Straightening After Heat Treat

Peter Atwood

Peter Atwood

Joined
Oct 26, 2000
Messages
2,468
I have read that you should straighten a warped blade immediately after heat treat. But I also am under the impression that a freshly quenched blade is fairly brittle and therefore subject to cracking. This would seem to be somewhat of a contradiction so what's the scoop? If I wait until after tempering will the blade then be harder to straighten?
 
Good Luck Peter,I usually don't have much luck straightening a blade either way...I end up just re grinding the blade to get rid of any warps and then do that blade again..
Bruce
 
Immediately after hardening, while the blade is still warm to the touch, it is remarkably flexible. After awhile that goes away and you have a hard brittle blade. As soon as you can touch the blade is the time to straighten it. After tempering, you have to usually apply some localized heat and lots of pressure.
 
Peter,
If your trying to straighten a blade there is an easier way. Leave your blade thickness oversize, say .030 When you grind, just do the lead in grind, say at 45deg. Just along the edge of the blade, then heat treat it and if it warps, you can surface grind it straight and finish the grind on a hard blade. This info is mentioned in one of the tutorial on the custom knife directory wegsite, the one by Jason Howell on how to build a linerlock. Check it out, it might help.
Chuck

www.customknifedirectory.com
 
Thanks guys. None of the 440C folder blades I did the other day warped. Even the one that was ground way too thin was perfectly straight and the others were only profiled so they were fine. But the O1 fixed blades all warped, every darn one. Some are curved enough to spin on a table. I think what I'll do is reharden the O1 blades next time I heat treat and then straighten as Jerry suggests. Maybe I'll get a quench press together by then and not have to worry about it.
 
I learned a trick during the Intro course at the Moran School of Bladesmithing from Kevin Cashen. Well, first I have to tell you this before I tell you that :)

Kevin taught a method of heat treating that was very unique and effective. I've been using it ever since with great results. He called it 'quenchus interruptus'(AKA Marquenching). When you go for the quench all you have to do it plunge the blade into your oil until the worst of the bubbling and smoking stops. Then pull the blade out and watch it. If the oil is just smoking but not vaporizing you're at the right point for the 'interruptus' part (about 350 - 400 deg F). If the blade is warped now is the time to straighten it out. According to Kevin, you have about a 10 second window of opportunity here. Just be sure and wear thick gloves (I use my welding gloves) and you can bend it in your hands. Then let it cool down to room temp and temper.

However, having said all that I'll say this: Since 99% of my quenches are edge quenches I don't have a problem with warping. Of course some of the reasons for that could be independant of the edge quench. Normalizing is probably the biggest one. I do occasionally have a little warpage during normalizing. That's when I do my straightening as a general rule. I keep a small farriers anvil, wood block and brass mallet at my heat treating station just for that very purpose. But I've learned that if you keep the blade straight during the forging process you won't usually have any warping to speak of. Forging at too low of a temp is a sure way to get warping in my experience.
 
I'm kinda like Bruce. I haven't had much luck doing it but I don't have one warp often. I did have one warp a month or so again. It was a tricky grind and I spent a lot of time on it. I built a jig to go in my vice like an arrow straightening jig on steriods. I was amazed at the flex the blade had and I had tempered it a little on the hard side. It worked without a hitch. Every now and then I just get lucky.
 
In my experience When heating with a torch, warped blades usually are caused by heating too much on one side. Be sure to heat both sides the same, alternating sides every couple of passes with the torch.

I believe that forging excessively on one side can cause some distortion in the hardening operation. Forge both sides the same number of hits by turning the blade over regularly. One time Rex called me on my forging practices, stated that I forged on the right side more than the left. He saw the results in his photomicrpgraphs. There may be a lesson to be learned here when forging chisels and the like.

Sometimes distortion (warp) happens anyway, then try grinding a convex in a 2 x 4 and place a 2 x 2 against the high side of the blade and squeze it in vice. Do this after tempering and then temper again.
 
Well for what it is worth, I seem to do it different
I forge to shape weaving it oversize, normlise 3
times, quench 3 times in ATF, temper imediatley. No
grinding is done befor heat treating unless it is
profiled, this means that all grinding is done after
heat treating. I have not had any warped blades
when using this method, I have bent blades but none
warped and I was plaged with it befor. Coments please
as I am alwayes looking for a better way Gib
 
That's the way that I would always do it if I had the time. It does take me much longer because I have to be concerned with heating the blade. The scratches seem to come out cleaner on a hard blade and the overall looks is just better.
 
I usually dont have a problem with warping, but when I do, I find it depends on the steel involved and thickness of the blade as to whether it can be sucessfuly straightened. You can tweak a thin, tempered 5160 blade in a vise sometimes.. a thicker 1085? Usually not. :( Even at a tempering temperature. I do save the broken blades to eventually weld up into "frontier" damascus. :)
 
Some one once said "If your gonna make knives, your gonna hafta learn to straighten them" or something to that effect.

I've never had much luck with O-1, but I have had to straighten a few others. The way I do it is with 3 larg pine dowls in a vise, put one on the high side of the blade, and the other two on eigther end of the warp, and then crank down with the vise. I've always done this after the final temper, and havent broken one this way yet.

On a side note, I was makeing a slip joint folder out of O-1 and the back spring was a little weak, so I decided to bend it slightly. It was spring tempered and flexed real good, but wouldn't bend. I kept trying a little more force untill ping! Some steels are just more prone to bend than others.
 
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