Another Warping Question

ddavelarsen

Well-Known Member
You guys have helped me determine when the best time is to correct a warped blade - right out of the quench - which seems to be working great. But I have an even more basic question - why the warp in the first place?

Here's my recent experience. I've been heat treating 3/16 O1 mostly. I use an Even Heat oven, preheat to 700 before applying PBC, normalize for half an hour at 1250, ramp up to critical at 1550 and soak for 10-15 minutes.

My quench tank is at about 140-150 degrees with circulating oil; I quench in Houghton Quench G.

When I'm ready to quench, I remove the blade with a large pair of pliers and immediately submerse in the tank, edge down, and move the blade forward - lengthwise - back and forth through the oil. No side to side movement at all, I'm very careful about that.

Here's the rub. Almost every blade comes out warped where I've held it with the pliers. Doesn't seem to matter what part of the blade I grab, that's where it warps. Sometimes its very minor, but other times it's pretty significant. The worrying part is that this almost always happens. And if it's a worse warp, I've cracked some blades trying to straighten them, even still somewhat hot from the quench. (I use a three-point system in my large vise to counter warping.)

Any ideas what's causing this? Is it the pliers having too much contact surface on the steel? I've ground the jaws smooth - no ridges - but they do pretty much grip the blade with the jaws flat on each side.

I have a pair of tongs that because of the angle of the jaws, would probably only grasp the top edge of the blade when I pull it out of the oven. Think using those would make any difference? A friend suggested lifting the blade from the oven with a wire through one of the holes in the tang. Think I might make something to try that, but I also think it might take enough time that I'd lose some heat in the process of getting hold of the blade.

Any and all ideas are really welcomed here. This is frustrating and disturbing. I never had this issue when I was heat treating in my propane forge and only bringing the edge up to critical before quenching in vegetable oil - those were the "old days" when things worked right... ;)

Well, thanks for listening to me whine, and I'd sure appreciate any insight you gents might have.
 
You are going much too hot for critical. 01 should not be taken beyond 1475° for the hardening, and soaked 15 to 20 minutes. I found that a separate stress relief of an hour at 1200° to 1250° and air cooled to ambient reduced warp, and quenching point down also helped greatly. Your oil might be a little on the hot side also. 125° to 130° should be plenty. I usually run three per batch, and of three, I may get one with minor warp that can usually be hand straightened during the first five minutes out of quench. Warp also seems to happen if I remove the blade from the oil to soon. My blades normally are in the thickness range of 3/32" to 5/32", finished to an edge thickness of about .010 to .015, and coated with ATP-640 for decarb protection. Double coated along the edge area. You need a set of tongs that will allow you to grip the tangs, and not the blade. I made a set from large electrician pliers, and long handles of electrical conduit. Light weight and easy to handle.
 
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... Maybe too much agitation. If your quench tank is circulating the oil already, no need to agitate the blade. You may also be able to slow the circulation down a bit. There really aren't too many places for vapors and bubbles to get trapped on most blades. Still quenching (no agitation or circulation) may also be a reasonable alternative, especially when edge quenching and/or with canola (no vapor or boiling phases).

The convective action (or current) itself offers a degree of circulation.

Once the blade gets past the vapor and boiling phases and around the pearlite nose, you'd probably be much better off turning the circulation and/or agitation off... You want the transformational phase of the quench slow to avoid unnecessary stress and warpage.

Holding the blades by the end of the tang would be better. It's probably a combination of the tongs hindering the quench on that spot and too much agitation/circulation.
 
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You guys have helped me determine when the best time is to correct a warped blade - right out of the quench - which seems to be working great. But I have an even more basic question - why the warp in the first place?

Here's my recent experience. I've been heat treating 3/16 O1 mostly. I use an Even Heat oven, preheat to 700 before applying PBC, normalize for half an hour at 1250, ramp up to critical at 1550 and soak for 10-15 minutes.

My quench tank is at about 140-150 degrees with circulating oil; I quench in Houghton Quench G.

When I'm ready to quench, I remove the blade with a large pair of pliers and immediately submerse in the tank, edge down, and move the blade forward - lengthwise - back and forth through the oil. No side to side movement at all, I'm very careful about that.

Here's the rub. Almost every blade comes out warped where I've held it with the pliers. Doesn't seem to matter what part of the blade I grab, that's where it warps. Sometimes its very minor, but other times it's pretty significant. The worrying part is that this almost always happens. And if it's a worse warp, I've cracked some blades trying to straighten them, even still somewhat hot from the quench. (I use a three-point system in my large vise to counter warping.)

Any ideas what's causing this? Is it the pliers having too much contact surface on the steel? I've ground the jaws smooth - no ridges - but they do pretty much grip the blade with the jaws flat on each side.

I have a pair of tongs that because of the angle of the jaws, would probably only grasp the top edge of the blade when I pull it out of the oven. Think using those would make any difference? A friend suggested lifting the blade from the oven with a wire through one of the holes in the tang. Think I might make something to try that, but I also think it might take enough time that I'd lose some heat in the process of getting hold of the blade.

Any and all ideas are really welcomed here. This is frustrating and disturbing. I never had this issue when I was heat treating in my propane forge and only bringing the edge up to critical before quenching in vegetable oil - those were the "old days" when things worked right... ;)

Well, thanks for listening to me whine, and I'd sure appreciate any insight you gents might have.

Dave, why warp…? The contributing factors are endless. The basic answer is uneven strain energy. Many think that a straight blade comes from the elimination of stress, not so. It is the enormous strain energy in the blade that makes a properly heat treated knife hard, so we must have “stress” to have a hard blade. The trick is to balance all that energy. Harden a sword blade and then grind down just one side to mess with that balance and you will see exactly what I mean.

Forging unevenly will lead to immediate warping and distortion during subsequent heat treatments if that uneven strain is not dealt with using appropriate normalizing, which when done correctly will completely correct or nullify the problem. But if one doesn’t normalize properly you can make problem worse. In the last class I taught I showed the class how to normalize with a scrap blade and then just tossed it on a steel table to show them another one. It was an excellent lesson in improper normalizing, the blade curled almost into a horseshoe from the strain of the uneven cooling as the steel table pulled the heat out of one side. The very definition of normalizing is even heating and even air cooling, to equalize both the blades matter and its stored energy.

Annealing is a matter of reducing energy levels evenly, and any work done afterwards must be done in a way that adds strain energy evenly, this is why I deal out fifty lashes to my students who turn blades blue on the grinder and force the others to watch as the punishment is administered. But in all seriousness, there are some folks who feel there is no problem with heat treating on a grinder or introducing strain in such a chaotic fashion after they when through the trouble of good normalizing and annealing, but those folks simply have a serious lack of understanding of strain in metallic crystalline systems; they obviously have never taken a pass to quickly on a surface grinder.

In your case, massive amounts of strain energy are being added to the blade, even if it is all perfectly even from all we have just discussed it can still get pretty out of whack here. Will your tongs/pliers affect things- you bet! Not only will you get uneven cooling from the oven to the quench, but you will get uneven cooling in the quench itself, and not just from the tong contact, but from the swirls and eddies in the oil caused by the tongs (which would be my first suspect in your problem). But your agitation is a good thing and I would not suggest eliminating it. Agitation is not just for defeating the vapor jacket but also in assisting in convection so that fresh cool quenchant is available to the cooling process. My low temp salts have virtually no vapor jacket when they are maintained correctly and yet agitation is critical to a proper quench to introduce unheated liquid to the blade surface. Agitation can be overdone when it is so violent that is introduces air into the quench so avoid splashing and aeration, if that is happening tone the action down a bit.

I am not surprised that you did not have as many issues when edge quenching since you were only heat treating perhaps 1/3 of the blade. Many bladesmiths make this observation but I am surprised at the number of them that don’t understand that only heat treating 1/3 or less of the blade isn’t the same as a full quench, I do, however understand that you no doubt get that point Dave. I always do all of my work holding at the tang, most often with a wire, but sometimes grab the tang with tongs when horizontal quenching, which I do spine down, but still always by the tang. I would not be surprised to find a deviation in Rockwell where your tongs/pliers were.

One other final note- if you cracked a blade during marquenching/interrupted quenching you were straightening way too late or interrupted at too low a temp. First, if you interrupt at or just above Ms you should not need a three point bender at all, your gloved hands are more than enough to move the blade around like putty; in fact I would strongly encourage you to lose the three point bender as it is not only overkill, it will not allow you to feel the stiffness setting in to tell you it is time to stop because of martensite. At around 50% martensite it gets riskier to push that steel, but when the steel is still more than 50% austenite you will be surprised at how you can move it with just your fingers. Three point benders can be used when the blade is annealed and cold, or when the blade is at tempering heat after hardening but is not necessary above the M50% temperature.
 
Agitation does a few things, helps defeat the vapor jacket, helps keep vapors and bubbles from getting trapped in places, and brings fresh fluid into contact with the blade. It can help keep the quench more even and speed it up a bit (if necessary).

However, too much agitation can lead to, or contribute to, distortion and warping, even when air is not being introduced into the quenching medium. Determining the proper amount of agitation for any specific part can be tricky and there isn't any hard absolute rule for that, but it can be especially problematic on distortion and crack prone parts in general such as knife blades.

Your engineered petroleum fluids already have accelerators to shorten the duration of the vapor phase. As mentioned also, not too many places for vapors and bubbles to get trapped on knife blades as they would on say on a gear or a part with lots of holes or piercings in critical areas, and the convective current itself does circulate the medium and brings fresh fluid into contact with the blade, in which the speed varies according to the viscosity of the quenching medium.

I'd be careful not to overstate or understate the role of agitation/circulation on knife blades in general, depending on a number of possible variables.

I'm personally of the philosophy and/or school of minimizing stress/strain during heat treating, which seems to be the conventional norm. I suppose the "Ride 'em cowboy" approaches have their place, (especially in marketing), but I'm not convinced of the metallurgical "soundness" of those types of approaches, especially with long, thin, pointy, triangular and diamond shaped cross sections which are geometrically problematic and complicated further by the use of high carbon and alloy steels...

I don't like warped and/or cracked blades.
 
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but sometimes grab the tang with tongs when horizontal quenching, which I do spine down, but still always by the tang.

Kevin,

Great info in your post and I will read it again several times. However, something jumped out at me. SPINE DOWN when horizontal quenching.

Can you give us a tad bit more info on why you quench spine down?

Thanks,

Robert
 
I too noticed the spine down! So now I got to be like my 4yr. old grandson, Why???????? Is it because of the quench pushing the heat out of the spine and into the blade, sort of like edge quenching and differential heat treating??? You know that all made sense till I went to put into writing, so I hope I was getting what I was trying to say across. So once again why???
 
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Theoretically, it helps compensate for the thermal gradient and helps minimize stress. The thinner edge section naturally cools faster than the thicker spine, so it's a way to help equalize the cooling rates between the edge and spine.

It's also about push and pull stress timing between the edge and spine, and how the associated stresses and strains express themselves.

It's not a common practice, but there may be something to it.

Probably the reason more people don't try it is because they are worried about not getting the edge into the quenching medium fast enough. However, as long as you don't waste any time, that shouldn't be a problem.
 
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I think that another reason that people feel that one has to get the steel into the quench in less than one second, I know it was the case with me, is that they haven't seen an IT diagram and don't understand that they have less than one second, with a shallow hardening steel, for the steel to stay near the nose of the curve. There is only a problem if the blade is allowed to cool slowly in this range by a) being allowed to air cool, assuming that we are not dealing with an air quenching steel, or b) the transfer of heat slows down in the quenchant at this range, usually somewhere around 1000°, for some reason. For example, using an improper oil for marquenching.

Doug
 
I tried "backward quenching" a few times and it didn't seem to make any difference with what I was doing at the time, but that doesn't mean that under a different set of circumstances it wouldn't. I suppose it could, one way or the other...

I'm sure it does a few things a little different to the steel.

I think it might be more of a sword thing than a knife thing,... but I wouldn't rule it out.
 
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Anyone else have an opinion on "backward quenching" ?

I’d like to hear more about push and pull timing, expansion/contraction sequence theories and how it all relates to stress/strain and warpage,... pros and cons, ups/downs, ins/outs etc.

... not sure where it's going, but it might be pretty heady and interesting. :)
 
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