In my experience, almost every blade has some degree of warpage/movement when hardened. For me this became evident when I discovered that the shoulders on a hidden tang blade will "move" during heat treating.... I used to always "finish" the shoulders on a hidden tang prior to the quench....only to find that once I started the guard/handle process, the guard would not fit correctly (one of the shoulders always seemed to be out of whack, and would require touching up).

There is also some very minor "warping" of the blade(s) that happens during the heat treat.....generally very minor, that the finish grind takes care of.

Now, directly to your question of residual stress....thats where the tempering step does the job. Personally I think many makers "short cut" this step too much. Not only is the tempering step a "controlled softening", but its also a stress relieving operation. I often read where makers say "temper at XXX degress for 1 hour".... personally I think they are short changing themselves. Any operation with steel requires both a specific amount of time at a specific temp to achieve the desired results. Personally ALL the blades I produce go through three, 2 hours cycles in the tempering oven. My reasoning for doing this is both from practical experience, and from spectrographing reports. I am fully convinced that AT LEAST two of the cycles are necessary....and I use the third as "insurance".

To illustrate, my mind goes to a time when it was acceptable to use a "hot bar of steel" to temper....placing a blade's spine on the hot bar of steel and watching the colors "run". After flexing/breaking many of these type blades, it was evident that only about .010" of tempered "skin" existed afterward. (it takes time AND temp) to fully temper the steel within a blade.

I could go on and on, but suffice to say that IF a blade is tempered in such a manner that the entire cross section of the blade(s) it tempered, the stress problem is effectively eliminated.

I have used the temper straightening process since I learned it more than a dozen years ago.

At the time, I knew few folks who used it.

For the most part, when we "put" a steel into a shape and condition using our heat treating process, we give the steel a 'memory', in that even though we may physically place it into a new position, it will want to return to the original location - hence - memory.

When we straighten a warped piece of steel in the tempering phase of our heat treating process, we give the steel "NEW"! memory.

I do so much as first give the warped blade its first temper. Not a good idea to bend un-tempered steel.

Correct the warp on the second temper - or the third!

I will remove the blade and straightening clamp device from the oven and quench it in my quench tank.

Is that necessary to quench it instead of just letting it air cool? I don't know.

But I do know the blades are more likely to retain their new location as opposed to slightly return to their last place of memory.

All that said, I often find a bit of warpage common, like Mr. Caffrey states.

In fact, when I end up with blade perfectly straight after hardening, I am a bit suspect of minimal martensite conversion until I check everything to be certain my knife got fully hard.

I may be happy that it's straight - but something gives me a bit of comfort when I see a bit of warp.

It means I got conversion - and that's a good thing.

Thank you Ed, that was the best explanation of the blade tempering process that I have ever read.

I used to try to straighten part way through the quench but was always concerned about it so I have gone to straightening after the final temper and during a draw back of the spine with a torch. You masters are welcome to correct me if I am mistaken but I have felt that by straightening while the martensite is at a blue heat that it is less likely to want to go back to the warped form. This may not be necessary but, "If it ain't broke....". <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' />

Gary

In SOME instances, if a blade goes really wonky, I will straighten immediately after the quench, but before the steel "sets".....theres usually about 45 seconds to a minute where the steel is so pliable, it seems like you have a chunk of clay in your hands. I have a "jig" of sorts setup in a vise right by the quench tank that I use for straightening.....I'll try to remember to get a pic and post it on this thread.

I like Gary's "If it ain't broke"....right there with ya pal! <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' />

Contrary to Derrick’s concerns about the plastic deformation of straightening rather than the initial deformation of hardening, it is that initial deformation that concerns me more. Strain energy- it is always there, it is stored energy that drives almost every change in steel, from recrystallization, pearlite formation or the hardening process itself- all states of higher energy going to more stable conditions. We like to say that we remove the “stress” from the steel with things like annealing, stress relieving or normalizing, but all we are really doing is homogenizing the “stress” that will always be there. If that strain energy gets out of balance (non-homogenous) the steel gets pulled or pushed in another direction.

The hardening operation involves at least two, and maybe three, allotropic shifts where the atomic stacking of the steel will totally reorder itself. When we heat to harden, the steel drastically contracts as it shifts from body centered cubic to face centered austenite. If we didn’t do our pervious thermal treatments well the rate of austenization will not be homogenous and parts of the steel will “shrink” out of synch with the rest and there will be warpage. If we heat unevenly or carelessly we will get the same. Improper soaking resulting in insufficient austenite solution or inhomogenous solution will set things up to go wrong in the quench.

When we quench there should be nothing to do but go along for the ride until we reach Ms, however, if the quench in insufficient we will get a massive and rapid expansion of parts of the steel at 1000F as we lose some of the face centered austenite to body centered pearlite. The chances of this happening in a completely even fashion are so remote that there will invariable be some degree of distortion; after all, this is how Japanese swords get their curve. But non-homogenous points (points of higher energy) in the steel will be the preferred spots of pearlite colony formation, so all of the previous mechanisms will once again come back to haunt you.

Finally we get to the martensite transformation. This is the SUPER-massive expansion from face centered cubic to body centered tetragonal, it happens at the speed of sound and it involves incredible amounts of strain energy (“stress” if you will but they are different things in the stress-strain system). Any and all of the issues that have cropped up to this point will now come into play as entire plains of atoms tilt and shear past each other to accommodate all of this strain. If the rate of cooling is not absolutely even there will be distortion. This is why a good quenchant is not just about speed.

At this point you can leave the fate of the blade in the hands of the quenchant or you can do an interrupted quench and literally take matters into you own hands. Around the 400F mark the martensite transformation will begin, if you switch from liquid to air the drastic cooling differential from the spine to the edge will be alleviated and if you see distortion start to occur you can guide it back straight again. The amount of strain energy introduced to the steel from this is negligible since the only reason you are able to do it is that a good amount of the steel is still face centered austenitic, which has so many available slip systems that it is what allows us to forge steel. The bad strain energy is the pile up of strain artifacts such as dislocations and vacancies which is the cause of work hardening.

Any distortion should be able to be handled by this method alone, but if it gets away from you the practice of straightening during tempering is the next best thing and also introduces very little strain energy in comparison to the huge amounts involved in the hardening process. What happens is that the tempering temperatures skew the modulus of elasticity to allow for gentle and slow plastic reshaping without the need for stressing the steel with higher loads.

Steel moves two ways- plastic and elastic. Elastic will just take the original form once the load is removed, plastic keeps its new shape after the load is removed, and assumes some of the load in the form of strain energy. This is all accomplished by planes of atoms sliding past each other in what I have referred to here as slip systems, the denser the stacking the more slip systems and the easier it is to deform the steel via slip. Body centered stacking systems (room temp stuff) offer very few slip systems and thus pile up much more strain energy artifacts when forced to plastically deform, the more force that goes into the move the more energy is left in the steel.

So methods that would induce detrimental strain energy levels would be any sort of sudden loading, like hammering or rapid tugging on cold steel. To move steel cold you have to force the plastic deformation by overcoming a total lack of slip systems. But even with something like straightening in the temper, the two or three degrees of movement to pull a badly warped blade straight is introducing so little strain over a very wide area of atomic stacking that it is still nothing compared to the stresses involved in the hardening operation itself.

Strain energy can be revealed via crystallography methods, and to some extent metallographic analysis, it may even reveal itself in various ways in an etch, but spectral analysis really only tells you the chemical elements present in a sample which is unaffected by levels of strain or even the various phases in the steel. Unfortunately to know so many of these things for sure there are many analysis methods that need to be used to get a snapshot of each specific aspect or property of steel that we may be interested in, as I am fond of saying- the big picture is like a jigsaw puzzle, made up of many little pieces that have to be accurately gleaned and put into place one at a time.

I could take several Charpy samples that I slightly deformed, and then straightened during marquenching or tempering and compare their impact values to untouched pieces to address Derrick’s intitail concerns, but I am VERY confident that the numbers would be so close that the margin of error itself would make them indistinguishable.

This has been a very enlightening discussion. Thanks to all who have contributed.

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Around the 400F mark the martensite transformation will begin, if you switch from liquid to air the drastic cooling differential from the spine to the edge will be alleviated and if you see distortion start to occur you can guide it back straight again. The amount of strain energy introduced to the steel from this is negligible since the only reason you are able to do it is that a good amount of the steel is still face centered austenitic, which has so many available slip systems that it is what allows us to forge steel. The bad strain energy is the pile up of strain artifacts such as dislocations and vacancies which is the cause of work hardening.

Any distortion should be able to be handled by this method alone, but if it gets away from you the practice of straightening during tempering is the next best thing and also introduces very little strain energy in comparison to the huge amounts involved in the hardening process. What happens is that the tempering temperatures skew the modulus of elasticity to allow for gentle and slow plastic reshaping without the need for stressing the steel with higher loads.

This passage, and the concept of "slip systems" in particular really brought it home for me. Moreover, if I understand it correctly, it seems that any "memory" that a bar of steel may retain should be completely erased through the normalization process, long before any hardening or straightening processes should ever even occur. Thanks, Kevin, for taking the time to describe these concepts in simple terms - I now have a much better understanding of what's going on here.