Joseph, a lot will depend on the heat treating itself. Quenchant? Did the blade get as hard as it could? Was the blade brought to 1500 and held at that temp for a certain time, also known as soak time. Was the blade edge quenched? And if so, how high up does the hardness zone go? Finally, what type of steel? Deep hardening steel like 5160, or a shallow hardening steel like 1095? A lot of variables.

I usually straighten any warps right out of the quench, when the blade is still hot, like 700 degrees. You have a couple of minutes to straighten. The blade is still moveable for this time. You can also quench the blade until below critical, say 1200, and clamp it between two aluminum or copper plates, thick plates like 1/2", to straighten. Allow it to cool between the plates. I will say that even forging and grinding, also correct thermal cycling will help a lot with warping.

As far as torching the back and quenching it, I guess you could, but it would be better to full quench the blade, temper, then clean it and draw the back or spine. This is done with the edge in a water bath, with the water level about 1/3 or less up the blade. You can then use the torch, o/a or propane, to draw the spine. Just be careful when you get towards the point area. I also draw the tang soft.

Hope this helps.

Brion

Joseph, pages could be written on the subject of you question, indeed entire books could be needed to cover it all. This is partly due to the fact that something that seems so simple and understood by all is vastly misunderstood by the majority of the population.

There are many things in play here but to keep it basic there is elasticity (the ability to stretch and return to original shape- springy) and plasticity (the ability to permanently deform without breaking- bendy).

All heat treating of any kind can do is give more or less plasticity by replacing degrees of elasticity with it. Tempering the spine induces a little more plasticity and allows the blade to bend just a little bit without breaking. Differential hardness replaces all of the elasticity with plasticity, except in the very edge, so such a blade will have very little spring and will bend long before it resists enough to break.

How much force it takes to flex any blade cannot, I repeat- cannot, be affected by any heat treatment, it is solely a function of the blades thickness. What the heat treatment does is determines how soon the blade bends permanently. Due to the thickness of the spine vs. the edge I would expect the spine to determine the behavior of the blade which in this case will be almost totally ductile (plastic), and thus it should bend in your straightening operation very similar to an annealed bar of steel.

What you are witnessing is the exact point I often make about hamons and differentially hardened blades, while they may exhibit a resistance to fail in a brittle fashion (soft steel seldom does) they have almost no laterally strength whatsoever. Making that feature almost solely a matter of aesthetics. Blades that are differentially hardened and are not really bendy tend to have thicker spines in order to compensate for the lost strength, have a look at the cross section of an old Japanese sword sometime.

If you like the ease of straightening, you can keep doing what you are doing, however I would suggest prevention may be the better course since with an edge quenched blade distortion issues should be but a fraction of a fully hardened blade. I would double down on your normalizing and stress relieving efforts first.

If the blade is edge quenched the way you are doing it is probably the best option. If the blade is clayed, then you can interrupt the quench at around 400F and quickly knock the clay off and straighten with your gloved hands before the blade cools to below 275F, as you can also do with a fully hardened blade.

There would be no way to successfully induce more hardness in the spine without redoing the entire blade. One can tempering in zones, but in order to harden a minimum of around 1400F must be obtained to notice any changes, this is very hard to do without undoing adjacent areas. And if you insulate well enough to protect the other parts, they will effectively become heat sinks interfering with properly heating the other parts.

Wow...great answers. Helped a lot!

Brion- what is the difference between shallow and deep hardening steel? Also, great answer!!

Kevin- I see....makes sense. Very detailed response; I was able to recognize that a lot of what you are saying directly applies to what I was doing. Cool!

Question for both of you: You both mentioned straightening while the blade was still hot- I thought I was supposed to let it cool in the quenchant to get ultimate hardness? Wouldn't that 'interuption' also be an interruption to the hardening stage? Lastly, how would I do that with gloved hands...I mean, would it really be that ductile?

Maybe I should just full quench....I would prefer a springy blade that has a breaking point vs. a ductile blade that stays bent. Idea- what about a really thin layer of clay? Like, a really thin layer...maybe that would be 'just enough' for a springiness?

What do you guys do? Do you prefer the full quench or diff. hardening?

Sorry for all the questions!! Last one: Isn't the differentially hardened blade what is required in the ABS tests? I was wondering...has anybody had such a springy blade, that it was able to bend the 95 degrees, and yet still bend back almost straight? Maybe that is impossible, but can't hurt to ask.

Thanks again so much for the answers!! I really appreciate it...you guys are really helping out. I wonder when I'll run out of questions and mistakes...

|quoted:

...Sorry for all the questions!!...

Shame on you for apologizing for asking good questions <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//wink.gi f' class='bbc_emoticon' alt=';)' /> Folks like you are the life blood of this forum and your questions are what it is all about! I hate logging into this forum and not seeing any new questions for days.

The process of hardening is a very complex one that has many stages, with some study into what is happening inside of the steel you can start to paly with the process in ways that seem magical at first but are way cool for having total control over your medium. The whole idea behind hardening is to avoid the steel shifting to a soft phase when you cool it. True the steel is really soft when it is glowing hot but simple steels want to be another soft phase (known as pearlite) at room temperature. This shift to pearlite happens at around 1000F and it takes a little bit of time. A little bit means around .75 seconds, but that is enough for us to mess with it. By quenching we plunge the steel too quickly through this range for the really soft stuff to turn to pearlite.

If we don't make pearlite at around 1000F we will have the same really soft stuff that the steel was when it was glowing, even though it is no longer glowing. Now here is the mind blower- the steel doesn't actually turn to the hard phase, known as martensite, until it reaches the 400F-500F range. So from 1000F to around 450F the steel is still the same phase that it was when it was glowing, we just never notice this in a traditional quench because it is over in seconds under the surface of the quenchant. Most folks traditionally just kept the blade in the quench until it was cold and hard.

But if you time your quench and interrupt it around 450F the blade will not be hard yet and you can straighten any warps before it does harden. The martensitic transformation is not like pearlite, it does not give a hoot about time, all it needs is dropping temperature, so be it the quenchant or be it air cooling, below 400F it will harden all the same. The secret to the quench is avoiding that pearlite, what is left has to harden.

One page at a time- now your other questions <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//wink.gi f' class='bbc_emoticon' alt=';)' />

Yes the blade is that soft to just move with gloved hands, I have left my finger impressions in thin edges when I got stupid with my griping. As the blade moves through 350F to 250F you will feel it stiffening and when it gets cooler than that you should probably stop messing with it. I love messing with students minds with this and bending their blades and telling them they have around 3 minutes to straighten them <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//biggrin.gi f' class='bbc_emoticon' alt=':D' /> . At my Demo at CanIron in Quebec earlier this summer I brought badly bent blades to harden and then straightened them for the crowd while they were hardening.

This process is no secret in industry it is called marquenching and has been a standard practice for around a century. You will notice that as-quenched blades done with marquenching will Rockwell around 1.5 to 2 points lower and folks who do not understand the process believe this is a loss of hardness. What it actually is are the results of what is known as auto-tempering. You see, as much as 40% of your martensite will form at temperatures within the tempering range, and in the slower air cooling the blades own thermal energy will have enough time to get a head start in tempering, so you do achieve full hardness, you just got a little bit of tempering it back done already.

The best way to go springy instead of ductile is to full quench the blade and then put the edge in water to protect it while drawing back the spine to a blue temper.

I have done a bit of testing of my own in the area of tensile yield vs. impact toughness and because of this I fully harden all of my blades for the maximum strength vs. toughness. Just ask yourself how many good prybars or car springs have you seen that easily bent? I prefer a blade that will break, but can handle more force than the human arm can generate to do it, to a blade that a medium sized guy can pull over with one arm in a vice. Now for very flexible blades that neither bend nor break and return to true, just make them thinner. Try this- take a soft bar of 1/4 x 1" steel and try to flex it, yes it will take a bit of force but it will take a permanent set at around 25 to 30 degrees. Now take a really thin piece of steel like 1/16" or even a steel ruler and pull it over to 90 degrees and watch it spring back to true. All steel has a yield strength at a certain amount of strain and force, a thinner piece of steel will never develop the force/strain to reach the yield point, but the thicker you make it the more force/stress will be involved until you get dangerously close to the yield point.

Glad to hear that you don't mind answering questions, cause I've got a ton more coming!

I have to admit that some of this is mind boggling....not sure if I'm processing it all. I've learned though that if I try to remember the words I don't remember, and learn the ones I do understand, I'll recognize the information as I start following it. Thanks for the good responses...I'm getting there 😉

I'll have to try making a full quench on a blade...will a normal soddering torch work? Also, how do I put the edge in the water if it is a largly curved bowie knife, or a kukri? Lastly, let's say I put half the blade in the water...do I temper the other half (the spine) to a deep blue? Or maybe just 1/3 of the spine?

thanks again!