Thread: Does steel have a correctable memory?

Originally Posted by floppyshoes

I beg to differ on this. Aluminum has a very small elastic region, and steel can be flexed only so many times before it fails from fatigue (normally tens of thousands of cycles, but it will fail eventually)

In the subject of warped blades, I have nothing more to add that was not already mentioned by our esteemed bladesmiths. Once that sucker is teat treated, you're pretty much limited to material removal for shaping. This is not a problem if there is steel where you don't want it, but a major issue if there is none where you do want it.

wished i were teat treated

Originally Posted by nun2sharp

wished i were teat treated

I'm not sure what shocks me more; that I made a spelling error that slipped past my proof reading or that somebody actually read my post

Originally Posted by JoshEarl

Chris,

I believe steel does have a memory, but it's possible the warp actually represents the steel returning to the shape it was "remembering." I've heard of blades made of straightened coil springs developing a curve when quenched. Not sure if it's true.

One other option for straightening blades would be to soften the back of the blade while keeping the edge cool. You might be able to keep the edge submerged in water and heat the spine with a torch. That would partially anneal the spine and allow you to straighten it.

Probably more trouble than it's worth, but it might be fun to try sometime.

Josh

I saw this done in my bladesmithing class for the ABS. Heating the spine while keeping the blade in the water to straighten out a warped blade. Michael Connor's from winters was teaching the class. I didn't get to see the whole process but no pressure was placed on the blade. It was gently heated and it straightened itself. The guy who did it said he could feel it straighten.

A blade could also warp because it was not stress relieved properly, or at all before harding. If using an oil quench if you move the blade around to much you could also cause a warp.

I would've like to have seen that.

Originally Posted by nun2sharp

I would've like to have seen that.

+1 on that. Now THAT would be a viable service to be offered if it worked on a consistent basis. Man alive, I could see myself sending a small box of blades to a "razor straightener". Seriously. Alas....if it sounds to good to be true.........well, you know the rest.

Chris L

Originally Posted by jimmyseymour

I saw this done in my bladesmithing class for the ABS. ... The guy who did it said he could feel it straighten.

A blade could also warp because it was not stress relieved properly, or at all before harding. If using an oil quench if you move the blade around to much you could also cause a warp.

I'd love to "see" him "feel it straighten" too. Must be using the Force. That and a coupla solid straight edges would show some good results.

Realistically, if the edge was flat and the blade honed properly, what difference, other than the aesthetic, would there be as far as the blade's ability to shave well? Now I suspect that razor manufacturers didn't much care that the spine had a warp as long as the edge was salvageable enough to hone/shave and sell.

Stress relieving: doesn't that occur after heat treatment? And, how will any quench cause warping by moving the blade around?

I think there's only one kind of steel that can morph back into its original state. Don't remember what it was called, but I saw a video of it once. Basically you can warp it into any shape, but once you apply heat, it will morph back into its original state. But it's not something you would use as a razor since it's so flexible. In the demo video, the guy used a spring, he twisted it in all sorts of ways, and then heated it with a hairdryer. He said he thought it could be used to erect buildings quickly. The buildings would be made at a hardware store modularly, then they could bend it into a compactable shape and move it to the site, and then heat it and have it erect itself. Making a whole building would take only a day!

I'm certainly no blacksmith, but I believe the only way to bring the steel back into shape is if you reheat it and reforge it. Simply applying a force could break it.

There are several alloys of stuff called memory metal. Here's one link: kinda cool really.

NiTi Memory Metal

But mostly small applications, and not blade steels.

Originally Posted by Mike Blue

I'd love to "see" him "feel it straighten" too. Must be using the Force. That and a coupla solid straight edges would show some good results.

Stress relieving: doesn't that occur after heat treatment? And, how will any quench cause warping by moving the blade around?

This wasn't a razor but a very big bowie knife. I was also incorrect in my writing. The convex side was heated and a little on the spine, not just the spine was heated. I'm sure with all that mass moving you would be able to feel it. It wasn't me who did it, so I can only repeat what he said. Maybe his feeling the blade straighten was more in his head while watching than anything.

stress relieving should be done during hardening especially for stainless steels. For stainless you let it soak for thirty minutes at a predetermined temperature. For a forged blade it could be argued that the normalizing process or the spheroidizing before intial heattreatment accomplish the same thing. There's no correct answer only personal opion on that one.

I was told again by Michael Connor that they moving the blade around (side to side motioin) during the quench causes the vapor barrier on one side of the blade to diminish causing one side to cool faster than the other. Up and down motion was ok and would help to break up the vapor barrier. If one side cools faster than the other then you will definitly have a warped blade. I've only had one blade warp on me but I believe it was due to poor grinding as I have just never moved a blade side to side when quenching. Remember the clay differiantial thickness is used to harden a samurai sword's also causing the curvature of the blade because the edge hardens far quicker than the spine. It's also why an uneven ground blade will warp. One side has less material than the other side and will cause one side to cool faster than the other and warp.

I have a book on making samurai swords and they talk about using heated copper blocks to change the curvature of the swords if it has to much or to little curvature. Heating the sides of the blades with the copper then laying it flat on the anvil and giving it a whack with a wooden mallet to straighten the blades, etc...

For sheetmetal and bodywork I've seen old timer heat the side of car panel up and mist water on the steel to cause it to shrink and straighten out some creases.

So depending on the steel type used and application will determine what is best for straightening a blade, or just throwing it away.

A few comments:

stress relieving should be done during hardening especially for stainless steels. For stainless you let it soak for thirty minutes at a predetermined temperature. For a forged blade it could be argued that the normalizing process or the spheroidizing before intial heattreatment accomplish the same thing. There's no correct answer only personal opion on that one.

Ahem. Stress relieving occurs in two types. The first is to relieve internal stresses generated by heavy machining of clamped pieces. The second is tempering which relieves the internal stresses in steel caused by martensite formation against areas of non-martensite. I could use your Japanese sword example because that is a perfect illustration. So, the fact is, not personal opinion, stress relief is desirable when one of the above conditions exists in the material.

... the vapor barrier on one side of the blade to diminish causing one side to cool faster than the other. Up and down motion was ok and would help to break up the vapor barrier. If one side cools faster than the other then you will definitly have a warped blade. I've only had one blade warp on me but I believe it was due to poor grinding as I have just never moved a blade side to side when quenching. Remember the clay differiantial thickness is used to harden a samurai sword's also causing the curvature of the blade because the edge hardens far quicker than the spine. It's also why an uneven ground blade will warp. One side has less material than the other side and will cause one side to cool faster than the other and warp.

Martensite formation occurs at the speed of sound (770 miles per hour at 68F at sea level, about 13548 inches per second). Even in a great big bowie knife (1/4-5/16ths of an inch thick at the spine), I don't think that any wiggling about a human being could do will cause a warp to occur in the short amount of time available between the initial quench and the formation of martensite, unless some very specific conditions are controlled. About the only way to significantly alter quenching both sides of a blade to cause one side to cool faster than the other, would be to lay only one side of the blade on the surface of the quench bath.

From what you describe, the vapor jackets are very transient and the bubbles would break down and be replaced by the liquid quenchant as fast as they form. That amount of time is not as fast as the formation of martensite. The more likely conditions that could be affected by martensite formation are the changes occuring in the grain structure as a result of the transition from austenite to martensite or pearlite. Those could easily be affected within the amount of time available and could contribute to warping. Another likely condition is that the steel is still in the metastable phase and has not completed conversion to martensite and is "more plastic" than hard yet. In that state banging off the inside of the tank or possibly even the resistance to motion afforded by a thick cold oil could bend the blade against the resistance of motion provided by your hand. It wouldn't be the motion itself, or the vapor jacket, but specific conditions in the steel though. I admit, it's a lot easier to think that it's the vapor jacket.

The speed of the quench is important in establishing the desired crystalline structure in the blade. It is the crystal's size caused by heat treatment or mechanical deformation that causes the internal stresses, not because the edge hardens quicker. For that matter, the sword blade is under a tension stress because of the size reduction of the blade's spine to pearlite from the larger austenitic crystal.

I have a book on making samurai swords and they talk about using heated copper blocks to change the curvature of the swords if it has to much or to little curvature. Heating the sides of the blades with the copper then laying it flat on the anvil and giving it a whack with a wooden mallet to straighten the blades, etc...
For sheetmetal and bodywork I've seen old timer heat the side of car panel up and mist water on the steel to cause it to shrink and straighten out some creases.
So depending on the steel type used and application will determine what is best for straightening a blade, or just throwing it away.

You were okay up to the point of whacking it with a wooden mallet. The copper block treatment is to relieve the sori, the formal curve that makes it a Japanese sword. That technique is not used to straighten a warped blade. I've seen 1/2 steel plate straightened with a rosebud torch and a little hammer. Good smiths have good techniques, but not all of them work in all conditions.

And not everything people think they know is true, even when it works. I'm still learning all the time.

Berticus has a better idea than throwing away the steel. Heat it up again and re-heat treat it.