Straightening and Steel Fatigue?

[Silky]

I've read that over time many sanmai blades will warp to some extent and may need to be straightened. Does anyone know how straightening blades affects the steel over long periods of time? For example, let's say over so many years you had to straighten a blade one, two, three, or more times in the same place. Would the blade become more prone to bending or even fail all together at some theoretical point? Would this be dependent on how far the blade needed to be bent to return it to true? Also, have any of the bladesmiths here had to straighten a blade post-heat treat and had it affect their blades in any way (other than just shattering)? Thanks KKF

 

[GoodMagic]

I have never had a san mai blade that warped to a significant degree if it was straight when i bought it. I have made blades that were bent slightly after heat treat, usually if they were too thin pretreat. I usually temper them and that has generally relieved the stress and fixed the bend. Iff the blade is still warped, I re-temper it and then if needed use a dowel jig to straighten. results never perfect. I have snaped a few and caused cracks in others. So now I just leave a bit more thickness pre heat treat.

 

[ian]

I’ve had some tip bends from a mag strip. Yea, I’ve wondered the same thing about what happens when you correct them.

 

[HRC_64]

I've read that over time many sanmai blades will warp to some extent and may need to be straightened. Does anyone know how straightening blades affects the steel over long periods of time? For example, let's say over so many years you had to straighten a blade one, two, three, or more times in the same place. Would the blade become more prone to bending or even fail all together at some theoretical point? Would this be dependent on how far the blade needed to be bent to return it to true? Also, have any of the bladesmiths here had to straighten a blade post-heat treat and had it affect their blades in any way (other than just shattering)? Thanks KKF

This whole issue is a red herrring...the knife is straightened during manufacture
Keep in mind that steel has fatigue-cycle-life in excess of 100,000 cycles IIRC
if you keep the steel away from plastic deformation.

That's the core. I think the real mis-understanding is that "iron" isn't steel,
and the wrought iron cladding is somewhat easily to deform plastically
(on the one hand) and also to confuse with "cast iron" (on the other).
Cas iron doesn't like plastic deformation, and is very crack prone...
(which is why its not used).

AFAIK the whole point about 'wrought' iron is you can deform it readily,
and repeatedly (without issue)...this and the fact that the core steel sometimes
is just "flexing" it like a spring, not plastically deforming...

Note that mono(steel) blades are a bit different in this regard.

 

[bahamaroot]

I've read that over time many sanmai blades WILL warp to some extent and may need to be straightened...

Change that "will" to "can" and you're closer to the right track. Sanmai blades are more likely to warp than monosteel but that doesn't mean they will, and more of them don't than do.

 

[HSC /// Knives]

where did you read this? I've never heard of such a thing.
It's true, they are flexible and will bend.
The only laminated steel blade I've ever bent is a chopper I made for a cutting competition and it had bends in it after chopping a 2x4.
I don't believe a kitchen san mai blade will warp on its own.

as far as in knifemaking, every laminated steel blade that I quenched has bends and even twists in it that I straighten with a brass mallet on wood. I don't think there is any concern with san mai, that is one of the benefits of the soft cladding which allows the straightening in this method vs a homogenous steel which is straightened differently .

For homogenous steels, there are several ways to straighten a blade, I've never experienced the straightening having any effects on the blade.

regards
Harbeer

 

[Silky]

Thank you so much for the responses so far, it's all very interesting. It's just something I've read here and there on the various knife and blade forums I browse. I don't know whether sanmai blades are more likely to warp over time but it was something I was also interested to learn if it was true or not. I've heard it in regards to either single-bevel blades or just sanmai blades in general that uneven tension in the cladding can lead to twists and bends.

Perhaps unrelated, but I also read a forum post about one makers experience hot vs cold stamping his maker's mark into his blades resulting in an apparent weakness in the blade at the point of the stamp when he did them cold. Specifically, the blades cold stamped were more likely to snap during a bend test at the site of the maker's mark.

This whole issue is a red herrring...the knife is straightened during manufacture
Keep in mind that steel has fatigue-cycle-life in excess of 100,000 cycles IIRC
if you keep the steel away from plastic deformation.

That's the core. I think the real mis-understanding is that "iron" isn't steel,
and the wrought iron cladding is somewhat easily to deform plastically
(on the one hand) and also to confuse with "cast iron" (on the other).
Cas iron doesn't like plastic deformation, and is very crack prone...
(which is why its not used).

AFAIK the whole point about 'wrought' iron is you can deform it readily,
and repeatedly (without issue)...this and the fact that the core steel sometimes
is just "flexing" it like a spring, not plastically deforming...

Note that mono(steel) blades are a bit different in this regard.

Do you mind explaining more about the terms fatigue-cycle-life and plastic deformation?

 

[RDalman]

I think bending is more the thing that happens over warping on itself.
A san mai blade will take bends and not spring back, this can be a blessing and a curse. It will also take bends from having material removed from one side (like during sharpening/grinding)
Most people will not notice slight, or even bigger, bends I think because the knife will work still. Thicker san mai blades requires more force to bend, over thinner, so blades like wat/shig/kato probably have issues more rarely than the very thin ones. Heard from guys about thinning/sharpening takeda blades that bent super easily with just two fingers and gave them quite the headache haha. Because during sharpening one can bend a thinner blade with both removal of material, and accidentaly pushing it too hard on the stones and bending it that way. Pretty sure every san mai knife I have had sent to me used, for sharpening/rehandling or such, the first thing I have got to do have been straightening.

 

[inferno]

I've read that over time many sanmai blades will warp to some extent and may need to be straightened. Does anyone know how straightening blades affects the steel over long periods of time? For example, let's say over so many years you had to straighten a blade one, two, three, or more times in the same place. Would the blade become more prone to bending or even fail all together at some theoretical point? Would this be dependent on how far the blade needed to be bent to return it to true? Also, have any of the bladesmiths here had to straighten a blade post-heat treat and had it affect their blades in any way (other than just shattering)? Thanks KKF

if you buy a knife you as a user will never ever have to straighten it!!
Bladesmiths that makes it might however have to, several times, before it gets to you.

 

[inferno]

silky! I'm glad to inform you of this fact:
if your knife is so thin and weak it deforms while you are using it; then its also so weak and shi**y so you can easily bend it back to shape using only 3 of your fingers!
this is a win/win situation. if it ever were to happen that is.

 

[Kippington]

I've read that over time many sanmai blades will warp to some extent and may need to be straightened. Does anyone know how straightening blades affects the steel over long periods of time? For example, let's say over so many years you had to straighten a blade one, two, three, or more times in the same place. Would the blade become more prone to bending or even fail all together at some theoretical point? Would this be dependent on how far the blade needed to be bent to return it to true? Also, have any of the bladesmiths here had to straighten a blade post-heat treat and had it affect their blades in any way (other than just shattering)? Thanks KKF

Jeez this is a deep series of questions, the answers could span a handful of scientific papers (which are already out there somewhere).
You won't get a good answer that's also short.

A few things come to mind:

• Work hardening/shifting dislocations in the cladding
• Thickness of the knife, ratio of cladding to core
• Expansion coefficients of the HT
• Time and temperature of the temper
• Areas to remove steel to get a good grind (and the...)
• Resulting internal-stress cancellation to a new net force

I'm too lazy to bother explaining any of it though.

The simple answer (not a good answer) is: Don't worry about anything going wrong when straightening a san-mai blade. Just get it done!

 

[Dan P.]

I think the best course of action is to worry about it if/when it happens.

 

[daizee]

I've straightened a number of blades as part of the heat-treating and tempering process...

Assuming it's already been hardened, you always want to straighten it at its highest planned tempering temperature. It's easier to bend, and less likely to crack. Then follow that bending with another temper to relieve stress you've put into the blade.

Note that in order to straighten, you have to bend the blade PAST it's spring recovery point - I'm sure the metallurgists know what the right term is. And even with a fairly thin (0.110") piece of a modestly tough stainless (154CM), I've had to put all my weight into it if just using a vise and the bend is not near the tip. You'll NEVER put that much force into a knife as a user, so you're not going to fatigue anything by bending it back and forth repeatedly. As long as it springs back into place, you haven't done any harm.

If you do have to straighten a thin tip, you might consider warming it to ~300F before (in oil, or on the back of a hot pan - use an IR thermometer) before you do so, and then again afterwards.

300F is *probably* at or below any blade's highest temper point, but as a user you probably won't know what it was, and want to be conservative.