Recommendation? Blade edge went from straight to wavy after grinding

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FunkCoaster

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Nov 2, 2010
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Highly related to this thread : https://www.bladeforums.com/threads/wavy-warped-edge-on-clay-quenched-blades.1751994/

The difference from the referenced thread is that everything was good and straight after heat treat and an initial 45 degree bevel on the edge. I put an s-grind into the blade and it "popped" into being wavy. I can now push the blade edge back and forth to alternate the wave.

The question I'm hoping to answer: Is there any recovery to this situation? Or is it bound for the 'oops' bucket?
I am considering tempering at a higher temperature (low expectations), or grinding away the entire spine and middle to remove all but the hardened edge. Drastic, but it might allow the edge to go back to straight.

My theory is the cutting edge was under compressive tension after hardening and trying to expand. But the spine (unhardened) was thick enough to keep the edge immobilized. Once enough mass was ground away, the hardened cutting edge was able to expand by buckling and relieving the compressing force. Like a spring going sideways.


The specs:
26C3 steel.
Basically a santoku profile.
Satanite applied 2/3 of the way from spine to edge. Symmetric on each side.
6.75" blade length.
2" blade width.

Using knife steel nerds recipe for hardening, with a relatively low tempering temperature (need to look it up).

After grinding:
Cutting edge measured 0.035" thick.
Halfway up measured 0.053" thick.
Spine measured 0.130" thick.

Before grinding was probably 0.06", 0.10", 0.13", but not measured.
 
Following. I tend to agree with your analysis … cant think of a way out of it without ruining the temper .. but am curious if others come up with suggestions
 
Or re temper at the same original temperature and clamped to a flat bar ?

Did you grind the hollows alternating sides?
 
HSC - If I understand your question, the hollows were ground in roughly symmetric... I would do ~3 passes on the left side, followed by the right. Repeat a dozen times until it reached the measured thicknesses. At the conclusion each side has the same depth of grind.

I expect a flat bar clamp along the edge can only help in the re-temper, so I'll plan to try that unless a brilliant alternative shows up.
 
How wavy is the edge? If clamping to a straight bar doesn't work, and there's only a couple waves, you can try this again, but instead of clamping to a straight bar, use dimes to shim the waves the other way and re-temper.

Or you might try the carbide bit hammer trick to stretch out the compressed side and push the wave back to straight.
 
Image attempted below, showing the edge-on view of the blade below the ruler:

PXL-20220928-204237151.jpg


I tried to re-temper at 350F while clamped between 2 angle irons. No change.

weo - The shims didn't seem possible, since pushing on the wave inverts it. Put another way: There is a high point at the 10cm ruler mark. If I push it downwards it becomes a low point, and the 7cm mark becomes a new high point. Makes a fun noise when it flip-flops. Something like 'Broing'.

Now that things are seemingly doomed, I may try the carbide hammering coupled with lots of prayer.
 
from what I have heard and see, it really looks like the edge is under compression due to relative expansion of the spine (like others have said, likely due to differential in the heat treat between the edge and spine).

perhaps t he carbide hammering will have some success, because it works by actually spreading out the metal away from the point of impact. I would strive to try to "stretch" the overall length of the edge ... perhaps by a series of hits going in a line lengthwise along the blade, at several distances from the edge. Also, dont forget to do the hits uniformly on both sides of the blade (otherwise, all you will do is curve the blade to one side).

there might well be an issue with getting the divots out after you do the work. will be interesting to see...
 
Cushing H. said:
I would strive to try to "stretch" the overall length of the edge ...
Click to expand...
I think it should be the other way around, shouldn't it? The way I see it, you want to stretch the spine so it no longer forces the edge into compression. If the spine is soft, you might be able to do it with a regular ball peen hammer and pretend like you had planned the textured look all along...
 
Hubert S. said:
I think it should be the other way around, shouldn't it? The way I see it, you want to stretch the spine so it no longer forces the edge into compression. If the spine is soft, you might be able to do it with a regular ball peen hammer and pretend like you had planned the textured look all along...
Click to expand...
On second thought, you probably have to stretch the neutral axis, not the spine or the edge. I can't think straight, I think the rona has addled my brain.
 
Hubert S. said:
I think it should be the other way around, shouldn't it? The way I see it, you want to stretch the spine so it no longer forces the edge into compression. If the spine is soft, you might be able to do it with a regular ball peen hammer and pretend like you had planned the textured look all along...
Click to expand...
Hubert, I think maybe you are right - I have been thinking about it in terms of if the spine is "stretched", then the torque from spine to edge would put the edge into compression .... but as I think about that condition, I can not see a way out of it (if you stretch the spine more you put the edge under more compression, whereas if you stretch the edge, you again just put it under more compression from the torque put on it from the spine.

On the other hand, if you think about it as the spine is "unstretched", but the edge is "stretched", the spine will hold the edge back and not allow it to physically stretch .... but it you hammer on the spine and allow it to stretch, then it no longer holds the edge back, and the compression along the edge should relax and allow the edge to go straight. Hammering along the spine also has a better chance of grinding out the divots.

So yeah, looks like hammering along the spine is the way to go.

(logically there should be some symmetry between the two ways of looking at it .... but for the life of me I can not see it)
 
Hubert S. said:
On second thought, you probably have to stretch the neutral axis, not the spine or the edge. I can't think straight, I think the rona has addled my brain.
Click to expand...
yeah - like I just wrote, it is kind of confusing how to think about it when you really try to do so :-(
 
I doubt it would help, but at this point I feel like it's a hail Mary situation, or the other options are to throw the blade away.

Maybe try heating the blade with a torch, very lightly, around the area where the hammon line will be. See if softening just that part of the blade provided any kind of help. Maybe keep the edge submerged in water while trying this.

I doubt it will help, but idk worth a shot

Edit: I'll add when I say heat, I don't mean until glowing, more just maybe back to a blue color or something similar.

Actually maybe clamp the edge, and submerge it.
 
With an 1/8" thick spine, and thin at edge, with the Satanite slowing the cooling up around the spine, and the thin lower half of blade allowed to cool quickly, could this have caused the "wavely" edge? Perhaps anneal the blade, then re-HT without the Satanite allowing the blade to cool more evenly?

Good luck and keep us posted as to results.
 
Good news! The carbide hammer was a success. Two blades were affected by this problem, and both look quite straight now.

What I did:
Use a carbide straight peen
Aligned to lengthen the blade (peen is perpendicular to blade edge)
Loads of light strikes. Maybe 200 in total.
Strikes were focused around the temper line (hamon) with some deviations to either side. The thought was that would enable the steel to lengthen where the most stress was located.
The carbide strikes resulted in some 'regular' bending that was quick to sort out using a mallet with strikes along the spine.

I haven't finished the blade out, but I suspect a few hammer marks will remain.

Ken: I think your description of what happened is basically right; I've seen more exaggerated waves happen straight away when an edge was thin going into a quench, but never this bad after grinding.
 
good news! I have not yet done my own heat treating, but if I ever go there, looks like I definitely will need to make a carbide peen :)

Im really glad this worked out for you.
 
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