Work hardening during stropping blade?

D

djomla

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In the last video on Karamat yt channel [video=youtube;QTx0lmwNkYc]https://www.youtube.com/watch?v=QTx0lmwNkYc[/video], Mors Kochanski talk about sharpening knives and he mentioned a very interesting statement (also mentioned in his booklet on sharpening). Namely, he says that stropping should be very quick to develop a heat, which eventually would "burn" the burr and RE-HARDENED blade. Since I know something about heat treatment and know how Mors meticulously approach to informations, I'm pretty sure that he didn't mean re-harden blade by re-quenching (blade must reach a critical temperature of about 800 celsius, and then suddenly be cooled). Actually, doubt at all that with stropping one can achieve such a temperature that would in any way affect the steel. What appeared to me as possible explanation is WORK HARDENING- strengthening steel by plastic deformation? It somehow makes sense, because a thin wire (burr) during stropping constantly shifts from side to side until abrasive effect of the stropping compound removed it. I wonder if someone has an explanation of the above phenomenon ??
 
I don't see how stropping could work-hardened knife steel.

Doesn't plastic deformation occur during a squeezing, bending, drawing, or shearing process?
 
You can only work harden non-hardened steel.

To work harden hardened steel is to fatigue it.
 
Any steel can be work-hardened, heat treated or not. And that is exactly what stropping a burr does-it bends and breaks off the wire edge, or "burr."
 
I don't think you are generating much heat with hand stropping, not enough to change the heat treat, but you can push the wire edge back and forth until it breaks away. At the microscopic level, I guess you are "work hardening" that bur until it breaks.

I can strop, and get the blade warm by hand, but it is not hot.
 
I'd say it possible. First the burr has to be small enough that when it breaks off the "edge" is usable. It implies the abrasive action of whatever is on the strop is so mild it cannot polish the burr off in the first place, so unlikely to do a good job of further refining after the burr snaps off.

The fold and snap method might induce some work hardening but I'm uncertain how one would test this. My own experience is that the edge lasts a lot longer if I don't fold the burr in the first place.

It is certainly possible to work harden/burnish HT'd steel, industrial uses have included steel run up to the low 60's Rockwell. This also requires the application of direct force such as a smooth wheel or rod of steel or glass, or some other means of locally overcoming the steel's resistance to deformation. Mild burnishing can be done on paper over a hard surface, hardwood etc. These means have in common the ability to exert a lot of pressure over a small surface area, something a leather strop really cannot.

Is possible mild abrasives in the leather heat might heat the steel up along the edge and it flows but then this would be readily observable and well understood in industry by now - it don't believe this to be the case.

All that said I don't have access to any sort of testing equipment that could determine what's going on at that level and have seen none of it with the gear I have. My gut says this mechanism is unlikely to work, but I couldn't say for sure.
 
I don't see how any heat generated during hand stropping could be of any real or significant benefit to the edge of the blade. Even if (theoretically) it gets hot enough, it's too random, uncontrolled or unregulated to be useful, and might do just as much bad as good. Absolutely no control over how hot it gets, or how long it stays hot. In that context, I'd consider such heating of the edge to be more 'work brittling' or 'work weakening' of a very thin, fine edge. Consider how many knives come from the factory with weakened steel at the edge, presumably from overheating during edge polishing or buffing/burr-removal (powered stropping, in effect). Those are usually 'fixed' by grinding off the damaged steel on a stone, then resharpening to restore a properly hard and strong edge. The bending back & forth of a burr, I'm sure, will heat the junction of metal at the base of the burr, but that's what makes it brittle enough to eventually be broken off, as it's then weakened (effectively) by the bending & heating.

I've occasionally noticed a warming of the blade's edge when stropping some knives on leather with green compound. I can sometimes even smell it, as it heats up; leather, green compound & steel makes a distinctive odor when stropping. But, I've also noticed it doesn't seem to change the behavior or edge-holding potential of the edge appreciably, or enough to be noticed. I wouldn't put much stock in it, as a means to strengthen or harden an edge. I tend to believe that the perceived 'strengthening' benefits to a stropped edge come only from the fact that the weakened steel (burr) has been removed from it, and not from any heating/hardening of the steel left behind after the burr comes off.


David
 
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How about this,
It is said (though I haven't found definitive proof) that the Spyderco Salt fully serrated knives have a higher HRC than their plain edge counterparts due to work hardening through the grinding of the serrations.

Which leads to my question, what temperature is required to actually consider "work hardening" a viability?

The gap between tempering down and burning an edge must be Very small in this process, right?
 
NJBillK said:
How about this,
It is said (though I haven't found definitive proof) that the Spyderco Salt fully serrated knives have a higher HRC than their plain edge counterparts due to work hardening through the grinding of the serrations.

Which leads to my question, what temperature is required to actually consider "work hardening" a viability?

The gap between tempering down and burning an edge must be Very small in this process, right?
Click to expand...

That's not exactly true,

H1 is rolled at the Mill and this is how it becomes work hardened, there is no heating of this steel. For example, if they start with 8mm of steel stock it is rolled to 4mm thickness and this work hardens the H1 steel. From a very good source it was explained to me that the H1 steel is harder toward the surface and has a lower hardness in the center, thus the reason the serrated blades have better edge retention than the plain edge version (the serrations are off center of the blade grind).
 
Heat treating of the H1 (or any) steel negates any work-hardening.
Work hardening has very little to do with temperature-think of bending a coat hanger until it breaks. That is work hardening.
 
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