Unwanted Tempering Whilst Sharpening

[John Harper]

Hi,

I read on another forums that Roman Landes said at ASHOKAN that he has found that sharpening a knife on a dry stone can induce unwanted tempering and lowering of the hardness in the vicinity of the edge and more so with abrasive belts.

Can anyone who attended confirm this and perhaps give a bit more by way of detail, especially how he measured any such drop in hardness?

Cheers
John

[Robert C]

It would be nice to have Roman make the statement direct instead of second or third hand comments.

[Kevin R. Cashen]

I know it seems unlikely at first that one could generate tempering temperatures by a swipe on a stone, but on the micron level it suddenly makes sense. I could give you the details but I agree with Robert that Roman himself could answer best.

[Mitch Rokicki]

Yup; Roman said it. I would like to hear him say it again.

[wick ellerbe]

Just heresay, but I was once told by a machinist that it is friction heat that acually wears out files. I don't really know, but have to think it probable.

[Ernie Balch]

I think he said that even sanding by hand creates super high temperatures on a microscopic scale where each tiny piece of grit scratches the blade metal. I guess it kind of explains how metal is removed. The influenced depth of heat penetration must be not much more than the scratch pattern depth. I guess it would be more of a tempering effect with a very thin edge than a typical edge.



ernie

[John Harper]

Hi,

Thanks for your replies.

I guess, I'll just have to wait until Roman sights this thread.

Cheers
John

[Roman Landes]

Hi guys,

sorry for delay but i was out of office.

I read on another forums that Roman Landes said at ASHOKAN that he has found that sharpening a knife on a dry stone can induce unwanted tempering and lowering of the hardness in the vicinity of the edge and more so with abrasive belts.

Yes that is true.

Grinding generates energy (heat) and every step of sharpening is grinding even the strop.
This heat can be sucked away by the right coolant such as water.
If the grinding action lacks the coolant, the heat goes mostly into the torn out part and the body (blade/edge) it was torn off.
It becomes obvious to see the induced energy when you see the sparks fly (Burning steel!!).

Depending on how hard you go over the piece the more energy is induced the hotter it gets, thats basic physics.
Than the guys come and say but I can do it so sensitive that the edge will not suffer and I'm dipping the blade each run into cold water....
Well that is a nice effort, but when it comes down to the very edge this tiny fraction is overheated faster, than the eye can see or the wrinkled fingers can feel.
Unfortunately the edge becomes thinner the close you come to the very edge/point means generated heat will get jammed in the tip.
In addition to that tempering colors that would visually proof this are ground away immediately when they appear.
and Stainless steels need a higher temperature to generate tempering colors and longer time to build them up.
Nevertheless one can do metallurgical examination that can proof the issue testing micro hardness
There are some old german study's that examined this issue in the very detail.

[Robert C]

Thanks Roman ! One of the reasons that you can see some beautiful delicate structures in a traditionally made katana is that hand polishing minimizes heat damage that would change those structures.

[Roman Landes]

And as you probably know Robert, every metallurgical sample preparation works the way, to avoid heat creation whist grinding.

I had a book dedicated to general grinding methods, in this book i found a test application.
A normal steel block apx. 2"x2"x4" that had a large number of highly sensitive thermocouples integrated in the surface.
The block was slit dry by hand over a 1000grit grinding paper.
The peak temps measured, walked up to 2000°C for split seconds in the very surface (some microns).
Of course the block did not melt since the volume fraction of induced heat was to tiny to affect such a large solid piece of steel.
But the effect was there and proofen.
In a edge we just talk about some microns of material, here the effect is solid an clear.
Every manufacturer of razorblades knows this and does excessive cooling whilst grinding and polishing edges, that need to hold an super sharp edge for very long.
It seems just some the magic makers out of the custom knife scene think, the physical principles like this, do not apply to them....