Tempering Technical Question

Greg Covington

Greg Covington

Joined
Oct 28, 1999
Messages
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I understand quite a bit about tempering in general and the benefits of multiple tempers, but here is a question.

Is tempering more a function of the temperature it is raised to, or is it the cooling that accomplishes the "softening"?

For instance, if a blade is heated to 350F and then taken out and placed in a freezer immediately, does it have the same effect as a leaving it in the oven and letting it slowly cool to room temp?


Greg Covington
 
It's the "heat up"/soak that causes the change within the steel's matrix. Rapid cool downs from a tempering heat can, and often do cause problems such as crystalization/brittleness. An example is when the "soft back draw" is accomplished and once the desired tempering color is reached.....the blade is quenched to prevent the colors from running any further. This is not a good thing to do! I have had several folks test for their JS whose bledes have broken during the bend test, and all have been guilty of soft back drawing and quenching to keep the heat contained to the spine. These blades have all shown a crystalized grain on the portion that has been quenched during the soft back draw process. So in reality, allowing the blade to cool to room temp is a desireable part of the process too. One other thing that I have seen happen when an individual takes a blade from tempering temp and cools it rapidly is micro cracks in the edge, that only become visible during final finishing.
 
Ed - The soft back draw I am familiar with is when you draw back the spine of a blade with a torch – this after the blade has been fully quenched . Are you saying that the makers you referred to again (quickly) quenched their entire blade to prevent the heat from migrating to the tempered edge? If so, is it the sudden change caused by heat migration of heat from the soft draw process that effects the edge/tempered portion?

I’ve seen examples knifemakers (Graymaker in Maine) water cool their edge while drawing back the spine, rather than quenching again. This seems more sensible to me if you are going to do the soft back draw. I do notice that the more traditional soft back draw is not endorsed so much any more (Wayne Goddard comes to mind) in favor of the edge quench. Still, it seems like it may be a sensible alternative if one can keep the heat away from the edge. Thoughts?

Though I’m relatively new to this, I also agree with your comments on the cooling process. Quite often, patience seems to be the byword. I let my blades cool on top of the oven for an hour or so and then let them stand some wher else for a while before doing the freezer thing. I may be a novice to steel in knifemaking, but I’ve seen and heard enough to fully realize that once you perform an action , molecular movement can occur for hours or days.

On last thing on the soft back draw – are you referring to 5xxx & 10xxx series forged steels? Can I assume that different things may happen with other carbon steels , particularly if they are formed by stock removal?

Thanks & I enjoy your posts & work

Bob
 
Your exactly right! I was referring to those who use a torch to heat the spine, (without having the edge in water) and then quickly quenching the entire blade to keep the colors from running to the edge. Eventhough the temp is no where near critical, a degree of thermal shock is realized in the blade, causing the crystalization. (usually visible in the spine area when the blade is broken)
The soft back draw can, and does work if the person takes the required care and time to do it correctly. Keeping the edge in water/oil to keep it cool, and SLOWLY bring up the heat in the spine. Most folks want to hurry up and get it done......and that's where the trouble begins........and just get worse from there. Keeping the edge in the coolant until there is no danger of the heat migrating further is a key to success also.

The same general thing will occur in a blade whether it's forged or stock removal (as far as the soft back draw is concerned). I can tell you that there is a big difference in performance of 5160/52100 that have been edged quenched, between forged and stock removal blades. I experimented with this several years ago, and was surprised to find that properly forged blades that are edge quenched tend to out perform forged blades that are treated the same way. Maybe the word "surprised" is not the right one......I suspected the forged blades would do better, but the degree of performace was more so than I had theorized.

You said it all with the word "Patience"! Get in a hurry.....mess it up!
 
Thanks for the info Ed.

I have been bringing my blades up to heat in the oven and then shutting the oven off and letting them cool to room temperature.

I have seen guys do the soft back draw by torch and also by a heated piece of steel, but never a quick quench from it.

Again, thanks for the info.

Greg Covington
My Website
 
since I'm mentioned here I'll tell you how I do it
I harden
two main ways with O1 and the 1095's of steels
one
on the big blades I will harden in the oven
then full quench for a full hardened blade.
two
the small blades I'll use the torch
by heating mostly the edge staying away from the spine.
but not worrying about it.
I do it by color by eye now for that just right time
you'll know it after doing enough of them

then I'll draw ether down in the house oven 2 hours
leaving it for the timer to kick it off, then
let cool in the oven with the door closed.
then I'll use a water tube, a copper tube slit down the middle
with the edge slid into the slit and the
tube bent up and around the tip. I connect a PVC hose
to the end of the tube and I gravity feed the room temperature
water to the tube controlling it with a valve.
with a fair stream of water you now can heat the spine
until your blue in the face or you run out of water:eek:
it leaves a nice temper line that you can see in a mirror
polish ,,
  this way it doesn't matter which way you harden it.
let the spine cool with the water running,
I'm not saying the temperatures because it differs
in what your using for steel and the use of the blade.
  this is basically the way I do it.

you can wait until the next day to draw the steel down
unlike the stainless steels.
I hope this adds to the help.
it may not be for you but it's fast and almost fool proof.
 
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