Heat Treating Blades That Have Already Been Heat Treated.

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gabbadj

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Oct 8, 2013
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I want to clay temper some blades that I have to create a hamon. Problem is that all the blades that I want to do this to have already been heat treated.
Is this something that can be done? Is it something that is not recommended? Maybe its something that is done often?

Most of the knives I want do this to are kitchen knives (at most 3.5mm thick) and some slightly thicker "bushcraft" knives.

Thank You
SMR
 
Hamon is not a beginner project

It requires specific alloys with specific qualities in the steel and specific temperatures to hit.
 
Well, I am a beginner and what you have said about specific alloys made me realize that most of my old kitchen knives are not ever going to see a hamon. I do however have a rather large collection of old blades that will work (10XX stuff). I have been retooling my old collection with new handles and polish for a while now but I want to do something more.

I just can't find any information on whether or not you can "re-temper" a blade.. As far as I know, once it has been through the process, the blade is done. Will a blade ever temper correctly a second time or is it just going to ruin the steel? Will an already treated blade ever be able to achieve a hamon by trying to treat it a second time?

Maybe it's just time for me to finally build that forge and experiment for myself?
 
I am rather new so take my advice with a grain of salt.

I think you could indeed go ahead and try to re heat treat the knives. If done correctly there shouldn't be any problem. It's common if a maker misses their target for HT they re do it.

Now it is possible to flub the HT, of course.

With a carbon blade though you will need some way to remove the decarb and that is a few thousandths you would have to take off so you should see if the blade has enough "meat" left to handle that.
 
You would need to anneal it and probably take it through some thermal cycles. You will need to know what steel you are working with. Then heat treat it again. Why not just get some 1084 or something and make a knife? It would probably be easier and you would have known good steel and it would be something you made. Your going to have to remove the handle anyway.
 
Carbon steel, right? If so, to re-heat treat them there is no need to "anneal" them and thermal cycle them. Annealing steel softens it so you can work on it (you aren't planning on grinding on it are you?). Thermal cycles are done after a normalizing heat to take care of aus grain that was blown up with the high normalizing heat (you weren't planning on doing a normalizing were you?). The knives in question are carbon steel blades that are already hardened. If you want to re-heat treat them.....go for it. Bring them up to 1475-1500, short soak if you can, quench in "fast" oil, and then temper them. (leaving out the usual suggestions of working with known steel, etc)

If the knives are 1075, 1095, W1, W2 or the like....you can get a proper hamon. If it's 1084, 52100, O1, etc....you will only get a differential hardening line.

Stainless steel is a different animal. Once the heat treat is done, you'll likely screw things up doing it again. That is NOT always the case tho. I know of some recent AEBL knives that were re heat treated, then sent off to a lab to see what was what, and there were no problems to speak of.
 
What about issues with warping? Most kitchen knives are ground pretty thin and I imagine you may have issues with this...
 
If these are already finished kitchen knives (ground thin), you will end up with this if you try:

ruffle-chips.jpeg


Not to mention, if you don't 100% know the steel you're working with, then even more problems.
 
Thanks for all the great advice everyone. I am going to jump into this and see how it goes. As far as ruining any of my knives, that isn't much of an issue. I have a few dozen of old dirty ones that I made myself 20 years ago as well as a box of others that I have collected. After watching some youtube videos on building a forge, this seems like it would be cheep and fun to try.
 
Re-heat treating the blade may effect the hamon because multiple quenching from austenitizing temp will refine grain to the point which is too much, and lower the hardenability of steel, result as the hamon may turn up too close to the edge.

To solve this kind of problem is to re-normalizing.
 
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The worse that will happen is that you will ruin the blade. Grind and smooth the edge back to @ 0.010" thickness to reduce the chances of cracking along the edge. Likewise smooth any other sharp edges. Take the clay coated blade up above critical, quench, snap temper @ 350 F.
 
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