carbide straightening

S

soundmind

KNIFE MAKER
I primarily use carbon steel, but a customer asked for a batch in stainless. I decided on magnacut because there was a lot of information on it, I couldn't see how I could really go wrong with it.

Anyway, I got them back from heat treating and they all had a small bow. I successfully temper straightened most of them, but I had a 3" wide veg clever that would not respond to that for some reason, not sure if it was because of the width. I tried carbide straightening today for the first time and it worked great. I was very pleased. The knife is saved and ready to be finished out.

But honestly, I thought as soon as I hit the knife it was going to crack right in half. So, my question is why wouldn't it? From my reading it seems like impact induces stress and hardens the steel.
 
They don't break often, but it can happen. I'm using a 3/8" carbide ball in my hammer. I did have one chef knife to break. I don't remember the details very well, perhaps I hit it too hard? Didn't have blade laying perfectly flat? Not sure but it sure did break in half.
 
I recently straightened a 2+ inch wide Nakiri, worked great. But it left marks on the blade in an area that i couldn't grind deep enough to remove them. Any ideas about to get around that problem?
 
KenH said:
They don't break often, but it can happen. I'm using a 3/8" carbide ball in my hammer. I did have one chef knife to break. I don't remember the details very well, perhaps I hit it too hard? Didn't have blade laying perfectly flat? Not sure but it sure did break in half.
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Yeah, it's good to know it can be a problem. Thanks Ken.
I wondered if 50 grit scratches would increase the potential for breaking but I was ok. But I bet deep scratches could play in as a factor.
 
MTBob said:
I recently straightened a 2+ inch wide Nakiri, worked great. But it left marks on the blade in an area that i couldn't grind deep enough to remove them. Any ideas about to get around that problem?
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I left mine a little thick for that reason. I had a couple divots hanging out, but I got them out.
 
I would think 50 grit scratches could be a stress point for breaks for sure. I usually grind to around 400 grit before HT'ing 'n tempering.
 
I leave the mill scale on the blades during the HT and the first tempering cycle (I do a 300-325 snap temper on everything after the quench/cryo). Then I straighten, clean up the blades and do the rockwell testing and re temper as needed, all before grinding bevels. The mill scale ensures that I will be able to grind to clean metal after straightening with the carbide hammer and hrc testing to remove the marks from that as well.

CPM steels often are oversized to account for grinding off the mill scale and imperfections from the rolling process, so even after everything is cleaned, I am often left with a blade just thicker than the actual dimensions. I have a 1/8" piece of Magnacut that is .145" even after most of the flattening and clean up was done! 1/16" Magnacut often gets down to around .070" after cleanup before grinding bevels.
 
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