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San-Mai knife bends when grinding bevels

Jeff Johnson
Active Member Apprentice Bladesmith

I made a San-Mai knife blade with 4"x1.25"x.25" bar of 1084. Mild steel was same dimension. Forge weld was good and I forged this into a knife with 15" blade and 6" handle. Blade width was 1.75" wide with .25" spine. 

I ground the forge scale off, thermal cycled 2 times and quench into 80-100 degree canola oil.  I had a small warp so I clamped it in a vice. File tested and it was hard. Because of the length, I would have to temper it in our kitchen oven. I decided to grind the final bevels and shape. As I was getting close to being done, I noticed the blade was flexing. I tested the flexibility of the blade to find out it did not flex back to true. I eventually ended up with a San-Mai horse shoe. I have used the above method to make blades up to 12" without failures.

Any suggestions on why this blade was able to bend?

Is there a better method forge, grind, heat treat?

Should I forge and grind closer to finish blade before heat treating? I do try to forge to a finish blade style.

Thanks, Jeff

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Topic starter Posted : 13/02/2022 12:13 pm
Karl B Andersen
Journeyman Bladesmith

Expect anything when you're working with San-mai.

I do pretty much exclusively stainless San-mai, but the principle is the same. You have a combination of hardenable and not hardenable steels in the same sandwich.

Many times when I'm grinding my blades pre-hardening the blade will warp - especially on the first heavy stock removal grind. This leave a large difference in the amount of not hardenable steel on each side. As I grind the second side - the blade straightens back out.

The issues is unequal stresses pushing back and forth.

Thermal cycling is important. So is ABSOLUTE equal amounts of not hardenable steel from one side to the other.

After I quench my blades for 14-15 seconds, they come out of the oil and get gently clamped straight until cool.

Then they get the first temper regardless of potential warping. Then I may need to clamp and straighten on a subsequent second temper. I have also found that I can give it a third temper if necessary at around 350 and accomplish further straightening without affecting the hardness - much.

If it was easy - everyone would do it.

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Posted : 14/02/2022 7:27 am
Jose Carlos Martinez
Active Member Apprentice Bladesmith

Wow, your work is excellent.

Ive never tried a SS San Mai.

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Posted : 17/05/2022 6:27 pm
Chris Young
Active Member Apprentice Bladesmith

Jeff, I am going to throw this out there as second hand info I learned from Dr. Larrin Thomas of "Knife Steel Nerds"  The culprit may be the canola oil.

He has a video on his you tube site where he tests 1084 using the same HT temp but different quenchants then tests the hardness at various depths of the steel by grinding into it.  Canola had the worst through hardening results.

It could be that your mild steel jacket prevented the core from hardening.  I am attaching a screen shot of his results.

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Posted : 20/05/2022 2:39 pm
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