Hardness inside of blade same as outside?

R

RedWolf4

Joined
Oct 20, 2019
Messages
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This is one question I've personally never seen discussed and am curious about. If you heat treat a blade (standard heat treat, oil quench) how does the inside (center) of the steel harden as much as the outside or layers close to the outside? The steel is hardened from the oil due to the rapid decrease in temperature, right? Then wouldn't the outer layer of the steel cool at a much faster rate than the inside of the steel? And I imagine on thicker blades this effect would be even more pronounced. Just as heating up a big chunk of steel takes longer, wouldn't it take longer to cool down as well? Never seen this addressed before. Wouldn't this be a problem because if you sharpen/reprofile/grind a significant amount of edge away, you would be exposing what I would think would be a much softer core.

I've never heard of this being a problem , I'm really curious about the science of what's going on here. Any insight appreciated. Thanks
 
I think there are a few threads about this. You can find some information by searching for "shallow hardening steel". Here is one thread where it was discussed in 2016: Shallow vs deep hardening steel.

It is my understanding that for the typical thickness range of knife blades, it is not a concern even with shallow hardening steels.
 
Medium hardenability/oil hardening steels like 52100 should still be pretty hard at the center for blades up to 1/4 inch thick.
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Source: http://www.mie.uth.gr/ekp_yliko/SteelsforBearings.pdf
 
Paul Bos met with us members of S, Cal Knife makers. He said there can be a minimum of 1 RC point in variation with air-hardening steels. This was years ago. Samurai swords are quenched For a hard edge & softer spine. As long as your edge is Hard! You should be fine..
 
Short answer:
At the thickness of a knife blade all the steels we use will through harden in a normal full quench. In many cases the outside would test softer because of decarb.
 
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