Soooo...I Have a Question for You All....

You're trying to make 4 aren't you?

 

Wellll...now that you say it....

 

Cris
What is going on here? would not san mai have three pieces forged together? am I seeing 2 here? and what is the reason that factory steel does not split but home made does? I thought that the forge welding actually caused the pieces to become one. Is it that the forge temp is not as high? this image puts me in mind of a bimetallic strip where one expands way more than the other and causes the strip to bend by design. Aren't the metals used in San Mai similar in their expansion in heat?

 

I'm sure whatever you decide to do will be awesome.
Big knifemaking world out there....kick it's a§...

 

San mai is three pieces yes. you're seeing two pieces because it literally split the core steel in two (this is fairly common as the weld holds, but the core steel does not). Factory steel can split as well, but it's less common because of the far more uniform weld. The stresses inherent in hardening san mai steel are pretty great. Yes the steel has become 'one', but that is no guarantee, as the pieces still aren't of the same chemical composition. So, you have a 'single' piece of steel, but only the middle hardens. When it does so, the crystalline structure takes on an entirely different form than the softer pieces. A very brittle form with slightly more overall volume (when I harden honyaki knives, the hard edge becomes 'proud' of the soft spine if you run the entire flat on the grinder). The cheeks (soft steel) return to the same state they were in prior to hardening. This puts ridiculous amounts of stress on the hard and brittle core steel. ANY flaw in the weld can perpetuate this kind of splitting. Hell, a longitudinal sanding mark along the core steel (spine or edge) can do the same.

So you see, it's not the expansion in heat. It's the crystalline changes upon rapid cooling that cause the problem.

Here's a quote on it by Howard Clark (whom most of you will not know, but many of you just might!), who is probably the greatest American swordsmith that has ever lived:

That pretty much says it all . Also, to clarify...'hardenability' does not refer to steel that will reach a high hardness, but rather high alloy steels that through harden readily. Steels with high manganese, or other alloys. Simple steels like W2 aren't as bad...which is what my san mai would be made from. This is one reason I'm not happy with Aldo's 52100/410SS options. 52100 has a ton of chromium and manganese, and thus is more likely to split as you've seen here.

Thanks brother...that's the plan!!