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San Mai Puukko blade help


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Hello all, I created an account here just for the sole purpose of picking your brains on some san mai I've made into a puukko style blade. Let me start out by introducing myself. I'm Keith from Indiana, I've been blacksmithing as a hobby for around 14ish years. I'm comfortable forge welding and I'd like to think decent with a hammer. I know there's always more to learn, but I just wanted everyone to know where I was coming from. 

 

My san mai construction is hardware store weldable mild on either side (the bar of stock that I got doesn't specify 1018 or A36), with a file for the core. Im using a coal fire pot btw with a hand crank blower, but burning charcoal. Kind of a pain to get that neutral fire with charcoal in a shallow pot. My steps were as follows, Welded a billet taking about 5 welding heats. It all seemed to have welded up just fine,  flattened out into a billet, normalized starting from welding heat and my 3rd normalizing heat was only brought up to critical and slow cooled in ashes. The next day I cut out two puukko blanks and ground a blade out of one of them. Sanded to 320 then quenched in water. After about 3 seconds in the water, I started to hear all sorts of ticking noises, and assumed the core was tearing itself apart. When I pulled it from the water I saw no obvious cracks so I tempered immediately and the cracks showed when I went to polish after tempering. It was the mild steel core that cracked!!! The blade did warp away from the side that cracked, and there is a touch more mild cladding on the side that didn't crack. I figured that side having more mild made the blade pull that way in the quench, but why on earth did the mild on the far side crack like it did? The only thing I can think is that it was A36 and with all the forge welding heats I took on it, some extra carbon migrated into it making some alloy of medium-ish carbon steel that couldn't handle the stress of the warp? Thoughts? The cracks run perpendicular to the cutting edge. Also, do I have the grind right for a puukko? A diamond cross section that follows the curve of the bevel along the belly? I have one more blank from this billet, thinking of grinding the 2nd blade and trying oil. But from what I've read, don't you want a faster quench medium to keep the san mai core from tearing itself apart. Especially with a combo like this that doesn't have similar heat treating properties?

 

Thanks, Keith 

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I meant to add my opinion earlier, but it's late for today and I would rather do it on a desktop than my phone, but the short version is never ever trust hardware store "weldable steel" for critical objects, and don't water quench san mai unless the outer layers are wrought iron.  Remind me tomorrow...

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On 8/6/2022 at 3:47 PM, Keith Bair said:

the mild steel core

I'm hoping this is a typo

“So I'm lightin' out for the territory, ahead of the scared and the weak and the mean spirited, because Aunt Sally is fixin’ to adopt me and civilize me, and I can't stand it. I've been there before.”

The only bad experience is the one from which you learn nothing.  

 

Josh

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4 hours ago, Alan Longmire said:

I meant to add my opinion earlier, but it's late for today and I would rather do it on a desktop than my phone, but the short version is never ever trust hardware store "weldable steel" for critical objects, and don't water quench san mai unless the outer layers are wrought iron.  Remind me tomorrow...

I figured for cladding on a san mai billet, hardware steel would be fine but I guess not. What I can't figure is how the mild cracked in the quench but the core steel is fine. I thought I had read that san mai needs the fastest possible quench medium the alloys can handle? I'm completely new to san mai though. Just a keyboard warrior for now

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We may never know, but yes, hardware store steel is A36 and may have enough copper contamination to cause those cracks when welded to high carbon and forged. 

 

 However, what I suspect happened is that the cladding had enough  carbon to harden, and did so, before the core had a chance to shrink on cooling.  The thicker cladding on the other side did then pull the hardened side until it cracked.  If the cladding had been thicker on both sides you'd have seen the reverse, the core would have split down the middle and peeled open.  I learned that one the hard way by water-quenching an axe head with 1095 bit and A36 cheeks.  Split the 1095 right down the center.  It hardened, the A36 shrank afterwards, and pop!  I felt it at the two-second mark.  

 

If you had been using wrought iron for the cladding you'd have been fine using a water or brine quench (maybe).  For any quenching, you actually want the slowest quench that still gets the job done.  That's why they developed fast oils, they cross the nose of the TTT curve as quickly as water, but then level out for a slow cool afterwards.  If you were to do the exact same process you used on this one but quenched in a fast or medium speed oil you would not have cracking issues.    

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7 hours ago, Alan Longmire said:

We may never know, but yes, hardware store steel is A36 and may have enough copper contamination to cause those cracks when welded to high carbon and forged. 

 

 However, what I suspect happened is that the cladding had enough  carbon to harden, and did so, before the core had a chance to shrink on cooling.  The thicker cladding on the other side did then pull the hardened side until it cracked.  If the cladding had been thicker on both sides you'd have seen the reverse, the core would have split down the middle and peeled open.  I learned that one the hard way by water-quenching an axe head with 1095 bit and A36 cheeks.  Split the 1095 right down the center.  It hardened, the A36 shrank afterwards, and pop!  I felt it at the two-second mark.  

 

If you had been using wrought iron for the cladding you'd have been fine using a water or brine quench (maybe).  For any quenching, you actually want the slowest quench that still gets the job done.  That's why they developed fast oils, they cross the nose of the TTT curve as quickly as water, but then level out for a slow cool afterwards.  If you were to do the exact same process you used on this one but quenched in a fast or medium speed oil you would not have cracking issues.    

Thanks so much for this. I'm gonna finish this blade out for myself to use and as a conversion piece, but its not getting my mark on it by any means lol. I'll try oil on the other half of the billet and see what happens. I also have a bunch of wrought, its my favorite material to work by far so I kinda horde it lol. Want to make a nice wrought iron clad san mai folder for my father in law for Christmas. Thanks again

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