Is there any way to STOP centreline cracking on San Mai?

[@sebastiannico7450]

I'm no metallurgist but I have hardened many stainless/wrought iron clad high carbon core - san mai blades. I've also had this splitting happen to me. The way we've managed to minimize it is by first exposing the core steel on the cutting edge of the knife profile, then we make sure to only edge quench the blade, about 40% of the way up the blade. We quickly correct any warps while the blade is still hot and then allow it to cool to the touch. As soon as we can comfortably pick up the blade we put it into the tempering oven and relieve the stress on that core steel. If the blade is left out too long after quench - that is when that splitting it going to occur. If you find that the splitting is occurring during the quench, then it's most likely due to too fast of a quench medium.

[@danielbottner7700]

Awesome, very few people have the mindset or patience to stay focused long enough to solve complex issues. Your insights are much appreciated.

[@clarkeknives4159]

The quench medium used has to match the core material and I've had splitting using the slowest oil. However, since this video was made I've heat treated a lit of susceptible blades with a thin coating of Satanite clay on the edges. None of them have split so that seems to be the easiest option if one prefers a fully hardened blade.

[@clarkeknives4159]

@@danielbottner7700 Thanks Daniel.

[@candyshop84106]

Anyhow.. Very much appreciate and like Mr. Graham Clarke..Thank you 😎

[@TheOGfrenchy]

Great video! Interesting topic! I believe I’ll be sticking with two Hardenable materials ! 👊

[@FredrikSpare]

Graham is looking well, great video!

[@clarkeknives4159]

Thanks Fredrik 😊😊😊😊

[@monroecustomknives5809]

I always figured it's from uneven amounts of mild steel causing the steel to pull more in different directions, could be a good thing to try just edge quenching on to see if there's still splitting that occurs

[@DietrichPodmajersky]

I think what is happening here is the core fails in tension as the outer skins shrink around the brittle core. It is probably easier to visualize in the case of differentially hardened blades. In the case of hamons gone wrong with the classic hockey stick crack that opens up, the shrinking spine rips the hardened edge apart. Think about what happens to a katana when properly heat treated. A straight blade goes into the quench, it then curves down, curves back up past its initial state and is pulled out with the expected curve. So the edge shrinks first, then the back shrinks and finally but it is hard to see the edge expands putting the whole blade in compression on the edge to spine axis. If the edge cools too fast and starts transforming too much into brittle martensite before the spine shrinks then it isn't able to plastically deform and fails in tension. In short the differential shrinking rates are dancing around the martensite formation in the hardenable section. As to the san mai, thicker skins = more force and thinner less so the ratio of core to sides is a variable. Putting clay on the spine would delay the formation of martensite giving more time for plastic deformation to equalize the forces, etc.

[@krissteel4074]

It doesn't happen to me enough of the time that I can really put much of a guess on 'why' it just sometimes, randomly seems to happen. Vast majority of the san-mai style I've made lately is 1018 cladding, layer of nickel (because shiny) and 125SC core, really fine slip of ATP over the whole lot, 10min at temp and a water quench. Wasn't a single crank, bang, delamination or warped one out of about 10 in a row. Now I'm not being a smartarse about it, but I fully expected half of them to pop on me! Literally if I got 40-50% usable out of that batch it was going to be good enough. Other side of the coin, whole heap of 1084 clad with 26C3 core, 10min soak in a fast-oil quench: almost guaranteed that it'll work as they're so close in terms of heat treatments Nope! A little over half of them had something completely crap-out on me. either an entire side peeled away on one, crack right down the middle or ended up with something going bananas. There was a lot of very short knives made that day and the art-bin got filled up with shame. Any advice? Yeah- make 2 or 3 of the same one and hope 1 survives! If they all survive that's great but I wouldn't count on it

[@clarkeknives4159]

Check those that cracked have failed along the centreline of the core. If not its a de-lam problem

[@Wiking419]

I had the same problem so I know the pain when you almost had it... you'll do all and at the most important part of the process, it fails. So the solution I come up with is to put less carbon steel near the core and then went with mild steel and then again with carbon steel and mild again at the end. So W1 or 52100 as a core, bandsaw blade next(which i was told is 15N20, so 2%Ni there) then mild steel and repeat or not. Good heat, none flux, no delamination no core cracking, no problems.

[@PsilocybeXIII]

ty for sharing

[@lasersailor184]

I occasionally get cracks from the knife edge up when I'm doing Hamons with W2 and sometimes other steels. Maybe 1/5 or 1/6? Any ideas?

[@normandbujold6677]

Very interesting video. I experienced this making sanmai with 1/4 “ stock mild steel and O1. But the crack happened while air cooling when the billet was approximately 1/2” thick after forge welding with hammer and anvil. I switched to W2 and it doesn’t happen anymore. However, I do get some perpendicular cracking when water quenching. Do you think that thining out the O1 billet before letting it cool would avoid this issue? Thanks for your videos, I am a fan

[@RainyDayForge]

How would compression affect this? Coming out of the oil and then clamping the steel between some plates. It might change the rate of success?

[@andyc750]

interesting, obviously a complex subject

[@danielbottner7700]

Always informative & scientific; info I find no where else. - The expansion of steel in the hardening/cooling process sheds light on steel cracking within & along crystal boundaries. - Micrographs reveal higher levels of cracking in outer layers of steel crystals of finished knife blades. - My thought & ( I may well be wrong ) is that this cracking may be related to the relative rapid cooling occurring in the blade grinding/profiling stage of machining. - I am also thinking rapid cooling occurring in the sharpening process may also result in steel crystal cracking along boundaries & within carbon structures near the cutting edge, diminishing cutting edge stability. - Can any one confirm or refute these concepts ?

[@clarkeknives4159]

Cracks during grinding are well known. Surface grinding too harshly generates heat under the grinding wheel and the adjacent cold metal coupled to the quenching effect of the grinding coolant causes cracks perpendicular to the direction of grinding that are as long as the grinding wheel is wide ☹☹☹☹

[@danielbottner7700]

@@clarkeknives4159 That's extremely logical, though I had never put the two together. Have you ever tried heat soaking the blade to minimize surface to core metal temperature differential & the corresponding cracking ?

[@BestKiteboardingOfficial]

If almost as if the Japanese put the Hamon on for a reason beyond decoration.....