What happens metallurgically when a blade burns in a kiln?

daizee

daizee

Knifemaker / Craftsman / Service Provider
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Dec 30, 2009
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Saturday night we did a batch of stainless wrapped in foil, in a Paragon kiln. The usual stuff. Apparently there was a flaw in one of my packets, and the tip of one blade was visibly burned. We've seen this before with a bad packet, and if it's thin it usually requires... uh, "aggressive" correction.

Steel is S35VN plate-quenched w/ sub-zero, tempered twice at 400F.

So I took this one into my shop figuring I'd be able to grind through the decarb to find good steel, since I'd left the tip a bit thick. But let's test with a file first, right? Unfortunately, my worn file that I use for hardness testing just kept on biting and biting... It skated off the rest of the edge. I kept filing and filing... Finally I took it to the 60grit ceramic over a contact wheel at high speed, and removed almost 3/4" before reaching a spot the file would skate. This roughly corresponded to the burn pattern on the surface. Needless to say, it's a different knife now... (nope, no pix).

So the question is: What's going on metallurgically when this happens??
I figured there would be some thousandths of surface decarb, but it really seemed like the whole tip was soft (though I couldn't easily bend it, in this case). Did it... burn carbon out all the way through? Did it cause some sort of non-martensite structure to form during quench? Did carbides form some weird uneven pattern?

I can speculate all day, so I'm looking for an actual technical answer if there's one to be had.

Thanks!
 
Willie71 said:
You get massive decarb, so not enough carbon to harden.
Click to expand...

So, like decarb all the way through, so re-hardening is not an option in that case, right?
(I'm hoping the answer is 'correct', because otherwise I could have cycled it again instead of grinding the top off! :oops:)

How does the *inside* carbon disappear? Does it migrate out when everything is in solution?
 
daizee said:
So, like decarb all the way through, so re-hardening is not an option in that case, right?
(I'm hoping the answer is 'correct', because otherwise I could have cycled it again instead of grinding the top off! :oops:)

How does the *inside* carbon disappear? Does it migrate out when everything is in solution?
Click to expand...

Yes, all the way through. The carbon moves through the solution, then into the atmosphere.
 
Yeah, I've gotta use it better, apparently!
Usually we have pretty good luck. The shop is in flux, being re-organized and all, makes it trickier to do "clean" work like packet prep.

Thanks for the help!
 
I tend to doubt that the decarb goes all the way through the blade, especially if it only affected the tip and was done for a normal HT time. The decarb from a non-sealed packet can be a problem, but not to the degree described.

When you ground the tip back at "High Speed" on a contact wheel, you were more likely continuously burning the tip as you ground. Grinding post-HT needs to be done slowly with lots of dunking in water to keep it cool.
 
Last edited:
Maybe the tip was overheated and there is a lot of retained austenite, so it is soft. I've done blades at 2200F held for 45 minutes, with air leak in the foil packet, but after grounding off a bit of scale and tempering I get 64 rc throughout the blade. Decarb wasn't an issue even at these extreme temperatures. The problem I had in my experiments was retained austenite with overheating and blades coming out at only 50 rc.
 
I had a knife recently that the tip burned back a couple milimeters, with no steel left at all. I ground back a quarter inch and was at good steel.
 
Some steels are more susceptible than others to damage at high heat.

Alloys containing moly are particularly prone to problems. Other elements may also contribute to this like tungsten and vanadium.

Hoss
 
cotdt said:
Maybe the tip was overheated and there is a lot of retained austenite, so it is soft. I've done blades at 2200F held for 45 minutes, with air leak in the foil packet, but after grounding off a bit of scale and tempering I get 64 rc throughout the blade. Decarb wasn't an issue even at these extreme temperatures. The problem I had in my experiments was retained austenite with overheating and blades coming out at only 50 rc.
Click to expand...

In my case, there was a small hole poked through at the tip, but the foil was snug behind the tip. The tip was basically exposed to open atmosphere at nearly 2000f for 30 min.
 
I think my case was similar to Warren's description. It's entirely possible the tip area I ground off was in the 50Rc range. It wasn't bendy-soft, but it sure wasn't file-skate hard like the rest. The blade did go through sub-zero (dry ice, not nitrogen) and two tempers before I bothered poking at it. Hold time was about 25 minutes at 1950F.
 
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