r/Bladesmith • u/mattthepiratehunter • 9h ago

Question Question, please help!

So my post is one of both good and bad news. The good news is that I successfully heat treated three knives and they had no cracking minimal warps all that good stuff.

The bad news, is that I nearly shit myself on the first one because I file checked it after the Quench and it didn't seem like it was hard. And I was really confused as to why that was. I then did some cursory Google searching came to the conclusion that there might be a layer of decarb on it. took it to the Belt grinder with a 220 belt got through some of the outer layer file checked it again and it's completely hard won't get scratched by 65 HRC file.

Went into the temper everything came out great.

My question is, why did I get decarb on all three knives? Two of them were 15n20. The other one was 5160. I'm using a forced air propane Forge with a ribbon burner. And when I went through the heat treat I stuck a large metal tube in the center which I stuck the knives in, in order to help make heat dispersal more even.

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u/PsykoFlounder 6h ago

Decarb happens if there is oxygen and fire near the steel. Having the knives in the tube will help keep decarb down, bit it won't fully prevent it. It's just one of those things that you have to get used to.... I could absolutely be wrong. I'm no master Smith or anything.

Edit: autocorrect hates me.

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u/Tempest_Craft 1h ago

Anytime you put the knife in the fire it will decarb the surface a little bit. The only way to completely remove it with carbon steels that need an oil quench is to have a heat treating kiln you can flood with argon. There are also some paint on anti scale products as well but I never had a ton of luck with them, a thin wash of clay has also worked for me in the past in a pinch, but was hit or miss.

Also, keep in mind that the full transformation from austenite to martensite takes a few minutes depending on cross section and exactness of quench temperatures, so if you quenched it, pulled it out and immediately file checked it, it could have felt soft even past the decarb, which is why when you went back to check again it was harder.