Hardening Thick Stock - How is it possible

[SteveS]

OK This is bugging me and I can't answer it, maybe someone here knows.

When you quench something thick, first the surface cools. When that happens, the martensite is formed - Like in sub-sub seconds. But, as the core cools it is passing it's heat thru the surface. Doesn't that temper the surface martensite?

On a something thin like a blade edge, I'm sure it's not a problem. But if it's thick 3/8" or 1", it seems to me you can't avoid tempering the surface.

Steve

[mete]

I don't know how to best explain it but even with large pieces of air hardening steel where the heat transfer rate is slow you still end up with untempered martensite .

[AwP]

Get a paper cup and fill it to the top with water. Put it in a campfire/stove/charcoal forge with blower off. Until the water boils out of that cup, the paper won't burn, it's a neat trick and I think demonstrates the same principles of heat transfer pretty well.

Basically my understanding is that once the surface is cooled, it won't go back above boiling temp as long as it's still in the water, as the center cools it pushes heat into the outside, which is then moved from the outside into the water. The heat moves from the outside into the water faster then it moves from the inside to the outside though, so it'll always be cooler on the outside until it stabalizes at the quenchant or room temperature.

[SteveS]

Thanks helps a lot. Plus that's a cool demonstration to do with the kids.

Steve