Kees,
It's not overheating. If anything, it's the opposite. The visual evidence of carbon pooling that you see in Larry's video happens at a lower temp than the loss of magnetism, which is what most people rely on to determine the correct temp. Larry's point, as I understand it, is that by watching for the carbon pooling instead of using a magnet or looking for color (definitely the worst way), you'll avoid overheating.
FWIW, the video makes it look a whole lot easier than it usually is in practice, though that depends on your method of heat treating. It takes some practice to see the carbon pooling when bits of scale are flaking off and ash is swirling around and you're nervous about the 1400° temp in front of your nose. I think most people will find it much easier to use a magnet, at least at first. And the risk of overheating or grain growth with the magnet method is very low.
I agree that decarb is really not an issue for most people. But for anyone who's concerned, Borax works very well as an anti-scale compound. It needs to be sprinkled on at the right temp, after the steel is no longer blue but before it starts to show any red. And it (the Borax) is tougher to remove than more expensive commercial anti-scale compounds. But it definitely works. I wouldn't bother in most situations, but i might use it for something like a float, where you don't want to have to remove a lot of material post HT.
Last edited by Steve Voigt; 03-19-2016 at 11:03 AM.
"For me, chairs and chairmaking are a means to an end. My real goal is to spend my days in a quiet, dustless shop doing hand work on an object that is beautiful, useful and fun to make." --Peter Galbert