[bindlestitch]

Originally Posted by Alan L

If you're in a dark room and you have good eyes, plus a forge that can be dialed way back, you can visually observe the transition point. I'm not talking about colors, either.

The phenomena of decalescence and recalescence are your friends here. With the forge barely running in the low orangy region, say 1500 or so, put a blade in and watch it as it comes up to temperature. It will go to dull red, but then as it starts to get into the orange range (as seen in darkness, it's barely red in bright light) you will see what looks like shadows swirling around inside the steel. as the blade heats a little more, the shadows will disappear, running up the blade towards the tang. At the edge of this shadow/no shadow zone there will be a dark line. What you are seeing is the energy being absorbed by the iron and carbon crystal lattice as the crystals change phase from body-centered cubic to face-centered cubic, which means you're ready to quench (or hold at that temperature for a soak) once all the shadows are gone.

If you are just normalizing, pull the blade as soon as the shadows disappear. Watch it as it cools, and you will see the shadows reappear, but this time there will be a bright line at the leading edge. This is the phase change back to body-centered cubic structure. It releases energy instead of absorbing it, thus the bright line.

I do this in a coal forge with the lights off, and no air blast. If you're using propane you just need to turn it down until it's only just hot enough to cause the transition.

Alan,
Thank you kindly. I've seen exactly the process you've described but just never knew what it was. I too use a coal forge with the lights off, actually outside at night is when I us it, so I can usually see what the steel is doing.
Much obliged again,
Israel