Treating D2 in a coal forge

[lyuv]

I need to HT a D2 blade (2mm at the thickest) in a coal forge. I read that idealy, it needs to soak at 1020c for 1/2-1 hour. But It seems very hard to maintain steady temp for such a long time, and I assume carbon loss will be significant.

How much "performance" (hardness? toughness?) do I lose, if I soak for only few minutes?

What do you suggest?

[Frazer]

There are certain aspects of your question that I can't offer much insight on since I have never heat treated D2. However, to maintain a more constant temperature while heat treating in my coal forge, I have a piece of square tube (it's actually welded angle iron since I had that in a thicker walled section) that I half bury in the coals and I place the blade in there. The walls of the tube will conduct the heat more evenly and hold the heat if I need to move it around in the coals or remove clinker or whatever while it soaks. If the forge gets too hot, you can turn off the air and that heat will remain in that steel for a while until you turn it back on. You may still have hot spots, but they will diminish substantially after being conducted through the tube, the thicker the walls the better (up to a point). 

Still an hour is a long time to try to maintain an even temperature no matter what, but this will help you maybe hit that half hour mark. I think to get the most out of your D2, you'll need an oven that can hold the proper temperature for the proper duration, but you will still be able to harden it if you deviate from the manufacturers suggestions. How much will the performance be impacted? I don't know.

As for the carbon loss, the smaller the tube (closer to size of the blade) the less air will get to it. The creation of excess scale will increase carbon losses, but as long as you don't significantly overheat the blade for a long time, I don't imagine the losses will be noticeable. I'm just thinking of something like damascus, where you're forge welding repeatedly at high temperatures, after all is said and done, I haven't noticed an appreciable drop in harden-ability. When heat treating in an oven I have seen people wrap the blade in steel foil with some sort of combustible inside to remove the air that comes into contact with the blade while soaking, but in a coal forge that isn't really an option since you cant easily tell the temperature of the blade once you've wrapped it up.

However, that has just been my experience, if someone else has more experience with D2 I would defer to their opinion on the matter. 

[Irondragon Forge ClayWorks]

Heat treating in a coal forge with a pipe, (called a muffle) is discussed in several threads. One is a sticky above, Question about HT with a pipe.