Craig;
I would have to dig out my old Machinery's Handbook to find the critical temps. They do chamnge acording to the % of carbon in the steel. The higher the amount of carbon the lower the critical temp. Actually the process of "Cooking" in the carbon is technically referred to as Carburizing. My experience with the process has been limited to the non-color process. Often times if a part was wanted with only a portion of it hardened it could have machine stock left in the area not to be hardened, the part carburized, but not quenched. The extra stock would then be machined off the area not to be hardened thus removing the carbon rich layer or case. Part would then be reheated & quenched hardening only the areas where the carbon was left.
I seem to recall the minimum heat range for hardening to take affect is above 1300°F but don't recall the exact temp. On the parts I was familar with the absolute minimum was not used but normally was above 1400°F.
The process of carburizing + quenching thus produces what is commonly referred to as a Case Hardened part. The colors are a By-Product, are very shallow & the loss of color has no affect on the hardened case, its still there. As with any heat treated part heating it can & will draw or temper the hardness. The higher the temp is raised the more it will be tempered until at just below the critical temp it will for all practical purposes be virtually annealed. Slight drawing normally starts at about 350°F. It can be given the color without reaching the critical temp, but the part will not be hardened & is certain to be left in its softest state for the carbon content. This is why the process needs to be done by someone who absolutely knows what they are doing.
