Surface hardening is a little more complicated than that, as the process typically involves the addition carbon to low carbon steel (pack hardening, or the use of a product such as Kasenit). If mild steel is treated as you suggest, because carbon content is too low, there is little, if any, change in surface hardness. Medium carbon steel, and other steel alloys harden as you suggest, but the hardening is not just at the surface, but through the part. Quench rate determines how deeply. Very heavy cross sections typically have softer cores.Wanna-Be wrote:On the topic of hardening. Flame or surface hardening is not too complicated for gears and smaller parts if you have a small furnace or forge to heat them to about ??? degs. (not sure of the exact temp. since I don't have a pirometor). You will be able to tell when it is at the right temp. by checking it with a magnet.
Surface hardening of medium or high carbon content alloys can occur by rapidly heating the surface of the material, whereby the core does not reach a critical temperature. That has been commonly accomplished on lathe beds, as an example, or it can be accomplished by induction heating, using the proper frequency, whereby heating occurs on the surface, alone.
I make mention of this so readers won't get the idea they can make gears from mild steel and achieve a degree of hardness that would be useful. That typically is not the case.