Post-weld heat treating is best done by experienced blacksmiths and metalworkers, as it requires an intimate knowledge of the properties of high-carbon steels. Both the welds and the heat treatment must be precise or the weld will fail. The simpler the steel, the better it will react during post weld heat treatment. It is best to use 1045 through 1095 steel, rather than 5160 or D2. Using 10-series steels will provide more consistent results. The welding wire or welding stick used must be compatible with the particular steel being welded.
Preheating the Forge
Preheat your forge prior to welding. The forge must be as hot as it ever gets before you begin your welds. The forge door must be closed during preheat. If you do not have a forge door, place fire bricks in front of the forge opening so that they also preheat. Failure to preheat the forge to its maximum temperature will result in less complete normalization of the part after welding. Welding causes a lot of stress and variation in the steel structure. Normalization returns the steel as close to its original state as possible.
Weld While It's Hot
Once the forge is completely hot, begin welding. There must be a clear path between the weld station and the forge. The very second you finish welding, place the part into the blazing forge. As soon as the part begins to glow dull red, shut the forge down. Close the doors or place the fire bricks in front of the opening to hold the heat in as long as possible. Let the part sit overnight in the forge to cool. In the morning, remove the part from the forge.
Reheat the forge to maximum temperature again, using the same method. Once at maximum, place the part inside the forge again. As soon as it glows dull red, shut down the forge and close the door or block it with fire bricks again. Allow the part to cool over the course of six hours.
Use Standard Heat Treatment
Follow the standard heat treatment for the type of steel used to make the part, using the slowest quench medium recommended. Temper the steel to the lowest acceptable hardness for the part. According to industrial blacksmith Gypsy Wilburn, "You want the softest temper and quench because of the stresses placed on the steel by welding. Do a triple-draw temper even if you do not normally do so. This makes the best and most complete martensitic transformation in the welded areas."
How Heat Treatment Works
According to metallurgist David Pye, "As heat is applied to iron...(t)he oxygen in the air will begin to react with the shiny surface of the iron to form Iron oxide. Once the steel reaches...1350 F, a change in the structure takes place as well as the phase. The phase changes from ferrite to austenite. An indication of the change occurring is that the ion will lose its magnetic property, which can be seen by testing the surface of the steel with a magnet."