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Good info. Probably best to use the propane torch vs. the oxy/acetylene? If you overheat the bolts could you possibly compromise the strength?
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Oxygen-Acetylene will heat the bolt quite fast, you would anneal it greatly. (Kinda like sitting in the sun in the desert) the pores in the skin will be very open, also the burn from the torch will effect the finish, Hardness will depend on curing. With propane you will anneal it enough to pull oil in for the finish needed, If you were to rockwell test it, it would be just slightly harder than original using this method.