Tempering Induction Hardened 42CrMo4 Steel Helical Gearwheels from Residual Heat Using Spray Cooling

Induction hardening is a surface hardening process in which severe temperature gradients occur due to limiting the heating to the sub-surface layer. High residual stresses result from these gradients, which can negatively influence the fatigue strength and induce cracking. To reduce the residual stresses, induction hardening is followed by a tempering, which is usually carried out for several hours in a furnace. Substituting furnace tempering by tempering from the residual heat motivated the current investigations. During induction hardening of helical gearwheels made of 42CrMo4 heat-treating steel using a controlled quenching by means of spray cooling, an additional tempering from the residual heat was performed. The result of the heat treatment was analyzed with the aid of hardness, residual stress, and distortion measurements. The obtained hardening results demonstrate the equivalence of the two tempering processes considered here.