Influences of Induction Tempering on the Fatigue Strength of Rapid Induction Quenched Steel.

When surface quenched steel is tempered through usual high frequency induction heating process, the hardened layer possesses a hardness distribution where hardness decreases as approaching the surface. This selective tempering owing to the principal reason of the induction heating process causes the reduction in the compressive residual stress at the surface which decreases the fatigue strength less than that in case of furnace tempering. However, as shown in this study, the above disadvantage of the induction tempering can be overcome if the following appropriate conditions are employed : (i) a low frequency to extend the heating area ; (ii) a low rate of rasing temperature ; (iii) cyclic heating to allow heat transfer to the inside. Using one of these conditions, the hardened layer is uniformly tempered and the remarkable decrease of the compressive residual stress at the surface is prevented. As a result, the fatigue strength of the induction tempered steel is maintained as high as that of the furnace tempered one with the same surface hardness.