Bending fatigue tests were made for VC coated steels which were coated by immersing in a molten borax bath and for hardened steels which were quenched and tempered, in order to clarify the effect of VC coating.
The results obtained are as follows;
(1) The endurance limit of VC coated steels was a little lower than that of hardened steels. It is considered to be mainly due to the decrease of hardness in the substrates. Accordingly, the endurance limit recovered almost to the level of hardened steels by an additional diffusion treatment.
(2) The initiation point of fatigue fracture of VC coated steel in reversed bending was on the substrate just under the VC layer. Hence, the endurance limit is correlated to the hardness of this part.
(3) However, there is a considerable scatter in this relationship and the endurance limit of VC coated steels was a little lower than that of hardened steels with equal hardness. These results suggest that the fatigue strength of VC coated steels is determined not only by the hardness but also by other factors. For example, the residual tensile stress in the substrate just under the VC layer is one of the factors besides hardness.