ENDURANCE LIMIT OF THIN HARD-COATED STEELS IN BENDING FATIGUE

Bending fatigue tests were carried out on steels coated by thermoreactive deposition and diffusion of vanadium carbide (VC) and chromium plating, as well as on ordinarily hardened steels, to clarify the effects of the coatings, VC coating thickness, post-heat treating conditions, surface roughness, and notches. The linear correlation between the endurance limit and the residual stress in the substrates was confirmed in relation to the substrate hardness in hardened and VC-coated steels. The correlation cannot be applied to chromium-plated steel, owing to cracks in the plating. It was recognized that VC coatings as thick as 18 mum, and a very rough surface with 11 mum R(max), did not necessarily reduce the endurance limit of VC-coated steels. The endurance limit of notched specimens decreased in proportion to the stress concentration factor in VC-coated steels as well as in hardened steels.