Shoji Hotta

Fatigue strength at a number of cycles of thin hard coated steels with quench-hardened substrates

Abstract Bending fatigue tests were carried out to clarify the effects of thin hard coatings on the fatigue strength at a number of cycles for steels. Coatings of carbides and nitrides produced by thermoreactive deposition and diffusion, chemical vapour deposition and physical vapour deposition, processes and of electroplated chromium on various steels were tested as well as hardened steels. The results thus obtained revealed that the fatigue strength at a number of cycles highly depended on hardness and residual stress at the substrate surface as in the case of the endurance limit and that the existence of the coatings suppressed the cracking in the substrates to increase the fatigue strength after loading cycles. Effects of coating thickness, surface roughness, notch shape, and post-heat treatment were also examined.

Fatigue strength of steels with thin hard coating

Abstract Bending fatigue tests were made on various steels coated with carbides (VC, NbC or CrC) by the molten salt bath immersion process, with TiC by CVD, with TiN by PVD and also on the same kinds of steels hardened, in order to clarify the effect of coatings. The results indicated that the endurance limits of carbide coated steels were slightly lower than those of hardened steels in some cases. The observations of specimens before and after fracture revealed that the fatigue fracture initiated not in the coatings but in the substrate just adjacent to the coatings. It was concluded from measurements of the residual stress and the hardness that the lower endurance limit of carbide coated steels mentioned above was due to the increase in tensile residual stress and the decrease in hardness of the substrate by consumption of carbon in the substrate for carbide formation. Furthermore, the bending endurance limit was linearly correlated to the residual stress in the substrate on the same hardness level, regardless of the substrate steels used in the tests. Diffusion soaking to recover the consumed carbon was effective in increasing the hardness and decreasing the tensile residual stress of the substrate, leading to an increase of the endurance limit.

Endurance limit of thin hard-coated steels in bending fatigue

Abstract 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 μm, and a very rough surface with 11 μm 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.