Cr-Mo-V鋼の熱疲労

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It is well known that the tool of extruding and the die of casting, of which the materials are Cr-Mo-V steel, are frequently broken by fire crack. It is believed that this is the result of thermal cycling during the hot working. In order to obtain basic information for a reasonable design and working of such material, a series of thermal fatigue tests have been made under large amplitude of temperature. In the present study,particular attention has been directed to the effect that the rapid change of deformability of the present material owing to the temperature level will have on the thermal fatigue characteristics. Attention has been paid also to microscopic observations of nucleation and propagation of thermal crack. From the present experiments the following conclusions are marked : (1) Thermal crack nucleates at the crevice of the scale on the surface of the specimen, in case of rich ductility and of fine structure (grain) of the material. The crack at early period grows by repetition of oxidation and at the latter period it propagates under thermal stress cycling. In case of coarse structure (grain) and of poor ductility, the crack propagates rapidly along the austenite grain boundary. (2) Thermal fatigue life of the material, of which ductility is much dependent on the temperature level, is approximately estimated from the information of static tensile tests of the same material at various temperature levels. This is expressed as [numerical formula] or [numerical formula] where Δε_p is plastic strain range, N_f the number of cycles to fracture, T_2 and T_1 the upper and lower temperature levels, ε_f(T) tensile elongation at temperature level T. The latter equation holds in case of constant range of plastic strain during the thermal fatigue test.
社団法人日本材料学会の論文
1965-02-15

Cr-Mo-V鋼の熱疲労

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