Effects of Tensile Stress on Diffusion-Transformation During the Course of Cooling of Steels

Abstract

It seems to be a useful information for heat treatment techniques, particularly hardening, to understand the effect of various tensile stresses applied during cooling upon the Ar′ transformation and the effect of stress upon the martensite transformation, as already reported in previous papers. Therefore, various experiments were conducted with the following results: (1) When tensile stress was applied in the cooling process, the upper critical cooling velocity in hardening increased almost in proportion to the square of stress and its increasing rate became larger as the temperature for applying the stress increased. For instance, when a tensile stress of 13.8 kg/mm2 was applied at 850°C to high carbon chromium steel, the critical cooling velocity became seven times as large as the value in the case of no load. (2) The tensile stress applied isothermally deviated the beginning and ending curves of the isothermal transformation diagram to the left side in accordance with the magnitude of stress. This occurred both in pearlite and bainite ranges. (3) As for the effect of stress upon continuous cooling transformation, it was also found that the stress moved the beginning curve of transformation diagram to the upper left side. (4) It was proved that the quenched hardness was considerably influenced by the tensile stress as a result of the promotion of Ar′ transformation as mentioned above. (5) The experiments were conducted only with the loading of tensile stress in this study. According to the results obtained previously, it may be reasonable to infer that compressive stress may exercise almost similar effects to tensile stress.