Effects of Elastic Tensile and Compressive Stresses on Phase Transformations in Steels

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In order to get information on microstructure controlling and elucidation of mechanism of bainitic transformation in steels, the starting temperature of the transformations during continuous cooling were examined through dilatometry, micrography and electric resistivity measurement.Results can be summarized as follows.(1)martensitic transformation---In Fe-30Ni and Fe-20Ni steels, the transformation is promoted by tensile stress. The increase in Ms temperature by the application of unit stress is comparing to the value from the theory by Patel and Cohen. The effects of compressive stress are small. In 0.4C-4Ni-0.25Mo steel, effects of tensile and compressive stresses are negligibly small. The reason why effects of stresses is very small has remained unsolved. It seems necessary in future to make clear whether Mo-carbide precipitates during cooling and affects the effects of stresses on the transformation.(2)austenite->ferrite transformation---In 0.1C-5Ni steel, effects of stresses are small.(3)bainitic transformation---In 0.4C-2Mo steel, tensile stress promotes the transformation, while effects of compressive stress are hardly recognized. In 0.4C-3NI steel, both stresses promote the transformation although the effects of tensile stress is larger. It is observed that stresses also affect to growth of bainite. IN 0.01C-3Mn-1Cr-0.0016B steel, effects of stresses are too small to be noticed. In this steel, the transformation tend to occur from surface region and grow to inner region. Such transformation behavior may be contributable to the very small effect of stresses on the transformation.