The Effects of Stress Induced by Preventing the Thermal Contraction on the Amount of Retained Austenite. (The Effects of Stress in Cooling on the Stability of Austenite: (I))

Abstract

Thermal contraction in cooling was prevented as a representation of a special thermal stress in the surface layer of steel induced by quenching. The effects of such thermal stress on retained austenite (γ_R) were as follows. (A) Experiments on high carbon alloy steels. (1) The tensile stress induced by preventing the thermal contraction increased linearly with decreasing temperature and indicated the maximum value at Ms temperature, that was about 10 kg/mm². (2) The amount of γ_R was markedly increased by preventing the thermal contraction. This is contrary to the expectation from the mechanism of martensite transformation which has been interpreted from the criterion of shearing stress. (3) The effective temperature range of stress on γ_R existed between 100∼150°C above Ms point and room temperature. Also the most effective temperature lies in the vicinity of Ms temperature. (4) The rise of cooling velocity at the range of above Ms temperature increased the amount of γ_R as the result of increment of induced tensile stress. (5) At below Ms temperature, the rise of cooling velocity decreased the amount of γ_R, similarly to free contraction cooling. Therefore it appears that there is no effect of stress on the thermal stabilization of austenite. (B) Stabilization of austenite by preventing the thermal contraction was not observed in medium and low carbon alloy steels, and in low carbon alloy steel the amount of γ_R was decreased, contrary to the case with high carbon alloy steels.