Effect of temperature and strain rate on strength and ductility of four alloy steels

Tensile stress-strain curves have been obtainedfor four alloy steels, BS 968 (BS 4360), QT 35, HY 80, and HY 100, at temperatures between 225 and 77 K and mean plastic strain rates from 10^-3 to 2500S^-1. The effect of temperature and strain rate on the upper yield stress, the flow stress at 2.5% plastic strain, and the elongation to fracture have been determined, and a comparison is made between the present results and those of an earlier investigation at room temperature. At high strain rates and low temperatures only a slight reduction is found in the rate sensitivity of the flow stress compared with that at room temperature. For two steels, BS 968 and HY 80, the results are analysed in terms of the theory of thermally activated flow. Although for all steels at the highest strain rates a marked reduction in the elongation to fracture is found when the temperature is lowered to 77 K, the only completely brittle failures were by cleavage in BS 968 specimens tested at temperatures below 110 Kat strain rates greater than 100S^-1