Temperature dependence of the strength-differential effect in hardened steels

The strength differential (SD) has been measured as a function of temperature in a fully hardened 0.2C, 6 Ni steel, quenched and then tempered at 250°C. It is found appropriate to express the results in terms of the intercept flow stresses as extrapolated back to zero plastic strain. The corresponding SD increases significantly with decreasing temperature below the ambient, and the data are well fitted by separating the SD into an athermal component (~3 pct) and a thermally activated component (up to ~15 pct at the liquid-nitrogen temperature). The latter type of contribution is thought to arise from a dilatation which occurs during the activation process for dislocation motion and which causes an increase in the activation energy under compressive loading and a decrease under tensile loading. The parameters obtained via this treatment are consistent with a double-kink Peierlsbarrier mechanism of plastic flow at low temperatures. This paper is based on a doctoral thesis presented in August 1972 by F. B. Fletcher to the Department of Metallurgy and Materials Science at the Massachusetts Institute of Technology.