Prediction of thermal stresses in quenched plates of En 308 (835 M30) steel

Abstract

A finite-difference method has been used to calculate the temperature distribution produced in a plate of En 30B steel during cooling from the austenitizing temperature in various quenching media. This information has been used to calculate the stress arising from the combination of that due to the temperature alone with that produced by the transformation of austenite to martensite. The range of quenching media investigated was such that the ratio of the surface heat-transfer coefficient to the thermal conductivity varied from 1.6cm^-1 (brine) to 0.05 cm^-1 (slow oil). Except at the lowest quenching rates, the surface stresses produced during cooling followed the sequence tensile - compressive - tensile. Plastic deformation was always produced during the first tensile stress stage, and may also be associated with the compressive and tensile stresses generated at the surface later in the process. The type of residual stress distribution present at the completion of the quench was dependent upon the cooling rate. Thus, at high quenching rates the residual stress at the surface was tensile, while the corresponding stress at the centre was compressive. However, the reverse stress pattern was produced by the slowest quench. The - technique used may be applied to other low-alloy steels and thus provide quantitative comparisons of the thermal stresses generated during the quenching of such materials.