Effect of aluminium (grain size), manganese and residual alloy content on the hardening response of plain-carbon steels

Throughout the world, heat treaters have reported occasional difficulties in hardening plain-carbon steels. The most frequent problem has been obtaining carburised case hardness, although poor through-hardening response has been reported with medium- and high-carbon steels. Most problems of case-hardening are related to the significant reduction in case hardenability when fine-grained (aluminium-treated) steels have been used instead of coarse-grained steels. Although the effect of grain size on through-hardening is well known, the effect on case-hardening is less well recognised, and new work has been carried out to study the phenomenon. Most importantly, the effect of Mn, Cr and residual alloying elements in compensating for a hardenability reduction has been examined. Quantitative data are presented describing the influence of grain size and small compositional changes on hardening response and limiting ruling section. These should be useful to steel specifiers in selecting/specifying the optimum steels for heat treatment. Furthermore, they could assist in the development of 'guidance for heat treatment' in future carbon-steel standards.