Influence of cooling rate from normalizing temperature and tempering on strength of ferrite–pearlite steels

Abstract

The influence of cooling rate from the normalizing temperature and tempering in the temperature range 450–600°C on the lower yield strength (LYS) and ultimate tensile strength (UTS) of two ferrite–pearlite structural steels has been examined. Falls in both LYS and UTS were obtained as a result of slow cooling or tempering; these falls were not related to grain-size changes. The processes controlling the falls in LYS and UTS were found to have activation energies of ∼170 kJ mol⁻¹ and ∼290 kJ mol^–1, respectively, and the kinetics of the processes obeyed t ^2/3 and t ^1/3 relationships, respectively (t being the time). The activation energy and kinetics for the process controlling the fall in UTS correspond to those associated with the spheroidization of pearlite, the rate-controlling factor being the self-diffusion of iron in the matrix, while the activation energy for the fall in LYS corresponds to that for diffusion of iron at the grain boundaries. An interesting relationship was also obtained between this fall in LYS and the thickening of the boundary carbides that occurred during tempering below the Ae ₁ transformation temperature. The activation energy for the growth of grain-boundary carbides was ∼170 kJ mol⁻¹ and the kinetics of growth also obeyed a t ^2/3 relationship. On the basis of these results, it is suggested that C that has become trapped at the boundaries on transformation can precipitate out on existing carbides at the boundaries during tempering. The removal of C from solution at the boundaries would then reduce the strength of the steel by lowering the stress required to transfer yielding across grain boundaries. Further work is suggested to examine this hypothesis in more detail.