Effects of Austenitising Conditions on the Microstructures and Mechanical Properties of Martensitic Steel with Dual Matrix Structure

Abstract

A new high strength steel with dual matrix structure and exceptionally high toughness plus ductility have been produced by intermediate quenching of 0.22_wt% C microalloyed steel. The treatment consisted of initial austenitization and rapid quenching of the steel samples to achieve a fully martensitic state followed by annealing in the intercritical (α+γ) region of 730℃-810℃ for the period of 30, 60 and 90_minutes. These samples were subsequently quenched to obtain dual phase microstructure containing varying proportions of ferrite and martensite constituents. The mechanical properties of the samples were measured according to ASTM standard and their microstructures were analyzed by optical microscopy. The experimental results show that martensitic dual phase (MDP) steel samples developed within the intercritical temperature range of 770–790℃ revealed finer martensite and precipitate-free ferrite microstructure. The tensile and impact properties of the developed HMDP steels increased with intercritical annealing (ICA) temperatures, with an optimum properties obtained at 790℃ mainly due to finer microstructure of the constituent phases and absence of carbide precipitate that permit ease of dislocation flow. A further increase in the intercritical annealing temperature beyond 790℃ led to general decrease in the mechanical properties. This is attributed to the formation of coarse structure in this region. The results further show that with increasing intercritical treatment time from 30 to 90 minutes, the general mechanical properties of the MDP steels were found to increase except at the higher temperature of 810℃ which showed decreasing values. In general, the tensile strengths and ductility as well as the impact properties of the developed dual phase steel samples are greatly improved with the intercritical heat treatment investigated.