Microstructure and Mechanical Properties of Martensitic High-Strength Engineering Steel

A promising martensitic steel with good hardenability is studied. In the cooling rate range 0.1-30 degrees C/sec and the only transformation recorded by a dilatometer starts at an M-s temperature of 355 +/- 10 degrees C. Microstructure and mechanical properties of the steel studied are analyzed after various heat treatment regimes: cooling from the austenitizing temperature at various rates (from 0.02 to 20 degrees C/sec), austempering at 280-380 degrees C, and oil quenching and tempering at 200-600 degrees C. It is shown that a 1000-fold change in the cooling rate has a weak effect on steel strength. Bainitic transformation in the steel in question occurs in the martensitic transformation temperature range, which leads to a reduction in impact strength compared with a martensitic microstructure both after slow cooling and isothermal hardening.