Effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel

The effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel has been investigated through optical microscopy, electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction analyses. A nano-level microstructure containing plate-like bainitic ferrite and film-like retained austenite is obtained by isothermal transformation at Ms+10 degrees C followed by tempering within 240-450 degrees C. Results show that the sample tempered at 340 degrees C occupies the optimal balance of strength and toughness by maintaining a certain level of plasticity; samples tempered at 320 degrees C and 360 degrees C with low and high yield ratio come second. The microstructure of the steel is not sensitive to tempering temperatures before 360 degrees C. When the temperature is increased to 450 degrees C, the significantly coarsened bainitic ferrite plate and the occurrence of a small quantity of carbide precipitation account for its low toughness. The amount of retained austenite increases with the tempering temperature before 400 degrees C, followed by decreasing with further increase in the temperature. This behavior is related to the competition between retained austenite further transforming into bainite and decomposing into carbide during tempering.