Effect of tempering temperature on microstructure and mechanical properties of nanostructured bainitic steel

The previous research on tempered nanostructured bainite has been mainly focused on the carbide precipitation at high temperatures with few details on nanostructured bainite tempered at lower temperatures. In this research, the microstructure evolution and the corresponding mechanical properties of the nanostructured bainite tempered at a temperature ranged from 180 °C to 400 °C were studied via X-ray diffraction, transmission electron microscope and high-resolution thermal dilatometer. Results show that retained austenite occurred bainitic transformation when the tempering temperature ranged from 210 °C to 320 °C. The amount of newly formed bainite increased at first and then decreased slightly with increasing tempering temperature. When the tempering temperature reached 380 °C and higher, the cementite precipitated from retained austenite and reduced the stability of over-stable retained austenite. As a result, it can be concluded that stress-induced martensitic transformation would occur easily in retained austenite during deformation process, leading to the optimum increase of retained austenite contribution to toughness. coefficient of carbon increased sharply, the dispersed cementite particles precipitated between the nanostructured bainitic plates, inhibiting coarsening of bainitic plates when the tempering temperature reached 380 °C. Finally, nanostructured bainitic plates only coarsened slightly. Therefore this high-carbon high-silicon nanostructured bainitic plate possess a high dimensional stability.