Effects of Mn and Cr contents on microstructures and mechanical properties of low temperature bainitic steel

The effects of Mn and Cr contents on bainitic transformation kinetics, microstructures and mechanical properties of high-carbon low alloy steels after austempered at 230, 300 and 350 °C were determined by dilatometry, optical microscopy, scanning electron microscopy, X-ray diffraction and tensile tests. The results showed that Mn and Cr can extend bainitic incubation period and completion time, and with the increase of Mn and Cr content, the bainitic ferrite plate thickness decreased and the volume fraction of retained austenite increased. TRIP (transformation induced plasticity) effect was observed during tensile testing which improved the overall mechanical property. The increase of Mn concentration can improve the strength to a certain extent, but reduce the ductility. The increase of Cr concentration can improve the ductility of bainitic steels which transformed at a low temperature. The low temperature bainitic steel austempered at 230 °C exhibited excellent mechanical properties with ultimate tensile strength of (2146 ± 11) MPa and total elongation of (12.95 ± 0.15) %.