Tempering response and improved mechanical properties in secondary hardened steel by introducing an optimized austempering process

The tempering response and mechanical properties of the austempered Cr4Mo4V steel with bainite/martensite multiphase structure were investigated. During the tempering, the retained austenite (RA) decomposed almost entirely in the specimens without or with a short austempering, while massive blocky RA still remained in the specimens with a long austempering process (4 h). Meanwhile, the significant coalescence and coarsening of bainite occurred during the tempering of the specimens with longer austempering. After the tempering, the diameter of packets in the specimens austempered for 1 h decreased from 1.95 to 1.06 μm, which could be attributed to the second division of austenite grain during austempering and the minimum degree of bainite coarsening during tempering. Additionally, the phase transformation texture was weakened while the dislocation density was increased, as compared with the martensitic quenched specimens. Due to the synergistic effect of the finer packet, higher dislocation density and weaker texture, the ultrahigh strength (2604 MPa) combined with an improved impact toughness (93 J) was achieved in the Cr4Mo4V steel austempered for 1 h without compromising hardness (63.0 HRC). However, with the increase of austempering time, the mechanical properties would deteriorate due to the coarsened packets and undecomposed blocky RA. These results indicate that by introducing an optimized austempering process, the mechanical properties of Cr4Mo4V steel can be dramatically improved, while the content of bainite should be strictly regulated to prevent blocky RA and bainite coarsening.