Improving mechanical and wear properties of multiphase M50 steel by tailoring bainite morphology

The martensite/bainite multiphase microstructure has shown excellent strength-toughness matching compared with the traditional martensitic microstructure. The ratio of martensite/bainite has always been the focus of performance optimization for the multiphase microstructure. However, the effect of bainite morphology on mechanical properties and wear properties remains unknown. In this work, the microstructure, mechanical, and wear properties of multiphase M50 steel with different bainite morphologies have been investigated. The austempering parameters were controlled to obtain a similar bainite fraction (around 10%) and different bainite morphologies. The microstructure characterization shows that the properly introduced lower bainite can significantly refine the lath structure due to the significant microstructure segmentation effect. When the bundle width and length of lower bainite are respectively 0.107 μm and 3.758 μm, the finest lath microstructure is obtained considering the balance of martensite growth space. The mechanical properties results show that the toughness increases without loss of strength under the tailored bainite. The high-temperature wear performance shows that the multiphase microstructure with tailored bainite results in the decrease of friction coefficient by 42% and a decline of wear loss by 37.9% due to the optimized lubrication layer. This work provides a novel perspective to further improve the mechanical and wear properties of multiphase M50 steel by finely tailoring the bainite morphology on the basis of fixed bainite content.