Effects of Q&P technological parameters on microstructures and mechanical properties of 34SiMn2CrNiMo steel

Abstract

A medium carbon steel 34SiMn2CrNiMo was designed and fabricated, and the relationships between processing, microstructures and mechanical properties were established through microstructure characterization and mechanical properties testing on the quenching and partitioning(Q&P) heat-treated samples. Based on the experiment results of thermal expansion transformation behavior, the effects of quenching temperature TQ, heating approach, partitioning temperature TP and partitioning time tP on microstructures evolution were investigated and mechanism of strength and toughness was illuminated as well. The results show that multiphase microstructures of martensite and retained austenite can be obtained by means of Q&P processing, as a result the match of strength and plasticity is greatly improved. In the optimal treatment state, the retained austenite volume fraction is about 24%, the yield strength is 1053 MPa, the tensile strength is 1607 MPa, the elongation is 24.9%, and the product of strength and elongation(PSE) is 40.0 GPa·%. To avoid forming the blocky secondary martensite, the best quenching temperature has to be set at least 150 ℃ below martensitic transformation starting temperature Ms. More retained austenite can be achieved by slow heating than fast heating upon partitioning after the steel is incompletely quenched, the plasticity and toughness are improved apparently. The microstructures and properties of Q&P treated steel keep relatively stable with prolonged duration when it is partitioned at 400 ℃. However, the volume fraction of retained austenite decreases with tP at 450 ℃, leading the yield strength and elongation decrease simultaneously.

Keywords

• ultra-high strength steel
• quenching and partitioning
• retained austenite
• microstructure and property
• strength and plasticity