Study of Quenching and Partitioning (Q&P) and Ultrasonic Surface Rolling (USR) Process on Microstructure and Mechanical Property of a High-Strength Martensitic Steel.

Steel with a combination of strength and plasticity is prevalently demanded for lightweight design and emission reductions in manufacturing. In this study, a high-strength Cr-Ni-Mo martensitic steel treated by quenching and partitioning (Q&P) and ultrasonic surface rolling (USR) processes was studied for both strength and plasticity enhancement. Specimens were austenitized at 850 °C and then quenched to 240 °C via cooling by water, oil, and normalization in quenching. This was followed by partitioning, in which two groups of specimens were heated to 370 °C and 350 °C for 45 min, respectively. At last, all the specimens were quenched to room temperature with the same methods of quenching. The highest tensile strength increased from 681.73 MPa to 1389.76 MPa when compared to as-received (AR) steel after the Q&P process. The USR process with a static force of 800 N further improved the tensile strength of specimens with high tensile strength after the Q&P process, which improved from 1389.76 MPa to 1586.62 MPa and the product's strength and elongation (PSE) increased from 15.76 GPa% to 15.9 GPa%, while the total elongation showed a mitigatory decrease from 11.34% to 10.02%. Tensile fractures were also studied and verified using a combination of strength and plasticity after a combined process of Q&P and USR.