Effects of tempering temperature on the microstructure evolution and mechanical properties of 16%Cr–5%Ni super martensitic stainless steel

Abstract

The effects of tempering temperature on the microstructure evolution and mechanical properties of 16%Cr–5%Ni super martensitic stainless steel (SMSS) were investigated in the present study. Reversed austenite was detected via X-ray diffraction when the as-quenched samples with full martensite were tempered above 500 °C, and its volume fraction attained a maximum of ∼17% at 620 °C. When tempered below 620 °C, all the reversed austenite formed was able to keep stable at room temperature, while partial or even entire reversed austenite formed above 640 °C with Ni content less than 8 wt% transformed back into fresh martensite during cooling, demonstrating that the enrichment of Ni determined the thermal stability of reversed austenite. EDS results revealed that the precipitation of M23C6 carbides caused Ni enrichment in the adjacent regions, promoting the nucleation of reversed austenite, and the growth of reversed austenite was mainly controlled by Ni diffusion. The results of tensile tests and instrumented impact tests indicated that stable reversed austenite effectively improved the ductility and toughness, and offset the deterioration of toughness caused by the M23C6 precipitating at grain boundaries. Tempering at 540 °C achieved an excellent combination of strength and toughness, while the best plasticity of samples was obtained by tempering at 620 °C.

Keywords

• Super martensitic stainless steel
• Tempering temperature
• Reversed austenite
• M23C6 carbides
• Mechanical properties