Martensite-austenite transformation during dual-stage tempering and work-hardening characteristics of Mn–Ni–Mo steel

The reactor pressure vessel (RPV) in nuclear power plants is mainly made from Mn–Ni–Mo steel. After quenching, this steel's microstructure features martensite-austenite (M-A) islands. Tempering above 600 °C poses a challenge, as these islands can transform into harmful rod-like Fe 3 C precipitates, adversely affecting the RPV's tensile properties and structural integrity. This study explores the effects of M-A transformation during dual-stage tempering on the tensile properties of Mn–Ni–Mo steel, focusing on two austenite grain sizes (5 μm and 47 μm). The dual-stage tempering included a 2-h pre-tempering phase at temperatures between 200 °C and 500 °C, followed by a 3-h final tempering stage at 650 °C. Results showed that M-A transformation is temperature-dependent. At 200 °C, partial transformation led to rod-like Fe 3 C precipitates during final tempering, similar to direct tempering at 650 °C, reducing the critical stress for micro-crack nucleation and compromising tensile properties. Pre-tempering at 350 °C produced fine, aggregated precipitates that improved tensile strength by inhibiting dislocation movement. Conversely, pre-tempering at 500 °C resulted in coarser, more dispersed spherical precipitates. The Kocks-Mecking plot indicated that the 350 °C pre-tempered sample had the highest strain hardening capacity, with the strain hardening rate curve farthest from the origin and the shallowest slope during stage IV hardening. However, the 500 °C pre-tempered samples showed the best combination of elongation and strength.