Martensite-austenite transformation during dual-stage tempering and work-hardening characteristics of Mn–Ni–Mo steel
Year: 2024
Type: article
Abstract: 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 Fe3C 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 two-hour pre-tempering phase at temperatures between 200°C and 500°C, followed by a three-hour final tempering stage at 650°C. Results showed that M-A transformation is temperature-dependent. At 200°C, partial transformation led to rod-like Fe3C 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.
Authors: Rahul Ranjan, Anil Meena
Institution: Indian Institute of Technology Madras
Language: English
Subfield: Mechanical Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,430
OpenAlex ID: W4401871399