Microstructure evolution and kinetics analysis during tempering of Mn–Ni–Mo nuclear power forging steel

Abstract

The mechanical properties of Mn–Mo–Ni nuclear power forging steel can be affected greatly by tempering processes. In the present paper, scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and hardness measurements were employed to correlate property variation with the evolution of microstructure during tempering of an Mn–Mo–Ni steel. Kinetics analysis based on hardness measurements was applied to isothermal tempering at temperatures in the range 600–660°C. The hardness variations during tempering are explained by decomposition of retained austenite and martensite–austenite (M–A), precipitation and coarsening of carbides within bainitic ferrite, and recrystallisation of bainitic ferrite. According to the kinetics analysis, the tempering transformation activation energy has been calculated as about 101·4 kJ mol⁻¹, which indicates a mechanism of carbon diffusion within ferrite. Finally, the considerable precipitation and coarsening of carbides within bainitic ferrite have been observed only at high isothermal temperatures (generally equal to or above 660°C), indicating a high tempering resistance of the Mn–Mo–Ni steel.

Keywords:

• Bainitic steel
• Tempering kinetics
• Hardness
• Microstructure
• Carbide

This work is supported by National Basic Research Program of China (grant no. 2011CB012904), and by ‘CNC machine tools and basic manufacturing equipment technology comments’ (grant no. 2012ZX04012011). The authors greatly acknowledge helpful discussions with Professor S. P. Chen. The paper is based on a contribution to the 20th IFHTSE Congress held in Beijing, China on 23–25 October 2012.