Precipitation behaviour of 20MnMoNi55 RPV steel in the temperature range of 630–670 °C

Abstract

The precipitation behaviour of 20MnMoNi55 grade RPV steel while tempering and PWHT in the temperature range of 630–670 °C was characterised through microstructural observation and CALPHAD based thermodynamic calculations. Steel samples were tempered at 630 °C from 1 to 6 h and further subjected to simulated post weld heat treatment (PWHT) between 650 °C and 670 °C temperatures, which is relatively higher than the conventionally employed PWHT temperature range. Microstructural observations revealed that tempering and subsequent simulated PWHT of 20MnMoNi55 grade RPV steel resulted in precipitation of Fe3C and Ksi carbides as Mo lean and Mo rich precipitates, respectively. CALPHAD based computations also suggested that Ksi carbide and Mo2C could form as metastable carbides, whereas Fe3C and M7C3 as thermodynamically stable carbides while heat treatment in the temperature range of 630–670 °C.