Effect of prior microstructure on carbide precipitation in a Cr-Mo-V pressure vessel steel at 650 °C

Cr-Mo-V steels are key materials used for constructing thick reactor pressure vessels (RPV) owing to an impressive combination of their mechanical properties and weldability. Conventionally, these steels are subjected to a quality heat treatment that comprises austenitization, quenching and tempering. Owing to its large thickness, an RPV experiences widely different cooling rates across its thickness during quenching, leading to considerable variation in the microstructure from the surface to the centre of the vessel. In this study, a Cr-Mo-V steel was austenitized at 950 degrees C for 1 h and cooled at two rates (water quenching and 10 degrees C/min), representative of the cooling rates at the surface and centre of a typical RPV, to study the effect of prior microstructure on the carbide precipitation during tempering of the steel at 650 degrees C. Electron microscopy of the as-cooled samples revealed the presence of martensitic microstructure in water quenched sample and predominantly granular bainitic microstructure in slow cooled sample. Synchrotron studies revealed the presence of retained austenite containing similar to 0.8% carbon in the slow-cooled sample. Tempering of the steel at 650 degrees C for 200 h precipitated M7C3 and MC (VC) carbides in the water-quenched sample, while the slow cooled samples precipitated M23C6 as an additional constituent. This difference in the carbide precipitation in the two samples has been attributed to the excess carbon in the martensite/austenite (M/A) regions in slow cooled samples and explained on the basis of carbide precipitation sequence predicted using Thermo-kinetic simulations. This analysis has shown that M23C6 and M7C3 carbides in the slow cooled sample precipitate simultaneously as critical radii of their nucleation as well as their incubation time become comparable when the matrix contains more than 0.25% C. The present study has demonstrated that the microstructure prior to tempering may significantly affect the precipitation sequence of carbides during tempering.