Effect of Ferrite on the Precipitation of σ Phase in Cast Austenitic Stainless Steel Used for Primary Coolant Pipes of Nuclear Power Plants

The effect of ferrite phase on the precipitation of sigma phase in a Z3CN20.09M cast austenitic stainless steel (CASS) used for primary coolant pipes of pressurized water reactor (PWR) nuclear power plants was investigated by using isothermal heat-treatment, optical microscopy (OM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA) techniques. The influence of different morphologies and volume fractions of ferrite in the s phase formation mechanism was discussed. The amount of sigma phase precipitated in all specimens with different microstructures increased with increasing of aging time, however, the precipitation rate is significant different. The formation of sigma phase in specimens with the coarsest ferrite and the dispersively smallest ferrite is slowest. The lowest level Cr content in ferrite and fewest alpha/gamma interfaces in specimen are the main reasons for the slowest s precipitation due to they are unfavorable for the kinetics and thermodynamics of phase transformation respectively. By contraries, the fastest formation of sigma phase takes place in specimens with narrow and long ferrite due to the most alpha/gamma interfaces and higher Cr content in ferrite which are beneficial for preferential nucleation and formation thermodynamics of sigma phase.