The Microstructure and Mechanical and Corrosion Behaviors of Thermally Aged Z3CN20-09M Cast Stainless Steel for Primary Coolant Pipes of Nuclear Power Plants

The effects of thermal aging time at 400 °C on the microstructure and mechanical and corrosion behaviors of Z3CN20.09M cast stainless steel were investigated; and the corresponding thermal aging mechanism was studied. It was revealed that the changes in mechanical properties after thermal aging were mainly caused by the iron-rich phase (α) and the chromium-rich phase (α’) produced by the amplitude-modulation decomposition of ferrite. A similar trend of thermoelectric potential during thermal aging was determined in relation to the Charpy impact energy. However, the corrosion resistance of Z3CN20.09M cast stainless steel deteriorates as thermal aging time increases. When the thermal aging is longer than 3000 h, the precipitation of G phase has a great influence on the corrosion resistance. The interfacial matching relationship between G phase and the surrounding ferrite was established by selected area electron diffraction of HRTEM. The relationship is of cube-on-cube phase boundary type. The impact fracture mechanisms in relation to thermal aging time were also studied and compared.