Effect of multiple heat treatments on fracture property of centrifugal casting stainless steels Z3CN20.09M with long-term thermal aging degradation

Abstract

It is of great significance to investigate effect of multiple heat treatments on fracture property of centrifugal casting stainless steels Z3CN20.09M cut from pump casing with long-term thermal aging degradation for nuclear power plants to consider actual operation of nuclear power plants. Both multiple heat treatments and accelerated thermal aging experiment at the same temperature of 400 °C for different time were successively carried out on centrifugal casting stainless steels Z3CN20.09M in order to examine the metallographic modification and impact properties. Finally, an additional investigation on the related fracture properties was carried out, in which the critical initial fracture toughness Ji was determined by stretch zone width and 0.2 mm offset line methods. These results indicated that the multiple heat treatments led to the dispersed distribution of ferrite phases in austenite matrix and thus microhardness increased, but impact energy exhibited a decreasing tendency significantly. After long-term aging, the metallographic structure remained almost unchanged, but the size of ferrite phases showed a slight increasing trend because of spinodal decomposition in ferrite phases and G-phase precipitation. In addition, centrifugal casting stainless steels Z3CN20.09M with multiple heat treatments exhibited the higher microhardness, Charpy impact toughness, critical initial fracture toughness JIC (J-integral determined by 0.2 mm offset line method), and JSZW (J-integral determined by stretch zone width method) than those with primary heat treatment, while the specific number of the heat treatment had a low influence on fracture toughness.