EFFECTS OF THERMAL AGING ON MICRO- MECHANICAL PROPERTIES AND IMPACT FRACTURE BEHAVIOR OF Z3CN20-09M STAINLESS STEELS

Abstract

Cast austenite stainless steels (CASS) were used in the primary circuit piping of pressurized water nuclear reactors (PWRs), because of their excellent strength, corrosion resistance and good weldability. However, after long-term service at mid-temperature, CASS would suffer a loss of toughness and Charpy impact energy due to thermal aging. The micro-mechanical properties and impact fracture behavior of Z3CN20-09M stainless steel after long-term thermal aging at intermediate temperature were studied in this paper. A nano-indenter was used to study the changes of mechanical properties in the ferrite phases during the aging process. The impact behavior of the aged material was investigated by an instrumented impact tester, and the impact fractures were observed with SEM. The results indicated that long-term thermal aging caused the declining plastic deformation ability of the ferrite phases and the decrease of the impact toughness. The loss of impact energy in the aging process was mainly due to the reduction of stable crack propagation energy. The impact fracture morphology changed from ductile dimple in the initial stage into a mixture of cleavage in ferrite along with tearing in austenite in the later stage. In the impact fracture process, cracks initiated firstly in the ferrite phases and fast propagated under impact loading, and then the ferrite phases fractured along the cleavage planes. At last, the cracks extended to the austenite phases and the cracks connected through the specimen. KEY WORDS Z3CN20-09M stainless steel, thermal aging, impact, fracture behavior, mechanical property