Effect of microstructure evolution on fracture toughness in isothermally aged austenitic stainless steels for cryogenic applications

Two types of austenitic stainless steels JJ1 and JN1 were isothermally aged at temperatures from 600°C to 900°C for 10-1000 min in order to study the micro structural evolution and its effect on fracture toughness at cryogenic temperatures. These steels were developed for applications in the superconducting magnets of a fusion experimental reactor by the Japan Atomic Energy Research Institute. The Charpy V-Notch (CVN) fracture energy at 77 K showed a significant decrease with aging time for both steels. The intergranular precipitation of carbides and nitrides is responsible for the fracture toughness deterioration. The scanning electron microscope (SEM) fractographs showed an intergranular brittle fracture and its fraction also increased with aging time and temperature. The presence of a more abundant intergranular precipitation resulted in a more rapid decrease in fracture toughness with aging time in JN1 steel due to its higher content of C and N, compared to that of JJ1 steel. The volume fraction of precipitates can be uniquely correlated with the reduction in toughness.