Effects of thermal aging and low-fluence neutron irradiation on the mechanical property and microstructure of ferrite in cast austenitic stainless steels

Abstract The effects of low-fluence neutron irradiation on hardening and microstructure evolution in ferrite of solution annealed or thermally aged CF3, CF3M, CF8 and CF8M cast austenitic stainless steels (CASSs) have been investigated by means of nanoindentation tests and atom probe tomography (APT). Thermal aging was performed at 400 °C for 500 h. Neutron irradiation was carried out to a fluence of 4.84 × 1018 n/cm2 (E > 1 MeV) at the temperature ranging from 289 to 292 °C in the LVR-15 research reactor. Irradiation hardening in thermally-aged specimens was found to be similar with or smaller than that in the corresponding solution annealed specimens. Phase decomposition and formation of solute clusters acted two major factors for the hardening in ferrite with thermal aging and/or neutron irradiation. The phase decomposition of ferrite increased with either the thermal aging or the neutron irradiation for the solution annealed materials; however, the change in the phase decomposition of ferrite was neither significant nor apparent with the low-fluence neutron irradiation for the thermally-aged materials. Ni–Si–Mn enriched solute clusters were observed in the matrix of ferrite in the aged specimens, and the irradiated specimens with/without thermal aging. Mo in the CASSs appeared to inhibit the formation of solute clusters under the neutron irradiations. In the thermally-aged specimen with low-C and without Mo, neutron irradiation enhanced the formation of solute clusters significantly. For the first time we discussed the relationship between hardening and microstructure evolution in ferrite of CASSs with consideration of both thermal aging and neutron irradiation.