Influence of Cold Work, Heat Treatment, and Chromium Content on the Fracture Properties of Irradiated Martensitic Steels

Abstract

Martensitic steels undergo a change in the fracture mode from ductile microvoid coalescence to brittle cleavage as temperature decreases. Neutron exposure can raise the ductile-brittle transition temperature and produce a condition of low toughness during service. This study examines the effects of cold-work, heat treatment, and chromium content on the postirradiation fracture behavior of selected steels to provide a better understanding of fracture resistance in hopes of improving it. Experimental materials included alloy HT9 containing 12 percent Cr, a vacuum remelt of HT9, modified HT9 containing 10 percent Cr, and 9Cr-1Mo. They were irradiated in the Experimental Breeder Reactor(EBR)-II at 400 to 550°C to fluences of 2.9 and 6 × 1022 n/cm2 (E > 0.1 MeV). Results of fracture toughness tests conducted at temperatures from 93 to 450°C with electric-potential single-specimen techniques were analyzed by means of the J-integral approach. The initiation fracture toughness of irradiated martensitic steels increased with increasing chromium content and tempering temperature. Remelting of HT9 slightly raised the fracture toughness of the irradiated materials. Cold work did not improve the fracture resistance of irradiated ferritic steels. Initiation fracture toughness decreased slightly while the tearing modulus increased significantly as irradiation temperature increased from 400°C to 500°C. Results also showed that neutron fluence had little effect on the fracture resistance of these steels, over the present range of fluences.