Effect of Neutron Irradiation on Yield Stress and Ductility of Reduced-Activation Martensitic 9Cr-WVTa Steels

Abstract

The tensile properties of reduced-activation martensitic 9Cr-1WVTa and 9Cr-3WVTa steels for fusion reactors were investigated over the temperature range from room temperature to 873 K after neutron irradiation in the Japan Materials Testing Reactor at 538 K to fast neutron fluences of 2×1023 and 3×1024 n/m2 (E>1 MeV). A conventional 9Cr-1MoVNb steel was also examined for comparison. The irradiation caused an increase in yield stress and a decrease in total elongation. Irradiation-produced defect clusters could not be seen directly by transmission electron microscopy. The increase in yield stress caused by irradiation Δσy was proportional to the 1/4 power of the neutron fluence. The Δσy was smaller in the 9Cr-1WVTa and 9Cr-3WVTa steels than in the 9Cr-1MoVNb steel and was smaller in the 1% W steel than in the 3% W steel. With increasing test temperature, the Δσy exhibited a further increase at 573∼673 K and then decreased to zero over the range from 673 to 873 K. The increase in irradiation hardening at 573∼673 K was postulated to be due to the formation of vacancy-carbon complexes, and the decrease in it at 673∼873 K was due to the annealing out of irradiation-produced defects. The decrease in total elongation was shown to be accompanied by a decrease in work hardening rate. © 1993, The Japan Institute of Metals. All rights reserved.