Influence of W concentration on the Charpy impact property and corrosion behavior of reduced activation ferritic-martensitic steels for fusion reactor

Abstract

Fe-9Cr-0.4Mn-0.2V-0.1Ta-0.02Ti-0.1C-W (all in wt%) alloys with different W additions of 1 and 2 wt% were prepared to investigate the effects of W content on the mechanical properties and corrosion behavior of reduced activation ferritic–martensitic steels. The base steel samples were made through a heat treatment involving normalizing and tempering after hot rolling. When the W content was 1 wt%, the microstructure of the base steel consisted of tempered martensite with two types of precipitates, intergranular Cr-enriched M23C6 carbide and Ta/Ti-enriched MC carbide, while δ-ferrite additionally formed in an alloy containing W at 2 wt%. The results of Charpy impact tests showed that the ductile-to-brittle transition temperature slightly increased with an increase in W concentration without degrading the upper shelf energy, implying that the impact property slightly deteriorated. In addition, potentiodynamic polarization tests showed that the resistance to pitting corrosion also decreased when the W concentration was increased from 1 to 2wt%. This unexpected degradation of the impact property and corrosion resistance in alloy containing 2 wt% W was caused by the formation of δ-ferrite. That is, the δ-ferrite provided a relatively easy path for crack propagation during the impact tests. Also, since the δ-ferrite is a W-enriched phase, the matrix adjacent to the δ-ferrite was electrochemically active and acted as a local anode and the δ-ferrite formation then decreased the resistance to pitting corrosion.