Strengthening Factors and Phase Relation in Ni-Cr-W Alloys Developed for Nuclear Steelmaking

Creep rate and creep rupture stength were measured in a serieS of five Ni-20 pet Cr alloys containing different amount of tungsten up to twenty per cent. Creep tests were conducted at 900 and 1 OOO'C in the stress range between 15 and 59 @'a in vacuum and in helium as we/L as in air. The creep rupture strength increased and the steady state creep rate decreased with increasing tungsten concentration. Factors affecting solid solution strengthening due to tungsten were clarified, using the first four alloys with Less than fifteen per cent tungsten. In the aZZoy with twenty per cent tungsten, a tungsten-rich bee ~12 phase precipitated at the grain boundary during testing. The precipitation of the a2 phase at the grain boundary together with the maximum solubiLity of tungsten substantiaZZy improved the creep properties of the Ni-20 pet Cr alloys with tungsten. The precipitation of the a2 phase eliminated the environmental suseeptibility inherent to the single phase aZZoys based on the pi-CT-W system in the reduced o;cygen potent&Z.