Effects of Ti and Al on Creep Rupture Strength and Microstructure of 30Cr-60Ni Alloys

Abstract

To improve creep rupture strength of a 30Cr-60Ni alloy which can be used for ultra super critical-pressure boiler tubes, the effects of Al and Ti additions on strength and microstructure were investigated.
Creep rupture strength is improved by increasing Ti content in the range from 0.4 to 1.0 wt%. The most effective range lies in the addition of 0.8 to 1.0 wt% Ti with 0.4 wt% Al. These additions promote γ' phase formation at 700 and 750°C and the amount of γ' precipitation increases with Ti content. However, at 700°C excess Ti addition beyond 0.8 wt% causes carbide coagulation on grain boundaries and in grains which leads to the lowering of creep rupture strength. Accordingly, creep rupture strength is dependent not only on γ' precipitation but also on carbide coagulation.