Effect of Nb, Ti and C Contents on Creep Rupture Strength of High Strength Austenitic Heat Resisting Steel Developed for Ultra Super Critical Pressure Boiler Tubes

Abstract

The effect of Nb, Ti and C contents on the creep rupture strength of a 20Cr-25Ni austenitic steel developed for ultra super critical pressure boiler tubes, has been investigated from metallographic viewpoints.
In the case where only Nb was added, the creep rupture strength of the steel reached its maximum when the contents of Nb and C coincided with the NbC solubility curve and the stoichimetric ratio of NbC. When Ti was added together with Nb, the creep rupture strength of the steel was higher than if only Nb was added, and the strength was maximized by adding to the steel, maximum amounts of Nb and Ti which could be dissoluable at the solution treatment temperature. These results were explained by the fact that fine Nb (C, N) and Ti (C, N), precipitated in the early stage of creep, subsequently induced a uniform and fine precipitation of M₂₃C₆ which was delayed in coalescence and growth.