Effect of softening temperature on microstructure and properties of s30432 austenitic heat-resistant steel tube for U.S.C boiler

摘要

By microstructure analysis,mechanical tests and short term,creep rupture tests,this paper presents comparative investigations on microstrucrural features,mechanical properties and creep rupture behaviors of S30432 austenitic heat-resistant steel tubes for ultra-supercritical (U.S.C) boiler,which are produced by lower temperature softening at 1170°C + cold-rolling + solution annealing at 1150°C and by higher temperature softening at 1250°C + cold-rolling + solution-annealing at 1150°C,respectively.The results show that both tubes have austenite + precipitates in their structures,and the precipitates are Nb(CN).However,their microstructures difference is due to their different treatments.The tube treated by lower temperature softening has no homogeneous austenite grain,and there are many larger and non-dispersed particles in its austenite matrix;while the tube treated by higher temperature softening has better micro-structure,which indicates there are fine equiaxed austenite grain,and uniformly dispersed particles in matrix.Meanwhile,the latter's properties are better than the former's ones.This demonstrates softening temperature has an important effect on the dissolution and precipitation behavior of Nb(CN) particles,and the size and dispersion of Nb(CN) play a special role in the tubes' microstructure and mechanical properties.This is caused by the dissolution and precipitation behavior of Nb(CN) particles taking place at different temperatures during different softening treatments.Since Nb(CN) can mostly be dissolved at the higher solution temperature,this will increase the saturation of Nb in austenite.Therefore,fine and uniformly dispersed particles would be precipitated during following cold-rolling and solution-annealing at a lower temperature than softening temperature,and these fine particles have an important pinning effect on boundary movement of the structure and prevent grain size of austenite from getting coarser during recrystallization in the solution-annealing so that the tube exhibits good combined properties.However,if the softening temperature of 1170°C is used,it is inadequate dissolving of Nb(CN) and insufficient saturation of Nb in austenite at the temperature,and it is possible gathering and growth of Nb(CN) during final solution treatment at 1150°C that result in the bad condition of final precipitation and distribution,namely fine effect of grain is not obvious and grain is not homogeneous.Finally,this leads to its worse properties.