Calculation and Analysis of Microstructure of Austenitic Steel for Supercritical Unit

Microstructure and the precipitated high temperature ferrite δ phase of an austenitic steel 10Cr18Ni9NbCu3BN tube was investigated. It reveals that segregation during solidification process results in the precipitation of high temperature ferrite. The calculated amount of δ phase was consistent with our XRD analysis. With decreasing solution treatment temperature, Nb-containing phase will be refined and the amount of δ phase as well as process-cost can be reduced. Because of the highest performance/cost ratio, austenitic super 304H steel is applied as pressure component under supercritical conditions. It was originally developed by Sumitomo Metal Industries, Ltd and Mitsubishi. Based on 304H, Super304H has lowered the upper limit of Mn, but added Nb, N and Cu. Elements Nb and N can form stable NbN,Nb(C,N)-phase, so as to refine the grain size and result in precipitation-hardening. Cu can form coherent segregation phase which also has the hardening effect, decreases the hardening rates in the cold-working process and improves the plastic formation of steel. In this kind of steel, the main strengthening phases are copper-rich phase, MCs. The alloying effect of elements Nb,N and Cu can increase allowable stress and service life of the steel under the working temperature[1]. In this paper, experimental and theoretical analysis was carried out in order to develop new 10Cr18Ni9NbCu3BN steel tube. In accordance with ASME code case 2328-1, the contents of steel 10Cr18Ni9NbCu3BN were listed as follows in Table 1.