Design And Precipitates Of Supercritical Steam Turbines Heat-resistant Steel

Alloying designed of heat-resistant steel for application at650℃with supercriticalsteam turbines by Thermo-Calc sofrware and alloying contribution. According to the designchemical constituents smelt alloy. The paper measured austenitic transition temperature andexpansion coefficient of material by thermal dilatometer to set up heat treatment temperature;the effect of different treatment on temperature material structure and mechanical propertywere studied; the forming reason of δ-ferritic were discussion and the effect of δ-ferritic andRE content on mechanical property were studied in detail. Then the type, morphology, size,distribution of precipitates were discussed through the XRD, OM, SEM, TEM, EDS.The results indicate that: the stable phase of ferritic heat-resistant wereα+MX+Laves+M₂₃C₆. Based on W-Mo co-addition, C-N co-addition, V-Nb co-addition,control the content of precipitates and restrain from δ-ferritic.The microstructure of material were lath martensite and δ-ferritic afternormalizing and high tempering treatment. The effect of heat treatment temperature onlath martensite morphology was significant. The tensile strength of material was higherthan the normal value and lower impact toughness. The δ-ferritic cause boundaryembrittlement and contribute to crack initiation and propagation.The optimal parameters of new ferritic heat-resistant heat treatment were exploredby test, when normalizing was done at1050℃-1100℃and tempering was done at670℃-700℃, the excellent lath martensite and mechanical property were obtain. Theprecipitates of material basically had M₂₃C₆and MX two types by EDS and select electrondiffraction analysis. The M₂₃C₆carbides precipitated in the prior austenite grain boundary andlath boundary and within lath, which morphology were different. The circle granular or chainof Cr₂₃C₆and the short columnar of （Cr，Fe）₂₃C₆precipitated in grain boundary and lath, theshort columnar of Cr₂₃C₆precipitated within lath and dispersive distribution. The V, Nbmainly existed in the form of the short columnar MX carbide and carbonitride dispersoidswhich could effectively pin dislocations and subgrain boundaries, strengthen material strengthand toughness.The expansion coefficient of material was12.75(10^-6·℃^-1) and accessed tosupercritical steam turbines index at650℃. The optimum content of rare earth was0.1%, RE were able to strengthen the grain boundary and increase the number of carbides ingrain boundary, improve the comprehensive mechanical properties of material, butoverdose would lead to reduced material toughness.