Influences Of High Temperature Softening,Cold-Rolling And Solid Solution Treatment On Intergranular Corrosion Of Super304H Steel

With the rapid development of the ultra-supercritical(USC) thermal power unit, the steam parameters in its boiler have been increasing constantly, which has brought up higher requirements for the properties of heat-resistant steels used in the boiler. As one of the preferred materials for the superheater and reheater tubes in USC boilers, Super304 H steel has excellent steam oxidation resistance and high temperature strength. However, its high carbon content and relatively complex processing technology has brought a serious problem of high intergranular corrosion sensitivity(IGCS). Reducing the carbon content of Super304 H steel to suppress its IGCS will result in the loss of its high temperature strength. Therefore, optimizing the processing technologies of Super304 H steel may reduce its IGCS and guarantee the stability of its chemical composition and strength as well. But research concerning this aspect is still in scarcity.In this paper, the processing technologies of Super304 H steel, such as high temperature softening,cold rolling and solid solution treatment were optimized respectively to suppress its IGCS. Optical Microscope(OM), X-Ray Diffraction(XRD), Scanning Electron Microscope(SEM), Electron Backscattered Diffraction(EBSD) were used to investigate the effect of high temperature softening on the dissolution of primary Nb-rich phase, the effect of cold rolling and solid solution on the microstructure and grain boundary engineering in Super304 H steel. The degree of sensitization(DOS) of Super304 H steel was tested by means of Double Loop Electrochemical Polarization Reactivation(DL-EPR) to reveal the comprehensive influence law of high temperature softening, cold rolling and solid solution and to determine the optimized process parameters.The research results show that the grade of the grain size of Super304 H steel ranged from 7 to 8 and the main precipitates were Nb-rich phases including coarse primary ones and dispersed secondary ones. After high temperature softening at 1300℃ for 5min-60 min, its grain size grew to the grade of 0.3 to 1.6 and the amount of primary Nb-rich phases decreased significantly by dissolving into the matrix and the DOS declined gradually over time. In addition, high temperature oxidation came out as a concern during softening. Therefore, taking the several aspects during high temperature softening into account, the optimum softening process was 1300 ℃/20-30 min followed by water quenching.Cold rolling with thickness reduction from 20% to 80% was applied on Super304 H steel after high temperature softening at 1300℃ for 30 min. With increasing thickness reduction of cold rolling, the saturation magnetization intensity increased showing the existance of martensite. After the cold rolled samples were solution treated at 1150℃ for 30 min, it was founded that only the 80% cold rolled specimen obtained uniform fine-grain microstructure. Hence, 80% thickness reduction was taken as the optimum parameter of cold rolling deformation.80% cold rolled specimens were solution treated at 1150℃ for different times from 5 to 80 min. After solid solution treatment, recrystallization occurred and the deformed grains gradually grew up to the as-received level with the extension of solid solution time. After sensitization treatment at 650℃ for 2h, the amount of M23C6 gradually was found to decrease over solid solution time and the IGCS of Super304 H steel declined at the same time. When solid solution time went to 80 min, the DOS reduced to 0.8% which was already lower than the level of as-received specimen of 1.7% and a lot of dispersed secondary Nb(C,N) precipitated uniformly and densely. The optimum parameters of solid solution treatment was therefore confirmed as 1150℃ for 80 min and water quenching.