| The super highly alloyed austenitic 254SMO stainless steel contains substantial amounts of chromium, nickel and molybdenum in addition to some nitrogen and copper, which are well-known alloying elements that enhance corrosion resistance. As compared to the common austenitic stainless steels such as AISI 304,316 and 316L, 254SMO is considered suitable for application in highly aggressive environments such as bleaching equipment in the pulp and paper industry, seawater desalination, heat exchangers and flue gas process equipment in chemical industry.254SMO stainless steel is a kind of steel that is high alloyed and is difficult to smelting, in which segregation and cracking are common. The inner factors that affect the thermo plasticity mainly are the precipitation of brittle phases and the remaining impurity in the smelting, the external factors include deformation temperature and strain rate and so on. This article have developed a basic theoretical and experimental research on the micro-structure, the high temperature precipitation kinetics and the thermal simulation fracture mechanism of the 254SMO super austenite stainless steel to provide theoretical guidance for smelting, hot-rolling production process. The mainly content can be concluded as follows.First, metallographic microstructure observation and SEM analysis showed that there were grain boundary precipitates of as-cast samples and a single austenite phase obtained by means of solution treatment at 1250℃for 0.5h. In addition, many inclusions were found in the microstructure of all the samples, mainly circular oxide (Al2O3, MnO, Cr2O3) and long strips of sulfide (MnS).Second, the type of precipitates and precipitation rule was obtained from solid solution treatment for as-cast samples(1#,2#) with different temperatures(950,1000,1050,1100,1150,1200℃) and times(1,3,5h) by SEM, EDS, XRD and microstructure observation. During 950℃to 1100℃, sample 1# has the maximum amount of precipitates, while that in sample 2# appears in the range of 950～1050℃,the precipitates turn out to be a phase, besides, the amount of precipitates grows as the aging time increased, precipitates firstly appear along the grain boundary then in the grain. It is found by comparison that the increase of the addition of Ni, N improves the stability of 254SMO austenitic stainless steel and suppresses the precipitates at the same time.Third, thermal simulation and the fracture mechanism, Under hot tensile deformation condition, it has nice thermal plasticity at 1200～1250℃. The SEM and microstructure analysis of tensile fracture surface shows that stretching in the range of 800℃～950℃, with the appearance of grain boundary brittleness resulted from precipitates along the grain boundary, the fracture mode changes from transgranular fracture to intergranular fracture, the percentage reduction of area(Z%) gradually reduces.When in 950℃～1150℃, the amount of precipitates is relatively high, the fracture mode is typical of the intergranular fracture, the percentage reduction of area(Z%) has the lowest value 3.96%at 1050℃, at this temperature,the fracture can be attributed to the combined effect of precipitates and impurities. At 1150℃～1250℃,as the precipitates decreases and dynamic recrystallization, intergranular fracture gradually disappears, the percentage reduction of area(Z%) gradually increases up to 53.76%at 1250℃. The optimum temperature for dynamic recrystallization is 1200℃,the basic reason for it's fracture at this temperature is the exist of inclusions. The temperature range 1200℃～1250℃is suitable for steel rolling.Finally, applying the Materials Studio software, the influence of Mo on the matrix and interface stability of stainless steel was calculated respectively. The one result showed that, matrix stability enhanced by solid dissolved Mo in the a-Fe and y-Fe, and the system of Mo existing in a-Fe is more stable. The other result found that, Mo is easy to make the interface segregation in ferritic and austenitic stainless steels. Both results are agreed with experiment.
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