Effect Of Ageing On Microstructures And Mechanical Properties Of Superaustenitic Stainless Steels

The super austenitic stainless steel （SASS） is developed in the1960s with a highcontent of chromium, nickel, molybdenum and special extra-low carbon stainless steel.Compared to the conventional austenitic stainless steel, super austenitic stainless steelhas excellent resistance to pitting and crevice corrosion resistance. Because of thesuperior corrosion resistance in chloride and many reducing acid medium, the superaustenitic stainless steel is more suitable for some harsh industrial working conditions,such as paper bleaching, seawater desalination, flue gas desulfurization, and some of themore severe corrosion environments, such as chemical waste gas treatment. However,austenitic stainless steels are easily embrittled by inappropriate heat treatment orprolonged exposure at elevated temperatures as a result of the formation of secondphases such as carbides, nitrides, and intermetallic phases. Among the various secondphases, the most commonly observed precipitates are M23C6carbide, intermetallicsigma, chi, and Laves phases, although other minor phases such as M6C carbide, Rphase, π phase and Cr₂N. These precipitation has a significant negative effect to themechanical properties, corrosion resistance and organizational stability of material, fornot a lot in the application of super austenitic stainless steel in high temperatureenvironment, the study of its precipitation behavior and the performance impact of thesuper austenitic stainless steel is more important than the engineering application, inorder to avoid the emergence of the second phase which harmful to the super austeniticstainless steel during hot working, heat treatment and welding high temperatureoperation of super austenitic stainless steel, which provides the theoretical foundationand its practical application in the future. In view of this, this article by optical microscopy, scanning electron microscopeand transmission electron microscope and X-ray diffraction inspecting technology, thesolid solution process after the two super austenitic stainless steel at differenttemperature of aging, and study the high temperature separation behavior andcomposition in super austenitic stainless steel high temperature separation phaseinfluence. The results show that:904L SASS are inclined to precipitate at thetemperature of900℃, and254SMo is sensitive to the temperature of950℃,precipitates of904L are mainly distributed in the grain boundary, while precipitates of254SMo are distributed in grain boundary and inter grain, under the same heat treatmentcondition, we found that254SMo with more content of molybdenum is easier toprecipitate than904L. The SEM and TEM analysis indicate that the precipitates of the904L SASS continuous distributed in grain boundary as a chain, mainly for the sigmaphase. While the precipitates of254SMo SASS is complicated, they mainly distributedin grain boundary with the shape of block and long strips, inside the grain with theshape of granular, TEM analysis show that the precipitates of grain boundary is σ phase,grain boundary strips and the granular phase inter grain are chi phase. Precipitates playa significant effect on the mechanics properties of super austenitic stainless steel, whichobviously reduces the ductility of super austenitic stainless steel at high temperature.When the aging temperature approach to sensitive temperature and prolong the agingtime, the corrosion resistance of the super austenitic stainless steel decreased obviously.