The Study On Sigma Precipitation Phase In 254SMo And 2507 Stainless Steel

254SMo is a kind of special stainless steel with high content of chromium, nickel, molybdenum and special extra-low carbon. Which been widely used in some harsh corrosive environment, because of its excellent corrosion resistance and comprehensive mechanical properties. However, the high alloying of 254 SMo SASS is easily embrittled by prolonged exposure at elevated temperature as a result of the formation of second phases. These precipitations have a significant negative effect to the mechanical properties, corrosion resistance and organizational stability. 2507 stainless steel can partially replace 254 SMo SASS. And the high chromium, nitrogen and low carbon, nickel was designed to promote the precipitation of sigma phase. Which leads to the plasticity, toughness and corrosion resistant performance fell sharply of this steel. Therefore, the study on sigma precipitation phase in 254 SMo SASS and 2507 SDSS has important significance to formulate the reasonable hot working institution and ensure the alloy has excellent corrosion resistance.This article first through JMat Pro software to implement the thermodynamic calculation of 254 SMo SASS and 2507 SDSS and made the different heat treatment system. Optical Microscope, X-rays Diffraction Spectrum, Scanning Electron Microscope and Transmission Electron Microscope were employed to investigate the precipitated phases of 254 SMo SASS and 2507 SDSS under different aging time. The type, morphology, distribution, structure and composition of the precipitated phases were revealed, and analyzed the separation mechanism of sigma phase.Metallographic analysis results shows that 254 SMo SASS has a clear second phase precipitation in boundary under different aging time at 1020℃. The precipitated phase number and size increase gradually with the extension of aging time, and the precipitated phases present short rod, long strips and cellular morphology. SEM and EDS analysis shows that the grain boundary phase rich in Cr, Mo and low Ni. TEM analysis shows that the precipitated phase is σ phase. Thermodynamic calculation and experimental study show that the sigma phase transform from austenite, mean γ→σ.The study shows that the precipitated phase priority in alpha/gamma interface under different aging time in 2507 SDSS at 950℃. The precipitated phase number and size increase with the aging time prolonged, but the ferrite contents reduce. XRD and EDS analysis show that the precipitate phase is sigma phase, which rich in Cr, Mo. Thermodynamic calculation and experimental study show that the precipitation mechanism of sigma phase in 2507 stainless steel is a eutectoid reaction, namely according to α→γ2+σ transformation.