The Effect Of Boron On The Precipitation Phase Of Super Austenitic Stainless Steel S31254

Super austenitic stainless steel S31254 has a high content of chromium,nickel and molybdenum,and it also contains copper and nitrogen.So it has excellent corrosion resistance in chlorine-containing solutions and other halide solutions,and widely used in seawater desalination,flue gas desulfurization,petrochemical and pulp,paper bleaching and other high corrosive environments.However,due to the variety and high content of alloy elements in the S31254,the element(Mo)segregation is severe during solidification.During the hot working process,the ? phase is sensitive to precipitation,the deformation resistance is large,the thermoplasticity is poor,and edge cracking and cracking are easy to occur.It has become a technical bottlenecks in the production and application of S31254.Therefore,how to effectively suppress the grain boundary segregation of Mo element and ? phase precipitation in S31254 steel is the key to solve the poor hot workability of S31254.In this paper,Thermo-Calc thermodynamics software was used to analyze the influence of different alloying elements on the microstructure of S31254 and the precipitation of the second phase.According to the existing industrial production process of S31254,combined with the phase diagram,the super austenitic stainless steel S31254 with different boron content was optimized.The effect of solution treatment temperature on the ? phase dissolution of boron-containing super austenitic stainless steel S31254 was studied,and the optimal solution treatment process specification was determined.The aging treatment of the samples after solution treatment was carried out to study the effect of aging temperature on the precipitation behavior of ? phase.The effect of boron on the compression and impact properties of S31254 was analyzed in combination with production.The following conclusions were obtained:(1)Based on S31254,the influence of main elements such as B,Cr,Ni and N on the austenite structure stability and precipitation phase was calculated theoretically.The optimal design ideas of boron,nitrogen and chromium-reducing molybdenum alloys were proposed.Super austenitic stainless steels with different boron contents were designed and prepared.The dissolution and precipitation mechanisms of the precipitates were studied.(2)By comparative analysis of the boron content(0,20,40 ppm)super austenitic stainless steel S31254 after 1180°C,1200°C and 1220°C solution treatment microstructure of the microstructure.It can be seen that the boron content has a significant effect on the re-dissolution of the precipitated phase;the precipitation rate of the precipitated phase of the boron-containing sample is significantly faster than that of the boron-free sample,the boron-containing 40 ppm super austenitic stainless steel,and the solid solution at 1180°C for 4 h.The second phase can be completely dissolved;the second phase is completely dissolved at 1220°C for 1h;the re-dissolution of the precipitate phase starts from the twin boundary and is followed by the grain boundary.(3)The evolution process of precipitated phase after aging treatment of different austenitic stainless steel S31254 with different boron contents at different aging temperatures(850°C,950°C,1050°C)was compared.It indicates that the addition of boron has a significant effect on the precipitation behavior of the sigma phase.In the boron-free austenitic stainless steel,the ? phase precipitation rate is fast and the quantity is large,and the grain boundary is preferentially precipitated.With the extension of time,the precipitation phase gradually cascades in the grain boundary.For samples containing 20 ppm boron,the ? phase precipitation time was delayed by 5 min compared to the boron-free sample when aging at 950°C,and the ? phase precipitation time of the boron-free sample was delayed by 25 min.(4)In the hot deformation process,super austenitic stainless steel S31254 not only has thermodynamic precipitation phase,but also deformation induced precipitation phase precipitation.When the boron-free sample is at 950°C,the larger the deformation amount,the more the precipitation phase is,and the precipitated phase is firstly precipitated in the grain boundary.When the deformation amount is 20%,the precipitation phase of the grain boundary is almost connected into a network,and the ? phase is precipitated at 40%.The grain boundaries have been connected to a coarse network,and a large number of precipitated phases have appeared in the crystal.The optimized sample obtained by adding 40 ppm of boron has only a small amount of ? phase precipitated when the deformation is 10%,and the precipitation phase of the grain boundary increases even when the deformation is 40%,but the precipitated phase particles are small and the dispersion is remarkable.(5)The super austenitic stainless steel with optimized composition was tested for hot tensile and impact properties.The boron-containing samples have higher reduction in area than the boron-free samples.The slight amount of precipitation has a significant influence on the impact performance.The industrial thermal deformation of boron-containing 40 ppm super austenitic stainless steel shows that there is no cracking phenomenon after 1100°C through 75% deformation,which effectively solves the problem of poor thermoplasticity and cracking in S31254 hot working process.In this study,the addition of trace amounts of boron in S31254 steel optimizes the heat treatment process in the industrial production process.It is optimized for the long-term solution treatment and is optimized for only 1 h at 1220°C or 1 h at 1180°C.Use provides positive guidance.