| 904L is a kind of super austenitic stainless steel with high Cr,high Mo and high Ni,which alloy content is more than 40%.High alloying content makes it have excellent mechanical properties and corrosion resistance,and has been widely used in petroleum,chemical and other industries.However,the higher alloy content makes the904 L solidification process more complicated and the cast structure microsegregation becomes serious,which brings considerable difficulty to the subsequent hot processing and heat treatment.Therefore,studying the solidification segregation behavior and homogenization process of 904 L super austenitic stainless steel,and formulating a reasonable homogenization system,can provide a theoretical basis and technical reference for the hot forming of steel.In this paper,based on Thermo-Calc thermodynamic calculation results,the high-temperature laser confocal scanning microscope was used to dynamically observe the solidification process of 904 L.With the help of optical microscope,scanning electron microscopy,electron probe quantitative qualitative analysis and other equipment for qualitative and quantitative analysing the impact of cooling rate(6?/min,50?/min and 100?/min)on the solidification process and solidification structure.In order to providing a data basis for further mastering the solidification and segregation behavior of 904 L super austenitic stainless steel.A reasonable heat treatment system was formulated according to the element diffusion dynamics,which systematically characterized the 904 L microstructure evolution and element diffusion behavior during the homogenization process.This study mainly draws the followingconclusions:1)Thermo-Calc calculation results show that the Scheil-Guliver simulation results under non-equilibrium conditions are basically consistent with the simulation results under equilibrium conditions.904 L super austenitic stainless steel chromium-nickel equivalent ratio is 0.98,solidification mode is type A,there will be generated Sigma phase and Laves phase at the end of solidification.In the solidification process,the solute partition coefficient of all the elements except Ni is less than 1 and they will aggregate into the liquid phase.Among them,the Mo element is the most severely segregated,meanwhile the Ni element hardly segregates.2)The solidification process of 904 L can be divided into three stages.As the cooling rate increases,the crystallization temperature and the secondary dendrite arm spacing both decrease with the increase of the cooling rate.The solute partition coefficient of different elements indicates that as the cooling rate increases,the degree of segregation is alleviated,and the precipitation amount of Sigma phase at grain boundary is reduced.3)The results of structural analysis of different parts in the width direction of the slab indicate that the edges,1/2 and core are all developed dendrite structures.From the edge to the core,not only the distance between the secondary dendrite arms gradually increases,but also the number and size of the Sigma phase at the grain boundaries are increased.The segregation coefficient of different elements increases and the segregation becomes heavier,indicating that the segregation of the core structure of the slab is the most serious.4)The results of homogenization experiment showed that when homogenized at1200? for 6h,the dendrite structure in different parts of the ingot were significantly ablated and all precipitated phases were completely dissolved back.The Mo element segregation coefficient of the core tissue was 0.191,and the residual segregation index was 0.12,reaching the industrial homogenization standard.Therefore,the optimal heat treatment process is 1200? homogenization for 6h. |
PDF Full Text Request