Study On The Microstructure Uniformity Of Co-W-Mo-V Series Ultra-high Strength Stainless Steel

With the continuous development of aerospace and aerospace science and technology,the requirements for the performance of load-bearing components have gradually increased,making ultra-high-strength steels with ultra-high strength and excellent comprehensive mechanical properties widely used in this field.Considering the life and safety of the material itself,high-strength stainless steel with ultra-high strength,good toughness and excellent corrosion resistance has become a research hotspot in this field.In response to the urgent demand for materials for key load-bearing components in service in the marine environment,the CENTRAL IRON&STEEL RESEARCH INSTITUTE has developed large-scale bars of 1900MPa grade ultra-high strength stainless steel.The steel has some problems of alloy element segregation and high hardness after annealing in the engineering stage.In view of the above problems,this paper has carried out research on the as-cast microstructure uniformity and the optimization of the preliminary heat treatment process.Thermo-Calc software was used to simulate the solidification process of vacuum consumable smelting ingots,and Melt Flow-VAR was used to study the evolution of cast structure and the segregation of alloying elements.Using phase analysis and scanning electron microscope to study the effect of heat treatment process on the microstructure.By studying the heat treatment process,the uniformity of the structure is improved,and at the same time,the best hardness matching state for mechanical processing is obtained.Provide reliable data support for the engineering of this steel.The room temperature structure of this steel in the equilibrium solidification state was composed of martensite,austenite,M₂₃C₆ type carbide,Laves phase,and?-Cr phase.Melt Flow-VAR simulation calculation results show that Cr and Mo elements was enriched at the core and top of the ingot,and Co and Ni elements was enriched at the sides and bottom.The primary dendrite spacing size was 230?290?m on the side of the ingot,the core was 520?540?m,and the secondary dendrite spacing size was150?145?m on the core.The metallographic results show that the edge structure of the ingot was fine,and the dendrite spacing is consistent with the simulation value;the structure of the core of the ingot was thick,and the actual value of the dendrite spacing was greater than the simulation result.This result provides a reference for the subsequent hot working deformation of the steel ingot to break the as-cast dendrites.In the actual measurement results,the content of gas elements in the core of the steel ingot is higher,and the total content of[O]+[N]+[H]in the core is 2ppm higher than that in the side.Harmful elements are distributed evenly in the ingot,the total content of[S]+[P]is 20ppm.The average size of the inclusions in the ingot is less than 4?m,the number of cores is more than the number of edges.Inclusions are mainly oxides,sulfides and salts.The oxide content exceeds 80%of the total inclusions,and the average size of the oxide gradually decreases from 3.7?m at the top of the ingot to 3.5?m at the bottom.The macro segregation of each alloy element in the ingot is light,and the concentration at the core and top of the ingot is slightly increased.After the ingot was forged,it was found that the content of the second phase gradually decreased as the normalizing temperature increased,M₂₃C₆ type carbide,Laves phase,M₆C type carbide gradually dissolved,austenite content increased,hardness gradually decreased.When the normalizing temperature is 1020?,the mass fraction of the second phase is 0.1%,the second phase at the grain boundary is completely dissolved,the mixed crystal phenomenon is basically eliminated,and the grains have not grown abnormally,this temperature is the best normalizing temperature.After normalizing,as the tempering temperature increases,the hardness decreases first and then increases.The hardness value is the lowest around 700?,the content of the second phase and austenite in the steel gradually decreases,but after the temperature is higher than 700?,the coarse second phase precipitation on the grain boundary affects the continuity and uniformity of the structure,resulting in an increased tendency to crack.When tempering at 680?,the hardness is low,and the precipitation phase is relatively uniform,especially there is basically no second phase precipitation on the grain boundary,so this temperature is the optimal tempering temperature.Therefore,the best preliminary heat treatment process is 1020?×1h+680?×30h to obtain a more uniform structure and lower room temperature hardness,which provides data support for the engineering of the steel.