Investigation Of Solidification And Homogenization Of Three 7Mo Super Austenitic Stainless Steels

With the development of high-tech manufacturing,the demand for low-cost and high-performance materials is becoming more and more urgent.As a combination of excellent mechanical properties and corrosion resistance,the super austenitic stainless steel(SASS)is expected to replace the expensive nickelbased alloys in extremely harsh environments.However,as the stainless steel with the highest alloy content(alloy content>50%),7Mo SASS is facing a variety of difficulties in the production process,such as the serious segregation and the formation of high content of brittle phase,resulting in the difficulties of subsequent processing.The research and development of SASS in Europe and the United States has been a century,and our studies on the steel is still at the initial stage,which has not been able to produce the SASS stably.As the main precipitation phase in SASS,σ phase has the characteristics of high melting point,high hardness with high content,which is the key factors affecting the processing of SASS.In order to effectively optimize the solidification microstructure,this thesis is based on the chemical composition of 654SMO SASS to regulate the Ce and Mn content in the steel,aiming to reduce the solidification segregation and inhibit the formation of σphase.We will investigate the formation and dissolution mechanism of σ phase in 7Mo SASS,exploring the effect of Ce and Mn element on the solidification and homogenization process of 7Mo SASS,to enrich the theoretical system of SASS.In this thesis,7Mo SASSs produced by Taiyuan Iron and Steel Group were used as the material of study:1)using thermodynamic calculations to reveal the influence of Ce and Mn on the evolution of solidification microstructure and alloy segregation in 7Mo SASS;on this basis,2)combining with the in-situ experiment to observe the solidification behavior of the steels,the formation mechanism of σphase in steel was clarified by exploring the characteristics of in-situ solidification microstructure,the effect of Ce and Mn on solidification path and alloy segregation behaviors were also explored;meanwhile,3)the orientation relationship between the eutectic σ phase and the matrix is clarified,as well as to clarify the microstructure of the σ/γ phase interface from the atomic scale and to analyze the interfacial strength;furthermore,4)Thermo-Calc thermodynamic calculations and DICTRA kinetic calculations were used to determine the parameters of the isothermal homogenization process,obtained the dissolution kinetic curves of σphase under isothermal conditions;5)the mechanism of σ→γ phase transition and its structural evolution at 1250℃ were investigated in depth by combining submicroscopic and microscopic characterization methods;finally,5)the optimized homogenization process of 7Mo SASS is proposed,by drawing on the stepped homogenization process of high-temperature alloys,demonstrating the advantages of the stepwise homogenization process..The study of the thermodynamic principles of solidification behavior and alloy segregation of 7Mo SASS showed that:the equilibrium solidification process is L→L+δ→L+δ+γ→δ+γ→γ→γ+σ,while the non-equilibrium solidification path occurred L→L+δ→L+δ+γ→L+δ+γ+σ→γ+σ→γ→γ+σ+HCP_A3 for 6Mn and 6Mn-Ce,L→L+γ→L+δ+γ→L+δ+γ+σ→γ+σ+δ→γ+σ→γ+σ+HCP_A3 for 3Mn steel;the addition of a small amount of Ce has little effect on the solidification path of 7Mo SASS,but can slightly reduce the precipitation temperature of the σphase;by investigating the effect of Ce on the Cr and Mo pseudo-binary phase diagram,it can be seen that with the increase of Ce content,the δ phase region expanded and reduced the Cr and Mo content required for the formation of σ phase.The effect of Mn content on the solidification path of 7Mo SASS is more obvious,when the Mn content is lower than 4.15 wt.%,the solidification primary phase is γphase,after the Mn content exceeds 4.15 wt.%,the primary phase will change from y phase to δ phase,and as the Mn content continues to increase,the solidification temperature gradually decreases,and the Mo element aggregates to the σ phase.The microstructure evolution and element segregation of 7Mo SASS during solidification process were investigated by the high-temperature laser confocal microscopy,and the results showed that:all three stainless steels with the actual solidification path is L→L+γ→L+γ+σ→γ+σ,where the faster the cooling rate,the finer the austenite particles;Ce elements in the form of atoms and Ce-O inclusions working together to increase the undercooling degree of the liquid,to promote austenite nucleation and refine the solidification microstructure,as well as reducing the volume fraction of the σ phase,while the addition of Ce increases the Mo element content in the σ phase;increasing the Mn content has the opposite effect.When the Mn content in steel is increased from 3 wt.%to 6 wt.%,the Mn-S inclusions increase,the Ce-O inclusions are reformed to Ce-Mn-O inclusions,and the nucleation sites in the liquid decrease,leading to coarsening of the solidification microstructure and higher contents of the σ phase,the segregation of Mo elements also tend to in the liquid and σ phases.The in-depth study of the σ phase shows that the eutectic σ phase is the main second phase in the steel,with a network-shape at faster cooling rates and a specific orientation relationship with the y phase:(001)σ//(111)γ,[112]γ//[010]σ,(001)σ//(111)γ,[110]γ//[100]σ,the eutectic interface(001)σ//(111)γ consists of several mismatched atomic layers,where the alloy composition gradually transitions,and the results of the nanoindentation tests show that the co-lattice interface has higher strength.The study of isothermal homogenization of 7Mo SASSs and the solid-state transformation of the σ phase showed that:the σ phase in the three steels could be largely eliminated by holding at 1250℃ for 30 h.The dissolution kinetic curves of the σ phase were obtained by fitting the experimental results according to the JMAK equation.Both DICTRA calculations and experimental results during isothermal homogenization show that the elimination of segregation is faster than the dissolution of σ phase,thus the elimination of σ phase is a decisive factor in determining the homogenization process.Increasing both Mn and Ce content in the steel can accelerate the homogenization process.The evolution of the microstructure in the σ→γ phase transition has been studied profoundly,and the results show that the atoms in the σ phase undergo short-range diffusion to produce small displacements,thus forming a unique modulated structure,the new y phase is nucleated on the dislocation of the modulated structure to grow along the domain boundary,merging with the adjacent γ particles,and finally forms a lamellar structure with growth direction,and the lamellar structure coarsens further to complete the σ→γ phase transition.According to the high-temperature alloy homogenization process,the new process of stepped homogenization treatment of SASSs is studied,results show that:due to the chemical components of the eutectic σ phase are seriously deviated from the equilibrium composition,the long-time insulation at 1250℃ will cause the uphill diffusion of Cr and Mn elements,leading to the accumulation of Cr and Mn elements at the phase interface and triggering the remelting of the phase interface.At the same time,with the extension of holding time,Cr elements will be enriched in the σ phase,reducing the homogenization efficiency.The stepped homogenization process adopts the method of "slow heating rate+multi-stage insulation+graded cooling" to avoid elemental up-hill diffusion and the remelting,which improve the homogenization efficiency to shorten the homogenization time from 30 h to 12～13 h.In this thesis,the solidification behavior and homogenization process of three 7Mo SASSs were studied systematically and deeply,focusing on the mechanism of the formation and dissolution of σ phase,investigating the effect of Ce and Mn on solidification and homogenization of 7Mo SASSs,and obtaining a series of innovative results:The specific orientation relationship between the σ phase and the γ phase in the solidified eutectic structure is revealed;It is reported for the first time that the transformation mechanism of σ→γ,which promotes the formation of a new γ phase inside the σ phase by producing a modulated structure;and on the basis of in-depth research on σ phase,a new homogenization process of stepped homogenization was proposed to improve the production efficiency greatly by shorten the homogenization time by more than half.