Research On Phase Transformation Behaviors Of Modified High Cr Ferritic Heat-Resistant Steels

High Cr ferritic heat-resistant steels are being considered as an attractive candidate material for high-temperature structural components such as main steam pipe, superheater tube and resuperheater tube in advanced power plants due to the good high-temperature endurance, creep resistance properties, excellent heat conductivity, low thermal expansion coefficient and high performance-cost ratio. Additionally, they are also the potential candidate for structural steel in nuclear reactors because of its outstanding irradiation resistance. Furthermore,under the pressure of energy shortage and environment pollution, the study on elevating the thermal efficiency of generating station and endure temperature of the boiler tube materials is also imperative.T91 steel is the representative of high Cr ferritic heat-resistant steels, which has been widely used as power plants materials, and regarded as research benchmark for the development of new ferritic heat-resistant steels with higher application temperature. Based on the previous research on phase transformations and strengthening processes of T91 steel by our group, and strengthening mechanism and alloying principle, four types of the modified high Cr ferritic heat-resistant steels have been developed. The microstructural analysis and mechanical properties testing has been also carried out to evaluate whether they have the superiorperformances for the 650℃grade power plant. Furthermore, to clarify the phase transformation process and mechanism, microstructural evolution and explore the controlled rolling and cooling process of the modified high Cr ferritic heat-resistant steels, the transformation behaviors during continuous heating and cooling, isothermal holding and tempering process were systematically investigated by means of microstructural observation, high-resolution differential dilatometric measurements, micro hardness testing and so on. On this basis, the models for phase transformation kinetics were developed to analyze the transformation behaviors. Results as following:(1) Microstructural analysis and mechanical properties testing of the modified high Cr ferritic heat-resistant steels was carried out. The normalized microstructure of the modified high Cr ferritic heat-resistant steels is composed of martensitic laths with high density dislocation, and a small quantity ofδ-ferrite. During tempering, the precipitates form on the boundary and inside of grain, the density of dislocation decreases and the width of martensitic lath increases. The optimal parameter of heat treatment of the modified high Cr ferritic heat-resistant steels is normalizing at 1100℃+ tempering at 750℃. The results of mechanical properties testing suggest that, the tensile and yield strength of the modified high Cr ferritic heat-resistant steels is remarkably higher than that of the conventional high Cr ferritic heat-resistant steels as T91 and T92 steels. The experiment of resistance to tempering indicates that the modified high Cr ferritic heat-resistant steels, the rate of recovery and recrystallization of which is slow, possess the more excellent resistance to tempering than the conventional high Cr ferritic heat-resistant steels.(2) The austenitic transformation behaviors and kinetics of the modified high Cr ferritic heat-resistant steel during continuous heating were studied. Variation of heating rates affects the austenitic transformation starting temperature, A_c1, and finishing temperature, A_c3. Both A_c1 and A_c3 increase as the heating rate applied is raised, which means that austenitic transformation is retarded to the higher temperature. The increase of heating rate results in the increase of diffusion rate, and thus promotes the diffusion-controlled austenitic transformation and shortens the transformation time. Based on the JMAK model, the isochronal austenitic transformation of the modified high Cr ferritic heat-resistant steel during continuous heating was described well by a phase-transformation model involving site saturation and diffusion-controlled growth. The precision and physical significance of the fitted kinetics parameters agree well with the actual austenitic phase transformation process. The increase of heating rate results in the contraction of austenitic transformation time and the incompletion of dissolution of alloying elements. Hence, the activation energy for diffusion during austenitic transformation decreases from 130.1 kJ/mol to 79.0 kJ/mol, while the heating rate increases from 10℃/min to 3000℃/min. After the accomplishment of austenitic transformation, the alloy elements continue to dissolve, which results in the deviation of thermal expansive curve.(3) Phase transformation during continuous cooling after austenization, of T91 steel and the modified high Cr ferritic heat-resistant steel, was investigated systematically. The model for isochronal martensitic transformation kinetics of the modified high Cr ferritic heat-resistant steel was developed, and extended to strain-induced martensitic transformation kinetics. In T91 steel, the needle-liked M₃C precipitates generate during air cooling after austenization, while water cooling suppresses the formation of the M₃C phase. Furthermore, it was found that, precipitation of the M₃C phase takes place in metastable austenite, before martensitic transformation, and thus leads to the splitting phenomena of the martensitic transformation. In the modified high Cr ferritic heat-resistant steel, high cooling rate also blocks the precipitation of M₃C particles. With the increase of cooling rate, the number of quenching vacancy and density of dislocation increases, resulting in the slighte levation of M_s. Yet, the increase of defection density improves strength of the parent phase, and thus slows down the growth rate of shear dominant martensitic transformation. The model for martensitic transformation in the modified high Cr ferritic heat-resistant steel was developed base on the classical Koistinen-Marburger model. The present model describes the feature of athermal martensitic transformation well. The analysis of the model suggests that, increase of cooling rate slightly causes the slight increase of nucleation rate for martensitic transformation and remarkable decrease of interfacial migration rate for martensitic growth, namely retarding the progress of martensitic transformation. The pre-stress loading leads to the broken grains and narrow martensitic laths, and raises M_s due to the increase of defection density. Besides, pre-stress loading also decreases the interfacial migration rate for martensitic growth by means of strengthen the parent phase, and thus results in decrease of transformation rate and elongation of transformation time.(4) Isothermal holding during cooling after austenization in the modified high Cr ferritic heat-resistant steel was studied by microstructural analysis and transformation behavior research. Furthermore, the model for bainitic transformation kinetics was developed. Isothermal holding at 550℃or 400℃results in the occurrence of bainitic transformation. The decrease of holding temperature promotes the bainitic transformation. The production of bainite causes the disappearance of thermal stabilization of austenite, namely the increase of M_s. Bainite is not observed in the sample after isothermal holding at 650℃. Nevertheless, thermal stabilization of austenite also does not occur because of the precipitation of carbide at the prior austenitic grain boundary. The isothermal bainitic transformation of the modified high Cr ferritic heat-resistant steel during isothermal holding was described well by an incomplete displacive bainitic transformation model in view of autocatalytic nucleation. The precision and physical significance of the fitted kinetics parameters agree well with the actual bainitic phase transformation process. According to the present model, the critical temperature for isothermal bainitic transformation is 687.5℃. With the decrease of isothermal temperature, the driving force of bainitic transformation and the amount of bainite increases. Reduction of holding temperature promotes to bainite transformation by means of increase of number of embryos for autocatalytic nucleation and decrease of activation energy.(5) Microstructural evolution and precipitation of the second phase during tempering in T91 steel and the modified high Cr ferritic heat-resistant steel were investigated by microstructural and kinetic analysis. The M₃C precipitates formed during air cooling affect the early-stage tempering in T91 steel. The existence of the M₃C phase results in that the M₂₃C₆ particles become finer and denser, and intend to be precipitated within the grains. However, with the elongation of tempering time, the influence of the M₃C phase on precipitation of the M₂₃C₆ phase becomes not remarkable. The two-step tempering treatment leads to more precipitates, higher dislocation density and smaller martensitic lath width than that obtained from the traditional tempering process. This is because that, the firstly tempering at a low temperature forms some precipitates, which would pin the dislocation and martensitic lath, and the subsequent secondly tempering at a high temperature complete the precipitation of particles while the recovery of lath and dislocation remain a low level as before. The investigation of tempering kinetics in the modified high Cr ferritic heat-resistant steel indicates that, the heterogeneous nucleation sites of the M₂₃C₆ particles decrease with the increase of number density of precipitates, which leads to the decrease of nucleation rate. At the early stage of tempering, the growth of the M₂₃C₆ particles accords with the diffusion-controlled model developed by Zenner. The growth rate of the M₂₃C₆ phase is related to diffusion coefficient of the solute atoms. With the process of tempering, solute concentration in matrix declines, leading to the decrease of growth rate of the M₂₃C₆ phaes. Furthermore, the migration rate of martensitic lath is controlled by thermally activated diffusion of the atoms at the grain boundary. With the elongation of tempering time, the width of martensitic lath increases. The research on the tempering of strain-induced martensite in the modified high Cr ferritic heat-resistant steel suggests that, in the sample after tempering, pre-stress loading results in the reduction of width of martensitic lath, and formation of sub-grain. This is because that, existence of residual strain energy increases the system free energy and lowers the activation energy for recovery, so the phenomenon of recovery appears in advance. Besides, pre-stress loading also decreases the size of precipitates, and increases the number density of that. This is ascribed to that increase of defection results in the increase of nucleation sites, and thus improves the nucleation rate of the precipitates.