Study And Application Of Strain-induced Precipitation Distribution Model For Austenitic Heat-resistant Steel

As a key material used in thermal power,nuclear power,chemical industry and other fields under high temperature,the service performance of heat-resistant steel has attracted much attention.There are a large number of alloying elements in heat-resistant steel,and more types and quantities of second phase particles are often precipitated in its service process.These precipitates have important effects on the thermal strength,toughness,microstructure stability and high temperature oxidation and corrosion resistance of heat-resistant steel.For example,the strain causes a large number of dislocations in the matrix,which promotes the precipitation of the second phase particles and is beneficial to improving the high-temperature strength of heat-resistant steel components.People often explore the precipitation behavior and mechanical properties of heat-resistant steel by means of aging,creep and other experimental methods as close as possible to the service situation.However,the service time of high-temperature members is long,which causes great difficulties in studying the effect of strain on the precipitation behavior of heat-resistant steel and the development of new steel grades.Along with the computer simulation in materials research in recent years unceasing progress,the advantages of simple and highly effective for the precipitated phase type heat resistant steel is more complex situation and the characteristics of precipitation rule provides a feasible way,the particularity of problem of heat resistant steel service process precipitation simulation,aiming at close to the actual process of precipitation precipitation under the condition of uneven distribution of solute elements,Monte Carlo method is used for simulation.Therefore,this paper establishes a strain-induced precipitation distribution model of heat-resistant steel by means of thermodynamics and kinetics of precipitate phase,in order to solve the study of the effect of strain on precipitation of heat-resistant steel.In this paper,three common precipitated phases(MX,M₂₃C₆ andσphases)in austenitic heat resistant steel are taken as the research objects.Based on Johnson-Mehl-Avrami equation,LSW theory and D-S strain-induced average size model,a strain-induced particle size distribution model for heat-resistant steel was established by combining Monte Carlo method.The size,quantity,volume fraction and size distribution of the three precipitated phases in Sanicro25 steel as an example were investigated with aging time,and the effects of strain,temperature and critical crystal embryo shape on the precipitation characteristics of the precipitated phases were studied.The samples of solid solution Sanicro25 heat resistant steel without deformation and 45%cold deformation were treated at 700℃for different time,and electrochemical extraction was carried out.Nano particle size test and XRD test were carried out for extraction solution and powder,respectively,to verify the model results.The research results show that promote aging temperature by influencing the diffusion rate of solute elements,led to by precipitation rate and the growth rate is higher,so compared with the aging temperature under the condition of low precipitation process speed up under the same aging time,precipitation volume fraction,the size of the precipitated phase distribution and the value of the average particle size is larger,smaller number of precipitated phase particles.By strain increases the dislocation density,make the small size of precipitated phase as a whole,narrower size range,the precipitation particles have a larger number of ascension,the more precipitation volume fraction under the same aging time,the strain effect on the size of the MX phase is small,stable in smaller size range,but the influence of the size of M₂₃C₆ and sigma phase,In addition,the precipitated phase extraction experiment of Sanicro25 steel with deformation and undeformation found that strain promoted the precipitation ofσphase,and also confirmed that strain-induced precipitation resulted in smaller particle size of the second phase,which verified that the model results were consistent with the experimental results.The experimental results of particle size analysis are in good agreement with the simulated curve.The study in this paper provides a model simulation technique for further understanding the growth law and mechanism of the second phase precipitation during the aging process of heat-resistant steel.