Reverse Austenite Transformation And Phase Field Study Of Fe-13Cr-5Ni Martensitic Stainless Steel

In this work,the Fe-13Cr-5Ni martensitic stainless steel,in which the reverse austenitic phase transformation from martensite to austenite and the austenite memory phenomenon have been systematically studied by experiments and phase-field modeling.The numerical accuracy and less-cost of phase-field method in modeling the martensite to austenite process(solid state phase transformation),has been deeply studied as well.The austenite behavior has been studied with different heat treatment conditions,thus reveal the effects of annealing temperature and holding time(at 650?)on the reverse austenitic phase transformation.The experimental results show that during the reverse phase transformation,austenite tends to nucleate at prior austenite grain boundaries with a globular shape,and at lath boundaries with an acicular shape.What's more,the acicular shaped austenite bore the Kurdjumov-Sachs(K-S)orientation relationship with the martensite lath.Moreover,annealing temperature and holding time have the same effects on the amount of retained austenite at room temperature.The volume fraction of retained austenite first increase with the raising of temperature or holding time,at 650? of annealing temperature or 20 minites of holding time gets to the peak,then decrease with their raising.Results of experiment and Dictra simulation show that,Mn and Ni diffusion happen during the reverse austenitic phase transformation,and have a great effect on the stability of reversed austenite.Phase-field modeling has been applied to directly study the austenite growth behavior at high temperature.A new phase-field model has been established to simulate the austenite to martensite and then to austenie phase transformation cycle.Based on the former experimental results,in our modeling,there are K-S orientation relationship and coherent interface between new austenite phase and martensite lath during the reverse austenitic transformation.The influences of initial martensite microstructure,the phase transformation induced strain and mechanical boundary conditions on the phase transformation thermodynamics,kinetics and microstructural evolution,have been deeply discussed.Together with the results of experiments,the following demonstrations can be made.The acicular austenite nucleates at martensite lath boundary and grows along the boundary,which bore the K-S orientation relationship with martensite matrix.With the phase transformation proceeds,the acicular austenite grows to meet each other and converge to a larger microstructure,thus leading to the austenite memory phenomenon.The simulation results also show us that both of increasing the defects energy in the initial martensite and exerting constraint mechanical boundary condition on the sample during reverse austenitic transformation,can impede the happening of austenite memory phenomenon.Finally,the numerical superiority of phase-field method in studying reverse austenitic transformation was investigated.Besides making up the shortness of general material characterization methods in studying reverse austenitic transformation at high temperature,the results indicate that the Fast Fourier Spectral Iterative Perturbation Method(FSIPM),which is used in phase-field modeling has high acurracy and low computational cost.