Numerical Simulation Of Martensitic Transformation Kinetics

Steel is the most widely used engineering material and almost spreads over every corner of industry and life.In order to improve its performance,it is indispensable to manipulate steel through thermal treatment.And the steel phase transformation takes place when it is quenched to a certain temperature,then,the generated martensite significantly enhances the strength and stiffness in steel,results in improved mechanical properties.Therefore,researchers have made consistent efforts in martensite(steel)all over the world.Martensite phase transformation including two research contents: martensite nucleation and martensite growth.The former can be observed in a static view while the latter cannot.Due to the high-speed growth process(about 1000 m/s),there are lots of difficulties in observing and recording its growth behavior.Herein,on the basis of the previous theoretical achievements around the world,as well as the experimental research in our group,we put efforts into the numerical stimulation of the homogeneous expansion model in martensite phase transformation dynamics,the main research contents are as follows:(1)A reasonable three-dimension model is established by finite element method in order to simulated the entire martensite-containing austenite.In respect to the external loads,its specific displacement constraint is related to the expansion values of martensite.Accordingly,the stress field is solved by numerical analysis method.To begin with,only one first-generationmartensite system was analyzed in each gradually growth step,and its strain energy of nucleation was calculated with the help of classical elastic theory.Thus,the quantitative analysis and investigation is carried out to martensite phase transformation.Results show that the strain energy per unit volume of the single first-generation martensite gradually increased,along with the continuous growth of the martensite core.(2)We also focus on the situation that two first-generation martensites simultaneously generating and then growing in the austenite,the three-dimension model that two firstgeneration-martensites arrange in different direction and distance was established by the finite element method.The phase transformation calculation was performed considering the volume expansion values as specific displacement constraint.Results show that the stress and the strain energy per unit volume increased,with the increasing angle between the two martensites arrangement.Besides,the stress induced interference range and the strain energy per unit volume gradually decreases when the distance between two martensites increased.(3)Considering the situation that several martensites simultaneously generating and growing in the austenite,the three-dimension model is set up with the finite element method including situations that austenite contain different quantities of first-generation-martensites,and both first-and second-generation-martensites exist in the austenite at the same time.For the three-dimensional model of the second-generation martensite.The phase transformation calculation was performed considering the volume expansion values as specific displacement constraint.Results show that its stress and strain energy per unit volume increased with larger amount of first-generation martensite.Moreover,the stress and strain energy per unit volume also increased with the larger amount and bigger size of the second-generation martensite.In our work,the rational martensite-austenite three-dimension models have been effectively developed,then the growth process has been quantitively calculated and analyzed according to the principle of martensite phase transformation,by means of numerical analysis method.In summary,our present work may provide the theoretical assistances for further indepth investigation of martensitic phase transformation dynamics,which is of great significance for improving the physical and chemical performances of steel,as a result developing a new generation of steels.