Effects Of Alloying Element Partition And Its Interfacial Segregation On The Thermodynamics And Kinetics Of Phase Transformation In Steels

The mechanical property of steels is closely related to its microstructure.Addition of alloying elements profoundly affects the thermodynamics and kinetics during phase transformation,and hence determines the microstructure evolution.Until now,controversy still lies on the physical essence and effects of alloying element partitioning and the interfacial segregation behavior.Undoubtedly,it is ultimate to gain a deep understanding and bring new insights.The current research systematically studies ferrite transformation,bainite transformation,pearlite transformation and its austenitization process.Theoretical models are built up in an attempt to clarify the interaction between microstructure evolution and alloying element partition/interfacial segrega tion,which are basis for the design of microstructure and enhancement of properties.Plate-lengthening rate,plate thickening rate and isotropic growth rate of ferrite are calculated based on traditional Gibbs Energy Balance model,and the corresponding microstructure evolution is plotted over time.Comparison is made on Energy reduction rate between plate-like and spherical morphology,with an aim to clarify the essence of morphology competition.Ferrite morphology according to GEB model is compared with experimental observation on the microstructure of Fe-0.23C-1.86 Mn alloy transformed at 650?C/600?C/500?C and Fe-0.38C-1.48 Si alloy transformed at 800?C/750?C/700?C.Besides,the physical mechanism of incomplete transformation of bainite is analyzed via thi s approach.The solute drag effect caused by interfacial segregation of alloying element results in a ‘steady state' on the thickening kinetics,which is related to the global transformation stasis phenomenon.Carbon supersaturation within bainitic ferrite plate is assessed by GEB model,and its combination with heterogeneous nucleation theory can be used to calculate the internal nucleation rate of carbides.According to microstructural difference between upper and lower bainite,the spatial distance betwe en neighboring carbides for the transition is set to be 100 nm.Afterwards,the critical transition temperature could be readily determined.Both theoretical model and experimental results show a similar trend of this critical temperature,i.e.,it first increases then decreases with the elevation of bulk carbon content.Moreover,the peak temperature and its corresponding carbon concentration via GEB approach agrees well with experimental values.The carbon supersaturation and velocity of ?/? interface are employed to reveal the physical essence of inverse ‘V' trend.The growth of pearlite is simulated by phase field method.In Fe-0.81 C binary alloy,the kinetics of transformation is controlled by an integrated effect of carbon diffusion through austenite matrix,ferrite matrix and phase boundary.For Fe-0.69C-1.80 Mn ternary system,pearlite growth is dominated by carbon diffusion at 650?C,while it is controlled by boundary diffusion of Mn at 670 ?C.In addition,there seems to be no sharp kinetic transition between these two modes.On the other hand,the curvature of ?/? interface changes with temperature,and it is explained from a perspective of alloying element partitioning.The critical temperature for partitioning of substitutional alloying element in the early stage of austenitization,namely PNTT-I,is calculated as a function of pearlite transformation temperature and the isothermal holding time in Fe-CM(M=Mn,Cr,Si,Co,No)ternary alloy.Dictra simulation is performed to verify the kinetic transition at temperatures above and below PNTT-I.However,the kinetics of dissolution of cementite particles in Fe-1.27C-0.36Mn-0.19 Cr alloy shows two stages,i.e.,a quite sluggish rate followed by the initial fast dissolution.The early stage is controlled by carbon diffusion,whereas final stage by M diffusion.In this regard,the concept of PNTT-II is put forward.When austenitization temperature is higher than PNTT-II,the growth of austenite is dominated by carbon diffusion from the beginning to the end,which effectively avoids the commonly observed incomplete dissolution of cementite particles.Lastly,a new design of heat treatment process is proposed for medium Mn steel.Full lamellar pearlite is selected to be the initial microstructure,with Mn partitioned between cementite and ferrite.Then the sample is austenitized at a temperature higher than PNTT-II,followed by water quench to room temperature.Alternating layers of martensite and Mn enriched retained austenite could be obtained,which is supposed to bear excellent mechanical properties.