CALCULATION OF AUSTENIZATION RATE OF LAMELLAR PEARLITE

In the field of phase transformation in steels,much attention has been paid to the austenite decomposition transformation,while austenization has not been vastly investigated.However, most industrial heat treatment is related to this process,which is strongly influenced by initial microstructure.The austenization of lamellar pearlite is relatively complicated compared with spheroidized pearlite due to that in spheroidized pearlite,the angular component can be neglected, so only radius growth is considered.During the past years,much work has been done for austenite's one dimensional growth in spheroidized pearlite.However,less attention has been paid to the austenization of lamellar pearlite,which is a two dimensional growth process.Since the growth rates are different along parallel direction and vertical direction to the lamellar,it is necessary to treat growth behaviors individually in these two directions.In order to further investigate effects of substitutional alloying elements,an Fe-0.6C binary alloy and Fe-0.6C-1M(M=Mn,Cr) ternary alloys were studied. Equilibrium composition and thickness ratio of initial ferrite/cementite mixture microstructure was calculated with software Thermo Calc,and a single-layer structure model is employed.Growth of austenite vertical to lamellar direction is simulated by software DICTRA.It is found that ferrite dissolution was controlled by carbon diffusion in Fe-0.6C-1Mn alloy in 1073 K,while that of Fe-0.6C-1Cr alloy was controlled by Cr diffusion.The simulation results qualitatively agreed with experimental observation result.Compared with the parabolic growth pattern,austenite's parabolic growth coefficient is calculated for various alloys.For growth along lamellar direction,parabolic shape phase boundary is deduced based on a simplified assumption and microstructure observation.A general form of growth rate in this direction is derived for Fe-C binary system,and phase boundary shape is also modified to some extent.If a stable parabolic shape interface is reached,the growth rate parallel to the lamellar direction can also be derived for certain values of parabolic shape parameter p and lamellar spacing A, which can be decided by microstructure observation.Given the pearlite colony size and ignoring the influence of nucleation incubation,it takes 0.053 s for a pearlite colony to transform to austenite,which is consistent with that pearlite austenization is a relatively fast process.In addition,taking interfacial energy into consideration,the formation of symmetric parabolic phase boundary is rationalized.