Research Of Diffusion And Phase Transformation Of Rare Earth Micro-Alloy Steel

It is well known that the micro-structure properties of steels were changedsignificantly by the changed content of Rare Earth in the steels. The dissolution,precipitation, recrystallization and phase transition were influenced by rare earth (RE). Theprimary reason of RE effect of recrystallization and phase transformation is RE effect ofiron atomic free enthalpy and effect of diffusion of RE in steel. Currently, mechanism ofaction and dissolution of RE in steels are rarely studied, especially dissolution coefficient.Effect of isotherml phase transformation performance on the micro-ally steel and pureiron with RE was investigated by L78type quenching dilatometer. Discussed the existingform of La in α-Fe according to density functional theory (DFT). The diffusion behavior ofLa in pure iron was studied by diffusion bounding experiment and metallurgicalmicroscope and scanning electron microscopy (SEM).Isotherml transformation curve of micro-alloyed steel were investigated by L78typequenching dilatometer in detail. The micro-structure was analyzed by metallurgicalmicroscope and OLS4000confocal laser scanning microscopy in different temperature.Compared to the previous research result of the micro-structure in without addition of REmicro-alloy steel, the surface micro-structure of steels added RE displays an irregularshape at725oC isothermal temperature. Pearlite structure was obtained along with thegrain boundary of austenite in650oC isothermal temperature. With the isothermaltemperature further decreases between625oC and525oC, the surface micro-structure ofsteels added RE changed into lath bainite and granular bainite.DFT is used to calculation the segregation energy of La in crystal, grain boundary andfree surface. The results show that La tend to be located at grain boundary in the α-Fe sincethe impurity formation energy keeps lowest. So La tend to segregation at grain boundary inα-Fe. Moreover, La addition also makes the atomic density of states on the grain boundarymove to the left, reduce the total energy of the system, and make the grain boundary morestable. The diffusion bonding of Fe and Fe-La was conducted by Gleeble-1500D thermalsimulation test machine. The diffusion of rare earth in the Fe matrix was observed by theAxiovert25metallurgical microscope and SEM. On the above basis, the diffusioncoefficient of La in the pure iron was calculated by the equation of diffusion coefficient. Itis found that La diffused toward the pure iron and along the grain boundary. According tothe experimental data and theoretical analysis, the grain boundary diffusion coefficient ofLa is Db=1.59×10-17m2/s.