Microstructure And Mechanical Properties Of Nanostructured Pearlite Rail Steels

Pearlitic steels are indispensible in high-strength rails for their high strength and excellent wear resistance. This study investigated the effects of different Al, Mn, Cr and Si contents on the formation and properties of a pearlitic microstructure in high-carbon steels treated through slack quenching after hot deformation. The microstructure, tensile plastic deformation behavior, and alloying element partitioning between ferrite and cementite were analyzed using scanning electron microscopy, transmission electron microscopy, and atom-probe tomography, respectively.The results show that the interlamellar spacing of pearlite remarkably decrease with increasing Al content and decreasing Mn content. A fully p earlitic microstructure with an interlamellar spacing of approximately 80 nm is obtained at a cooling rate of 100 °C/min. The nanoscaled microstructure is responsible for the high tensile strength of 1670 MPa and elongation of 11.2 %. The increase in Al content and decrease in Mn content promote the nucleation density of pearlite colony and decrease the interlamellar spacing by raising the free energy during the transformation of pearlite. Al and Si partition to the ferrite phase, whereas Mn and Cr present high concentrations in the cementite phase. The redistribution of alloying elements decelerate pearlite growth rate.In addition, the cooling rate affects the microstructure and properties of pearlite. Low cooling rates produce low undercooling; the density of nucleation sites is lower. The cementite lamellae regularly grow and interlamellar spacing is coarse which leads to the low hardness and strength. The undercooling increases with increasing cooling rate. The density of pearlite nucleation increases and the interlamelar spacing becomes finer, which results in the higher hardness and strength. The longitudinal growth of cementite lamellae, which alternately form and grow with ferrite lamellae, is retarded.