Improving the Mechanical Properties and Wear Resistance of a Commercial Pearlitic Rail Steel Using a Two-Step Heat Treatment

The possibility of improving the mechanical and wear performance of steel rails with conventional compositions, near-eutectoid and without special alloying elements, using a two-step heat treatment was investigated. The heat treatment involved a first step involving quenching to a low temperature, near the Ms, maintaining at this temperature for a short time, followed by a second step at higher temperatures until complete transformation. The microstructures, tensile, and wear properties of the obtained products were characterized. The dilatation data confirmed that the remaining austenite was completely eliminated by the bainitic transformation at 400 °C for only 300 seconds, in the absence of alloying elements such as silicon without formation of carbide precipitates. The refined bainite-ferrite microstructure obtained by the two-step heat treatment significantly increased mechanical properties, as well as wear resistance measured using tensile and pin-on-disk tests. The bainitic ferrite structure exhibited approximately 20 pct higher hardness and about 53 pct less mass loss on pin-on disk test than the as-received pearlitic sample. Dilatometric and microstructural analysis using EBSD-electron backscattered diffraction techniques provided evidence that the two-step heat treatment increased the nucleation rate of the bainitic transformation and shortened the incubation time for transformation at the second step, at the same time increasing the density of crystallographic defects such as dislocations and grains boundaries. The proposed heat treatment, besides improving the mechanical properties and wear resistance, avoids the technological difficulties of using molten salt or metal for isothermal heat treatment of long products as rails.