The Structural Formation in Differentially-Hardened 100-Meter-Long Rails during Long-Term Operation

By using modern physical-materials science methods, the roll surface of structural-phase states and mechanical properties has been analyzed at a distance ranging from 0 to 22 mm with respect to the central axis and to the fillet of differentially-quenched DT 350 grade 100-meter-long rails. These rails have been manufactured at Evraz United West Siberian Metallurgical Plant after long-term operation on an experimental test ring (passed gross tonnage amounting to 1411 million tons). According to chemical composition, the rail metal satisfies the requirements of TU (Engineering Specifications) 0921-276–01124323–2012 for steel E76HF grade steel. The impact strength and hardness on the head’s roll surface and throughout the cross section meet the TU (Engineering Specifications) requirements. The microstructure of the rail metal is represented by finely dispersed lamellar pearlite—1.5 points of scale No. 7 according to GOST (State Standard) 8233—with inclusions of excess ferrite along the grain boundaries. The interlamellar distance in the railhead ranges from 0.10 to 0.15 μm. The long-term operation of the rails is accompanied by the formation of a gradient structure presented by a regular change in hardness, microhardness, and impact strength throughout the railhead section. The microhardness at a depth of 2 mm counted from the roll surface amounts to 1481–1486 MPa. At a depth of 10 mm, the microhardness decreases to 1210–1385 MPa, which is caused by an increase in the interlamellar distance and by a decrease in the level of metal cold-work strengthening in the course of long-term operation. It has been suggested that this could be caused by an increase in the interlamellar distance and by a decrease in the level of cold-work strengthening in the course of long-term operation.