COMPREHENSIVE ASSESSMENT OF HEAT TREATMENT EFFECT ON MICROSTRUCTURE AND SERVICE PROPERTIES OF RAIL STEELS

Abstract

This study investigates the effect of heat treatment regimes on the formation of microstructure and service properties of eutectoid and hypereutectoid rail steels. Particular attention is paid to the relationship between fine- structural parameters (interlamellar spacing, grain size) and subgrain features (mosaic block size, dislocation density) with the fatigue limit, which is a key factor in ensuring rail durability and reliability under real operating conditions. Using X-ray diffraction and metallographic techniques, it was established that single hardening from induction heating produces a mixed-grain structure with increased dislocation density but insufficient austenite homogeneity. In contrast, double heat treatment combining spheroidizing annealing for fine-grained pearlite with subsequent induction hardening significantly refines the grain size, reduces grain inhomogeneity, and forms a highly dispersed and uniform structure. This structural refinement leads to a considerable increase in the fatigue strength compared to single heat treatment. It was revealed that the most critical factor in improving fatigue resistance is the reduction in mosaic block size and an increase in dislocation density, whereas the role of average austenite grain size is less decisive. The strongest improvement was observed in hypereutectoid steels with higher carbon content, where an increased carbide fraction and reduced intercarbide spacing provide additional resistance against fatigue crack propagation. The results obtained have direct practical relevance for the development of next-generation rail steels with improved durability, strength, and reliability, which are crucial for modern rail transport systems operating under intensive loading and variable climatic conditions.