Improvement of microstructure and mechanical properties in U75V rail electroslag welding joints via temperature field control

Ideally, the electroslag welding (ESW) of heavy-haul railway rails requires a critical balance between maintaining high weld formability and refining the microstructures. In this study, the temperature field evolution was deduced by integrating infrared thermography with three-dimensional finite element simulation, and the influence of temperature field evolution on the microstructure and mechanical properties of U75V pearlitic steel joints was systematically investigated. Four distinct cooling curves were established, corresponding to t 8/5 cooling times of 120 s, 100 s, 75 s, and 62 s, respectively. Dilatometric (DIL) testing was employed to determine the pearlite transformation temperatures of the weld metal (WM). Multi-scale characterization revealed a pearlitic structure at t 8/5 intervals of 120 s, 100 s, and 75 s, while a mixed bainite-pearlite structure emerged at t 8/5 =62 s. Accelerated cooling refined pearlite colonies and decreased interlamellar spacing. The fraction of high- angle grain boundaries (HAGBs) increased from 26.2% to 37.5%, reducing crack propagation. A nucleation mechanism involving undercooling and (Ti, V) C precipitation at dislocations and grain boundaries was identified. Mechanical testing showed improved strength with higher cooling rates but deteriorating impact toughness when t 8/5 dropped below 75s due to brittle bainite formation. Therefore, a cooling window of t 8/5 = 75-100s is proposed for optimizing the balance of strength (> 1100 MPa) and toughness in heavy-haul railway joints.