Hydrogen Anti-flaking Heat Treatment in VAR89S Rail Steel

Hydrogen content in the diffusible range and beyond a critical level in steel is detrimental to the formation of hydrogen flakes that show up in ultrasonic testing. It is observed that the hydrogen content in any hot-rolled steel has a gradient with lowest hydrogen at the surface and higher towards the core. This is associated with the secondary phase transformation of high hydrogen-soluble austenite phase to a low-soluble ferrite phase, occurring last towards the core. Anti-flaking heat treatment was carried out in a flake-sensitive pearlitic rail steel grade, VAR89S, with an initial hydrogen level of 1.86 ppm at the core and 1.58 ppm in the surface of a 160-mm diameter hot-rolled bar. The heat treatment involved holding at 650 °C for times varying between 8 and 36 h, which decreased the hydrogen content. The decrease in hydrogen during anti-flaking heat treatment was correlated using Fick’s law model which took into account the initial hydrogen gradient across the bar cross section. The model predicted the experimental trend for hydrogen distribution at surface, mid-radius, and core. The hydrogen distribution at surface showed higher than predicted values, which could be attributed to the carbide spheroidization at the surface.