TWI has been working with Railway Safety to exploit an idea for using friction transformation hardening as the basis for a method of repairing rails containing shallow surface defects such as rolling contact fatigue cracks.
The concept of using friction heating as a method of modifying the microstructure of steels is not a new one. There are many publications, mostly from the former Soviet Union, which describe the concept and its application to a number of tasks.
When applied to rails, the outer circumference of a rotating disc or wheel is rubbed against the upper surface of a rail, under a controlled pressure. The wheel is moved along the rail at a pre-determined rate. Frictional heat develops, which heats the rail surface rapidly to high temperatures, converting the microstructure close to the surface to austenite. Once the wheel has passed by, the rail surface then transforms to martensite during the rapid cooling induced by the large heat sink of the rail.
Conditions can be set up such that immediately below the rail surface, material is plastically deformed, and this feature can be exploited to heal shallow cracks, or at least seal deeper ones. The surrounding microstructure can also be controlled by applying suitable pre-heat or post-heat, in order to slow down the cooling rate and prevent the surface layer from being too hard.