Laser Applications Laboratory (LAL)

Laser Glazing of Railroad Rails

Laser glazing treatment is applied on the gage face of the rails to help minimize flange and rail wear, reduce fuel consumption, and decrease the likelihood of derailments caused by flange climb and low-rail rollover.

To maintain the toughness and relative inexpensiveness of rail steel while reducing wheel/rail friction, an ideal approach is to modify the loading surface of existing rail in a way that renders it substantially harder yet leaves the substrate unaffected. Laser treatment can accomplish this by changing the surface microstructure, and doing so in a such a way that the surface layer undergoes a smooth transition into the substrate and is well bonded to it. One way to harden the surface of rail steel is by laser surface hardening, i.e. selective austenitization and martensitization of the local surface region of rail steel by rapid heating and cooling. The hard surface layer will resist plastic flow and reduce friction and wear. This simple process has some minor disadvantages. Unlike in the case of hard bulk materials, like diamond, o thin hard surface when loaded substantially will transmit these loads to the soft substrate, which can deform even if the surface does not. If the surface layer has the same elastic modulus as the substrate, it can not mitigate stress concentrations any differently than an untreated surface. However, a surface layer with a smaller modulus than the substrate can distribute surface stress concentrations elastically within the layer and mitigate their effect on the substrate. Also, if the hard surface layer does deform plastically, it will do so in the same manner as untreated rail steel, producing a damage layer along which separation can eventually occur leading to delamination. What is needed then is a surface layer that is hard with respect to plastic flow, is elastically compliant, and, when it does deform plastically, does not produce a damage layer, Laser glazing can be an ideal treatment to achieve this goal.

In laser glazing, a thin surface layer is melted and rapidly solidified to produce an amorphous or quasi-amorphous surface film. The lack of crystalline order in such materials makes them substantially resistant to plastic flow, and hence very hard. They exhibit about a 30% or greater reduction in elastic modulus. If they do deform plastically, localized defects are not involved, and the more open structure tends to be self healing. Consequently, repeated plastic flow is less likely to result in debris being deposited in a subsurface damage layer. For these reasons, the laser glazing process alone is sufficient to provide satisfactory reductions in friction and wear.