Additive manufacturing for railway track component life extension with premium laser clad coating

The lifespan of conventional grades of rail steel is limited by wear and rolling contact fatigue and can be costly to repair or replace. Additive manufacturing of premium rail materials with higher yield points which are more resistant to plastic damage could potentially increase the life of rails across the network. Laser cladding offers a feasible method of in-situ application on targeted areas which are more susceptible to damage. This study assesses laser clad low carbon Martensitic Stainless Steel (MSS) alloy with 14.64% chromium as a material for repair of railway track components considering the effect of the substrate rail.

The laser clad coatings resistance to strain presents challenges in characterisation, a novel method was developed to extract the Shear Yield Stress – Plastic Shear Strain (SYS-PSS) relationship from minimal twin-disc tests. A further set of low cycle twin-disc tests were conducted to determine the rate of strain accumulation within the materials. These experiments provided the data required to quantify plasticity using an empirical ratcheting model, a development to the modelling technique was made with the incorporation of a surface roughness model to generate the contact pressures which cause the plastic deformation in the top few microns of the laser clad coated rails. Hertzian contact pressures were assumed due to the low levels of plasticity within the novel rail materials. It was found that due to the higher yield stress of the new and novel rail materials that smooth Hertzian contact pressures were not sufficient to predict the depth and magnitude of ratcheting in the laser clad coating and substrate rail. A study into the surface topography and its effect of subsurface shear stress was conducted. The asperity peak wavelength and asperity
tip radius were calculated from these measurements and further used in numerical simulations to calculate the asperity contact half width and asperity maximum Hertzian contact pressure, which was shown to be up to six times higher than the smooth contact.

In conclusion laser clad coating is considered to be a viable enhancement process for railway track component life extension using Martensitic Stainless Steel (MSS) alloy with 14.64% chromium. The laser process parameters and the depth of laser clad coating must be carefully controlled to achieve the desired quality, if the coating is too thin and the peak contact stress occurs below it,
plastic shear strain accumulates within the substrate. The low levels of plastic shear strain observed in this work indicates that the wear and RCF performance would be improved compared to conventional rail steels. There is potential to reduce rail replacement frequency if applied to areas in track prone to damage. It is shown in the repair tests that this can cause the coating to become elongated and creep along the surface. There is further potential to utilise the laser clad coating method for in-situ repairs. A series of twin-disc tests were designed to test the integrity, wear and RCF of laser clad repairs using three candidate materials. A repair with a homogeneous material to the parent rail provided the most effective repair as the comparable ratcheting rate prevented material flow of the parent rail over the repair site, reducing crack initiation points. The geometry of the repair must be carefully controlled to avoid a thin coating at the surface interface as it is shown in the repair tests that this can cause the coating to become elongated and creep along the surface.