Improving fatigue resistance of railway axles by cold rolling: Process optimisation and new experimental evidences

Cold rolling is a relatively simple process known as a method to increase the surface hardness and the mechanical fatigue resistance of metal components by the introduction of residual compressive stresses. Cold rolling is also particularly convenient as it is applied in the final part of the manufacturing process. Despite the process being quite common in the manufacturing of various mechanical components, very limited information is available about the actual advantages that are achievable on safety critical components like railway axles. This paper focuses on both optimisation of the process parameters adopted on the Lucchini RS and on the advantages that are obtained when a well calibrated process is applied. The process is optimized by an analytical model, validated by means of an extensive number of residual stress measurements using the X-ray Diffraction method, that estimates the residual stress profile that is generated under the surface depending on different parameters (force, roller geometry, roller feed, axle geometry and steel grade). An overview of various advantages, demonstrated by an extended series of experimental full-scale tests, is then shown: an increase in the fatigue limit even in the presence of artificial micro-notches and corrosion fatigue. An analysis of the driving force at the crack tip is able to confirm the experimental evidences.