Fatigue behavior and microstructural characterization of a high strength steel for welded railway rails

Railway rails are subjected to complex dynamic loading, which promotes the fatigue crack propagation phenomenon. As newer demands arise for increasingly faster and more heavily burdened trains, the need for rails with improved mechanical properties increases as well. In this work, a high strength TMCP steel of S700MC grade aimed for rail production is evaluated regarding welding ability. Steel joints were MAG welded and characterised regarding fatigue life. Fatigue tests were carried out using 0.1 stress ratio. Paris Law was assessed using Digital Image Correlation method. The weld seam and heat affected zone were characterised regarding microharciness variation throughout, and results were interpreted based on microstructural features. The produced welds show different microstructure depending on the cooling rate from weld pass temperature. In the centre of the seam, weld root presents fine grain bainitic structure with HV0.1 around 336, while weld face shows coarse grain ferritic structure with HV0.1 around 307. Experimental data from fatigue tests were used to validate a numerical simulation; a difference below 7% was obtained. Experimental data were used to evaluate a case study regarding crack propagation in a railway rail. Numerical simulation showed that only 718,320 cycles are required to increase a crack with initial length 40 mm in 4 mm. The results obtained from the joint experimental and numerical approach show that assessing the material properties and correlating them with the material microstructure is fundamental to develop applications for new materials, while simulation of the crack propagation phenomenon can be used to compare material performance.