Deep rolling is a powerful tool to increase the service life or reduce the weight of railway axles. Three fatigue-resistant increasing effects are achieved in one treatment: lower surface roughness, strain hardening and compressive residual stresses near the surface. In this work, all measurable changes introduced by the deep rolling process are investigated. A partly deep-rolled railway axle made of high strength steel material 34CrNiMo6 is investigated experimentally. Microstructure analyses, hardness-, roughness-, FWHM- and residual stress measurements are performed. By the microstructure analyses a very local grain distortion, in the range < 5 µm, is proven in the deep rolled section. Stable hardness values, but increased strain hardening is detected by means of FWHM and the surface roughness is significantly reduced by the process application. Residual stresses were measured using the XRD and HD methods. Similar surface values are proven, but the determined depth profiles deviate. Residual stress measurements have generally limitations when measuring in depth, but especially their distribution is significant for increasing the durability of steel materials. Therefore, a numerical deep rolling simulation model is additionally built. Based on uniaxial tensile and cyclic test results, examined on specimen machined from the edge layer of the railway axle, an elastic–plastic Chaboche material model is parameterised. The material model is added to the simulation model and so the introduced residual stresses can be simulated. The comparison of the simulated residual stress in-depth profile, considering the electrochemical removal, shows good agreement to the measurement results. The so validated simulation model is able to determine the prevailing residual stress state near the surface after deep rolling the railway axle. Maximum compressive residual stresses up to about -1,000 MPa near the surface are achieved. The change from the induced compressive to the compensating tensile residual stress range occurs at a depth of 3.5 mm and maximum tensile residual stresses of + 100 MPa at a depth of 4 mm are introduced. In summary, the presented experimental and numerical results demonstrate the modifications induced by the deep rolling process application on a railway axle and lay the foundation for a further optimisation of the deep rolling process.

No abstract

According to the IIW recommendation, the fatigue strength of welded steel joints is defined as independent of the base material in case of the as-welded condition. However, post-treatment techniques can improve the fatigue performance of welded structures, especially for increased base material strengths. Therefore, this paper investigates the effect of TIG dressing, as common post-weld treatment method, on the fatigue strength of high-strength steel S700 cruciform joints. The statistically evaluated fatigue test results reveal a significant increase of the nominal fatigue strength from FAT 90 for the as-welded up to 182 MPa for the TIG-dressed state. The experiments are further compared to recommended and suggested design curves applying both nominal as well as local stress approaches. Focusing on the TIG-dressed state, the suggested increase in nominal stresses is well validated leading to a conservative assessment. In addition, the proposed slope in the finite life region with a value of m1 = 4 shows a sound fit to the statistically evaluated value of m1 = 4.7 for the test results. The local fatigue strength estimation is performed based on a recent proposal using the theory of critical distances. Therefore, linear elastic numerical analysis of the investigated specimens is performed. Again, the resulting S-N curves agree well to the experiments validating the proposed local design approach.