Pipeline transport uses millions of kilometers of pipes worldwide to transport liquid or gas over long distances to the point of consumption. High demands are placed, especially on the transport of hazardous substances under high pressure (gas, oil, etc.). Mostly seamless steel pipes of various diameters are used, but their production is expensive. The use of laser-welded pipes could significantly reduce the cost of building new pipelines. However, sufficient mechanical properties need to be ensured for welded pipes to meet stringent requirements. Therefore, laser-welded 10 mm thick pressure vessel steel plates were subjected to various mechanical tests, including high-cycle fatigue tests. Furthermore, the microstructural parameters and the state of residual stresses were determined using X-ray and neutron diffraction, which could affect fatigue life, too. The critical areas for possible crack initialization, especially in and near the heat-affected zone, were found using different tests. The presented results outline the promising application potential of laser welding for the production of pipes for high-pressure pipelines.

An investigation of nitriding effect on fatigue behavior of small-size specimens of a 30CrMoV9 (ČSN 15330) nitriding steel heat treated to high strength was carried out, and fatigue damage mechanisms were evaluated. Circular cross-section specimens either smooth or notched, with the diameter approximately 2 mm in gauge section were used. Axial fatigue tests were carried out at constant load amplitude, loading was of repeated type—load asymmetry R = 0. The program was carried out as a separate part of the complex evaluation of fatigue resistance of gears against failure. The nitriding process resulted in unforeseen, mostly negative effects—in a strong reduction of fatigue life and fatigue limit in most cases. Fatigue limit only was slightly increased, namely, by 10% in case of smooth specimens. However, further small increase of load amplitude resulted in strongly reduced fatigue strength in comparison with specimens without nitriding. Nitriding applied on notched specimens reduced their fatigue limit by more than 40%. Fractographical and metallographical analyses showed an extensive brittle cracking of nitriding layer particularly in the vicinity of fatigue crack initiation sites and an existence of multiple deep fatigue cracks besides the main crack. A likely reason of premature failure was a superposition of external stress, which were close to yield strength, with tensile residual stresses under the nitrided layer, resulting in a contribution of the small specimen size in a local subsurface plastic deformation and brittle cracking of the thin nitrided layer. Once brittle cracks in the layer arose, fast fatigue crack growth occurred.

Railway wheels are usually attached to axles by press-fitting; therefore, the mechanical processes taking place during operation can result in failure, with fatal consequences for the axle seats. This manuscript describes the effect of laser hardening on the residual stress state, microstructural parameters (lattice defects—dislocations, crystallites, microstrains, etc.), and mechanical properties of laser-hardened EA1N steel railway axles under fatigue life conditions. Differences were found between ground, single-track, and multi-track hardened surfaces. Tensile residual stresses, low dislocation densities and hardnesses, and different microstructures (tempered cubic martensite) were found at the overlapped tracks and at the boundary of the heat-affected zone and bulk surface compared with the hardened zone. As a result, the surface treatment of axle seats by laser hardening improved the fatigue failure resistance compared with untreated seats. Optimal properties of the integrity of the axle seat surface were achieved, including fatigue resistance, which seems to be positively influenced mainly by sufficient hardness and the appropriate microstructure. The influence of the other investigated parameters was not evident, and was reduced by the presence of fretting corrosion and press-fitting.

The authors would like to correct the sentence in the “Materials and Methods” section of their article