Very high cycle fatigue properties of SMA490BW steel welded joints for train bogie

Very high cycle fatigue properties of SMA490BW steel welded joints for train bogie were investigated by the ultrasonic fatigue testing method. The residual stress of the welded joint was tested through X-ray stress tester. Fatigue crack initiation, propagation and fatigue fracture mechanism were observed and analyzed using scanning electron microscopy. Experimental results show that fatigue property of SMA490BW steel base metal is much higher than that of the welded joint. Under the condition of 1 × 10⁸ cycles fatigue life, the fatigue strength of welded joint is 141 MPa, while that is only 44.2% of base metal. Fatigue cracks of welded joint initiated mainly at surface defects of welded toe, and the fatigue fracture mechanism is exhibited as quasi-cleavage fracture, which accompanied by the traces of plastic deformation in the fracture surface. Residual tensile stress and stress concentration at welded toe, and non-uniformity of microstructure and properties in welded zone are the main factors causing low fatigue property of the welded joint.