Grain Refining Mechanism and Fatigue Properties of Bogie Welded Cruciform Joints Treated by Ultrasonic Impact
Surface treatment was carried out on the welded cruciform joint of 16MnR steel by means of ultrasonic impact treatment. The effect of ultrasonic impact on the fatigue properties of welded cruciform joint of 16MnR steel was studied. For comparison, fatigue experiments were carried out with both treated and un-treated specimens. The ultrasonic impact current was 1.2A, the impact amplitude was 30 microns. The grain refining mechanism was investigated using high resolution transmission electron microscope. The experimental results show that the fatigue performance of the welded cruciform joint of 16MnR steel is significantly improved through the ultrasonic impact treatment. The fatigue limit (2x106) of un-treated and treated joint is about 157MPa and 235MPa, respectively. Compared to the un-treated joint, the conditions of fatigue limit of the treated joint increased by 50% and the fatigue life increased 45-52 times. The equiaxed crystal nanostructured surface layers were fabricated on the surface of welded cruciform joint of 16MnR steel by ultrasonic impact, with the grain size being less than 100nm. The main reason for improving the fatigue performance is that the ultrasonic impact can reduce the stress concentration in the weld toe, decrease the tensile stress, and even change to compressive stress in the weldment, with the grain size in the welded joint being greatly refined. It indicates that ultrasonic impact treatment has remarkable effects on the fatigue life of 16MnR steel.
Keywords: Fatigue strength, ultrasonic impact, grain refining, cruciform joint, 16MnR steel, transmission electron microscope, fatigue limit, crystal nanostructured surface
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