Study on microstructure and mechanical properties of continuous drive friction-welded joints of bainitic rail steel

The U20Mn bainitic rail steel rods with a diameter of 15mm were joined by continuous drive friction welding (CDFW) below the A₁ temperature. The microstructure characteristics of the welded joints without post weld heat treatment (PWHT) were investigated by metallographic observation, scanning electron microscope investigation, and electron backscattered diffraction analysis. Their mechanical properties were evaluated through hardness, tensile, and impact tests. The results show that the welded joints are well-formed without metallurgical defects. They manifest a dense martensitic-bainitic composite phase structure with good mechanical properties attributed to fine-grain and second-phase strengthening effects. The maximum impact energy is 29.2J, with peak hardness increasing to about 124% compared to the base material (BM). Tensile strength reaches 1267±15.1 MPa (102% of BM), and elongation reaches 13.8±0.2% (97% of BM), realizing quasi-equal strength and toughness matching with the BM. These findings provide a basis for the realization of friction welding for bainitic steel rails and the future development of novel rail welding techniques and equipment.