Improving the microstructures and mechanical properties of U71Mn rail steel liner friction welded joint by normalizing treatment

Abstract

A large amount of martensitic and elemental segregation exert negative effects on the mechanical properties of U71Mn steel LFW joints. In this study, the normalizing treatment was applied to promote martensite phase transformation and eliminate the element segregation area, thereby improving the mechanical properties of the joints. Specifically, after normalizing treatment, both the tensile strength and impact toughness of the joints were improved. The highest tensile strength could reach 900.1 MPa, about 89% of the BM. The impact energy could be increased from 2.1 J•mm2 to 12.1 J•mm2, indicating a significant enhancement. Compared with LFW joints, the recrystallized grain proportions in the WCZ and TMAZ of the N-LFW joint showed a smaller difference, and the microhardness values of the N-LFW joint were almost identical. This illustrates that normalizing treatment was beneficial to improve microstructural uniformity (about 315Hv). Meanwhile, the strain energy stored in the grains was released, which improved the failure resistance of the joints. This study provided an efficient and stable method for improving the microstructures and mechanical properties of U71Mn LFW joints.