Surface treatment of rail welded joints to balance wear in different zones: Laminar plasma inhomogeneous quenching method

The rail welded joints represent critical components and potential weak points within seamless rail lines. The differences in microhardness and microstructure across different zones of these joints contribute to the occurrence of uneven wear. This study proposes a laminar plasma inhomogeneous quenching (LPIQ) method to address this issue and comprehensively investigates the effects of LPIQ on wear and rolling contact fatigue (RCF) behaviors of two types of rail welded joints. The results indicated that LPIQ treatment induced grain refinement (1.63–2.04 μm) in different zones of two types of welded joints. Simultaneously, this treatment facilitated the recrystallization process dominated by continuous dynamic recrystallization and geometric dynamic recrystallization. The proportion of recrystallized grains in the quenching zone exceeded 55 %, with the difference between various zones being less than 1 %. The high uniformity of the microstructure following LPIQ treatment contributes to balancing the wear across different zones of the welded joints, yielding wear homogenization index of 20.75 % for quenching thermite welded joint (QTW) specimens and 4.96 % for quenching flash welded joint (QFW) specimens. Furthermore, the RCF damage observed in QFW specimens is more complex and severe compared to that in QTW specimens. The quenching zones of the specimens following the LPIQ treatment are rich in high-angle grain boundaries, whereas the substrate predominantly consists of low-angle grain boundaries. This differentiated distribution of grain boundaries enhances the joint's resistance to damage and inhibits crack propagation towards the substrate. Additionally, the kernel average misorientation (KAM) value in the quenching zone significantly increases compared to that of the substrate, and its distribution exhibits a high degree of uniformity.