In Situ Fatigue Crack Analysis of 590 Wheel Steel in Different States

Fatigue crack is the main failure course of wheel steels, but its relationship with microstructure, texture, welding process and deformation remains unclear. Herein, a laser confocal microscope integrated with a tensile testing apparatus was adopted to in situ investigate the crack initiation and propagation of a 590 MPa strength wheel steel during hot-rolled, cold-deformed, and welded states based on the complex fabrication and welding processes. Further, by utilizing Electron Backscattered Diffraction (EBSD) technology, the relationship between the crack initiation and propagation process and the structure type, crystal orientation and texture strength were analyzed to reveal the underlying mechanism of the fatigue failure. It was found that the earliest crack initiation occurred in the 10% cold-deformed plate, followed by the hot-rolled plate, while the welded joint had the latest cracking onset. The crack propagation speed was highest in the cold-deformed sample, intermediate in the welded one, and lowest in the hot-rolled one. The crack propagation rate increased gradually in hot-rolled state. At the beginning stage, the crack propagation rate was low, and then increased slowly. With the extension of crack length, the stress intensity factor increased, accelerating the crack propagation. The EBSD results confirmed that the crack initiation was closely related to the dislocation slip process and the initiation in ferrite depended on the strength of {001} <110> texture. Accordingly, it was concluded that the resistance of wheel steels to fatigue crack initiation can be improved by reducing the cold-deformed structure or deformation degree during the fabrication process, as well as lowering {001}<110> texture strength of the hot-rolled one. © 2023 Editorial Department of Journal of Sichuan University. All rights reserved.