Fatigue crack growth resistance of welded joints simulating the weld-repaired railway wheels metal

Abstract

Purpose: The aim of the paper is to study the structure and fatigue crack growth resistance characteristics of weld metal (WM), and heat affected zone (HAZ) under cyclic loadings for the development of railway wheels weld-repairing technology. Design/methodology/approach: WM and HAZ of the welded joint were investigated. The welded joint of 65G steel (0.65 mass.% C; 0.19 Si; 0,91 Mn), which is a model material for high-strength railway wheels, was received by welding Sv-08HM wire per linear welding energy of 10 kJ/cm. Regimes of welding were selected so that the cooling rate of the metal in the temperature range 500-600°C was 5°C/s. As a result, the bainite structure in WM and bainite-martensite one in HAZ are formed. To eliminate the residual stresses generated after the weld cooling, heat treatment was proposed: holding at 100°C for 2 hour after cooling under temperature below then that at the beginning of martensite transformation. Fracture resistance under cyclic loading was estimated by fatigue crack growth rates diagrams (da/dN vs. ΔK) according to standard method for compact tension samples testing. The microstructure and fracture surface were investigated using an optical, and electronic scanning and transmission microscope. Findings: Microstructure parameters and fatigue crack growth resistance characteristics of WM and HAZ after the proposed heat treatment, and also residual stresses of the second kind and local strains in the bulk of bainite and martensite are obtained. Research limitations/implications: Investigations were conducted on samples that simulate the structure and properties of real renovated railway wheels made of steel with high content (0.65%) of carbon. Practical implications: Service durability and safety of weld-repaired railway wheels under high service loadings is increased. Originality/value: HAZ is the most dangerous zone in terms of fatigue cracks initiation and propagation in elements repaired by surfacing method. The positive result on the proposed heat treatment influence is received since the fatigue crack growth resistance characteristics of HAZ metal with bainite-martensite structure raise to the level of weld metal.