Fatigue crack growth behavior of railway wheel steel modified by sulfides enveloping oxides inclusions

Abstract

This study aims to investigate the fatigue crack growth (FCG) behavior of a railway wheel steel, which has undergone inclusion modification through sulfides enveloping oxides. It involves experimental and finite element (FE) investigations, including microstructural and inclusion analysis, mechanical testing (tensile, hardness, fracture toughness, and FCG tests), and FE analysis based on tessellated stress model and fracture mechanics method. The test results reveal that the microstructure of wheel steel remains almost unchanged as the sulfur content increases from 0.002% to 0.008%. However, the inclusion area fraction increases, the average inclusion size decreases, and most of the oxides are modified to be the duplex inclusions in the form of sulfide enveloping oxide. Moreover, the FCG resistance and fracture toughness are improved. The FE analysis results indicate that the tessellated stress around the inclusion, which is caused by the thermal expansion mis-match between inclusions and matrix material, can be reduced by the coverage of hard oxides by soft sulfides. This reduces the driving force of FCG at low ΔK ranges and results in improved FCG resistance. Furthermore, due to the lower driving force of FCG, fatigue cracks in the modified steel are more prone to deflect at cementite lamellas at low ΔK ranges, which leads to a rougher fracture surface. This further improves the FCG resistance by the mechanism of roughness-induced closure. As ΔK increases, FCG is gradually controlled by the remote applied loading, and the influence of the inclusion tends to disappear. In addition, the thermal expansion mis-match between inclusions and matrix material is found to have a more significant effect on FCG of wheel steel than the stiffness mis-match.