Effects of Service on Residual Stresses in Sub-Critically Quenched Rail Car Axles

Abstract

Abstract Sub-critically quenched axles have been used in passenger rail cars for many years to obtain improved fatigue resistance. While the formation of axial compressive residual stresses by sub-critical quenching is well documented, it has been theorized that bending from service loads and high forces from mounting and demounting pressed-on axle components could possibly relax or relieve some or all of the beneficial stresses from the sub-critical quench. To test this theory, an axle with 11 years and an estimated 660,000 miles (1,056,000 km) of service was obtained for residual-stress measurements. A section of this axle was re-sub-critically quenched by the same procedure as when new. Residual stresses in this comparison section and the service axle were measured by both X-ray diffraction and ASTM E837 hole drilling methods. Axial compressive stresses found at the surface of the service axle were 15 to 18 percent lower than those on the surface of the re-heat treated section, and residual stresses levels tapered off with depth. After making an allowance for final machining of the service axle when it was manufactured, it is concluded that the 11 years of service and maintenance operations caused no significant reduction in axial residual compressive stresses.