Integrity of Modern Head Hardened Rails - Defect Growth Characterization

To enhance their durability, modern steel rails undergo a head-hardening process that results in greatly improved wear resistance. The head-hardening process, which includes a thermal quench and subsequent roller straightening procedure, results in a rail that exhibits significant spatial material property and residual stress variations. These two variations significantly affect the rate at which fatigue cracks develop and grow in the rail, making it crucial to characterize the material properties and residual stress distribution of the head-hardened rail to properly evaluate fatigue life. The main objective of this combined experimental/computational study was to investigate the microstructural gradient and residual stress effects on the fatigue and fracture properties of modern, head-hardened rail steels and compare these results with legacy (circa 1970 – 80) rail steels.

To achieve this objective, a methodical and repeatable testing protocol was established to characterize and quantify the mechanical behavior of three different types of head-hardened rails to contrast with the legacy rail steels. The testing protocol involved a sequence of mechanical tests, including the characterization of hardness distribution, uniaxial tensile properties, fracture toughness, and fatigue crack growth behavior.

This study of head-hardened rail material properties, and the contrast with legacy rail steels, provides valuable insights into how improved material properties of rail steels can beneficially impact rail transportation, with fewer derailments and reduced rail inspection intervals.