Microstructure and Mechanical Properties of Continuous Welded 50N Rail

Korean Journal of Metals and Materials > Volume 0(0); > Article

Research Paper

Wookjin Choi1, Min Ji Song1, Nam-Hyoung Lim2, and Soo Yeol Lee1,*

Microstructure and Mechanical Properties of Continuous Welded 50N Rail

Published online: 30 November 1999.

ABSTRACT

Rails are subjected to repeated stresses due to wheel-rail contact during train service. Rails under stress conditions undergo microstructural changes, and these cause degradations of the structural integrity and lifetime of rails. In this study, three different rails (newly-manufactured rail, newly-manufactured headhardened rail, and worn (used) rail) were compared to examine the effects of heat treatment and repeated wheel-rail contact stress on the microstructure and mechanical behavior of continuous welded rail. The crystal structure, constituent phase distribution, tensile property, and hardness were investigated at various locations along a cross section of the rails. All three rails consisted of a mixture of BCC and FCC crystal structures as a majority phase with a very small amount of cementite (Fe3C) as a minor phase. Rietveld analysis revealed that the weighted fractions of the BCC crystal structure were approximately 74%, 64%, and 85% for the new rail, head-hardened rail, and worn rail, respectively. While the web and foot areas of the three rails showed no significant differences in mechanical properties, the railheads of the three rails revealed much higher yield strength, tensile strength, and hardness. The highest tensile strength and hardness were measured at the railhead in the head-hardened rail, and were attributed to the evolution of the bainite phase, generated by additional heat treatment. The higher mechanical strength of the railhead of the worn rail is thought to have resulted from a combination of work hardening and smaller lamellar spacing of the pearlite phase, induced by repeated wear processes during train operation.

Keywords: continuous welded rail, microstructure, mechanical property, X-ray diffraction, crystal structure