Effect of carbon content and hardness on rolling contact fatigue resistance in heavily loaded pearlitic rail steels

Since its inception, railway transport has remained a preferred mode of transport, especially for bulky goods.  Even though heavy freights are encouraged for efficient transportation, they increase the wheel load, which results in rolling contact fatigue. Typical rolling contact fatigue damage involves crack propagation, rail breakage and spalling, which make maintenance of tracks and railway lines difficult and costly. Recently, preventive care characterized by inhibition of the rolling contact fatigue damages has been proposed from different viewpoints. Most importantly, the development of high-strength rails with pearlitic structures has significantly improved the service life of rails. The lamina spacing (the space between the cementite and ferrite phases) affects the hardness of pearlitic structures and can potentially inhibit rolling contact fatigue damages caused by plastic deformation. Similarly, the effects of carbon content on the resistance to rolling contact fatigue damages have attracted significant research attention.

To this note, Dr. Masaharu Ueda from Nippon Steel Corporation in collaboration with Dr. Kenji Matsuda from Kyushu Institute of Technology investigated the effects of carbon content and initial hardness on the rolling contact fatigue characteristics of pearlitic steel used in heavy haul railways. Specifically, they used a two-disk machine model of heavy haul rail/wheel contact to compare and contrast the rolling contact fatigue damage of pearlitic steels having different initial hardness and carbon content values. Additionally, they investigated the relationship between the carbon content and the degree of spalling based on fracture mechanics approach.

The authors observed that even at almost the same initial hardness, an increase in the carbon content of pearlitic steels resulted in a subsequent decrease in the number of spalls and crack depth. Similarly, the rolling contact surface became harder with the increase in the carbon content level. This notably contributed to the suppression of the development of plastic flow. Furthermore, the correlation between the inclination angle and the development of plastic flow was revealed. Pearlitic steels with high carbon content produce large crack inclination angles and exhibit a high potential of suppressing the crack propagation than those with low carbon content. This was attributed to the increase of stress concentration at the tip of cracks due to water penetration and decrease in the inclination angle. As such, the rolling contact fatigue resistance of pearlitic steels was significantly improved.

In a nutshell, the research team evaluated the rolling contact fatigue resistance of rails using pearlitic steels with varying hardness and carbon content values. Based on the results, rolling contact fatigue resistance characteristics of pearlitic steels with high carbon content significantly improved compared to those with low carbon content due to efficient suppression of fatigue crack propagation at high carbon content. According to the authors, the study presents useful insights that will pave the way for more detailed investigation of rolling contact fatigue damages of rails and their possible prevention measures, thus ensuring sustainable railway transport.