Rolling Contact Wear Characteristics of Locally Overlay-Repaired U75V Steel Rails

Literature Overview

This paper by Rong Bin, Wang Yongqiang, Zhao Huoping, Liu Shaopeng, and Shen Mingxue (2023) published in Materials in Mechanical Engineering (Volume 47, Issue 2, pp. 67-72) investigates the rolling contact wear behavior of U75V steel rails under two-stage conditions—dry followed by wet—comparing specimens with and without local overlay repair. The study was funded by the National Natural Science Foundation of China (Grants 52061012, 51805170) and the Jiangxi Provincial Natural Science Foundation (Grants 20212ACB214003, 20224ACB204012). The research originates from the Key Laboratory of Ministry of Education for Vehicle and Equipment at East China Jiaotong University, and is directly relevant to railway infrastructure maintenance engineering.

Core Technical Findings

The experimental design employed a two-stage rolling contact wear test—first under dry conditions, then transitioning to wet conditions—on both as-received and locally overlay-repaired U75V rail specimens. The adhesive coefficient behavior reveals critical tribological transitions:

• During the dry wear stage, both specimen types exhibited adhesive coefficients around 0.60.
• Upon entering the wet wear stage, the adhesive coefficient dropped sharply to approximately 0.25.
• In the dry stage, the overlay-repaired rail showed a slightly higher adhesive coefficient than the unrepaired rail.
• In the wet stage, the relationship reversed—the overlay-repaired rail showed a lower adhesive coefficient.

The overlay layer itself was analyzed through metallographic examination and hardness testing, revealing three distinct zones with markedly different microstructural compositions and wear behaviors.

Microstructural Analysis and Wear Behavior by Zone

The overlay layer, composed predominantly of martensite, demonstrated the highest hardness and the least surface damage after rolling contact wear, with no observable cracks or plastic deformation. This is consistent with the well-established principle that martensitic microstructures provide superior resistance to abrasive and adhesive wear due to their high dislocation density and carbon content in the body-centered tetragonal lattice.

The HAZ presented the most problematic behavior—highest surface roughness, significant plastic deformation, and numerous large-angle propagating cracks. This is a critical finding for engineering practice, as the HAZ represents a transition zone where thermal cycling during overlay welding creates a mixed microstructure that is mechanically less coherent than either the overlay or the base material. The presence of large-angle cracks suggests that the HAZ is the critical weak link in the repaired rail section.

The non-overlay base area, composed primarily of pearlite, exhibited the lowest hardness but showed a flat wear surface with only moderate plastic deformation and few cracks, indicating that the base U75V material itself possesses reasonable wear tolerance under the test conditions.

Engineering Practice Integration

From a railway maintenance engineering perspective, this study carries several important implications:

1. Overlay repair as a viable field intervention: The overlay layer's superior wear resistance confirms that local overlay repair is a technically sound approach for extending rail service life, particularly for high-traffic routes where full rail replacement is impractical.
2. HAZ as the critical control zone: The finding that the HAZ exhibits the worst wear behavior—highest roughness, most cracking—demands that welding procedure specifications (WPS) for rail overlay repair focus heavily on HAZ control. Preheating strategies, interpass temperature management, and post-weld heat treatment (PWHT) must be optimized to minimize the extent and adverse metallurgical transformation of the HAZ.
3. Environmental condition sensitivity: The dramatic drop in adhesive coefficient from dry to wet conditions (from 0.60 to 0.25) highlights the importance of environmental factors in rail wear. In real-world railway operations, rain, snow, and track lubrication agents all play roles in modifying the tribological interface. Maintenance schedules should account for seasonal environmental variations.
4. Inspection priority: Given that the HAZ is the most crack-prone zone, ultrasonic testing (UT) inspection protocols for overlay-repaired rails should prioritize the HAZ region, particularly using phased array ultrasonic testing (PAUT) or time-of-flight diffraction (TOFD) methods capable of detecting angular crack initiation.

Key Questions and Reflections

The study raises an important question: why does the overlay layer perform better in the wet stage relative to the base material, while showing slightly higher adhesion in the dry stage? This suggests that the martensitic overlay surface may benefit more from the lubricating effect of water than the pearlitic base surface, possibly due to differences in surface roughness, chemical reactivity, or the formation of protective oxide films. This warrants further investigation through surface analysis techniques such as X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS).

Another reflection concerns the long-term durability of the overlay repair. While the study demonstrates excellent short-term wear resistance of the overlay layer, the HAZ cracking behavior raises concerns about fatigue failure under repeated rolling contact loading. In railway applications, rails are subjected to millions of load cycles, and crack initiation in the HAZ could propagate through the rail section, potentially leading to catastrophic failure. Future research should incorporate fatigue testing protocols to evaluate the long-term reliability of overlay-repaired rails.

Summary

This study provides valuable experimental evidence supporting the use of local overlay repair for U75V steel rails, demonstrating that the martensitic overlay layer offers superior wear resistance under both dry and wet rolling contact conditions. However, the HAZ emerges as the critical weak zone with the most severe cracking and plastic deformation, demanding careful welding process control and enhanced inspection protocols. The findings underscore the importance of holistic repair strategy design—not merely achieving a hard overlay surface, but ensuring metallurgical compatibility and mechanical integrity across the entire repair zone including the HAZ.