Rolling Contact Wear Characteristics of Locally Overlay-Welded U75V Rails

Literature Overview and Research Background

This study by Rong Bin, Wang Yongqiang, Zhao Huoping, Liu Shaopeng, and Shen Mingxue from East China Jiaotong University, published in Materials in Mechanical Engineering (2023, Vol. 47, Issue 2, pp. 67–72), addresses a critical engineering challenge in railway maintenance: the performance evaluation of overlay-welded repair on U75V high-carbon manganese rail steel under rolling contact conditions. The research was supported by the National Natural Science Foundation of China (Grants 52061012, 51805170) and the Jiangxi Provincial Natural Science Foundation (Grants 20212ACB214003, 20224ACB204012). In railway operations, rail spalling, corrugation, and head check are prevalent failure modes that necessitate periodic repair. Overlay welding offers a cost-effective alternative to full rail replacement, but the long-term reliability of the repaired zone under actual rolling contact loading remains a subject of concern among railway engineers worldwide.

Core Technical Findings

The researchers conducted a two-stage continuous rolling contact wear test—dry followed by wet—on both unwelded and locally overlay-welded U75V rail specimens. The key quantitative findings are summarized below:

The transition from dry to wet wear conditions caused a sharp drop in the adhesion coefficient from approximately 0.60 to 0.25 for both specimen types, which is consistent with the lubricating effect of water reducing friction at the contact interface. During the dry stage, the overlay-welded specimen exhibited a marginally higher adhesion coefficient, likely attributable to the harder martensitic overlay layer generating greater frictional resistance. Conversely, during the wet stage, the overlay-welded specimen showed a slightly lower adhesion coefficient, suggesting that the hardened surface may have facilitated a more effective lubricant film under wet conditions.

Microstructural Analysis and Defect Evaluation

The most significant insight from this study lies in the differential behavior of the three distinct metallurgical zones under rolling contact wear:

1. Overlay layer: Composed primarily of martensite, this zone exhibited the highest hardness and the most favorable wear performance. The wear surface showed only minor damage with no significant cracks or plastic deformation observed. This indicates that the overlay welding process successfully deposited a wear-resistant layer that can withstand repeated rolling contact loading.
2. Heat-affected zone (HAZ): This zone, consisting of a mixture of ferrite, pearlite, and martensite, displayed intermediate hardness but the poorest wear performance. After wear testing, the HAZ exhibited the greatest surface roughness, significant plastic deformation, and a high density of large-angle propagating cracks. The heterogeneous microstructure in the HAZ creates stress concentration points that promote crack initiation and propagation under cyclic contact stress.
3. Unwelded zone: Composed predominantly of pearlite, this zone had the lowest hardness. The worn surface appeared relatively flat with moderate plastic deformation and a small number of cracks, indicating acceptable but not optimal wear resistance.

Engineering Practice Implications

From a practical standpoint, this study highlights a critical concern that railway maintenance engineers must address: while the overlay layer itself performs well under rolling contact conditions, the HAZ represents the weakest link in the repair system. The high density of large-angle cracks in the HAZ suggests that this zone may be the primary origin of subsequent fatigue failure, potentially leading to rail breakage under service loading. This finding has direct implications for welding procedure qualification and post-weld heat treatment requirements.

The following engineering recommendations can be derived:

• Post-weld heat treatment (PWHT) should be optimized to homogenize the HAZ microstructure and reduce hardness differential between the overlay layer and the base metal.
• The welding heat input must be carefully controlled to minimize the extent of the HAZ and reduce the proportion of brittle martensite in the transition zone.
• Residual stress measurement and relief procedures should be incorporated into the repair protocol to mitigate the risk of crack initiation at the weld-heat affected boundary.
• Non-destructive testing (NDT) protocols should specifically target the HAZ region using phased array ultrasonic testing (PAUT) or time-of-flight diffraction (TOFD) to detect subsurface cracks that may not be visible on the surface.

Key Reflections and Study Insights

This research provides valuable data for the qualification of overlay welding procedures in railway rail repair applications. However, several aspects warrant further investigation. The study does not report the specific welding parameters (heat input, interpass temperature, number of passes) used for the overlay welding, which limits the ability to directly translate the findings into production welding procedures. Additionally, the rolling contact test was conducted under laboratory conditions that may not fully replicate the complex loading spectrum experienced in actual railway service, including thermal cycling, chemical contamination, and variable contact pressure. Future work should incorporate thermal-mechanical fatigue testing and extended service-life validation to establish comprehensive repair criteria for overlay-welded rails.

The observation that the HAZ is the weakest zone reinforces the principle that welding repair quality is not determined solely by the weld metal properties but is critically dependent on the metallurgical compatibility and microstructural uniformity of the entire weld assembly. This principle is equally applicable to overlay welding repairs on other critical structural components such as pressure vessels, pipelines, and rotating machinery.