Rolling Contact Wear Characteristics of U75V Rail Steel Repaired by Local Cladding

Literature Overview

This 2023 study by Rong Bin, Wang Yongqiang, Zhao Huoping, Liu Shaopeng, and Shen Mingxue from East China Jiaotong University investigates the rolling contact wear behavior of U75V high-carbon high-chromium rail steel after local repair by weld overlay cladding. Funded by the National Natural Science Foundation of China (Grants 52061012, 51805170) and Jiangxi Provincial Natural Science Foundation (Grants 20212ACB214003, 20224ACB204012), the research addresses a critical infrastructure maintenance challenge in railway engineering.

Core Technical Content

U75V is a high-carbon high-chromium bearing rail steel widely used in Chinese railway networks, characterized by a carbon content of approximately 0.75% and chromium content of 0.5–0.7%. The material exhibits a martensitic microstructure with dispersed carbides, providing excellent resistance to rolling contact fatigue and wear under normal service conditions. However, when surface damage occurs—such as squats, head checks, or localized wear—the traditional approach of grinding or rail replacement is often impractical for heavily loaded routes.

Weld Overlay Repair Strategy

The local cladding repair approach involves depositing a compatible alloy layer over damaged areas of the rail head. The key technical challenges include:

Thermal matching: The cladding layer must have thermal expansion and thermal conductivity properties compatible with U75V to avoid excessive residual stresses during welding and subsequent thermal cycling in service.
Microstructural compatibility: The deposited layer should develop a microstructure similar to or harder than the base U75V material to ensure uniform wear behavior across the repaired and unrepaired zones.
Bond strength: The metallurgical bond between the cladding layer and the base rail must withstand cyclic rolling contact loads of 200–300 MPa.

Rolling Contact Wear Test Results

Test Parameter	Value
Contact stress	200–300 MPa
Sliding distance	10–50 km equivalent
Temperature	Ambient to 80 °C
Wear rate (cladding layer)	0.5–1.2 × 10⁻⁶ mm³/N·m
Wear rate (base U75V)	0.3–0.8 × 10⁻⁶ mm³/N·m
Bond strength	>30 MPa

The study reveals that the cladding layer exhibits slightly higher wear rates than the as-rolled U75V base material, which is expected given the differences in microstructure between the cast/welded deposit and the rolled rail steel. The rolled U75V possesses a refined, elongated microstructure with aligned carbides that provide superior rolling contact wear resistance compared to the equiaxed microstructure of the weld deposit.

Defect Analysis and Countermeasures

Common Defects in Cladding Repair of Rails

Defect Type	Cause	Countermeasure
Cracking at weld toe	Excessive residual stress, poor heat input control	Reduce heat input, apply pulse welding, post-weld stress relief
Insufficient bond strength	Contamination, inadequate penetration	Surface preparation, increased preheat, multi-pass welding
Hardness mismatch	Excessive dilution, improper filler selection	Multi-layer approach, progressive alloy increase, hardness profiling
Subsurface fatigue cracking	Microstructural discontinuity at interface	Control cooling rate, optimize post-weld treatment

Process Parameters for Rail Cladding Repair

The recommended welding process for U75V rail repair typically employs either submerged arc welding (SAW) for thick deposits or gas metal arc welding (GMAW) with flux-cored wire for precision repair. Key parameters include:

Preheat: 200–300 °C to reduce cooling rate and minimize cracking susceptibility
Heat input: 15–25 kJ/mm to balance dilution control with adequate fusion
Interpass temperature: 250–350 °C to prevent excessive thermal cycling
Post-weld treatment: Tempering at 550–650 °C to relieve residual stresses and optimize microstructure

Engineering Practice Integration

The practical implementation of cladding repair for railway rails must comply with relevant standards including TB/T 1632 (rail welding), TB/T 2344 (rail head profile), and EN 13676 for rail material specifications. The repair procedure must be qualified per NB/T 47014 or equivalent railway-specific qualification requirements.

A critical consideration is the post-repair rail profiling. After cladding, the rail head must be ground to the standard profile per TB/T 2344, which removes a portion of the deposited layer. The cladding thickness must therefore be designed to allow for adequate material removal while maintaining a minimum functional thickness of 3–5 mm.

Study Insights and Implications

This research represents a significant advancement in railway maintenance technology, offering an alternative to traditional grinding-and-replace strategies. The findings suggest that while cladding-repaired rails cannot fully replicate the rolling contact wear performance of new rails, they provide acceptable service life extensions that can defer expensive rail replacement cycles.

The economic analysis favors cladding repair for heavily loaded routes where rail replacement frequency is high. However, the technology requires rigorous quality control, including non-destructive testing of the repair zone using magnetic particle testing (MT) and ultrasonic testing (UT) per JB/T 4730, to ensure no subsurface defects compromise the integrity of the repaired section.