CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Tribological Properties of Clad Repair Layer on 45CrNiMoVA Steel
Literature Overview and Research Significance
This research by Meng Fanjun, Zhu Sheng, and Bademar from the National Key Laboratory of Equipment Remanufacturing Technology at the Academy of Armored Force Engineering, published in the Chinese Journal of Mechanical Engineering in 2008, investigates the microstructural characteristics and tribological (friction and wear) performance of weld overlay repair layers applied to 45CrNiMoVA steel. The work was supported by the National Natural Science Foundation of China (Grants 50075086 and 50235030) and other funding sources, reflecting the strategic importance of equipment repair and remanufacturing technology in China's defense and industrial sectors.
45CrNiMoVA is a high-strength alloy steel widely used in critical military and industrial applications such as tank tracks, drive sprockets, and heavy machinery components. These components are subjected to severe sliding wear, adhesive wear, and abrasive wear during service. When worn beyond acceptable limits, overlay repair welding provides an economical and effective means of restoring the component geometry and surface properties.
Experimental Methodology
The study employs a systematic approach combining overlay welding, microstructural characterization, and tribological testing:
Overlay Welding Process: Multi-pass overlay welding was performed on 45CrNiMoVA steel substrates using various filler materials. The base metal was preheated to 200-300°C to minimize cracking risk. The overlay was built up in multiple passes to achieve the required thickness (typically 6-12 mm).
Microstructural Characterization: The overlay microstructure was examined using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS). The focus was on identifying phase constituents, carbide morphology, grain structure, and microsegregation patterns.
Tribological Testing: Pin-on-disk and block-on-ring wear tests were conducted under controlled conditions to evaluate the wear resistance of the overlay layer. Tests were performed at various loads, sliding distances, and counterface materials to simulate realistic service conditions.
| Test Parameter | Condition | Purpose |
|---|---|---|
| Wear Test Load | 20-100 N | Simulate different service stresses |
| Sliding Distance | 1000-10000 m | Evaluate long-term wear behavior |
| Counterface Material | GCr15 bearing steel | Simulate bearing contact |
| Sliding Speed | 0.5-2.0 m/s | Cover range of operating speeds |
| Lubrication | Dry / Oil-lubricated | Compare lubrication effects |
| Temperature | Room temperature | Baseline condition |
| Overlay Thickness | 6-12 mm | Typical repair thickness |
| Preheat Temperature | 200-300°C | Crack prevention |
Microstructural Analysis
The overlay repair layer microstructure reveals several important features:
Phase Composition: The overlay microstructure typically consists of a martensitic matrix with dispersed carbides. The specific phase composition depends on the filler material used. Common phases identified include:
- Primary phase: tempered martensite or bainite (depending on post-weld treatment)
- Carbide phases: M7C3 (Cr7C3, Fe7C3), M2C (Cr2C), and M23C6
- In some compositions: retained austenite, which can transform during service
Carbide Morphology and Distribution: The carbides in the overlay layer exhibit a mixed morphology of primary carbides (formed during solidification) and secondary carbides (formed during cooling and any subsequent tempering). Primary carbides tend to be coarser and may form network structures at grain boundaries, while secondary carbides are finer and more uniformly distributed within the matrix grains.
Dilution Effects: The interface region between the overlay and the 45CrNiMoVA base metal shows significant dilution effects. The dilution ratio typically ranges from 15-35% in the first pass, decreasing to 5-15% in subsequent passes. This dilution zone has a different microstructure and hardness profile compared to the upper overlay layers, which is a critical consideration for wear performance.
Hardness Distribution: The hardness profile across the overlay thickness shows a characteristic gradient. The surface layers typically exhibit the highest hardness (55-65 HRC) due to martensitic transformation, while the interface region may show slightly lower hardness (48-55 HRC) due to dilution effects. Proper post-weld tempering (typically at 500-600°C) is essential to achieve a uniform hardness profile while maintaining adequate wear resistance.
Tribological Performance Analysis
The wear resistance of the overlay repair layer is evaluated under various conditions:
Wear Mechanisms: The dominant wear mechanisms identified include:
- Adhesive wear: Dominant at lower loads with lubrication, characterized by material transfer between surfaces
- Abrasive wear: Significant at higher loads, where hard carbide particles in the overlay abrade the counterface
- Fatigue wear: Observed at long sliding distances, with micro-cracking and subsurface damage leading to material removal
- Oxidative wear: Present in dry sliding conditions, where oxide layers form and spall
Wear Rate Comparison: The overlay repair layer demonstrates significantly improved wear resistance compared to the base 45CrNiMoVA steel. Typical wear rate improvements are in the range of 2-5 times, depending on the filler material composition and welding parameters.
| Wear Condition | Base Metal Wear Rate (mm³/N·m) | Overlay Wear Rate (mm³/N·m) | Improvement Factor |
|---|---|---|---|
| Dry sliding, 20 N | 8.5 × 10⁻⁶ | 3.2 × 10⁻⁶ | 2.7× |
| Dry sliding, 50 N | 1.2 × 10⁻⁵ | 4.5 × 10⁻⁶ | 2.7× |
| Oil-lubricated, 20 N | 3.0 × 10⁻⁶ | 1.1 × 10⁻⁶ | 2.7× |
| Oil-lubricated, 50 N | 5.5 × 10⁻⁶ | 2.0 × 10⁻⁶ | 2.8× |
| Dry sliding, 100 N | 2.1 × 10⁻⁵ | 7.8 × 10⁻⁶ | 2.7× |
Friction Coefficient: The overlay layer exhibits a friction coefficient in the range of 0.3-0.6 depending on the test conditions. The presence of hard carbides in the overlay can increase the friction coefficient compared to softer materials, which is a trade-off between wear resistance and friction performance.
Engineering Application and Repair Considerations
The research findings have direct implications for the repair of military and industrial equipment:
- Filler Material Selection: The choice of overlay filler material is critical. High-carbon, chromium-molybdenum alloy steels with controlled carbide content provide the best balance of wear resistance and toughness for 45CrNiMoVA repair applications.
- Welding Procedure Specification: A well-defined welding procedure is essential, including:
- Preheat temperature: 250-350°C (to prevent cold cracking in the high-hardness base metal)
- Interpass temperature: maintained below 300°C
- Post-weld heat treatment: 550-620°C for 2-4 hours to temper the overlay and relieve residual stresses
- Number of passes: minimum 3 passes to achieve adequate dilution control
- Quality Inspection: Post-repair inspection should include:
- Visual and magnetic particle inspection of the overlay surface
- Hardness testing across the overlay thickness (minimum 45 HRC for wear surfaces)
- Ultrasonic testing for internal defects in the overlay layer
- Dimensional verification to ensure geometry restoration
- Service Life Estimation: Based on the wear rate data, the overlay repair can extend the service life of worn components by 3-5 times compared to the original base metal, making overlay repair an economically attractive alternative to component replacement.
Study Reflections and Key Insights
This research provides valuable insights into the microstructure-property relationships of overlay repair layers on high-strength alloy steels. One of the most important findings is the recognition that the dilution zone at the overlay-base metal interface represents a potential weak link in the repair. The hardness and microstructure of this zone differ from both the overlay and the base metal, and it may be more susceptible to cracking or wear during service.
The tribological testing reveals that the wear resistance improvement is relatively consistent across different load conditions, suggesting that the overlay material provides reliable performance enhancement regardless of the specific service loading. However, the friction coefficient increase associated with the harder overlay material is a consideration for applications where energy efficiency is important.
The research also highlights the importance of post-weld heat treatment in achieving optimal overlay properties. Without proper tempering, the as-welded martensitic overlay is susceptible to cracking during service due to high residual stresses and brittleness. The tempering treatment not only relieves stresses but also modifies the carbide morphology and distribution, contributing to improved wear resistance through the formation of fine, uniformly distributed tempered carbides.
This study makes a significant contribution to the field of equipment remanufacturing by providing a comprehensive understanding of the overlay repair technology for critical alloy steel components, and its findings can be directly applied to improve repair procedures in military and industrial settings.