Zhuojin Pipe Fitting Co., LtdMicrostructure and Sliding Wear Performance of High-Chromium Bimetallic Wear-Resistant Plate Overlay Layer
Literature Overview
Published in Materials in Mechanical Engineering, Volume 50, Issue 5, pages 79–85, this study by Liu Tianjun, Li Xintong, and Wang Qingliang from Hebei Iron and Steel Group Mining Co., Ltd. and China University of Mining and Technology investigates the sliding friction and wear behavior of high-chromium bimetallic wear-resistant plate overlay layers under different loading and environmental conditions. The research is supported by the Central Universities Basic Scientific Research Fund.
High-chromium cast irons and their welded overlay equivalents are widely used in applications subject to severe abrasive wear, including mining, cement, and material handling industries. The bimetallic construction — a tough base with a hard, wear-resistant overlay — represents an engineering solution that balances wear resistance with impact toughness. Understanding the wear mechanisms and performance limits of these overlay layers under different service conditions is essential for proper material selection and life prediction.
Core Technical Findings
The study examines a high-chromium bimetallic wear-resistant plate under two loading conditions (200 N and 600 N) and two environmental conditions (dry friction and quartz sand abrasion). The overlay layer microstructure is characterized, and the wear performance is compared with a conventional martensitic wear-resistant steel plate.
Microstructural Analysis
| Feature | Description | Engineering Implication |
|---|---|---|
| Primary carbide phase | Cr₇C₃ | High hardness (~1800-2000 HV), good wear resistance |
| Matrix phase | α-Fe (ferrite) | Provides toughness and ductility |
| Secondary carbide | Cr₂₃C₆ (minor) | Moderate hardness, may contribute to brittleness |
| Fusion zone hardness gradient | Increases from base to overlay | Gradual transition reduces thermal stress concentration |
The presence of Cr₇C₃ as the primary carbide phase is significant. Cr₇C₃ carbides are known for their high hardness and good thermal stability, making them effective against abrasive wear. Their formation is favored in high-chromium alloys with appropriate carbon content and cooling rates. The ferritic matrix provides the necessary toughness to prevent catastrophic fracture under impact or cyclic loading, which is critical for mining and material handling applications.
Wear Performance Comparison
| Condition | Overlay Layer | Martensitic Wear-Resistant Steel | Relative Performance |
|---|---|---|---|
| Friction coefficient (dry) | Comparable | Comparable | Similar tribological behavior |
| Mass loss (dry friction) | Lower | Higher | Overlay superior |
| Volumetric wear rate (dry) | Lower | Higher | Overlay superior |
| Mass loss (quartz sand) | Lower | Higher | Overlay superior |
| Volumetric wear rate (quartz sand) | Lower | Higher | Overlay superior |
The consistent superiority of the high-chromium overlay layer over the martensitic steel in both mass loss and volumetric wear rate metrics is significant. The friction coefficient being comparable suggests that the improved wear resistance is not achieved at the expense of tribological efficiency — the overlay layer does not generate excessive frictional heat, which could accelerate wear through thermal softening.
Interpretation of Technical Points
Wear Mechanism Analysis
The study identifies two primary wear mechanisms operating in different environments:
Dry friction environment:
- Dominant mechanism: micro-asperity cutting
- Secondary mechanism: fatigue spalling
- The hard Cr₇C₃ carbides resist ploughing by the counterface asperities, reducing the volume of material removed per unit sliding distance
- Fatigue spalling occurs due to cyclic plastic deformation at the contact interface, leading to the formation and propagation of microcracks that eventually cause material detachment
Quartz sand environment:
- Dominant mechanism: combined micro-asperity cutting and hard quartz sand micro-cutting
- Secondary mechanism: fatigue spalling
- The presence of hard quartz sand particles (Mohs hardness ~7) introduces a third-body abrasion mechanism
- The quartz particles act as independent cutting agents, ploughing through the overlay surface
- The high hardness of Cr₇C₃ carbides provides resistance to quartz particle cutting, but the ferritic matrix is more susceptible to abrasive attack
Hardness Gradient at the Fusion Zone
The observation that hardness increases progressively from the base metal through the fusion zone to the overlay layer is a critical finding. This gradient has several important implications:
- Stress distribution: The gradual transition in hardness means that thermal stresses induced during welding are distributed over a larger volume, reducing the peak stress at any single location and lowering the risk of cracking.
- Wear resistance transition: The hardness gradient ensures that the wear-resistant zone extends beyond the nominal overlay thickness, providing a margin of safety against localized wear breakthrough.
- Thermal compatibility: The gradient reflects a gradual change in thermal expansion coefficient, which reduces residual thermal stresses and improves the fatigue resistance of the joint.
Comparison with Martensitic Wear-Resistant Steel
The comparison with martensitic wear-resistant steel is instructive. Martensitic steels achieve their wear resistance primarily through high hardness (typically 50-60 HRC) derived from the hard, supersaturated martensitic structure. However, this high hardness comes at the expense of toughness, and martensitic steels are susceptible to cracking under impact loading or in the presence of residual tensile stresses.
The high-chromium overlay achieves comparable or superior wear resistance through a different mechanism — the presence of extremely hard Cr₇C₃ carbides dispersed in a tougher ferritic matrix. This architecture provides a more favorable balance between hardness and toughness, making it more suitable for applications subject to both abrasive and impact loading.
Process Analysis
The overlay welding process used to produce the high-chromium bimetallic plate involves several critical parameters:
| Parameter | Typical Range | Effect on Performance |
|---|---|---|
| Welding current | 200-400 A | Controls dilution and cooling rate |
| Travel speed | 100-300 mm/min | Affects heat input and microstructure |
| Shielding gas | Ar or Ar/CO₂ mix | Controls oxidation and nitrogen pickup |
| Wire composition | High Cr (20-30%), moderate C | Determines carbide type and volume fraction |
| Preheating | 100-300°C | Reduces cracking susceptibility |
| Interpass temperature | <250°C | Controls solidification rate and grain size |
The dilution ratio between the base metal and the overlay material is a critical factor in determining the final overlay composition and, consequently, its microstructure and properties. Excessive dilution reduces the chromium and carbon content of the overlay, leading to the formation of less hard carbide phases and reduced wear resistance. Conversely, too little dilution may result in excessive brittleness and cracking susceptibility.
Integration with Engineering Practice
Application Selection Guide
Based on the findings of this study, the following guidance can be offered for material selection:
| Application | Recommended Material | Rationale |
|---|---|---|
| Severe abrasive wear, low impact | High-Cr overlay | Maximum abrasion resistance |
| Moderate abrasion with impact loading | High-Cr overlay | Better toughness than martensitic steel |
| High impact, low abrasion | Martensitic steel or toughened alloy | Higher toughness |
| Slurry erosion | High-Cr overlay with appropriate Cr content | Resists both abrasion and corrosion |
| Dry abrasive wear | High-Cr overlay | Superior to martensitic steel |
Wear Life Prediction Considerations
For engineers responsible for maintenance planning, the following factors should be considered when predicting wear life:
- Load level: Higher loads accelerate wear through increased plastic deformation and fatigue spalling. The transition from mild to severe wear regimes may occur at specific load thresholds.
- Environment: The presence of abrasive particles (such as quartz sand) significantly accelerates wear compared to dry friction conditions. The hardness and angularity of the abrasive particles are critical factors.
- Temperature: Elevated temperatures can soften the matrix and reduce the effectiveness of the carbide reinforcement. Thermal cycling can also induce thermal fatigue cracking.
- Operating duration: Cumulative sliding distance is a more meaningful wear life parameter than calendar time, as wear is distance-dependent rather than time-dependent.
Key Questions and Reflections
The study raises several important considerations for further investigation:
- Effect of Cr₇C₃ vs. Cr₂₃C₆ ratio: The relative proportion of Cr₇C₃ to Cr₂₃C₆ carbides is determined by the carbon content and cooling rate. A systematic study of how this ratio affects wear resistance would provide valuable guidance for alloy optimization.
- Effect of overlay thickness: The study does not address the relationship between overlay thickness and wear life. In practice, the overlay must be thick enough to withstand the expected wear depth during the service interval, but thicker overlays increase cost and may have different residual stress profiles.
- Multi-body wear scenarios: The laboratory wear tests simulate two-body (dry) and three-body (quartz sand) wear. In actual service, the wear environment may be more complex, involving a combination of abrasive, adhesive, and corrosive mechanisms. The relative contribution of each mechanism to total wear life depends on the specific application.
- Effect of matrix microstructure: The ferritic matrix provides toughness but may be susceptible to abrasive wear. Investigating the effect of matrix microstructure (e.g., martensitic vs. ferritic vs. pearlitic) on wear resistance could lead to further performance improvements.
Study Insights and Implications
This study provides valuable quantitative data on the sliding wear performance of high-chromium bimetallic overlay layers under different loading and environmental conditions. The consistent superiority over martensitic wear-resistant steel in both mass loss and volumetric wear rate metrics supports the widespread use of high-chromium overlays in severe abrasive wear applications.
The identification of the dominant wear mechanisms — micro-asperity cutting in dry conditions and combined asperity and quartz particle cutting in sand-laden environments — provides a mechanistic basis for understanding and predicting wear behavior. This understanding is essential for developing wear life prediction models and for optimizing overlay composition and process parameters for specific applications.
The hardness gradient at the fusion zone is a particularly important finding for engineering practice. It demonstrates that the overlay/base metal interface is not a sharp discontinuity but a gradual transition zone, which has implications for residual stress distribution, fatigue resistance, and the effective wear-resistant thickness of the overlay. Engineers should consider this gradient when determining the minimum overlay thickness required for a given service life.