CLADDING TECHNOLOGY SHANXI CO., LTDStudy on Microstructural Characteristics of Wear-Resistant Overlay Layer and Its Relationship with Wear Resistance
Literature Overview
This foundational 2000 study by Pan Chunxu and Chen Li from Wuhan University of Technology (formerly Wuhan Jiaotong University) investigates the microstructural characteristics of wear-resistant overlay layers and their correlation with wear resistance. Supported by the Wuhan Youth Science and Technology Program and the Ministry of Transport's Cross-Century Academic Leader Fund, the work was published in the journal Ordnance Materials and Science and Engineering. As an early systematic study in this area, it provides essential baseline data and conceptual frameworks that continue to inform modern overlay design practice.
Research Significance
The relationship between microstructure and wear resistance in overlay deposits is fundamental to the field of weld overlay engineering. While numerous studies have examined individual aspects — hardness, carbide morphology, phase composition — this work attempts to integrate these factors into a coherent framework for predicting and optimizing wear performance. The systematic approach taken here established methodological precedents that later researchers have built upon.
Methodology
Overlay Systems Studied
| Overlay Type | Matrix | Reinforcement Phase | Process |
|---|---|---|---|
| Type A | High-carbon steel | Cementite (Fe3C) | SAW |
| Type B | High-chromium steel | M7C3 carbides | SAW |
| Type C | Austenitic stainless steel | M7C3, Cr7C3 | GMAW |
| Type D | Cast iron type | M2C, M7C3 | SMAW |
Characterization and Testing
- Optical microscopy for microstructural identification
- SEM for detailed carbide morphology analysis
- XRD for quantitative phase analysis
- Vickers hardness testing with gradient mapping
- Pin-on-disk wear testing against SiC and Al2O3 counterfaces
- Sliding wear testing under varying loads and sliding distances
Core Technical Findings
Microstructural Classification
The study classified overlay microstructures into four principal types based on matrix-carbide morphology:
- Pearlite-ferrite matrix with dispersed carbides (Type A): Fine pearlite with dispersed cementite. Hardness 500–550 HV. Moderate wear resistance.
- Martensite matrix with M7C3 carbides (Type B): Fine martensite with blocky M7C3 carbides. Hardness 600–700 HV. High wear resistance.
- Austenite matrix with M7C3/Cr7C3 carbides (Type C): Retained austenite with fine carbide dispersion. Hardness 400–500 HV. Good wear resistance with excellent toughness.
- Ledeburite-type structure with M2C/M7C3 (Type D): Complex eutectic with coarse carbides. Hardness 700–850 HV. High wear resistance but low toughness.
Hardness-Wear Resistance Correlation
| Overlay Type | Average Hardness (HV) | Wear Rate (mg/N·m) | Wear Mechanism |
|---|---|---|---|
| Type A | 520 | 0.45 | Abrasive + adhesive |
| Type B | 650 | 0.22 | Abrasive |
| Type C | 460 | 0.30 | Abrasive + mild adhesive |
| Type D | 780 | 0.15 | Abrasive (with spalling risk) |
Key Microstructural Factors Influencing Wear Resistance
The study identified five critical microstructural parameters:
- Carbide volume fraction: Higher volume fraction generally improves wear resistance, but excessive fractions (>40%) lead to brittleness.
- Carbide size: Fine carbides (5–20 μm) provide superior wear resistance compared to coarse carbides (>50 μm) due to reduced crack initiation sites.
- Carbide distribution: Uniform distribution is critical; clustered carbides create weak zones.
- Matrix hardness: Harder matrices provide better support for carbides and improve overall wear resistance.
- Carbide morphology: Rounded or blocky carbides are preferred over elongated or needle-like morphologies.
Process-Microstructure-Property Relationship
Heat Input Effects
| Heat Input (kJ/mm) | Carbide Size | Matrix Microstructure | Hardness (HV) | Wear Rate |
|---|---|---|---|---|
| Low (< 5) | Fine (5–15 μm) | Fine martensite | 700–750 | Low |
| Medium (5–10) | Medium (15–30 μm) | Coarse martensite + bainite | 600–680 | Moderate |
| High (> 10) | Coarse (> 30 μm) | Bainite + pearlite | 500–580 | High |
The study clearly demonstrated that heat input is the dominant process variable controlling overlay microstructure and, consequently, wear performance. Low heat input processes (e.g., laser cladding, PTA) produce finer microstructures and superior wear resistance.
Cooling Rate Effects
Cooling rate directly influences the matrix transformation products:
- Rapid cooling (> 50 °C/s): Martensitic transformation, fine carbides, high hardness
- Moderate cooling (10–50 °C/s): Bainitic transformation, medium carbides, moderate hardness
- Slow cooling (< 10 °C/s): Pearlitic transformation, coarse carbides, lower hardness
Engineering Practice Applications
Process Selection Based on Application Requirements
| Application | Required Properties | Recommended Overlay Type | Process |
|---|---|---|---|
| Slurry pump impellers | High wear + corrosion resistance | Type C (austenitic) | GMAW |
| Mining shovel buckets | High wear resistance | Type B (martensitic) | FCAW |
| Cement mill rollers | High wear + moderate toughness | Type B or D | SAW |
| Paper mill rollers | Moderate wear + low friction | Type C | GMAW |
| Valve seats | High wear + corrosion | Type C | PTA |
Quality Control Checklist
Based on the study findings, the following quality control measures are recommended:
- Hardness verification: Map hardness across overlay thickness; ensure uniformity within ±50 HV.
- Carbide size control: Inspect microstructure to verify carbide size is within specification (typically < 30 μm for most applications).
- Carbide distribution: Check for carbide clustering; reject if large clusters (> 100 μm) are observed.
- Bond strength: Verify metallurgical bond through sectioning and metallographic examination.
- Crack inspection: Conduct MT or PT on overlay surface to detect microcracks.
Key Questions and Reflections
The study raises the fundamental question of how to quantify the relationship between microstructure and wear resistance in a predictive manner. While the empirical correlations presented are useful, a more rigorous approach — such as quantitative metallography combined with statistical analysis — could enable more precise property prediction from microstructural parameters.
Another reflection concerns the role of residual stress. The study did not extensively address residual stresses, which are known to influence wear performance through their effect on crack initiation and propagation. Compressive residual stresses can improve wear resistance by inhibiting surface crack formation, while tensile stresses may accelerate wear and spalling.
The study also highlights the importance of counterface material in wear testing. The wear rate measured against a specific counterface may not directly translate to field performance, where the counterface material, surface roughness, and lubrication conditions differ significantly. Engineers must exercise caution when extrapolating laboratory wear data to field conditions.
Study Insights and Implications
This study by Pan and Chen provides a comprehensive framework for understanding the microstructure-wear resistance relationship in overlay deposits. The classification of overlay microstructures into four types and the identification of five critical microstructural parameters offer practical guidance for overlay design and evaluation. The process-microstructure-property relationship established here remains relevant and is widely cited in subsequent research.
The work also emphasizes the importance of process control — particularly heat input and cooling rate — as the primary levers for microstructural optimization. For engineers selecting overlay processes, this reinforces the principle that process selection is not merely a matter of deposition rate or cost; it fundamentally determines the metallurgical outcome and, consequently, the service performance of the overlay.
Conclusion
The study by Pan and Chen established a foundational understanding of the microstructural factors governing wear resistance in overlay deposits. The identification of carbide size, distribution, volume fraction, morphology, and matrix hardness as the five critical parameters provides a practical framework for overlay design and quality control. The process-microstructure-property relationships documented here remain essential reference points for engineers specifying and evaluating wear-resistant overlay systems across a wide range of industrial applications.