CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of High-Chromium Alloy Overlay Materials
Literature Overview
This study published in Hot Working Technology in 2011 by Cao Zhuoyuan, Li Chunquan, Jiang Jianxian, Xiang Chao, and Li Zhou from Sany Heavy Industry Pumping Research Institute investigates the microstructure and mechanical properties of high-chromium alloy overlay materials used in pump components. The research addresses the persistent challenge of balancing wear resistance with toughness in slurry pump applications where high-chromium white iron overlays are commonly employed.
High-chromium alloys (typically 20-30% Cr) are the material of choice for severe abrasive and corrosive wear environments due to their exceptional hardness and corrosion resistance. However, their inherent brittleness limits direct application, and overlay technology provides a viable solution by combining a tough substrate with a hard, wear-resistant surface layer.
Core Technical Content
The research systematically evaluates several high-chromium overlay compositions and their resulting microstructures:
- Base composition: Fe-28Cr-2.5C-1.5Mo-0.5V with variations in alloying additions
- Microstructure: Predominantly Cr7C3 carbides in a martensitic matrix, with secondary phases of M7C3 and retained austenite
- Hardness: 850-950 HV for as-welded condition, decreasing to 750-850 HV after tempering
- Wear resistance: 3-5 times that of conventional 45 steel in dry sliding tests
Microstructural Characterization
The overlay microstructure exhibits a dendritic pattern with primary Cr7C3 carbides forming at dendrite boundaries and secondary carbides within the interdendritic regions. The matrix consists primarily of martensite with varying amounts of retained austenite depending on cooling rate and composition.
| Overlay Composition | Hardness (HV) | Wear Rate (mg/N·m) | Impact Energy (J) | Corrosion Rate (mm/y) |
|---|---|---|---|---|
| Fe-28Cr-2.5C | 920 | 0.8 | 8 | 0.15 |
| Fe-28Cr-2.5C-1.5Mo | 940 | 0.6 | 10 | 0.10 |
| Fe-28Cr-2.5C-1.5Mo-0.5V | 950 | 0.5 | 12 | 0.08 |
| Fe-25Cr-2.0C-1.0Mo-1.0Nb | 880 | 0.7 | 15 | 0.12 |
Heat Treatment Effects
Post-weld heat treatment significantly influences the final properties:
- Tempering at 550°C: Reduces hardness by 10-15% but increases toughness by 40-60%
- Tempering at 650°C: Further hardness reduction (20-25%) with substantial toughness improvement
- Solution treatment: Dissolves secondary carbides, requiring subsequent aging for optimal properties
The optimal tempering temperature of 550°C provides the best balance between wear resistance and impact toughness for pump application conditions.
Process Parameters and Welding Considerations
High-chromium overlay welding presents unique challenges due to the high carbon and chromium content of the filler material:
- Welding method: Submerged arc welding or flux-cored arc welding preferred for thick deposits
- Current density: 15-20 A/mm² to ensure adequate penetration without excessive dilution
- Shielding: Argon or argon-helium mixtures for gas shielded processes
- Preheat: 100-150°C to minimize cracking susceptibility
- Dilution control: Critical to maintaining overlay composition; target ≤15% dilution
Common Defects and Countermeasures
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking | High carbon content, rapid cooling | MT/PT inspection | Preheat, low current, post-weld tempering |
| Porosity | Gas absorption from flux | RT/UT inspection | Flux drying, clean base metal |
| Excessive dilution | High heat input | Hardness mapping | Low travel speed, proper electrode angle |
| Undercut | Excessive travel speed | Visual inspection | Adjust parameters, proper technique |
Engineering Application in Pump Manufacturing
The practical application of these findings in slurry pump manufacturing demonstrates significant performance improvements. Pump impellers and casings with high-chromium overlay exhibit 3-5 times longer service life compared to uncoated counterparts in abrasive slurry applications. The overlay thickness of 3-5 mm provides adequate wear reserve while maintaining structural integrity.
A key engineering insight is the importance of dilution control. Excessive dilution (above 20%) significantly reduces overlay hardness and wear resistance, negating the benefits of the high-chromium composition. Welding procedure qualification should include dilution verification through chemical analysis of the overlay layer.
Study Insights and Implications
This research provides comprehensive guidance for selecting and applying high-chromium overlay materials in pump manufacturing. The systematic evaluation of composition-property relationships enables rational material selection based on specific service conditions. The recommendation of 550°C tempering as the optimal post-weld treatment temperature offers a practical processing parameter that balances competing property requirements.
The engineering significance extends beyond pump manufacturing to any application requiring hard, wear-resistant surfaces on tough substrates. The principles established here — controlled dilution, appropriate heat treatment, and composition optimization — form the foundation for successful high-chromium overlay implementation across industries.