Zhuojin Pipe Fitting Co., LtdEffect of Heat Treatment on D707 Surfacing Layer Microstructure and Properties
Literature Overview
This study by Peng Jixiang, Wang Shunxing, and Liu Yong from Luoyang Institute of Technology, published in Materials Development and Application in 2002, investigates the effect of heat treatment on the microstructure and properties of D707 carbide-based hard alloy surfacing layers. The research was supported by the Henan Provincial Department of Education Key Project and provides valuable insights into optimizing the performance of carbide-based surfacing layers through post-weld heat treatment.
Background on D707 Surfacing Material
D707 is a tungsten carbide-based hard alloy surfacing material designed for applications requiring extreme wear resistance. The material contains:
- Tungsten carbide (WC) particles as primary hard phases
- Iron-nickel-cobalt matrix providing toughness and bonding
- Additional alloying elements for enhanced properties
The surfacing layer is typically applied to low-alloy steel substrates such as 35CrMo, which may undergo carbon and nitrogen co-infiltration (C-N co-infiltration) treatment to improve compatibility and performance.
Heat Treatment Conditions Investigated
The study examines several heat treatment conditions for D707 surfacing layers:
| Heat Treatment Condition | Temperature | Cooling Medium | Tempering | Purpose |
|---|---|---|---|---|
| Air cooling | 850°C | Air | None | Baseline condition |
| Oil quench + temper | 850°C | Oil | 400°C | Enhanced properties |
| Substrate C-N co-infiltration | 850°C | Oil | 200°C | Substrate modification |
Microstructural Analysis
The heat treatment conditions significantly influence the microstructure of the D707 surfacing layer:
- Air-cooled condition: Produces a relatively coarse microstructure with larger carbide particles and potentially more retained austenite in the matrix.
- Oil-quenched and tempered condition: Produces a refined microstructure with better dispersion of carbide particles and reduced retained austenite, leading to improved hardness and wear resistance.
- Substrate C-N co-infiltration: Modifies the substrate surface composition, improving compatibility with the surfacing layer and reducing dilution effects.
Hardness and Wear Resistance Results
The study demonstrates that heat treatment significantly affects the mechanical properties of the surfacing layer:
| Condition | Hardness (Relative) | Wear Resistance (Relative) | Notes |
|---|---|---|---|
| Air-cooled D707 | Baseline | Baseline | Reference condition |
| Oil-quenched + 400°C temper | Significantly higher | Significantly higher | Optimal condition |
| Substrate C-N co-infiltration + 850°C oil quench + 200°C temper | 6-12x higher | 6-12x higher | Enhanced substrate |
The optimal condition of 850°C oil quench followed by 400°C tempering produces the highest hardness and wear resistance for the D707 surfacing layer. This condition also provides excellent corrosion-wear resistance, which is critical for applications in aggressive environments.
Substrate Modification Effects
The study also investigates the effect of carbon and nitrogen co-infiltration on the 35CrMo substrate. This treatment:
- Increases the carbon and nitrogen content at the substrate surface
- Improves compatibility with the D707 surfacing material
- Reduces dilution during surfacing
- Enhances the overall performance of the surfacing system
The combination of substrate C-N co-infiltration with appropriate heat treatment (850°C oil quench + 200°C temper) produces a system with 6-12 times the hardness and wear resistance of the untreated substrate.
Engineering Applications
The D707 surfacing material with optimized heat treatment is suitable for:
- Mining equipment: Excavator buckets, conveyor rollers, and crusher components.
- Cement industry: Mill liners, rollers, and grinding elements.
- Power generation: Coal handling equipment and boiler components.
- Steel industry: Rolling mill components and furnace fixtures.
- Steel pipe manufacturing: Wear-resistant components in pipe processing equipment.
For steel pipe applications, D707 surfacing could be applied to:
- Pipe mill rollers and guides
- Cutting and forming tools
- Wear-resistant components in pipe handling systems
- Components exposed to abrasive media in processing lines
Process Optimization Considerations
The study highlights several important considerations for optimizing D707 surfacing performance:
- Heat treatment temperature: 850°C appears to be optimal for achieving the desired microstructure transformation.
- Cooling rate: Oil quenching provides the appropriate cooling rate for maximizing hardness without excessive residual stresses.
- Tempering temperature: 400°C provides the optimal balance between hardness retention and toughness improvement.
- Substrate preparation: C-N co-infiltration improves compatibility but requires additional processing time and cost.
Key Questions and Reflections
One important consideration is the effect of heat treatment on residual stresses in the surfacing layer. While the study focuses on hardness and wear resistance, residual stresses can significantly affect the long-term performance of surfacing layers, particularly in applications subject to cyclic loading.
Another consideration is the scalability of the heat treatment process. For large components, achieving uniform heat treatment throughout the surfacing layer may be challenging, and this could lead to property variations across the surface.
The study also raises questions about the long-term stability of the optimized microstructure under actual service conditions. Thermal cycling and mechanical loading may cause microstructural changes that affect performance over time.
Study Insights and Practical Recommendations
This study demonstrates that post-weld heat treatment is a critical factor in optimizing the performance of D707 carbide-based surfacing layers. The optimal condition of 850°C oil quench followed by 400°C tempering produces significant improvements in hardness and wear resistance. Engineers should incorporate appropriate heat treatment into their surfacing specifications to maximize component life and performance.
The study also highlights the importance of substrate preparation in surfacing applications. The C-N co-infiltration treatment of the substrate provides substantial improvements in overall system performance, suggesting that surface preparation should be considered an integral part of the surfacing process rather than a separate step.
For steel pipe manufacturing and related industries, the D707 surfacing technology with optimized heat treatment offers a reliable solution for extending the life of wear-critical components. The significant improvement in wear resistance (6-12 times compared to untreated substrate) translates directly to reduced maintenance frequency and extended equipment availability, providing substantial economic benefits.