CLADDING TECHNOLOGY SHANXI CO., LTDInterface Diffusion Analysis of WC-Co Cemented Carbide TIG Welding Joints
Literature Overview
The research by Xu Peiquan, Yang Dexin, Zhao Xiujuan, Lu Fenggui, and Yao Shun, published in 2005 in the Journal of Shanghai Jiao Tong University, presents a detailed analysis of interface diffusion behavior in tungsten carbide-cobalt (WC-Co) cemented carbide joints produced by gas tungsten arc welding (GTAW/TIG). This study was conducted at Shanghai Jiao Tong University and Dalian Railway Institute, addressing a critical challenge in hardfacing and overlay welding applications.
WC-Co cemented carbides are extensively used in mining, drilling, cutting tools, and wear-resistant components due to their exceptional hardness (HRA 90-95) and compressive strength (3000-5000 MPa). However, welding cemented carbides to steel substrates presents unique challenges due to the vast differences in thermal conductivity, coefficient of thermal expansion, and melting behavior between the two materials.
Core Technical Analysis
The study investigated the microstructural evolution and elemental diffusion at the interface between WC-Co cemented carbide and various steel substrates (low carbon steel, medium carbon steel, and stainless steel) after TIG welding. The welding parameters were optimized to minimize thermal damage to the carbide while achieving adequate bond strength.
The interface region after welding exhibits a complex microstructure consisting of multiple diffusion zones. The study identified three distinct zones:
| Zone | Location | Composition | Microstructure | Hardness (HV) |
|---|---|---|---|---|
| Zone 1 | Carbide side | WC + Co + Fe diffusion | Partially dissolved WC grains in Co-Fe matrix | 1200-1500 |
| Zone 2 | Interface | Fe-W-C-Co compound | Mixture of Fe₃W₃C, Fe₅W₂C, and Fe₇W₆C | 800-1200 |
| Zone 3 | Steel side | Fe-C + Co diffusion | Martensite + retained austenite | 300-500 |
The diffusion analysis revealed that cobalt (Co) acts as a diffusion medium, facilitating the dissolution of tungsten carbide (WC) and the formation of iron-tungsten carbide compounds at the interface. The diffusion depth was found to be 10-30 μm on the carbide side and 20-50 μm on the steel side, depending on welding heat input.
The elemental diffusion profiles, measured using energy-dispersive X-ray spectroscopy (EDS), showed characteristic concentration gradients:
- Tungsten (W): High concentration at the carbide side, gradually decreasing toward the steel side, with a diffusion depth of 15-25 μm.
- Cobalt (Co): Relatively uniform distribution across the interface due to its liquid phase behavior during welding.
- Iron (Fe): Diffuses into the carbide matrix, forming iron-tungsten carbide compounds.
- Carbon (C): Redistributes during the thermal cycle, with some loss from the carbide side due to carburization of the steel side.
Welding Process Parameters and Their Effects
The study systematically varied welding parameters to optimize the interface quality and minimize thermal damage to the cemented carbide. Key findings included:
| Parameter | Low Value | Optimal Value | High Value | Effect on Interface |
|---|---|---|---|---|
| Welding current | 30 A | 50-70 A | 100 A | Higher current increases diffusion depth |
| Travel speed | 50 mm/min | 100-150 mm/min | 300 mm/min | Faster speed reduces thermal exposure |
| Heat input | 0.3 kJ/mm | 0.8-1.5 kJ/mm | 3.0 kJ/mm | Higher heat input deepens diffusion zone |
| Interpass temperature | 25°C | 50-100°C | 200°C | Higher temperature accelerates diffusion |
| Shielding gas | Ar | Ar + 2% H₂ | Ar + 5% H₂ | Hydrogen addition reduces carbide dissolution |
The optimal welding strategy involved low heat input (0.8-1.5 kJ/mm), moderate travel speed (100-150 mm/min), and controlled interpass temperature (50-100°C). This approach minimized carbide dissolution while maintaining adequate bond strength.
Mechanical Properties and Failure Analysis
The bond strength of the WC-Co to steel joints was evaluated using three-point bending tests and shear tests. The results showed that the interface diffusion zone is typically the weakest link in the joint, with failure occurring preferentially at or near the interface.
| Substrate Material | Bond Strength (MPa) | Failure Location | Hardness Profile |
|---|---|---|---|
| Low carbon steel | 150-250 | Interface (Zone 2) | Gradual transition |
| Medium carbon steel | 120-200 | Interface (Zone 2) | Sharper transition |
| Stainless steel (304) | 100-180 | Interface (Zone 2) | Moderate transition |
The lower bond strength of stainless steel joints is attributed to the higher chromium content, which promotes chromium carbide formation at the interface, reducing the ductility of the diffusion zone. The addition of small amounts of hydrogen to the shielding gas (2% H₂) was found to reduce carbide dissolution and improve bond strength by 10-20%.
Engineering Practice Implications
For hardfacing and overlay welding applications involving WC-Co cemented carbides, the findings have direct practical implications:
- Use low heat input welding processes (GTAW, plasma arc welding) to minimize thermal damage to the carbide.
- Consider using filler metals with controlled carbon and cobalt content to manage interface composition.
- For critical applications, consider post-weld heat treatment (stress relief at 400-500°C) to reduce residual stresses without significantly affecting interface properties.
- Implement microstructural examination as part of quality control to verify interface quality and diffusion zone characteristics.
The study also highlights the importance of substrate selection. Low carbon steel substrates provide the best bond strength and most gradual hardness transition, while stainless steel substrates, though offering better corrosion resistance, produce weaker interfaces.
Key Questions and Reflections
A critical question arising from this study is the long-term stability of the interface diffusion zone under service conditions. The diffusion zone, being a metastable mixture of iron-tungsten carbide compounds, may undergo further microstructural evolution during heat exposure in service, potentially affecting bond strength and wear resistance.
Another consideration is the effect of welding sequence on multi-pass hardfacing applications. The thermal history of subsequent passes affects the microstructure of previously deposited layers, potentially altering the interface diffusion characteristics. This suggests that welding sequence optimization is essential for multi-pass hardfacing operations.
The study also raises questions about the role of residual stresses in interface integrity. The mismatch in thermal expansion coefficients between WC-Co (6.0-6.5 × 10⁻⁶/°C) and steel (12-14 × 10⁻⁶/°C) generates significant residual stresses at the interface, which may contribute to cracking and delamination during service.
Study Insights and Reference Value
This research provides fundamental understanding of interface diffusion behavior in WC-Co cemented carbide welding, which is directly applicable to hardfacing, overlay welding, and bimetallic component fabrication. The systematic approach to parameter optimization, combined with detailed microstructural and elemental diffusion analysis, offers a comprehensive framework for developing reliable welding procedures for cemented carbide applications.
The broader implication is that welding of dissimilar materials with vastly different properties requires careful control of thermal input and interface composition. The findings are transferable to other challenging welding applications, including welding of ceramics to metals, superalloy to steel joints, and refractory metal to structural steel connections.
For engineers involved in hardfacing and overlay welding, the key takeaway is that interface quality is determined by the diffusion zone characteristics, which are governed by welding heat input, substrate composition, and filler metal selection. Quality control should include microstructural examination of the interface, not just mechanical testing of the bulk weld metal.