Zhuojin Pipe Fitting Co., LtdCobalt-Chromium-Tungsten Hard Alloy Overlay Welding Process on Low Alloy Steel
Literature Overview
This 2022 paper from China Chemical Equipment, authored by Teng Fei of Dalian Jinzhou Heavy Machinery Group Co., Ltd., presents a systematic study of cobalt-chromium-tungsten (Co-Cr-W) hard alloy overlay welding on low alloy steel substrates. The research focuses on determining optimal preheat temperature, post-weld cooling temperature and time, and measuring overlay layer chemical composition and hardness to establish a reliable welding process specification for industrial applications. Co-Cr-W alloys are widely used in applications requiring extreme wear resistance, high-temperature strength, and corrosion resistance, such as pump impellers, valve trim, and mining equipment.
Material System and Welding Challenges
The Co-Cr-W overlay alloy is a cobalt-based hardfacing material that contains chromium and tungsten as primary alloying elements. The alloy forms a hard carbide network in a cobalt-chromium matrix, providing exceptional wear resistance at both ambient and elevated temperatures. However, welding cobalt-based alloys onto low alloy steel substrates presents several challenges:
- High thermal conductivity mismatch between the cobalt overlay and steel substrate
- Significant dilution of the overlay composition by the steel substrate
- Susceptibility to cracking due to the brittle carbide network
- Thermal stress development during cooling due to coefficient of thermal expansion differences
Overlay Alloy Composition
| Element | Weight Percent (%) | Role |
|---|---|---|
| Cobalt (Co) | 65-70 | Matrix element, high-temperature strength |
| Chromium (Cr) | 17-20 | Carbide former, corrosion resistance |
| Tungsten (W) | 8-12 | Carbide former, wear resistance |
| Iron (Fe) | Balance | Dilution element from substrate |
| Carbon (C) | 4-6 | Carbide formation |
Process Parameter Determination
Preheat Temperature Optimization
Preheat temperature was varied from 100 °C to 400 °C to determine the optimal value that minimizes cracking while preventing excessive substrate softening. The study found that a preheat temperature of 250-300 °C provided the best balance. Below 200 °C, the thermal gradient between the molten pool and the substrate was too steep, leading to cracking in the overlay layer. Above 350 °C, the low alloy steel substrate experienced excessive softening, reducing its load-bearing capacity.
| Preheat Temperature (°C) | Cracking Susceptibility | Substrate Softening | Overall Assessment |
|---|---|---|---|
| 100 | High | None | Poor - cracks in overlay |
| 200 | Moderate | Minimal | Acceptable but marginal |
| 250-300 | Low | Moderate | Optimal balance |
| 350-400 | Very Low | Significant | Substrate too soft |
Post-Weld Cooling Control
The cooling rate after welding significantly affects the microstructure and hardness of the overlay layer. Rapid cooling leads to a fine but brittle microstructure with high hardness but poor toughness. Excessively slow cooling allows coarse carbide formation, reducing wear resistance. The study determined that cooling to 300-350 °C before allowing air cooling provided the optimal balance.
| Cooling Method | Cooling Rate | Overlay Hardness (HRC) | Microstructure | Assessment |
|---|---|---|---|---|
| Free air cooling | Fast | 65-70 | Fine, brittle carbides | Too brittle |
| Controlled cooling to 350 °C | Moderate | 58-62 | Balanced carbide distribution | Optimal |
| Furnace cooling | Slow | 50-55 | Coarse carbides | Too soft |
Heat Input Management
The welding heat input was controlled to limit dilution while ensuring adequate fusion. The recommended heat input range was 1.0-1.8 kJ/mm, achieved through careful control of welding current, voltage, and travel speed. Lower heat input reduced dilution but risked incomplete fusion, while higher heat input increased dilution and reduced overlay hardness.
Results and Process Specification
The final recommended welding process specification is summarized as follows:
| Parameter | Recommended Value |
|---|---|
| Welding process | GTAW (Tungsten Inert Gas Welding) |
| Shielding gas | Argon (99.99%) |
| Wire diameter | 1.6 mm Co-Cr-W alloy |
| Welding current | 100-150 A |
| Arc voltage | 12-16 V |
| Travel speed | 4-8 cm/min |
| Preheat temperature | 250-300 °C |
| Interpass temperature | 250-300 °C |
| Post-weld cooling | Cool to 300-350 °C, then air cool |
| Overlay hardness | 58-62 HRC |
| Number of passes | 2-3 depending on required thickness |
Study Insights and Reflections
This study provides a practical framework for establishing reliable Co-Cr-W overlay welding processes on low alloy steel substrates. The key finding is that preheat temperature and post-weld cooling control are the most critical parameters for preventing cracking and achieving optimal overlay properties. Engineers should adopt the recommended process window as a starting point and refine parameters based on specific substrate thickness and geometry. The work also highlights the importance of balancing overlay hardness with toughness, as excessively hard overlays may be brittle and prone to spalling under impact loading. For industrial applications, this process specification provides a validated baseline that can be adapted to different component geometries while maintaining overlay integrity.