CLADDING TECHNOLOGY SHANXI CO., LTDCladding of Cobalt-Based Hard Alloys
Introduction and Industrial Applications
Cobalt-based hard alloys, also known as cemented carbides or cermets, are among the hardest engineering materials available and are widely used for applications requiring extreme wear resistance and thermal stability. You Guangwei and Dong Anxia from Sinopec Nanjing Chemical Industry Co., Ltd. published a study in Pressure Vessels (2007) on the cladding of cobalt-based hard alloys, addressing the practical challenges of applying these materials to pressure vessel components and other industrial equipment.
Cobalt-based hard alloys typically consist of a matrix of cobalt or a cobalt-based alloy with dispersed hard particles of tungsten carbide (WC), chromium carbide (Cr3C2), or titanium carbide (TiC). These materials can achieve hardness values exceeding 1500 HV and retain their hardness at elevated temperatures, making them ideal for applications such as valve seats, pump impellers, extrusion dies, and chemical processing equipment.
Cladding Process Selection
The cladding of cobalt-based hard alloys can be achieved by several processes, each with its own advantages and limitations. The choice of process depends on the geometry of the component, the required overlay thickness, the base metal, and the production volume.
| Process | Deposit Rate | Overlay Thickness | Cost | Typical Application |
|---|---|---|---|---|
| Plasma transferred arc (PTA) | High | 1-5 mm | Medium-high | Large components, thick overlays |
| Laser cladding | Medium | 0.5-3 mm | High | Precision components, thin overlays |
| TIG cladding | Low-medium | 0.5-2 mm | Medium | Small components, repair |
| SAW cladding | High | 2-10 mm | Low-medium | Large flat surfaces |
| Electroslag welding (ESW) | Very high | 5-20 mm | Low | Thick overlays on large plates |
| Hot-wire TIG | Medium | 0.5-3 mm | Medium | Thin overlays, complex geometry |
For pressure vessel applications, the cladding process must be qualified in accordance with applicable codes such as ASME Section IX or NB/T 47014. The qualification must demonstrate that the process produces a sound, defect-free overlay with adequate bond strength and mechanical properties. The overlay composition must also be verified to ensure compliance with the specified material requirements.
Typical Cobalt-Based Alloy Compositions
| Alloy Type | WC (%) | Co (%) | Cr (%) | Hardness (HV) | Application |
|---|---|---|---|---|---|
| WC-Co (standard) | 60-80 | Balance | 0 | 1200-1500 | General wear parts |
| WC-Co-Cr | 50-70 | Balance | 5-15 | 1400-1800 | High-temperature wear |
| WC-TiC-Co | 50-70 | Balance | 0-5 | 1500-1800 | High-temperature wear |
| Cr3C2-Co | 0 | 50-70 | 25-40 | 1000-1300 | Corrosive wear environments |
| TiC-Co | 0 | 50-70 | 0-5 | 1200-1500 | High-temperature wear |
Technical Challenges and Solutions
The cladding of cobalt-based hard alloys presents several technical challenges. The first challenge is the dilution of the base metal into the overlay. Cobalt-based alloys typically have a high melting point and a low wetting ability on carbon steel, which can lead to poor fusion and inadequate bond strength. To address this, a multi-pass approach is often used, with a transition layer of a lower-melting-point alloy deposited first to improve wetting and fusion, followed by the cobalt-based overlay.
The second challenge is the prevention of cracking in the overlay. Cobalt-based alloys are inherently brittle and have limited ductility, which makes them susceptible to cracking under restraint. The thermal expansion mismatch between the cobalt-based overlay and the carbon steel base metal can generate significant residual stresses that promote cracking. To mitigate this, the base metal should be preheated to 200-300°C, and the interpass temperature should be maintained above 150°C to reduce thermal gradients and residual stresses.
The third challenge is the control of overlay thickness and composition. The high hardness of cobalt-based alloys makes them difficult to machine, and the overlay thickness must be controlled to minimize machining effort. The overlay composition must also be uniform to ensure consistent mechanical properties across the overlay. This requires careful control of welding parameters and the use of compatible filler materials.
Defect Analysis and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking | Thermal stress, low ductility | Preheat, control interpass temperature |
| Poor fusion | Low wetting, high melting point | Transition layer, increase heat input |
| Porosity | Gas pickup, flux contamination | Clean base metal, dry flux |
| Excessive dilution | High heat input, slow travel speed | Multi-pass, lower heat input |
| Delamination | Poor bond strength, thermal mismatch | Increase bond pass, stress relief |
Inspection and Quality Assurance
The inspection of cobalt-based alloy cladding is essential to ensure the quality and reliability of the overlay. Visual inspection is performed after each pass to detect surface defects. Magnetic particle testing (MT) is used to detect surface and near-surface cracks, while ultrasonic testing (UT) can be used to detect subsurface defects and measure overlay thickness. Radiographic testing (RT) may be used to detect internal porosity and lack of fusion.
The bond strength of the overlay must be verified by a bond strength test, which can be performed by the shear test method specified in ASTM A265 or by the bend test method. The overlay thickness must be measured at multiple locations to ensure uniformity and compliance with the specified minimum thickness. The hardness of the overlay should be measured at several locations to verify that it meets the specified minimum hardness.
Study Insights and Practical Implications
The cladding of cobalt-based hard alloys is a specialized technology that requires careful process development and strict quality control. The study by You and Dong provides valuable practical insights into the challenges and solutions associated with cobalt-based alloy cladding in industrial settings. The key lessons from this study are that the base metal preparation, welding parameters, and post-weld treatment must all be carefully controlled to produce a sound and reliable overlay. The use of a transition layer is often essential to ensure adequate fusion and bond strength, and the preheat and interpass temperature must be carefully managed to prevent cracking. Engineers working with cobalt-based alloy cladding should also consider the long-term performance of the overlay under thermal cycling and mechanical loading, as the brittle nature of the overlay can lead to spalling or delamination under severe conditions. A comprehensive approach to process development, including weld procedure qualification, trial builds, and thorough inspection, is essential for ensuring the success of cobalt-based alloy cladding projects.