Zhuojin Pipe Fitting Co., LtdOverlay Welding of Cobalt-Based Hardfacing Alloys for Pressure Vessel Applications
Literature Overview
This technical paper, published in Pressure Vessel Technology (2007, Vol. 24, Issue 8, pp. 55-58) by You Guangwei and Dong Anxia from Nanjing Chemical Industry Company (Sinopec Group), documents the practical experience gained in overlay welding cobalt-based hardfacing alloys (Stellite 6) on pressure vessel components. The work was driven by the needs of the coal chemical industry, where pressure vessels are required to withstand combined loading conditions including corrosion, high pressure, fatigue stress, impact, and wear.
Core Technical Content
Cobalt-based hardfacing alloys, particularly Stellite 6 (Co-Cr-W), are among the most widely used materials for severe wear and corrosion applications in the chemical and petrochemical industries. The alloy contains approximately 59% cobalt, 27% chromium, 5% tungsten, 3% molybdenum, and 2% carbon, providing an exceptional combination of hardness, corrosion resistance, and high-temperature strength.
Why Cobalt-Based Alloys for Pressure Vessels
The coal chemical industry presents unique challenges for pressure vessel design:
- Corrosive media: Hydrogen sulfide, ammonia, and various organic acids require corrosion-resistant surfaces.
- High pressure and temperature: Operating conditions can reach 30-50 MPa at 350-450°C.
- Cyclic loading: Start-up and shutdown cycles induce thermal fatigue.
- Abrasive wear: Slurry flows and solid particle erosion are common in coal chemical processes.
Cobalt-based hardfacing alloys address these challenges by providing:
- Excellent resistance to hot corrosion and oxidation up to 1,100°C.
- High hardness (HRC 40-45 as-cast, HRC 50+ after cold work).
- Good resistance to thermal fatigue due to their high thermal conductivity and low thermal expansion coefficient.
- Superior wear resistance in both sliding and abrasive conditions.
Welding Process and Parameters
The authors employed SMAW (Shielded Metal Arc Welding) for the Stellite 6 overlay application. Key process parameters and considerations include:
| Parameter | Value/Range | Rationale |
|---|---|---|
| Electrode type | EStellite 6 (SMAW) | Cobalt-based hardfacing |
| Current | 80-120 A | Low current to minimize dilution |
| Travel speed | Moderate | Controlled heat input |
| Preheat | 150-250°C | Reduce thermal gradients |
| Interpass temperature | <250°C | Prevent excessive grain growth |
| Pass thickness | 2-3 mm | Multiple thin passes |
| Post-weld cooling | Controlled | Air or furnace cooling |
Dilution Control
Dilution is the most critical factor in cobalt-based hardfacing welding. The base metal (typically carbon steel or low-alloy steel) dilutes into the overlay, reducing the cobalt and chromium content and consequently the corrosion and wear resistance. Strategies for minimizing dilution include:
- Using low current settings to reduce heat input.
- Applying multiple thin passes rather than a single thick pass.
- Using a backing plate or pre-welded buffer layer to isolate the overlay from the base metal.
- Employing a drag welding technique with minimal arc travel on the base metal.
Typical Dilution Rates
| Pass Number | Approximate Dilution (%) | Hardness (HRC) |
|---|---|---|
| First pass | 30-50% | 35-42 |
| Second pass | 15-25% | 42-47 |
| Third pass | 5-15% | 45-50 |
| Final pass | <5% | 48-52 |
Microstructure and Properties
The as-welded microstructure of Stellite 6 overlay typically consists of:
- Matrix: M7C3 type chromium carbides dispersed in a solid solution matrix of cobalt, chromium, tungsten, and molybdenum.
- Carbide morphology: Primary carbides (larger, irregular) and secondary carbides (finer, dispersed) depending on cooling rate.
- Grain structure: Columnar grains growing from the fusion boundary, with equiaxed grains in the center of the overlay.
Mechanical Properties
| Property | As-Welded | After Solution Treatment |
|---|---|---|
| Hardness | HRC 40-45 | HRC 42-47 |
| Tensile strength | 620-700 MPa | 650-720 MPa |
| Elongation | 20-30% | 25-35% |
| Impact energy (25°C) | 30-50 J | 40-60 J |
Engineering Practice Experience
The authors documented several practical challenges encountered during the manufacturing of two pressure vessels with cobalt-based hardfacing:
Challenge 1: Cracking at the Fusion Boundary
- Cause: Thermal stress concentration due to coefficient of thermal expansion mismatch between the cobalt overlay and steel substrate.
- Solution: Controlled preheating (200°C), slow cooling, and post-weld stress relief treatment at 700-800°C.
Challenge 2: Uneven Overlay Thickness
- Cause: Operator technique variation and complex vessel geometry.
- Solution: Establishing standardized welding procedures with specific travel speeds, current settings, and pass sequences.
Challenge 3: Porosity in the Overlay
- Cause: Hydrogen pickup from moisture in the flux or surface contamination.
- Solution: Strict electrode drying procedures (250°C for 2 hours) and thorough surface cleaning before welding.
Key Questions and Reflections
The paper does not provide detailed information on the long-term service performance of the hardfacing overlays. For coal chemical pressure vessels operating at high temperatures and pressures, the stability of the carbide structure over extended periods is a critical concern. Prolonged exposure to temperatures above 700°C can cause carbide coarsening and matrix softening, reducing the wear resistance of the overlay.
Another important consideration is the compatibility of the cobalt-based overlay with the vessel's pressure-containing function. The overlay must be free of defects (porosity, lack of fusion, cracking) that could initiate pressure boundary failures. Non-destructive testing (NDT) requirements for hardfaced pressure vessels should be more stringent than for conventional welded joints, potentially requiring 100% ultrasonic testing or radiographic examination of the fusion boundary.
The economic aspect of cobalt-based hardfacing is also worth noting. Cobalt is a critical metal with significant price volatility, and the cost of Stellite 6 electrodes is substantially higher than iron-based or nickel-based alternatives. Engineers must carefully evaluate whether the superior performance of cobalt-based alloys justifies the additional material and fabrication costs for each specific application.
Study Insights and Implications
This work provides valuable practical experience for engineers and fabricators working with cobalt-based hardfacing alloys on pressure vessel components. The emphasis on process parameter control, dilution management, and defect prevention is directly applicable to similar hardfacing applications in the petrochemical and coal chemical industries. For specification engineers, the key takeaway is that cobalt-based hardfacing requires careful attention to welding procedure qualification, thorough NDT, and potentially post-weld heat treatment to ensure long-term service reliability in demanding pressure vessel applications.