Zhuojin Pipe Fitting Co., LtdCladding of Cobalt-Based Hardfacing Alloy on Pressure Vessel Components
Literature Overview
The paper by You Guangwei and Dong Anxia, published in Pressure Vessel (Vol. 24, Issue 8, 2007, pp. 55-58), describes the practical experience gained in cladding cobalt-based hardfacing alloy (Stellite 6) onto pressure vessel components for coal chemical applications. The work was conducted at the Chemical Machinery Factory of Nanjing Chemical Industry Company, China Petrochemical Corporation, and addresses the manufacturing challenges associated with producing pressure vessels that require both corrosion resistance, pressure containment, and wear resistance.
Core Technical Content
The paper reports on the fabrication of two pressure vessels that required cobalt-based hardfacing cladding in addition to conventional pressure vessel requirements. The key challenges addressed include:
- Understanding the weldability of cobalt-based hardfacing alloys
- Developing appropriate welding procedures for Stellite 6 cladding
- Controlling welding parameters to minimize defects
- Ensuring the integrity of the pressure vessel during and after cladding
Stellite 6 Properties and Characteristics
Stellite 6 (also known as Cobaltite 6 or UNS R30006) is a cobalt-chromium-tungsten-molybdenum alloy with the following characteristics:
| Property | Value |
|---|---|
| Co content | Balance (~58%) |
| Cr content | 28-30% |
| W content | 10-12% |
| Mo content | 2.5-3.5% |
| C content | 1.2-1.6% |
| Hardness (as-cast) | 400-500 HV |
| Hardness (heat treated) | 450-550 HV |
| Maximum service temperature | ~1100°C |
| Corrosion resistance | Excellent in oxidizing environments |
| Wear resistance | Excellent at high temperatures |
Weldability Challenges of Cobalt-Based Alloys
Cobalt-based hardfacing alloys present several welding challenges:
- Cracking susceptibility: The high carbon content and presence of carbide-forming elements (Cr, W, Mo) increase the susceptibility to cracking during solidification and cooling.
- High melting temperature: The melting range of Stellite 6 is approximately 1300-1350°C, requiring high heat input.
- Poor thermal conductivity: Cobalt-based alloys have low thermal conductivity, leading to high temperature gradients and residual stresses.
- Carbide formation: Excessive cooling rates can lead to the formation of brittle intergranular carbides.
- Dilution sensitivity: The properties of the overlay are highly sensitive to dilution from the base metal.
Process Development and Welding Parameters
Welding Procedure for Stellite 6 Cladding
Based on the experience described in the paper, the following welding parameters are recommended for Stellite 6 cladding:
| Parameter | Recommended Range | Notes |
|---|---|---|
| Welding process | TIG or SMAW | TIG preferred for thin sections |
| Current (TIG) | 100-200 A | DCEN polarity |
| Current (SMAW) | 120-200 A | Depends on electrode size |
| Arc voltage (TIG) | 16-22 V | Maintains stable arc |
| Travel speed | 40-80 mm/min | Slow speed for good fusion |
| Shielding gas | Argon (99.99%) | High purity required |
| Preheat temperature | 200-400°C | Critical for crack prevention |
| Interpass temperature | 300-400°C | Maintain throughout welding |
| Post-weld heat treatment | 850-900°C, 1-2 h | Carbide precipitation; stress relief |
| Layer thickness | 2-4 mm per pass | Multi-pass for thick overlays |
Preheat and Interpass Temperature Control
Preheat is critical for successful Stellite 6 cladding. The recommended preheat temperature of 200-400°C serves several purposes:
- Reduces cooling rate: Slower cooling reduces the susceptibility to cracking and minimizes the formation of brittle carbides.
- Reduces thermal stress: Lower temperature gradients reduce residual stresses in the weld and base metal.
- Promotes carbide dissolution: Higher temperatures promote the dissolution of carbides in the heat-affected zone, improving ductility.
The interpass temperature should be maintained at 300-400°C throughout the welding sequence to ensure consistent thermal conditions for each pass.
Post-Weld Heat Treatment
Post-weld heat treatment is recommended for Stellite 6 cladding to:
- Relieve residual stresses: Reduce the risk of delayed cracking and distortion.
- Promote carbide precipitation: Form fine, evenly distributed carbides that improve wear resistance.
- Temper the base metal: Reduce hardness and improve ductility of the base metal in the heat-affected zone.
The recommended heat treatment is 850-900°C for 1-2 hours followed by air cooling. This temperature range is below the melting point of the cobalt-based alloy but high enough to promote carbide precipitation and stress relief.
Engineering Practice Implications
Application in Coal Chemical Industry
The coal chemical industry presents unique challenges for pressure vessel design:
- Abrasive media: Coal slurry and coal-derived products contain abrasive particles.
- Corrosive environments: Coal chemical processes involve aggressive chemicals (acids, alkalis, and organic compounds).
- High temperatures: Many coal chemical processes operate at elevated temperatures (200-400°C).
- Cyclic loading: Pressure vessels experience cyclic pressure and temperature loading.
The combination of Stellite 6 cladding with conventional pressure vessel materials (such as carbon steel or low-alloy steel) provides a cost-effective solution that meets all performance requirements.
Comparison of Hardfacing Materials for Pressure Vessels
| Material | Hardness (HV) | Wear Resistance | Corrosion Resistance | High-Temp Performance | Cost |
|---|---|---|---|---|---|
| Stellite 6 (Co-based) | 450-550 | Excellent | Excellent | Excellent | High |
| Cr-Cr3C2 cast iron | 400-500 | Good | Moderate | Moderate | Low |
| High-carbon martensitic | 450-550 | Good | Poor | Poor | Low |
| WC-Co alloy | 1200-1500 | Very high | Poor | Moderate | Very high |
| Ni-based alloy | 300-400 | Moderate | Excellent | Good | High |
Quality Control Considerations
For pressure vessel applications, the quality control of Stellite 6 cladding must meet the following requirements:
- Non-destructive testing:
- Penetrant testing (PT) for surface cracks
- Ultrasonic testing (UT) for subsurface defects
- Radiographic testing (RT) for volumetric defects
- Destructive testing (on coupon samples):
- Hardness testing (Vickers or Brinell)
- Metallographic examination
- Tensile testing (if required)
- Documentation:
- Welding procedure specification (WPS)
- Welding procedure qualification record (WPQR)
- Welder qualification records
- Material certification (base metal and overlay)
- NDT reports
- Regulatory compliance:
- ASME Section VIII, Division 1 or 2
- TSG 21 (Chinese pressure vessel regulations)
- Applicable material and welding standards
Connection to Pressure Vessel Standards
The cladding of pressure vessels must comply with relevant standards:
- ASME BPV Section VIII: Requires specific procedures for overlay welding on pressure vessels
- ASME Section IX: Qualification of welding procedures and welders
- ASTM B887: Standard specification for cobalt-chromium-tungsten alloy castings
- AWS A5.13: Specification for cobalt-chromium-based welding electrodes
- NB/T 47014: Chinese standard for welding procedure qualification
- TSG 21: Chinese regulations for stationary pressure vessels
Critical Analysis and Reflections
Strengths of the Research
The paper provides valuable practical experience in Stellite 6 cladding for pressure vessel applications, which is often not well-documented in the academic literature. The emphasis on process parameter control and the challenges encountered during production reflects real-world manufacturing conditions.
The application of Stellite 6 to coal chemical pressure vessels is particularly relevant given the growing importance of coal-to-chemicals and coal-to-liquids technologies in China. The wear and corrosion resistance of Stellite 6 is well-suited to the harsh service conditions in these applications.
Limitations and Open Questions
- Long-term performance data: The paper does not provide long-term service performance data for the cladded pressure vessels. Field experience over several years would be valuable for validating the design approach.
- Thermal cycling effects: The behavior of