CLADDING TECHNOLOGY SHANXI CO., LTDCobalt-Based Hardfacing Overlay Welding for Pressure Vessel Components
Literature Overview
This technical article, published in Pressure Vessels in 2007 by You Guangwei and Dong Anxia from the Chemical Machinery Factory of Nanjing Chemical Industrial Group Company (Sinopec), addresses the application of cobalt-based hardfacing alloys in the overlay welding of pressure vessel components. The work provides practical insights into the welding technology, process parameters, and quality control of cobalt-based hardfacing for critical pressure equipment used in chemical processing.
Cobalt-Based Hardfacing Alloys: Classification and Properties
Cobalt-based hardfacing alloys are classified into three main groups based on their matrix composition and hard phase type:
| Group | Matrix | Hard Phase | Typical Alloy | Hardness (HRC) | Key Properties |
|---|---|---|---|---|---|
| Co-Cr-C | Cobalt-chromium | Cr7C3, Cr23C6 | Stellite 6, Stellite 21 | 40–50 | High temperature wear resistance |
| Co-W-C | Cobalt-tungsten | Co3W, CoW | Stellite 26 | 35–45 | High temperature strength |
| Co-Mo-C | Cobalt-molybdenum | Mo2C, MoC | Stellite 25 | 45–55 | High temperature oxidation resistance |
The most widely used alloy in pressure vessel applications is Stellite 6 (Co-28Cr-6W-5Mo-1.5Fe), which offers an excellent combination of:
- High-temperature strength retention (up to 800 °C)
- Excellent resistance to abrasive and adhesive wear
- Good corrosion resistance in oxidizing and reducing environments
- Resistance to thermal fatigue and thermal shock
- Low thermal conductivity (reduces thermal stress at the interface)
Properties Relevant to Pressure Vessel Service
| Property | Stellite 6 Value | Significance |
|---|---|---|
| Density | 8.8 g/cm³ | Higher than steel; affects weight calculations |
| Thermal expansion | 13.5 × 10⁻⁶ /°C | Higher than carbon steel (12 × 10⁻⁶ /°C); creates thermal mismatch stress |
| Thermal conductivity | 11 W/(m·K) | Lower than steel (50 W/(m·K)); creates thermal barrier effect |
| Creep strength at 650 °C | ~150 MPa (10⁵ h) | Excellent for high-temperature service |
| Thermal fatigue life | >10,000 cycles (RT to 600 °C) | Suitable for cyclic heating/cooling service |
Welding Process Selection
For cobalt-based hardfacing overlay on pressure vessel components, the following processes are commonly used:
| Process | Application | Advantage | Limitation |
|---|---|---|---|
| SAW | Large flat surfaces, thick deposits | High deposition rate, deep penetration | High dilution, difficult for small areas |
| GTAW | Small areas, thin deposits, precision work | Low dilution, high precision | Low deposition rate, high cost |
| PTA | Large areas, thick deposits, high quality | Excellent control, low dilution | High equipment cost |
| Oxy-fuel | Field repair, small areas | Portable, simple equipment | High dilution, poor surface quality |
| SMAW | Field repair, emergency applications | Portable, simple | Inconsistent quality, high dilution |
For pressure vessel applications, GTAW and PTA are preferred due to their low dilution and high quality. SAW is used for large-area applications where deposition rate is critical. The selection depends on the specific component geometry, service conditions, and applicable code requirements.
Process Parameters for Cobalt-Based Hardfacing
GTAW parameters (Stellite 6 on carbon steel):
| Parameter | Value | Notes |
|---|---|---|
| Arc current | 120–180 A | Depends on electrode diameter |
| Arc voltage | 10–14 V | |
| Travel speed | 50–100 mm/min | Slower for thicker deposits |
| Shielding gas | Ar (99.99%) | 10–15 L/min |
| Preheat | 150–250 °C | Reduces cracking risk |
| Interpass temp | <200 °C | Prevents sensitization |
| Post-weld heat treatment | 850 °C × 2 h + air cool | Solution treatment for Stellite 6 |
SAW parameters (Stellite 6 on carbon steel):
| Parameter | Value | Notes |
|---|---|---|
| Arc current | 400–550 A | |
| Arc voltage | 30–38 V | |
| Travel speed | 150–300 mm/min | |
| Flux | Low-hydrogen basic flux | HJ431 or equivalent |
| Preheat | 200–300 °C | |
| Post-weld heat treatment | 850 °C × 2 h + air cool |
Quality Control for Pressure Vessel Hardfacing
Pressure vessel components are subject to strict code requirements (ASME VIII Div.1, GB/T 150, NB/T 47002). The following quality control measures are mandatory:
Non-Destructive Testing (NDT)
| NDT Method | Coverage | Purpose | Reference Standard |
|---|---|---|---|
| VT (Visual) | 100% | Surface defects, undercut, excessive reinforcement | ASME V, JB/T 4730 |
| PT (Liquid Penetrant) | 100% | Surface-breaking cracks, porosity | ASME V, JB/T 4730 |
| MT (Magnetic Particle) | 100% | Subsurface defects (if ferromagnetic) | ASME V, JB/T 4730 |
| UT (Ultrasonic) | 100% | Bond interface, lack of fusion | ASME V, JB/T 4730 |
| RT (Radiographic) | Spot check | Internal porosity, inclusions | ASME V, JB/T 4730 |
Destructive Testing (DT)
| Test | Requirement | Reference Standard |
|---|---|---|
| Hardness | Overlay: HRC 40–50; HAZ: <HRC 40 | ASTM A262 |
| Bond strength | Peel test: >200 MPa | ASTM A263 |
| Chemical analysis | Verify alloy composition | ASTM B625 |
| Intergranular corrosion | 100 h in 65% HNO3, no IGC | ASTM A262 Practice E |
| Tensile strength | Overlay: >600 MPa | ASTM A370 |
Engineering Challenges and Solutions
The welding of cobalt-based alloys presents several unique challenges:
1. Cracking susceptibility: Cobalt-based alloys have limited solid solubility for carbon, leading to carbide precipitation at grain boundaries during solidification. This promotes intergranular cracking. Solutions include:
- Adding carbon to the weld metal (0.5–1.0%) to promote grain boundary carbide formation in a controlled manner
- Using a two-layer approach: a transition layer of lower carbon content, followed by the hardfacing layer
- Controlling the cooling rate through preheating and interpass temperature control
2. Thermal mismatch stress: The coefficient of thermal expansion of cobalt-based alloys is higher than that of carbon steel. This creates tensile residual stress at the bond interface during cooling, which can lead to cracking. Solutions include:
- Preheating the base metal to 200–300 °C to reduce the thermal gradient
- Using a multi-pass technique with alternating heat input directions
- Post-weld stress relief at 650 °C for 2 h per 25 mm thickness
3. Dilution and composition control: High dilution compromises the wear resistance and corrosion resistance of the overlay. Solutions include:
- Using consumable inserts or backing rings to reduce base metal melting
- Applying a multi-pass technique with the first pass at lower current
- Using a wire with higher alloy content to compensate for dilution
4. Surface quality: The as-welded surface of cobalt-based hardfacing is rough and may contain slag inclusions. Solutions include:
- Post-weld machining to achieve the required surface finish
- Using a finishing pass with GTAW for a smooth surface
- Applying a grinding and polishing operation after welding
Practical Application Cases
In the chemical industry, cobalt-based hardfacing is commonly applied to:
- Reactor internals: Agitator shafts, impellers, and baffles in hydrogenation reactors
- Heat exchanger tubes: Tube sheets and channel covers in high-temperature service
- Valve components: Valve seats, plugs, and stems in corrosive and abrasive service
- Pump components: Impellers, wear rings, and shaft sleeves in slurry service
- Furnace components: Burner tips, fuel nozzles, and radiant tubes in high-temperature oxidation environments
A typical case study involves the hardfacing of a hydrogenation reactor agitator shaft. The shaft is made of 12Cr1MoV steel, with a Stellite 6 overlay applied to the shaft surface in contact with the catalyst bed. The overlay thickness is 3 mm, applied by SAW with a multi-pass technique. The welding procedure includes:
- Surface preparation: Grinding to remove rust and scale, cleaning with acetone
- Preheating: 250 °C using induction heating
- First pass: GTAW at 150 A, 80 mm/min, using Stellite 6 wire
- Second pass: SAW at 450 A, 250 mm/min, using Stellite 6 wire and HJ431 flux
- Third pass: SAW at 450 A, 250 mm/min, same parameters
- Post-weld heat treatment: 850 °C × 2 h + air cool
- Machining: Turn to final dimensions, Ra ≤ 3.2 μm
- NDT: VT, PT, MT, UT per code requirements
The resulting overlay achieves a hardness of HRC 45–50, with a bond strength exceeding 250 MPa. The component passes all NDT and DT requirements and is certified for service at 400 °C and 15 MPa.
Study Insights and Recommendations
The research highlights the importance of process control and quality assurance in cobalt-based hardfacing for pressure vessel applications. The key takeaways for engineering practice are:
- Always qualify the welding procedure per the applicable code (ASME IX, NB/T 47014)
- Control dilution through process selection and parameter optimization
- Perform comprehensive NDT and DT to ensure code compliance
- Implement a post-weld heat treatment to relieve residual stress and optimize the microstructure
- Maintain detailed welding records for traceability and quality assurance
The economic justification for cobalt-based hardfacing is compelling: the cost of the overlay material is significantly lower than replacing the entire component with a cobalt-based alloy, and the service life extension is substantial. For critical pressure vessel components, the investment in high-quality hardfacing is justified by the reduced risk of catastrophic failure.