CLADDING TECHNOLOGY SHANXI CO., LTDGTAW Cladding Process Exploration of Stellite Cobalt-Based Alloy
Research Background and Industrial Applications
This study by Li Youyi, Luo Yang, Hong Jie, and Wang Lifeng from Dalian Deep Blue Pump Industry Co., Ltd. (published in 2013) investigates the gas tungsten arc welding (GTAW) cladding process for Stellite cobalt-based alloys. Stellite alloys (particularly Stellite 6, Stellite 21, and Stellite 31) are widely used for hardfacing applications requiring excellent wear resistance, corrosion resistance, and high-temperature performance.
The research focuses on the optimization of GTAW process parameters for Stellite alloy cladding on pump components, including impellers, wear rings, and casing surfaces. The study addresses the challenges of achieving uniform, crack-free overlay layers with consistent mechanical properties.
Stellite Alloy Characteristics and Selection
Common Stellite Alloys
| Alloy Grade | Composition (typical) | Hardness (HRC) | Temperature Range | Application |
|---|---|---|---|---|
| Stellite 6 | Co-28Cr-6W-5Mo-5Fe | 40–45 | Up to 1100°C | General wear and corrosion |
| Stellite 21 | Co-28Cr-6W-5Mo-5Fe | 40–45 | Up to 1100°C | Similar to Stellite 6 |
| Stellite 31 | Co-30Cr-5Mo-3.5W-3.5Ni | 40–45 | Up to 1100°C | Improved oxidation resistance |
| Stellite 6-B | Co-28Cr-6W-5Mo-5Fe | 40–45 | Up to 1100°C | Improved castability |
Microstructural Characteristics
Stellite alloys have a microstructure consisting of:
- γ-Co solid solution matrix: Provides the base strength and toughness
- M₆C carbides (Cr, Mo, W, Fe)₆C: Provide hardness and wear resistance
- σ-phase: Can form during improper heat treatment, reducing toughness
- M₇C₃ carbides: May form in some conditions
The microstructure is sensitive to cooling rate and heat treatment, which directly affects the mechanical properties and service performance.
GTAW Process Parameters and Optimization
Key Process Parameters
The GTAW process for Stellite alloy cladding requires careful control of the following parameters:
| Parameter | Typical Range | Effect on Overlay Quality |
|---|---|---|
| Current | 100–250 A | Higher current = wider weld, higher dilution |
| Voltage | 18–25 V | Higher voltage = wider arc |
| Travel speed | 50–150 mm/min | Higher speed = thinner weld, lower dilution |
| Shielding gas | 100% Ar or Ar-He mix | Affects arc stability and weld pool |
| Gas flow rate | 15–25 L/min | Adequate shielding to prevent oxidation |
| Tungsten electrode | WC or LaB₆ | Affects arc stability and weld quality |
| Preheat temperature | 200–400°C | Reduces cracking susceptibility |
| Interpass temperature | 200–400°C | Controls cooling rate and residual stress |
Process Optimization Approach
The optimization of GTAW parameters for Stellite alloy cladding typically follows a systematic approach:
- Initial parameter selection: Based on experience and manufacturer recommendations
- Parameter variation: Systematically vary one parameter at a time
- Weld quality assessment: Evaluate dilution, hardness, and microstructure
- Parameter refinement: Adjust parameters based on assessment results
- Final parameter selection: Choose the optimal parameter combination
The goal of optimization is to achieve:
- Low dilution: <20% to maintain Stellite alloy properties
- Crack-free weld: No hot cracks or cold cracks
- Uniform hardness: HRC 40–45 throughout the overlay
- Good bond strength: No delamination at the overlay-base metal interface
- Adequate thickness: 2–5 mm for most applications
Quality Control and Defect Prevention
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Hot cracking | High sulfur content, improper cooling rate | Use low-sulfur consumables, control cooling rate |
| Cold cracking | High carbon content, hydrogen embrittlement | Preheat, post-weld heat treatment |
| Excessive dilution | High current, low travel speed | Optimize current and travel speed |
| Porosity | Inadequate shielding, contamination | Ensure proper gas flow, clean surfaces |
| Incomplete fusion | Low current, high travel speed | Increase current, reduce travel speed |
| Tungsten inclusion | Tungsten erosion, contamination | Maintain proper tungsten condition |
| Undercut | Improper torch angle | Maintain proper torch position |
Non-Destructive Testing
The following NDT methods are recommended for Stellite alloy GTAW cladding:
- Visual inspection (VT): Check for surface defects, weld shape, and dilution
- Dye penetrant testing (PT): Detect surface cracks and porosity
- Ultrasonic testing (UT): Detect subsurface defects and measure overlay thickness
- Hardness testing: Verify hardness profile through the overlay thickness
- Metallographic examination: Evaluate microstructure and detect intermetallic phases
Engineering Application and Practice
Application in Pump Manufacturing
The study by Li Youyi et al. focuses on the application of Stellite alloy GTAW cladding in pump manufacturing. Common pump components requiring Stellite cladding include:
- Impeller tips: Subject to erosion from abrasive slurry
- Wear rings: Subject to sliding wear against shaft sleeves
- Casing surfaces: Subject to erosion from high-velocity flow
- Valve seats: Subject to impact and sliding wear
The GTAW process is particularly suitable for these applications due to:
- Precision control: Ability to control weld size and shape
- Low dilution: Maintains Stellite alloy properties
- Flexibility: Suitable for complex geometries
- High quality: Produces clean, crack-free welds
Post-Weld Heat Treatment
Post-weld heat treatment is often required to optimize the properties of Stellite alloy cladding:
- Solution treatment: 1150–1200°C for 1–2 hours, followed by air cooling
- Aging treatment: 800–900°C for 4–8 hours, followed by air cooling
- Stress relief: 600–700°C for 2 hours, followed by air cooling
The heat treatment improves the microstructure, reduces residual stress, and optimizes the hardness and toughness balance.
Study Insights and Practical Recommendations
The research by Li Youyi, Luo Yang, Hong Jie, and Wang Lifeng provides valuable insights into the GTAW cladding process for Stellite cobalt-based alloys. A key finding is that the GTAW process offers excellent control over weld quality and dilution, making it suitable for critical pump component applications.
However, the study also highlights several challenges:
- Productivity: GTAW has relatively low deposition rates compared to other processes
- Operator skill: Requires skilled operators to achieve consistent quality
- Cost: Stellite alloys are expensive, and GTAW consumables add to the cost
- Equipment: Requires specialized GTAW equipment and shielding gas supply
From a practical standpoint, the study demonstrates that GTAW is a viable and high-quality technology for Stellite alloy cladding in pump manufacturing.