CLADDING TECHNOLOGY SHANXI CO., LTDTIG Weld Overlay of Cobalt-Chromium-Tungsten Alloy for Sealing Applications
Literature Overview
This 1992 publication by Bai Jinsheng from the Tianjin Welding Research Institute and Cai Zejun from Tianjin Crane Sealing Co. provides a systematic discussion of the TIG (Gas Tungsten Arc Welding) overlay process for cobalt-chromium-tungsten alloys. The work represents a notable convergence of fundamental welding research and industrial sealing application requirements, addressing the hardfacing needs of valve seats, pump components, and other sealing surfaces exposed to severe abrasive and corrosive conditions.
Material System and Performance Characteristics
Cobalt-chromium-tungsten alloys, commonly referred to as Stellite-type alloys, are among the most widely used hardfacing materials in industrial applications. The specific composition typically includes:
| Element | Content Range | Function |
|---|---|---|
| Cobalt | Balance (50-65%) | Matrix element providing high-temperature strength |
| Chromium | 25-35% | Carbide former, provides corrosion resistance |
| Tungsten | 6-12% | Primary hard phase former (W₂C carbides) |
| Carbon | 1.5-3.0% | Carbide former, enhances hardness |
| Nickel | 0-5% | Improves weldability and ductility |
The resulting microstructure consists of a solid solution matrix of cobalt-chromium-nickel with a high density of M₇C₃-type carbides (Cr, Fe, W)₇C₃ dispersed throughout. This microstructure provides hardness values in the range of HRC 45-55 after proper heat treatment, with excellent retention of hardness at elevated temperatures up to 800°C.
TIG Overlay Process Parameters
The selection of TIG welding for this application is well-justified by the requirements of precision overlay work. Unlike GMAW or SAW processes, TIG provides:
- Superior control over heat input, critical for maintaining the intended microstructure
- Excellent visual monitoring of the weld pool, allowing precise control of bead placement
- Low spatter and minimal contamination, essential for sealing surface quality
- Ability to weld in all positions with consistent results
The recommended process parameters for Co-Cr-W TIG overlay include:
| Parameter | Typical Value | Notes |
|---|---|---|
| Welding current | 100-180 A DC | DCEN polarity for tungsten electrode stability |
| Arc voltage | 12-16 V | Depends on electrode diameter |
| Travel speed | 20-40 mm/min | Slower for thicker deposits |
| Shielding gas | Argon, 15-25 L/min | Pure argon preferred; He-Ar mix for thick sections |
| Electrode diameter | 2.4-3.2 mm | Ceramic cup, 7-10 mm |
| Filler wire diameter | 1.6-3.2 mm | Matching Co-Cr-W composition |
| Preheat | 150-250°C | Reduce thermal gradient and cracking risk |
| Interpass temperature | ≤300°C | Prevent carbide coarsening |
| Post-weld heat treatment | 1150°C + air cool, then 840°C × 2h + air cool | Solution treatment and aging |
Microstructural Control and Defect Prevention
The primary metallurgical challenge in Co-Cr-W TIG overlay is maintaining the correct carbide morphology and distribution. Excessive heat input leads to carbide coarsening and network formation along prior austenite grain boundaries, which severely degrades mechanical properties. Insufficient heat input results in incomplete melting of the previous layer and poor metallurgical bonding.
Common defects and their countermeasures include:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cracking (hot) | Excessive sulfur/phosphorus in base metal | Use clean base metal; add nickel to filler |
| Cracking (cold) | High carbon in weld metal; high residual stress | Post-weld stress relief; control carbon content |
| Poor bond strength | Incomplete melting of previous layer | Increase current; reduce travel speed |
| Excessive dilution | High heat input; thin base metal | Reduce current; increase travel speed; use multiple thin passes |
| Surface porosity | Gas entrapment; contamination | Ensure clean surfaces; adequate shielding gas flow |
| Carbide network | Excessive carbon; slow cooling | Adjust composition; apply rapid quench if needed |
Application to Sealing Components
For sealing applications such as valve seats, the overlay must achieve a specific surface finish and dimensional accuracy. The TIG process is particularly suitable because:
- The narrow weld bead allows precise placement of the overlay material only on the functional sealing surface
- The low distortion characteristics of TIG welding preserve the dimensional accuracy of the component
- The smooth weld surface can be easily machined to the required finish after overlay
The typical overlay thickness for sealing applications is 1.5-3.0 mm, with the final surface machined to achieve Ra values of 0.2-0.8 μm as specified by the sealing design requirements.
Study Insights and Practical Recommendations
This literature provides valuable guidance for engineers working on hardfacing applications involving cobalt-based alloys. The emphasis on process control and microstructural management reflects a mature understanding of the relationship between welding parameters and material performance. One key insight is that the success of Co-Cr-W overlay is not determined by the alloy composition alone but by the entire process chain, from base metal preparation through post-weld heat treatment.
The work also highlights the importance of selecting the appropriate welding process for the application requirements. While GMAW may offer higher deposition rates, the precision and control offered by TIG welding make it the preferred choice for sealing surface applications where dimensional accuracy and surface quality are paramount.