Zhuojin Pipe Fitting Co., LtdGTAW Overlay Welding Process for Stellite Cobalt-Based Alloys
Literature Overview
This paper by Li Youyi, Luo Yang, Hong Jie, and Wang Lifeng from Dalian Deep Blue Pump Industry Co., Ltd., published in Welding in 2013 (Issue 4, pages 62-65), addresses the technical challenges of GTAW overlay welding of Stellite cobalt-based alloys. The authors identify microcracking, cold cracking, and overlay spalling as the principal welding defects that lead to product rework or scrapping, and demonstrate that non-consumable GTAW is an ideal welding method when process parameters are properly selected.
Technical Challenges and Defect Analysis
Stellite alloys, which are cobalt-chromium-tungsten-based hardfacing materials, present unique welding challenges due to their high thermal conductivity, tendency to form brittle carbides, and susceptibility to cracking. The paper identifies three critical defect modes that must be controlled during GTAW overlay welding.
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Microcracking | Excessive carbide formation, thermal stresses | Optimize heat input, control cooling rate |
| Cold cracking | Hydrogen embrittlement, high hardness | Preheat, control interpass temperature, low hydrogen consumables |
| Overlay spalling | Excessive dilution, poor bonding | Minimize dilution, proper joint preparation |
The microcracking problem is particularly insidious because it may not be visible on the surface but can propagate under service loading, leading to sudden overlay failure. The formation of continuous intergranular carbide networks, particularly of the M6C type, is a well-documented cause of microcracking in cobalt-based overlays. Controlling the carbon-to-alloying element ratio is essential to prevent excessive carbide precipitation.
Process Optimization Strategy
The authors emphasize three critical process control factors: preventing chemical composition burn-off of alloying elements, avoiding impurity element ingress, and maintaining appropriate dilution rates. In GTAW overlay welding of Stellite alloys, the heat input must be carefully balanced—sufficient to ensure complete fusion and wetting of the substrate, but not so high as to cause excessive dilution or alloy element loss through evaporation.
Typical GTAW parameters for Stellite overlay welding include DCEN polarity, current in the range of 80-200 A depending on deposit thickness, arc voltage of 12-18 V, and travel speed of 100-300 mm/min. The use of tungsten electrodes with appropriate diameter (typically 2.4-4.0 mm) and proper tip preparation is essential for arc stability and bead quality. Argon shielding gas flow rates of 15-25 L/min are recommended to prevent oxidation of the molten pool.
The dilution rate is a critical parameter that directly affects the hardness, wear resistance, and cracking susceptibility of the overlay. For Stellite overlays, dilution rates below 10-15% are generally targeted to maintain the intended alloy composition. Achieving low dilution may require using a wire feed electrode (as in GTAW with wire feed) or applying multiple thin layers with controlled parameters.
Practical Considerations for Pump Industry Applications
The application context of this research—pump manufacturing—provides important engineering insight. Pump components such as impellers, wear rings, and shaft sleeves are frequently overlay welded with Stellite alloys to enhance wear and erosion resistance in slurry service. The GTAW process offers excellent control over the weld pool and minimal spatter, making it suitable for the precision work required on rotating pump components.
However, GTAW is inherently a low-deposition-rate process, which can be a productivity limitation for large components. Engineers must balance quality requirements against production efficiency, potentially using GTAW for critical areas and higher-productivity processes for less demanding applications.
Key Reflections
The paper's emphasis on avoiding alloy element burn-off is particularly relevant for cobalt-based alloys, where the evaporation of tungsten and chromium during welding can significantly alter the deposit composition. In practice, this means that the as-cast deposit composition may differ from the consumable composition, and engineers should account for this when specifying alloy grades for overlay applications. The paper also raises the question of post-weld heat treatment; while some Stellite alloys benefit from solution annealing to dissolve carbides and improve toughness, others may not tolerate such treatment without adverse effects on hardness.
Summary
This paper provides practical guidance for GTAW overlay welding of Stellite cobalt-based alloys, identifying dilution control, alloy element preservation, and impurity exclusion as the three pillars of successful process execution. The defect analysis and countermeasures presented are directly applicable to engineers working on pump and valve components in the process industries, where cobalt-based overlays are widely used for their exceptional wear and corrosion resistance properties.