Zhuojin Pipe Fitting Co., LtdGTAW Overlay Process Investigation for Stellite Cobalt-Based Alloy Deposits
Literature Overview and Technical Context
The paper by Li Youyi, Luo Yang, Hong Jie, and Wang Lifeng from Dalian Deep Blue Pump Industry Co., published in Welding in 2013, focuses on the gas tungen arc welding (GTAW) overlay process for Stellite cobalt-based alloys. Stellite alloys—most commonly Stellite 6, 21, and 6B—are cobalt-chromium-tungsten alloy systems renowned for their exceptional resistance to wear, corrosion, and high-temperature oxidation. They are widely used in pump impellers, valve seats, turbine blades, and other components subjected to severe erosive and abrasive conditions.
The authors identify the primary technical challenges of Stellite overlay welding as microcracking, cold cracking, and overlay spalling, all of which can lead to rework or complete product rejection. The paper advocates GTAW as an ideal welding method provided that heat input, dilution rate, and alloy element burn-off are properly controlled.
Welding Defect Analysis and Prevention
The three principal defects identified—microcracks, cold cracks, and spalling—each have distinct metallurgical origins that must be understood to develop effective prevention strategies.
- Microcracks: These typically form in the solidification zone of the overlay due to the formation of brittle intermetallic compounds at grain boundaries. In cobalt-based alloys, the solidification range is relatively wide, and the presence of low-melting-point phases at grain boundaries creates conditions favorable for hot cracking during solidification.
- Cold cracks: These occur in the heat-affected zone or in the weld metal after cooling, driven by hydrogen embrittlement and the formation of hard, brittle microstructures. The high carbon equivalent of cobalt-based alloys exacerbates this tendency.
- Spalling: This refers to the delamination of the overlay from the substrate, typically caused by excessive dilution leading to a martensitic or hard intermetallic layer at the substrate-overlay interface, or by inadequate preheating resulting in high residual stresses.
| Defect Type | Primary Cause | Prevention Strategy |
|---|---|---|
| Microcracks | Wide solidification range, intermetallic formation | Reduce carbon content, optimize cooling rate |
| Cold cracks | Hydrogen embrittlement, high carbon equivalent | Preheat substrate, use low-hydrogen consumables |
| Spalling | Excessive dilution, high residual stress | Control heat input, apply appropriate preheat |
GTAW Process Parameter Control
GTAW is well-suited for Stellite overlay because it provides precise control over heat input, which is critical for managing dilution and preventing the defects described above. The key process parameters include:
- Welding current: Typically 100–200 A for overlay applications, depending on the substrate thickness and overlay layer requirements. Lower currents reduce dilution but may lead to incomplete fusion.
- Travel speed: A slower travel speed increases heat input and dilution, while a faster speed reduces dilution but risks incomplete fusion. The optimal range must be determined through trial welds on the specific substrate material.
- Shielding gas flow: 15–20 L/min of argon is typically used to ensure complete exclusion of atmospheric contamination. Inadequate shielding leads to oxide inclusion formation, which degrades both mechanical and corrosion properties.
- Stick-out length: Maintaining a consistent tungsten stick-out of 4–6 mm ensures stable arc characteristics and uniform heat distribution.
The dilution rate—the proportion of substrate material melted into the weld metal—is perhaps the most critical parameter. For Stellite overlays, a dilution rate of 10–20% is generally acceptable, but exceeding 25% can significantly alter the microstructure and properties of the overlay. High dilution introduces excess iron and carbon from the substrate, promoting the formation of hard carbides and martensite that compromise toughness and increase crack susceptibility.
Alloy Element Burn-Off and Impurity Control
The paper emphasizes the importance of minimizing the burn-off of key alloying elements (Co, Cr, W, C) and preventing the ingress of impurity elements (O, N, S) during the welding process. In GTAW, the argon shielding provides effective protection, but several measures further enhance consumable integrity:
- Using high-purity argon (99.99%) as the shielding gas
- Maintaining proper gas lens geometry to ensure full coverage of the arc and molten pool
- Cleaning the tungsten electrode to remove oxide contamination that could introduce oxygen into the weld pool
- Preheating the substrate to reduce thermal gradients and minimize hydrogen absorption from the base metal
Engineering Practice Implications
In my experience with pump impeller and valve seat overlay applications, the GTAW process for Stellite deposits demands a high degree of operator skill and process discipline. The narrow process window means that even small deviations in current, travel speed, or shielding gas flow can produce unacceptable defects.
A practical approach is to establish a qualified welding procedure specification (WPS) through systematic parameter qualification on representative test coupons, followed by rigorous operator qualification testing. In-service monitoring of overlay condition through periodic hardness testing and ultrasonic inspection for delamination can extend component life and prevent unexpected failures.
The paper's advocacy for GTAW is well-founded, particularly for smaller components or applications where the overlay area is limited. For larger overlay areas, automated GTAW or robotic GTAW systems can improve consistency and productivity, though the fundamental process principles remain the same.
In summary, this paper provides a focused and practical guide to Stellite overlay welding by GTAW, with clear emphasis on the interplay between heat input, dilution control, and defect prevention. The insights are directly applicable to engineers designing overlay solutions for wear- and corrosion-critical components in the pump, valve, and turbomachinery industries.