GTAW Cladding Process Exploration for Stellite Cobalt-Based Alloy

Literature Overview

This paper investigates the gas tungsten arc welding (GTAW) cladding process for depositing Stellite cobalt-based alloy overlay on carbon steel and low-alloy steel substrates. Stellite alloys (such as Stellite 6, Stellite 21, and Stellite 25) are widely used for their exceptional wear resistance, high-temperature strength, and corrosion resistance. The study examines the welding parameters, microstructure, mechanical properties, and wear performance of the GTAW-deposited Stellite overlay, providing valuable insights for optimizing the cladding process.

Core Technical Points

GTAW Cladding Process Parameters

GTAW (TIG) is a precise welding process that offers excellent control over the heat input and weld geometry, making it suitable for depositing thin overlay layers of expensive alloys like Stellite. The key process parameters include:

Parameter	Typical Range	Effect on Overlay
Welding current	80–150 A	Controls dilution and weld width
Arc voltage	14–20 V	Affects penetration and weld shape
Travel speed	30–80 mm/min	Controls heat input and dilution
Shielding gas flow	8–15 L/min	Protects weld from atmospheric contamination
Wire feed rate	0.5–1.5 m/min	Controls deposit thickness and dilution
Preheat temperature	150–300 °C	Reduces cracking tendency
Interpass temperature	< 200 °C	Controls microstructure and residual stress

Microstructural Characteristics

The microstructure of the GTAW-deposited Stellite overlay is characterized by:

A dendritic structure with primary dendrites and interdendritic regions.
Carbide precipitation (M₆C and M₂₃C₆) in the interdendritic regions, which provide wear resistance.
A dilution zone at the interface between the overlay and the base metal, where the Stellite composition is diluted by the base metal.
A heat-affected zone (HAZ) in the base metal with grain growth and possible phase transformation.

The dilution rate is a critical parameter that affects the mechanical and wear properties of the overlay. For GTAW cladding, the dilution rate is typically 10–25%, which is lower than that of other processes such as SAW (20–40%) or FCAW (25–50%). The lower dilution rate of GTAW is advantageous for maintaining the high performance of the Stellite alloy.

Mechanical and Wear Properties

The mechanical properties of the GTAW-deposited Stellite overlay include:

Hardness: 350–450 HV (depending on Stellite grade and dilution).
Tensile strength: 600–800 MPa.
Elongation: 10–20%.
Bond strength: > 150 MPa (shear test).

The wear resistance of the overlay is evaluated using standardized tests such as ASTM G99 (pin-on-disk) and ASTM G65 (block-on-ring). The wear rate of Stellite 6 overlay is typically 0.5–2.0 × 10⁻³ mm³/N·m, which is significantly lower than that of the base metal (10–50 × 10⁻³ mm³/N·m).

Engineering Practice Considerations

Common Defects and Countermeasures

Defect Type	Cause	Countermeasure
Cracking	High dilution, low ductility of Stellite	Reduce heat input, increase preheat, use low-dilution consumables
Porosity	Inadequate shielding, contaminated surfaces	Improve shielding gas coverage, clean surfaces thoroughly
Lack of fusion	Low current, high travel speed	Increase current, reduce travel speed
Excessive dilution	High heat input, thin deposit	Reduce current, increase travel speed, use multiple thin passes
Surface roughness	Irregular wire feed, arc instability	Stabilize wire feed, use consistent travel speed

Application in Pressure Vessel Fabrication

Stellite GTAW cladding is commonly used for:

Valve seats and stems in high-temperature and high-pressure service.
Heat exchanger tube sheets and channel covers exposed to erosive fluids.
Pump impellers and casings in mining and chemical processing.
Turbine blades and vanes in power generation.
Pressure vessel internals exposed to abrasive and corrosive environments.

In pressure vessel fabrication governed by ASME VIII Div.1 and NB/T 47002, the cladding procedure must be qualified in accordance with ASME IX and NB/T 47014. The qualification includes weldability testing, mechanical property testing, and non-destructive testing of the overlay.

Key Questions and Reflections

The GTAW cladding process offers excellent control and low dilution, but it is relatively slow and expensive compared to other cladding processes such as SAW or FCAW. The study raises an important question about the selection of cladding process based on the specific application requirements, considering factors such as deposit thickness, production volume, and cost.

Another reflection is the importance of consumable selection for Stellite GTAW cladding. The choice of filler metal (wire or rod) significantly affects the dilution rate, microstructure, and mechanical properties of the overlay. The study recommends using low-dilution consumables (e.g., Stellite 6 wire with low carbon content) to minimize dilution and maintain the high performance of the overlay.

Study Insights and Implications

The investigation of GTAW cladding for Stellite cobalt-based alloy provides valuable insights for optimizing the process parameters and achieving high-quality overlay deposits. The key findings are that GTAW offers low dilution and excellent control, making it suitable for applications where maintaining the high performance of the Stellite alloy is critical. For engineering practice, the process should be used for thin overlay deposits (1–5 mm) on critical components where wear resistance and corrosion resistance are paramount.

The study also highlights the importance of process qualification and quality control in Stellite cladding fabrication. The overlay must be thoroughly inspected for defects such as cracking, porosity, and lack of fusion, and the mechanical properties must be verified through hardness testing and bond strength testing. Future work should focus on developing automated GTAW cladding systems that can maintain consistent process parameters and improve productivity while maintaining quality.