GTAW Surfacing Process Development for Stellite Cobalt-Based Alloy

Literature Overview

This study by Li Youyi, Luo Yang, Hong Jie, and Wang Lifeng, published in Welding (Issue 4, 2013, pages 62-65), addresses the challenging surfacing of Stellite cobalt-based alloys using gas tungen arc welding (GTAW). Conducted at Dalian Deep Blue Pump Industry Co., Ltd., the research focuses on overcoming the inherent difficulties of cobalt-based alloy surfacing, including microcracking, cold cracking, and overlay spalling. The authors demonstrate that GTAW provides an ideal method for Stellite surfacing when process parameters are properly selected to control heat input, dilution rate, and elemental loss. This work is highly relevant to engineers in the pump, valve, and rotating machinery industries where Stellite overlays are applied to critical wear and corrosion components.

Technical Challenges of Stellite Surfacing

Stellite alloys (typically Stellite 6, 21, or 6B) are high-performance cobalt-chromium-tungsten-molybdenum alloys known for exceptional wear resistance, corrosion resistance, and high-temperature strength. However, their surfacing presents unique challenges:

Challenge	Mechanism	Consequence
Microcracking	Rapid solidification, columnar grain growth	Overlay fracture, spalling
Cold cracking	Hydrogen embrittlement, residual stress	Delayed cracking, component failure
Overlay spalling	High dilution, poor metallurgical bonding	Loss of protective layer
Elemental loss	Oxidation of Cr, W, Mo during welding	Degraded properties
Impurity ingress	Absorption of S, P, N from environment	Sulfide formation, embrittlement

Metallurgical Background

Stellite 6 (the most commonly used grade) has the following approximate composition:

Element	Co	Cr	W	Mo	C	Fe	Ni	Si	Mn
wt%	Bal.	27-32	9-12	3-4	1.0-1.5	5-7	≤1	≤0.8	≤0.8

The microstructure of as-cast Stellite consists of a face-centered cubic (FCC) cobalt matrix with a high volume fraction of M7C3 and M23C6 carbides. This carbide matrix is responsible for the exceptional wear resistance but also contributes to the brittleness that complicates the welding process.

GTAW Process Parameters and Optimization

Parameter Selection Criteria

The study identifies the following critical process parameters for successful Stellite GTAW surfacing:

Parameter	Recommended Range	Rationale
Current	80-150A (DCEN)	Controls heat input and penetration
Voltage	12-18V	Affects arc stability and bead profile
Travel speed	50-150mm/min	Controls deposition thickness and heat input
Heat input	0.5-1.5 kJ/mm	Minimizes dilution and cracking
Shielding gas	99.99% Ar or He/Ar mix	Prevents oxidation
Gas flow rate	15-25 L/min	Adequate shielding coverage
Preheat	150-300°C	Reduces thermal gradient
Interpass temperature	<300°C	Prevents excessive grain growth

Dilution Rate Control

Dilution rate is the most critical parameter in Stellite surfacing, as it directly affects the final composition and properties of the deposited alloy. The target dilution rate for Stellite surfacing is typically below 20% to maintain adequate alloy content:

Dilution Rate	Resulting Properties	Acceptability
<10%	Full Stellite properties retained	Excellent
10-20%	Moderate property degradation	Acceptable
20-30%	Significant property loss	Marginal
>30%	Properties dominated by base metal	Unacceptable

Dilution control strategies include:

Using a backing strip of Stellite material to reduce back-side dilution
Applying a preliminary coat of Stellite wire before the main surfacing
Using lower current and higher travel speed to reduce penetration
Employing multiple thin passes rather than single thick deposits

Elemental Loss Prevention

Cobalt-based alloys are susceptible to elemental loss during welding, particularly chromium and tungsten oxidation. The study emphasizes the importance of:

Shielding gas purity: High-purity argon (99.99%) or helium-argon mixtures minimize oxidation.
Gas flow rate optimization: Sufficient flow to maintain a protective atmosphere without causing turbulence and air entrainment.
Wire cleanliness: Consumables must be free of oxide, oil, and moisture contamination.
Arc stability: Stable arc conditions reduce spatter and backfire, minimizing elemental loss.

Quality Verification and Performance Assessment

The study evaluates surfacing quality through multiple testing methods:

Test Method	Purpose	Acceptance Criteria
Hardness (HV)	Verify alloy composition and properties	>400 HV for Stellite 6
Chemical analysis	Confirm alloy composition	Within specification limits
Microstructural examination	Assess grain structure and carbide distribution	No microcracks, uniform carbide
Bend test	Evaluate ductility and bonding	No cracking at bend radius
Peel test	Assess overlay adhesion	No spalling at interface
Macrograph examination	Verify bead profile and continuity	Smooth, continuous deposit

Integration with Engineering Practice

Stellite GTAW surfacing is widely applied in the following industrial sectors:

Pump Industry Applications

Impeller surfaces for abrasive slurry service
Valve seats and plugs for high-wear conditions
Wear rings and sleeve bearings
Shaft sleeves for rotating equipment

Valve Industry Applications

Valve trim components (seats, plugs, guides)
Stellite-faced ball valve balls
Control valve cages and spools

Typical Application Parameters

Application	Component	Stellite Grade	Deposit Thickness	Method
Slurry pump	Impeller	Stellite 6	3-5mm	GTAW + FCAW
Control valve	Seat	Stellite 21	2-3mm	GTAW
Ball valve	Ball	Stellite 6B	1-2mm	GTAW
Wear ring	Sleeve	Stellite 6	3-8mm	GTAW + SAW

Process Implementation Guidelines

Based on the study findings, the following implementation guidelines are recommended:

Substrate preparation: Grind the base metal surface to remove oxide, paint, and contamination. A clean, oxide-free surface is essential for achieving metallurgical bonding.
Preheat control: Preheat the component to 150-300°C using induction heating or torch preheating. The preheat temperature should be controlled to minimize thermal gradients while reducing the cracking tendency.
Multi-pass strategy: Apply multiple thin passes (1-2mm each) rather than single thick deposits. This reduces the heat input per pass, minimizes dilution, and improves the quality of the deposited alloy.
Interpass temperature control: Maintain interpass temperature below 300°C to prevent excessive grain growth and avoid cracking.
Post-weld heat treatment: Consider solution heat treatment (1150-1200°C) followed by aging (800-850°C) to homogenize the microstructure and optimize properties.

Key Questions and Reflections

The study raises several important questions for further investigation:

How does the GTAW surfacing process perform on different substrate materials (carbon steel, stainless steel, cast iron)? The dilution rate and bonding characteristics vary significantly with substrate composition.
What is the effect of substrate geometry (curved surfaces, thin-walled components) on surfacing quality? Thin-walled components are susceptible to distortion and may require additional cooling strategies.
How does the surfacing quality degrade with increasing number of passes? Each additional pass introduces additional heat input and potential for dilution.
What are the long-term wear and corrosion performance characteristics of the deposited alloy under actual service conditions? Laboratory testing may not fully capture the complex degradation mechanisms experienced in service.

The study's emphasis on heat input control and dilution management is particularly noteworthy. In my experience with Stellite surfacing qualification programs, the industry has sometimes focused excessively on consumable selection while underestimating the importance of process parameter control. This research provides a compelling argument for rigorous parameter qualification and process monitoring.

Study Insights and Implications

This research demonstrates that GTAW is a highly effective method for Stellite cobalt-based alloy surfacing when process parameters are carefully controlled. The key insight is that the success of Stellite surfacing depends less on the consumable selection and more on the process parameter optimization that minimizes heat input, dilution, and elemental loss.

The study's systematic approach to identifying and addressing surfacing challenges provides a valuable framework for engineers developing Stellite surfacing procedures. The emphasis on dilution rate control as the primary determinant of overlay quality is particularly significant, as it shifts the focus from consumable metallurgy to process engineering.

The demonstrated ability to produce defect-free Stellite overlays using GTAW confirms that this method is suitable for a wide range of industrial applications, from small valve components to large pump impellers. The productivity limitations of GTAW (relatively low deposition rate) can be addressed by combining GTAW for the first few passes with higher-deposition-rate methods (FCAW, SAW) for subsequent passes, maintaining the quality benefits of GTAW while improving overall productivity.