CO₂ Gas Shielded Arc Welding (GMAW-CO₂): Technical Review, Process Fundamentals, and Integration into Cladding Manufacturing

1. Definition and Fundamental Principles

CO₂ gas shielded arc welding, formally designated as Gas Metal Arc Welding with Carbon Dioxide shielding (GMAW-CO₂), is a process variant of the broader GMAW family (ISO 4063:135) in which a consumable solid or flux-cored wire electrode is fed continuously through a nozzle, while pure carbon dioxide gas is supplied to protect the arc zone and molten weld pool from atmospheric contamination. The process operates on the principle of short-circuiting transfer or spray transfer, depending on the combination of wire diameter, current density, and voltage settings. When pure CO₂ is used as the shielding medium, the arc characteristics differ markedly from those observed with mixed-gas (Ar/CO₂) shielding, necessitating distinct parameter optimization and consumable selection.

The fundamental electrochemical mechanism involves the ionization of CO₂ at the cathode (wire tip) to produce CO⁺, O⁺, and O₂ species, which participate in the arc plasma and influence the weld pool chemistry. The oxide-forming nature of CO₂ contributes to a certain degree of deoxidation of the weld metal, particularly when combined with appropriate wire compositions containing manganese (Mn) and silicon (Si) deoxidizers. Understanding these metallurgical interactions is essential for predicting weld metal properties, porosity susceptibility, and spatter behavior in production environments.

2. Category and Business Positioning Within Cladding Technology Shanxi Co., Ltd.

Within the company's three principal technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—CO₂ gas shielded welding falls squarely under the TIG/MIG weld overlay domain. It represents a cost-effective, high-deposition-rate complement to TIG overlay and mixed-gas MIG overlay operations. The strategic positioning of GMAW-CO₂ within the company's capability portfolio is threefold:

• Volume production capability: CO₂-shielded processes deliver deposition rates typically 1.5–3× higher than TIG overlay, making them suitable for large-area corrosion-resistant or wear-resistant cladding on bulk carbon steel substrates.
• Cost efficiency: Pure CO₂ shielding gas is significantly less expensive than argon or argon/CO₂ mixtures, reducing consumable costs by 30–50% on high-volume overlay campaigns.
• WPS qualification breadth: Demonstrating competence in GMAW-CO₂ expands the company's qualified WPS portfolio, enabling qualification under AWS D10.9, ASME Section IX, and NB/T 20710 for a wider range of customer specifications.

The "Research Status of CO₂ Gas Shielded Welding" learning initiative undertaken by the company's technical team represents a systematic effort to deepen process knowledge, identify optimization opportunities, and ensure that operators and engineers possess current awareness of the state of the art in GMAW-CO₂ technology.

3. Technical Purpose and Value in Cladding Applications

3.1 Primary Technical Objectives

• Achieve controlled dilution of overlay layers to maintain required alloy composition in the cladding material (typically ≤30% dilution for single-pass overlay, ≤15% for multi-pass overlay on ferrous substrates).
• Minimize porosity and inclusions inherent to CO₂ shielding, particularly nitrogen pickup and oxide inclusions, through parameter optimization and consumable selection.
• Maximize deposition rate while maintaining acceptable weld geometry, fusion ratio, and mechanical properties of the overlay.
• Reduce spatter to improve surface quality and minimize post-welding cleanup requirements.

3.2 Value Contribution to Product Delivery

For customers specifying overlay cladding on large-diameter carbon steel pipes, pressure vessels, or structural components, GMAW-CO₂ overlay provides a practical pathway to deliver corrosion-resistant (e.g., 309L, 316L, Inconel 625) or wear-resistant (e.g., Stellite, hardfacing) surfaces at competitive cost and schedule. The process is particularly advantageous for:

• Transition layers between carbon steel base and austenitic/nickel-based overlay alloys
• Build-up welding and repair of worn components
• Multi-layer overlay where the first pass serves as a dilution buffer and subsequent passes deliver the final cladding composition

4. Key Process Parameters and Implementation Points

4.1 Critical Parameter Ranges for CO₂-Shielded Overlay Welding

Parameter	Typical Range (Solid Wire)	Typical Range (Flux-Cored Wire)	Notes for Overlay Applications
Shielding Gas	100% CO₂, flow 15–25 L/min	100% CO₂, flow 12–20 L/min	Higher flow rates needed in outdoor or drafty environments
Wire Diameter	1.0 mm, 1.2 mm, 1.6 mm	1.2 mm, 1.6 mm	1.2 mm preferred for overlay to control dilution
Current (DC+)	120–350 A	150–450 A	Short-circuit transfer at lower current; spray transfer above ~250 A
Voltage	18–28 V	22–32 V	Lower voltage reduces spatter but increases dilution
Travel Speed	150–400 mm/min	200–500 mm/min	Slower speed increases penetration and dilution
Stick-out Length	12–18 mm	15–20 mm	Shorter stick-out reduces spatter and improves arc stability
Wire Feed Speed	4–12 m/min	5–15 m/min	Must be calibrated against current for consistent arc length

4.2 Transfer Mode Selection for Overlay

The choice between short-circuiting transfer and spray transfer fundamentally affects dilution, bead geometry, and mechanical properties of the overlay:

• Short-circuiting transfer (current <250 A, voltage <22 V): Produces lower penetration, reduced dilution (typically 15–25% for austenitic overlay on carbon steel), and lower heat input. Preferred for the first overlay pass on carbon steel substrates to minimize dilution. However, higher spatter rates (10–20% of deposited metal) and increased porosity susceptibility are observed.
• Spray transfer (current >280 A, voltage >24 V): Produces higher deposition rates (up to 8 kg/h), smoother bead surfaces, and reduced spatter. However, increased penetration leads to higher dilution (25–40%), which may compromise overlay alloy composition. Suitable for subsequent overlay passes where the previous layer acts as a dilution buffer.

4.3 Consumable Selection for Cladding

Wire electrode selection is critical for achieving the desired overlay composition and weld quality:

• E309L (AWS A5.9) / ER309L (AWS A5.18): Standard austenitic transition and overlay wire for carbon steel substrates. Provides good resistance to cracking and moderate corrosion resistance.
• E316L / ER316L: Molybdenum-bearing austenitic wire for enhanced pitting and crevice corrosion resistance in overlay applications.
• E309Mo / ER309Mo: Intermediate composition between 309 and 316, suitable where moderate pitting resistance is required.
• ERNiCrMo-3 (Inconel 625 equivalent): Nickel-chromium-molybdenum overlay for severe corrosion environments and high-temperature service.
• Flux-cored wires (AWS A5.20): Provide higher deposition rates and better penetration control in some overlay configurations, particularly for thick multi-pass builds.

4.4 Multi-Layer Overlay Strategy

Effective CO₂-shielded overlay welding on carbon steel substrates typically employs a multi-layer strategy:

1. Transition layer (Pass 1): Use E309L or equivalent with short-circuit transfer at reduced current (120–180 A) to achieve dilution ≤30%. Bead width should be controlled to 15–20 mm.
2. Intermediate layer (Pass 2): Use the target overlay alloy (e.g., E316L, ERNiCrMo-3) with controlled dilution ≤15%. Slightly higher current (180–250 A) acceptable.
3. Final overlay layers (Pass 3+): Achieve near-100% overlay composition with dilution <5%. Parameters optimized for surface quality and uniform composition.

5. Applicable Standards and Acceptance Criteria

5.1 Welding Procedure and Qualification Standards

• AWS D10.9M/D10.9 (2018): Qualification and Performance Requirements for Welding Procedures for Cladding and Surfacing. Defines essential variables, procedure qualification tests, and performance qualification requirements for weld overlay.
• ASME BPVC Section IX, QW-250 through QW-254: Governs qualification of welding procedures for cladding and surfacing in pressure vessel and piping applications.
• NB/T 20710-2015 (Welding Procedure Specification and Welder Qualification for Nuclear Power Plant Piping): Applicable when overlay welding is performed on nuclear-grade components.
• GB/T 985.1-2008 (Welding Procedure Specification and Welder Qualification): Chinese national standard governing WPS qualification, including GMAW processes.
• API 1104: Welding of Pipelines and Related Facilities—includes requirements for overlay welding on piping systems.

5.2 Material and Consumable Standards

• AWS A5.9/A5.9M: Specification for Covered Electrodes for Shielded Metal Arc Welding (stick electrode equivalents for reference)
• AWS A5.18/A5.18M: Specification for Welding Wires for Gas Shielded Arc Welding and Flux-Cored Arc Welding
• ISO 14341: Welding Consumables—Welding Wires for Gas Shielded Arc Welding
• GB/T 8110: Welding Consumables—Solid Wire for Gas Shielded Arc Welding

5.3 Non-Destructive Testing and Acceptance Criteria

• GB/T 3323.1-2017: Radiographic testing acceptance criteria for weld overlay (visual evaluation of radiographs)
• GB/T 11345-2013: Ultrasonic testing of welds—technique and acceptance levels
• ISO 17637: Non-destructive testing of welds—ultrasonic testing
• ASTM E2354: Standard Practice for Surface Preparation for Ultrasonic Testing
• EN ISO 17640: Magnetic particle testing acceptance criteria

5.4 Overlay-Specific Acceptance Criteria

• Overlay composition: Verified by optical emission spectroscopy (OES) or XRF at representative locations; must meet minimum alloy specification (e.g., Cr ≥ 22%, Ni ≥ 12% for 309L overlay).
• Dilution: Measured by cross-section metallographic examination; must not exceed the WPS-specified maximum (typically ≤30% first pass, ≤15% subsequent passes).
• Hardness: Overlay hardness verified by Rockwell C or Vickers testing; must be within specified range (e.g., 20–30 HRC for 309L overlay).
• Corrosion resistance: Salt spray testing per ASTM B117 or ISO 9227 for minimum 500 hours without pitting in overlay material for marine applications.

6. Common Risks, Defects, and Controls

6.1 Defect Spectrum Specific to GMAW-CO₂ Overlay

Defect Type	Cause	Detection Method	Control Measures
Porosity (CO₂ gas porosity)	Inadequate shielding gas flow, wind exposure, excessive arc length	RT (GB/T 3323), UT (GB/T 11345)	Maintain minimum 15 L/min flow; use wind shields; reduce stick-out
Nitrogen pickup	Shielding gas interruption, poor nozzle alignment	UT, macrographic examination	Pre-weld gas purge; ensure proper nozzle-to-workpiece distance (8–12 mm)
High dilution	Excessive current, high travel speed, wide bead pattern	Macrographic sectioning, OES	Use stringer beads; reduce current; maintain ≤20 mm bead width
Spatter	High voltage, long stick-out, improper wire feed	Visual inspection	Reduce voltage; shorten stick-out to 12–15 mm; use anti-spatter agent
Hot cracking	High sulfur/phosphorus in base metal; excessive restraint	MT (EN ISO 17640), UT	Preheat base metal to 100–150°C; use low-S wires; control interpass temperature
Oxide inclusions	Insufficient deoxidation, CO₂ dissociation products	Macrographic sectioning	Use Mn-Si deoxidized wires; ensure adequate wire composition
Undercut	Excessive travel speed, high current	Visual, PT	Reduce travel speed; adjust current/voltage ratio

6.2 Process Risk Management

• Shielding gas quality: CO₂ purity must be ≥99.5% (per GB/T 8176 or equivalent). Moisture content should be <50 ppm. Gas cylinders must be inspected for contamination, and regulators must be equipped with moisture traps.
• Base metal preparation: Surface must be free of rust, scale, oil, and paint to within 25 mm of the weld area. Grit blasting to Sa 2½ (ISO 8501-1) is recommended for overlay applications.
• Interpass temperature control: Maintain interpass temperature ≤200°C for austenitic overlay wires to prevent sensitization and grain boundary carbide precipitation.
• Thermal management: For thick-section components, employ back-gassing or backing bars to ensure full penetration and minimize back-side oxidation.

7. Application Scenarios Across the Company's Technology Routes

7.1 Integration with TIG/MIG Weld Overlay Route

GMAW-CO₂ is the primary high-volume complement to TIG overlay within this route. Typical deployment scenarios include:

• Large-diameter pipe overlay (OD >500 mm): CO₂-shielded overlay provides the deposition rates (4–8 kg/h) necessary to cover large surface areas within project timelines. TIG overlay is reserved for tight-geometry areas, small-bore piping, and final surface finishing passes.
• Transition layer for hydraulic explosive bonding: In some hybrid approaches, a GMAW-CO₂ transition layer is applied to the base metal prior to hydraulic explosive bonding of a stainless steel strip, ensuring metallurgical compatibility at the interface.
• Repair and build-up: CO₂-shielded GMAW is the preferred method for restoring dimensions on worn carbon steel components prior to final overlay or bonding operations.

7.2 Support Role in Hydraulic Explosive Bonding Route

While hydraulic explosive bonding relies on controlled mechanical impact for metallurgical bonding, GMAW-CO₂ serves as a preparatory and post-bonding process:

• Substrate preparation: Base metal surfaces may require GMAW-CO₂ build-up to correct geometric irregularities before bonding.
• Edge sealing: Post-bonding, edge sealing welds using GMAW-CO₂ with appropriate consumables (e.g., E309L) prevent ingress of corrosive media at the bond interface.
• Transition layer for hybrid cladding: When explosion-bonded strips are subsequently overlay-welded for additional thickness, GMAW-CO₂ provides an efficient means to build up the overlay layer.

7.3 Support Role in Explosion Welding Route

In explosion welding, where the flyer plate is accelerated to supersonic velocities for metallurgical bonding, GMAW-CO₂ is employed for:

• Post-weld edge sealing: Sealing the perimeter of explosion-welded clad plates to prevent environmental attack at the bond interface.
• Overlay build-up on explosion-welded surfaces: Adding additional corrosion-resistant or wear-resistant layers on top of explosion-welded clad plates for enhanced performance.
• Pipe fitting fabrication: When explosion-welded clad pipe requires additional overlay for specific service conditions, GMAW-CO₂ provides efficient deposition.

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

The systematic study of CO₂ gas shielded welding technology directly contributes to the company's qualification infrastructure:

• WPS expansion: Each qualified GMAW-CO₂ procedure adds to the company's portfolio, enabling coverage of additional material combinations, thickness ranges, and service conditions.
• Welder qualification: Operators trained in GMAW-CO₂ techniques can be qualified under AWS D10.9, ASME Section IX, and GB/T 985.1, broadening the skilled workforce available for overlay projects.
• Third-party certification readiness: Comprehensive GMAW-CO₂ qualification supports certification audits by ASME (N-stamp, R-stamp), API (Q1), and ISO 3834 (Level 2/3).

8.2 Product Delivery Enhancement

• Schedule compression: Higher deposition rates of GMAW-CO₂ (compared to TIG) enable 30–50% reduction in overlay welding time for large components, directly translating to shorter project delivery times.
• Cost competitiveness: Lower shielding gas costs and higher productivity enable more competitive pricing for high-volume overlay work, expanding the company's addressable market.
• Quality consistency: Well-documented GMAW-CO₂ procedures with controlled parameters yield repeatable overlay properties, reducing rework rates and improving first-pass yield.

8.3 Customer Value

"The study of CO₂ gas shielded welding research status enables our engineering team to make informed decisions on process selection, ensuring that each customer project receives the optimal combination of quality, schedule, and cost performance. By maintaining current knowledge of GMAW-CO₂ advancements—including wire developments, wire-feed technology improvements, and monitoring system innovations—we deliver overlay products that meet or exceed customer specifications while providing technical confidence in long-term service performance."

• Technical advisory capability: Engineers trained in GMAW-CO₂ can provide customers with credible process recommendations, dilution predictions, and performance data, strengthening technical credibility.
• Customized solutions: Understanding the full parameter envelope of GMAW-CO₂ enables tailored overlay specifications for diverse service environments (marine, chemical, oil & gas, power generation).
• Traceability and documentation: Rigorous process documentation per AWS D10.9 and ASME Section IX provides customers with complete quality records, supporting asset integrity management and regulatory compliance.

9. Current State of the Art and Future Directions

9.1 Recent Advances in GMAW-CO₂ Technology

• Advanced wire designs: New flux-cored wire compositions with tailored slag systems reduce spatter by 40–60% and improve weld surface quality, directly benefiting overlay applications.
• Wire-feed system improvements: Dual-drive and servo-controlled wire feeders provide ±1% current stability, reducing dilution variability and improving overlay composition consistency.
• Real-time monitoring: Arc voltage and current monitoring systems enable automatic parameter adjustment, maintaining consistent weld quality throughout long production runs.
• Robotized overlay: Integration with robotic systems enables repeatable multi-pass overlay with precise travel speed and wire feed control, reducing dilution and improving bead uniformity.

9.2 Recommendations for Company Implementation

1. Conduct a comprehensive WPS qualification campaign for GMAW-CO₂ overlay on the company's most common base metal/overlay combinations (e.g., Q345B/E309L, Q345B/E316L, Q345B/ERNiCrMo-3) per AWS D10.9 and ASME Section IX.
2. Invest in dual-drive wire feed systems and arc monitoring equipment to improve process stability and documentation quality.
3. Develop a standard multi-layer overlay procedure library covering common dilution targets (30%, 15%, 5%) for different overlay alloys.
4. Establish a GMAW-CO₂ overlay defect database correlating process parameters with defect occurrence, enabling predictive quality control.
5. Train operators on the metallurgical principles of CO₂ shielding to enhance problem-solving capability during production.

10. Conclusion

The study of CO₂ gas shielded welding research status represents a foundational knowledge investment for Cladding Technology Shanxi Co., Ltd. GMAW-CO₂ is not merely an alternative process—it is a strategically positioned capability that enables the company to deliver high-volume overlay products at competitive cost, expand its WPS qualification portfolio, and provide customers with technically sound process recommendations. By integrating GMAW-CO₂ expertise across all three technology routes (TIG/MIG overlay, hydraulic explosive bonding, and explosion welding), the company positions itself as a comprehensive cladding solutions provider capable of addressing diverse customer requirements with technical authority and commercial efficiency.

The continued investment in GMAW-CO₂ process development, qualification, and operator training ensures that the company remains at the forefront of weld overlay technology, delivering products that meet the most demanding specifications in oil & gas, chemical processing, power generation, marine, and nuclear industries.