CO2 and N2 Mixed Gas Shielded Arc Cladding Metal Performance Study

Literature Overview

This 1999 study by Yang Yuanxiu and Kang Fuyi from Hebei University of Science and Technology and Shijiazhuang Valve Factory No. 3 investigates the use of CO2 and N2 mixed shielding gases for arc cladding applications in valve manufacturing. The research is notable for exploring non-traditional shielding gas combinations that deviate from the conventional argon-based approach, potentially offering cost advantages while maintaining acceptable cladding performance.

Technical Background and Motivation

Valve manufacturing frequently requires cladding of wear-resistant or corrosion-resistant surfaces on carbon steel or low-alloy steel components. Traditional GMAW cladding with pure argon shielding provides excellent results but is costly. The study explores whether CO2-N2 mixtures can serve as economically viable alternatives while producing acceptable metallurgical outcomes in the cladding layer.

The valve applications targeted in this research typically require:

• Hardness in the range of 400-600 HV for wear resistance
• Adequate toughness to withstand pressure cycling
• Corrosion resistance in specific process media
• Dimensional stability after thermal processing

Shielding Gas Composition and Performance Results

The study evaluated several CO2-N2 mixture ratios and compared them against pure argon baselines:

The results demonstrate a clear trend: increasing CO2 content in the mixture increases weld hardness and dilution while decreasing arc stability and increasing porosity. Pure nitrogen alone provides poor arc stability and is generally unsuitable for GMAW cladding. However, the CO2-N2 mixtures offer significantly lower costs than argon-based mixtures, making them attractive for high-volume valve manufacturing where cladding quality requirements are moderate.

Microstructural Characteristics

Metallographic examination revealed distinct microstructural differences between argon-shielded and CO2-N2-shielded cladding layers:

• Argon-shielded deposits exhibited fine, equiaxed ferrite-pearlite structures with minimal grain boundary oxidation.
• CO2-N2-shielded deposits showed coarser grain structures with increased grain boundary segregation of manganese and carbon.
• The CO2 component contributed to carburization of the weld metal, increasing carbon content and promoting cementite formation.
• Nitrogen dissolved in the weld metal contributed to nitride formation, which increased hardness but reduced ductility.

The dilution rate was consistently higher with CO2-N2 mixtures due to the deeper penetration characteristics of these gases. Higher dilution means more base metal alloying elements are incorporated into the cladding layer, which can be beneficial when the base metal contains desirable alloying elements but detrimental when the cladding alloy is intended to provide specific composition.

Engineering Applications and Practical Considerations

For valve manufacturers, the selection of shielding gas must balance cost, quality, and productivity. The CO2-N2 mixtures identified in this study are suitable for applications where:

• High hardness is the primary requirement (e.g., valve seats, guides)
• Moderate porosity is acceptable
• Cost reduction is a significant driver
• Post-weld machining is planned to remove surface defects

These mixtures are not recommended for applications requiring:

• High corrosion resistance
• Low porosity (pressure-containing applications)
• High toughness (impact-loaded components)
• Precise dilution control (where base metal contamination is unacceptable)

Process Optimization Recommendations

Based on the study findings, the following process recommendations emerge for CO2-N2 mixed gas cladding:

1. Maintain CO2:N2 ratio between 50:50 and 70:30 for the best balance of hardness and weldability.
2. Use lower welding currents (150-200 A for 1.0 mm wire) to reduce porosity formation.
3. Apply a backing plate or gas backing to minimize back-side oxidation.
4. Maintain wire stick-out length between 10-15 mm to stabilize the arc.
5. Implement post-weld machining to remove surface porosity and oxide inclusions.
6. Conduct batch-level hardness testing to monitor process consistency.

Key Reflections and Study Insights

This study represents an early exploration of cost-effective shielding gas alternatives for industrial cladding applications. While the metallurgical compromises are significant, the economic argument is compelling for high-volume manufacturing environments where absolute quality is secondary to cost-effectiveness.

The finding that CO2 contributes to carburization is particularly important from a metallurgical standpoint. In valve applications where the cladding material is intended to provide specific corrosion resistance, the additional carbon from CO2 can shift the microstructure toward more carbide-rich phases, potentially degrading corrosion resistance. This is a critical consideration that the study acknowledges but does not fully quantify.

The dilution rate increase observed with CO2-N2 mixtures is another concern. For valve applications requiring precise cladding composition, the higher dilution means less control over the final alloy chemistry. Process qualification should include chemical analysis of the cladding layer to verify that composition remains within acceptable limits.

Summary

The investigation of CO2-N2 mixed gas shielding for arc cladding provides valuable data for cost-conscious valve manufacturers. While the metallurgical quality is inferior to argon-based shielding, the significantly lower gas costs make these mixtures viable for applications with moderate quality requirements. Engineers should carefully evaluate the trade-offs between cost savings and quality degradation, ensuring that the specific application requirements are not compromised by the shielding gas selection.