Properties of Arc Cladding Metals with CO2 and N2 Mixed Gas Shielding

Technical Context and Process Rationale

The use of mixed gas shielding (CO2 + N2) in arc cladding represents a cost-effective alternative to pure argon shielding, particularly for iron-based overlay systems where the primary requirement is hardness and wear resistance rather than corrosion resistance. This study systematically examines how the CO2:N2 ratio in the shielding gas mixture influences the chemical composition, microstructure, mechanical properties, and corrosion behavior of GMAW cladding layers. The economic motivation is significant—argon costs 3–5 times more than CO2 and 8–12 times more than N2, making mixed gas shielding attractive for large-scale overlay operations such as pipeline repair, tank lining, and wear plate production.

Experimental Matrix and Key Variables

Effects on Microstructure and Phase Composition

Carbon Activity and Carbide Formation

The CO2 content directly controls the carbon activity in the weld pool through the dissociation reaction: CO2 → CO + ½O2. Higher CO2 percentages increase carbon pickup in the overlay, promoting carbide formation:

• Pure CO2: Carbon content in overlay = 0.35–0.45%, predominantly M7C3 and M3C carbides, hardness HV 450–520
• 70/30 mix: Carbon content = 0.20–0.28%, mixed M7C3 and M3C with some M23C6, hardness HV 400–480
• 30/70 mix: Carbon content = 0.10–0.15%, primarily ferrite with sparse carbides, hardness HV 320–380
• 10/90 mix: Carbon content = 0.05–0.08%, essentially ferritic structure, hardness HV 280–330

Nitrogen Influence on Microstructure

Nitrogen acts as an interstitial hardening element and promotes the formation of nitride phases (Fe4N, Fe2-3N) and austenite stabilization:

• N2 content above 30% begins to noticeably increase retained austenite content (from 5% to 15–20%)
• Nitrogen solubility in the solidified overlay is limited (~0.03–0.05% N), with excess nitrogen forming nitride precipitates
• High N2 content (70–90%) produces a dual-phase ferrite + austenite microstructure with fine nitride dispersion

Mechanical Property Trends

Hardness and Strength

The hardness trend with gas composition follows a parabolic pattern:

• Peak hardness occurs at 70/30 CO2/N2 ratio (HV 480–520), benefiting from optimal carbon activity and moderate nitrogen hardening
• Yield strength follows a similar trend, peaking at 70/30 (YS = 650–720 MPa)
• Tensile strength is relatively insensitive to gas composition (YS/TS ratio varies by < 10%)

Toughness and Ductility

• Elongation decreases monotonically with increasing CO2 content (from 22% at 10/90 to 8% at pure CO2)
• Impact energy (Charpy V-notch at 25°C) drops from 45 J at 30/70 to 12 J at pure CO2
• The 50/50 and 70/30 ratios offer the best toughness-hardness balance for most wear applications

Corrosion Behavior

• CO2-rich shields produce overlays with higher carbon content but better oxidation resistance due to chromium carbide formation
• N2-rich shields produce overlays with lower carbon but potentially better resistance to chloride-induced pitting due to stabilized austenite
• The 50/50 ratio provides the most balanced corrosion performance across multiple environments

Process Stability and Weld Quality

Arc Stability Assessment

Dilution and Bond Strength

• Dilution rate increases with CO2 content due to higher arc energy density (CO2 dissociation adds thermal energy)
• Bond strength (tensile lap test) remains above 350 MPa for all gas compositions tested
• The 70/30 ratio provides optimal dilution control (20–25%) for most carbon steel substrates

Engineering Application Guidelines

Based on the study results, the following selection criteria are recommended:

1. Wear-critical applications (mining, cement, material handling): Use 70/30 CO2/N2 for maximum hardness with acceptable toughness
2. General-purpose overlay (tanks, pipelines): Use 50/50 CO2/N2 for balanced properties
3. Toughness-critical applications (impact loading, low-temperature service): Use 30/70 CO2/N2 for maximum ductility
4. Cost-sensitive large-area cladding: Pure CO2 is acceptable if toughness requirements are not stringent
5. Corrosion-wear dual service: 50/50 ratio provides the best compromise

The study also highlights important practical considerations for field implementation:

• Gas mixing equipment: Proper gas blending requires calibrated mixing valves and flow controllers; inconsistent mixing ratios lead to property variations between batches
• Wind sensitivity: N2-rich mixes are more susceptible to atmospheric contamination in outdoor conditions; wind speeds above 1 m/s can compromise shielding effectiveness
• Wire compatibility: The gas composition must be matched to the wire chemistry; stainless steel wires require different gas ratios than low-alloy steel wires
• Porosity control: N2-rich mixes increase porosity susceptibility; increased gas flow rate (20–25 L/min vs. 15–18 L/min for CO2-rich) is necessary to compensate

Study Insights and Practical Reflections

The most significant practical insight is that the 70/30 CO2/N2 ratio represents a near-optimal compromise for most iron-based cladding applications, providing hardness within 5–8% of pure CO2 results while maintaining significantly better toughness and process stability. This finding has direct economic implications: using 70/30 mixed gas instead of pure argon reduces shielding gas costs by 80–85% while achieving comparable or superior wear performance for carbon and low-alloy steel overlay systems.

A critical engineering consideration that the study reinforces is the interaction between gas composition and wire chemistry. The carbon pickup from CO2 dissociation must be accounted for in the wire specification; if the target overlay carbon content is 0.30%, the wire should be specified at 0.15–0.20% C when using 70/30 shielding, accounting for the additional 0.10–0.15% carbon contributed by the gas. Failure to account for this gas-wire interaction is a common cause of overlay property failures in production.

The research also demonstrates that N2, often considered merely an inert diluent, actually plays an active role in overlay property development through nitrogen hardening and austenite stabilization. This means that N2 content should be treated as a design variable rather than a cost-reduction measure, and gas composition should be specified with the same rigor as wire chemistry and welding parameters in overlay specifications.