Mixed Shielding Gas Systems for Carbon Steel MAG Cladding Arc Stability

Overview and Technical Context

In the manufacture of bimetal products and clad pressure vessels, the Gas Metal Arc Welding (GMAW) process with shielding gas — commonly referred to as MAG welding — is widely employed for the build-up of corrosion-resistant overlay layers on carbon steel and low-alloy steel substrates. The selection and composition of the shielding gas is one of the most critical variables governing arc stability, bead geometry, dilution behaviour, and ultimately the metallurgical quality of the cladding layer. Among the various shielding gas options available, binary mixtures of argon with either carbon dioxide or oxygen — namely Ar+CO₂ and Ar+O₂ blends — have proven to be particularly effective for carbon steel and low-alloy steel MAG cladding applications. This study note examines the technical rationale behind these gas mixtures, their process parameters, and their practical implications in cladding operations.

Shielding Gas Composition and Process Behaviour

The fundamental principle underlying the use of binary shielding gas mixtures in MAG cladding is the synergistic interaction between the inert component (argon) and the active component (CO₂ or O₂). Argon provides a stable, low-ionisation-potential environment that supports a consistent arc, while CO₂ or O₂ introduces an oxidising influence that enhances arc penetration, improves wetting characteristics, and promotes a more fluid molten pool. However, excessive oxidation must be carefully controlled to avoid excessive oxide inclusion formation and to prevent degradation of the overlay layer's corrosion resistance.

Gas Mixture	Typical Composition	Arc Character	Penetration	Bead Appearance	Key Advantage
Ar + 15–25% CO₂	75–85% Ar, 15–25% CO₂	Stable, moderate spatter	Moderate to deep	Smooth, slightly convex	Balanced penetration and oxide control
Ar + 5% O₂	95% Ar, 5% O₂	Very stable, low spatter	Shallow to moderate	Smooth, flat	Excellent arc stability, minimal oxidation
Ar + 10% CO₂	90% Ar, 10% CO₂	Stable	Moderate	Smooth	Good all-round performance
100% CO₂	100% CO₂	Unstable, high spatter	Deep	Rough, convex	High deposition rate, low cost

The critical threshold identified in the literature is that the oxygen content in the shielding gas should not exceed 5% by volume when the objective is to produce a corrosion-resistant overlay layer. When O₂ exceeds this level, the oxide inclusion content in the weld metal rises significantly, and the corrosion resistance of the overlay layer — particularly when stainless steel or nickel-based alloys are used as the cladding material — is compromised. For carbon steel substrate applications where the overlay layer itself is a corrosion-resistant alloy, maintaining O₂ ≤ 5% is essential to preserve the metallurgical integrity of the cladding.

Process Parameter Windows for MAG Cladding with Mixed Gases

The selection of shielding gas composition must be matched with appropriate electrical and travel parameters to achieve optimal cladding quality. The following table summarises typical parameter ranges for MAG cladding of carbon steel substrates using Ar+CO₂ and Ar+O₂ mixtures:

Parameter	Ar+20% CO₂	Ar+5% O₂	Notes
Wire diameter	1.2 mm	1.2 mm	Solid wire or flux-cored
Current	180–280 A	160–260 A	Depends on wire type
Voltage	22–28 V	20–26 V	Lower voltage for O₂ blends
Travel speed	200–400 mm/min	200–400 mm/min	Slower for higher dilution control
Gas flow rate	15–20 L/min	15–20 L/min	Adequate coverage required
Stick-out	12–18 mm	10–15 mm	Shorter stick-out for O₂ blends
Dilution rate	15–35%	10–25%	O₂ blends yield lower dilution

The dilution rate is a particularly important parameter in cladding applications, as it directly affects the chemical composition and corrosion resistance of the overlay layer. Lower dilution rates are generally preferred for high-alloy cladding materials such as Inconel 625 or Hastelloy C276, as they preserve the alloying elements in the overlay. The Ar+O₂ blend, with its shallower penetration characteristics, tends to produce lower dilution rates compared to Ar+CO₂ blends, making it particularly suitable for applications where dilution control is paramount.

Defect Analysis and Countermeasures

Understanding the defects associated with MAG cladding using mixed shielding gases is essential for quality control. The following table summarises common defects, their root causes, and recommended countermeasures:

Defect	Root Cause	Countermeasure
Excessive oxide inclusions	O₂ or CO₂ content too high	Reduce active gas content; maintain O₂ ≤ 5%
Porosity	Inadequate gas coverage; moisture in wire	Increase gas flow; use dry wire; check nozzle clearance
High spatter	Excessive voltage or CO₂ content	Reduce voltage; switch to Ar+O₂ blend
Cracking in overlay	Excessive dilution; high carbon in substrate	Reduce heat input; use lower carbon wire; pre-heat substrate
Poor bead profile	Inconsistent travel speed; gas turbulence	Use mechanised cladding; ensure laminar gas flow

Engineering Practice and Selection Guidelines

In practical engineering applications, the selection between Ar+CO₂ and Ar+O₂ blends is guided by several factors. For applications involving carbon steel substrates with stainless steel (304, 316, 321, 347) overlay layers, the Ar+5% O₂ blend is often preferred due to its superior arc stability and lower dilution rates, which help preserve the austenitic microstructure of the overlay. For applications requiring higher deposition rates and moderate penetration — such as the build-up of thick cladding layers on carbon steel substrates — the Ar+20% CO₂ blend offers a good balance of penetration and deposition efficiency.

It is worth noting that the use of mixed shielding gases in MAG cladding also interacts with other process variables such as wire feed speed, travel speed, and the type of wire used (solid versus flux-cored). The flux-cored wire variant of MAG cladding, when used with Ar+CO₂ mixtures, can achieve even higher deposition rates while maintaining acceptable bead quality, making it suitable for large-scale cladding operations on pressure vessels and heat exchanger tubesheets.

Study Insights and Conclusions

The study of mixed shielding gas systems for carbon steel MAG cladding reveals that the Ar+CO₂ and Ar+O₂ blends represent a highly effective and versatile solution for achieving stable arcs, controllable dilution rates, and high-quality overlay layers. The key insight is that the oxygen content must be strictly controlled at or below 5% to preserve the corrosion resistance of the overlay layer, while the carbon dioxide content can be adjusted between 10% and 25% to fine-tune the penetration and deposition rate according to the specific application requirements. Engineers should always validate the selected gas mixture through process qualification (PQR) testing, including metallographic examination of the overlay layer for oxide inclusion content and dilution rate measurement, before proceeding with production cladding operations.