The Effects of Shielding Gas Composition on Stainless Steel Weld Metal Composition

Q: What effect does shielding gas composition have on stainless steel weld metal composition, and can 316L be gas metal arc welded with C-25 (75% Ar/25% CO₂) shielding gas? 

A: ARC Specialties, Houston, Tex., performed some preliminary research on this topic recently, enabling a good opportunity to present results.

While specifications such as AWS A5.18, Specification for Carbon Steel Electrodes and Rods for Gas Shielded Arc Welding, include effects of shielding gas on weld properties, AWS A5.9, Specification for Bare Stainless Steel Welding Electrodes and Rods, only has brief mentions of gas, with no effects on weld composition or properties.

Weld pads were produced using the same 0.045-in.-diameter ER316LSi wire with two welding processes and several shielding gases to assess their effects on weld metal chemical composition. All welds were made on a single heat of 1/2-in.-thick 316L plate. Welds were three or four beads wide and three layers high. The wire feed speed was the same for hot wire gas tungsten arc welding (GTAW-HW), pulsed gas metal arc welding (GMAW-P), and short circuiting gas metal arc welding (GMAW-SC), so the deposition rate would be the same for the five welds. For the GMAW-spray (spray transfer) welds, the wire feed speed was increased to exit globular transfer and enter true spray transfer.

Pure Shielding Gases

Argon (Ar): Pure argon is used for GTAW-HW as it is inert, so little change in weld composition is expected. Weld beads should be clean with low welding fumes. The wire is fed directly into the back of the weld pool.

Pure argon is not used for GMAW because it creates an unstable arc, the weld bead wanders, and the weld bead is too cold to be acceptable in the GMAW-SC mode.

Helium (He): Like argon, helium is inert, but with a higher ionization potential, so the arc voltage is higher, producing more heat into the weld pool. The downside is its much higher cost.

Oxygen (O₂): Oxygen is added to argon for GMAW to stabilize the arc. It stops arc wander, reduces the surface tension of the weld pool so it wets and flows, reduces transition current so it goes into a spray arc at a lower amperage, and reduces the size of the weld droplet across the arc. The negatives would be greater oxidation of elements such as Fe, C, Si, and Mn. The weld surface will be more oxidized with more soot on the plate and more welding fumes. The weld composition will have less carbon, which is good if low carbon is needed for corrosion resistance.

Carbon Dioxide (CO₂): For carbon steel, CO₂ is usually added to a GMAW shielding gas to increase heat input, resulting in improved fusion characteristics and flatter weld beads. When welding stainless steel, CO₂ is added to get some oxidation, stabilize the arc, and improve wetting. The CO₂ has about 1/10 the oxidizing potential of oxygen, so it’s much easier to control the gas mixture. When CO₂ breaks down in the arc, there’s pickup of oxygen and carbon in the weld.

Gas Mixtures

O-2 (98% Ar/2% O₂): The 2% oxygen gas is a standard for GMAW-spray. It provides excellent arc stability and helps to remove carbon from the weld metal.

C-2 (98% Ar/2% CO₂): This gas can be used for GMAW-P and spray arc, providing enough oxidation for wetting and low enough CO₂ to keep carbon content reduced for a low-carbon stainless steel weld deposit. It is too cold for good short circuiting welding. For this study, it was used as the baseline shielding gas for all gas metal arc welds.

C-25 (75% Ar/25% CO₂): Many people familiar with welding carbon steel ask whether they can use C-25 for stainless steel welding to avoid buying the expensive helium tri-mix gas. Yes, stainless steel can be welded with C-25, but it will not produce a low-carbon deposit. If corrosion resistance is a key factor, C-25 is not the right choice.

A-1025 (90% He/7.5% Ar/2.5% CO₂): The 90% helium in this gas adds heat to the weld pool while remaining inert. The argon adds arc stability, and the 2.5% CO₂ gives the oxidation potential needed without adding too much carbon to the weld pool. It is the gas to use for high-
quality GMAW-SC on stainless steel.

Wire Composition

It should be noted that the wire used in the research contains Si and Mn as deoxidizers, which improve weld pool fluidity and wetting, resulting in a smoother, flatter weld bead. The higher the Si and Mn content, the more surface oxides appear on the weld surface. The higher Mn content also improves weld pool wetting and results in a smoother weld profile.

Results

Table 1 shows the chemical compositions of the weld wire and the weld metal with the various gas mixtures.

There were small changes in the content of Cr, Ni, Mo, and Mn in the resulting weld beads, but changes in C and Si are the bigger story. Carbon increased with CO₂. The higher the gas’s oxidation potential, the lower the Si levels. The list below contains more details.

• O-2 – lowest carbon content, oxidized weld surface, excellent spray transfer.
• C-2 – increased carbon content due to the 2% CO₂ in the shielding gas, still a low C deposit.
• A-1025 – better fusion characteristics and weld bead wetting, slightly higher carbon content than C-2, but still low C deposit, optimized for quality GMAW-SC.
• C-25 – no longer a low-carbon deposit. C-25 can be used for welding stainless steel, but the weld metal’s corrosion resistance will be reduced.

Summary

While most are aware of the effects of different shielding gases on the arc and the resulting weld bead appearance/profile and fusion/penetration characteristics, the effects on the composition of stainless steel weld metal are often overlooked. In carbon steels, these effects are minor because the carbon levels are much higher. However, because stainless steels have lower carbon content, these minor changes become significant, as carbon content strongly impacts corrosion resistance. For applications where corrosion resistance is critical, the choice of shielding gas is an important variable.

This was not an exhaustive test, but it verified the effects of O₂ and CO₂ on weld composition.

This article was written by David A. Hebble, technical services manager at ARC Specialties; Richard L. Holdren, senior welding engineer at ARC Specialties; and Richard D. Campbell, American Welding Society Fellow, for the American Welding Society.