MIG Welding for Stainless Steel

MIG welding stainless steel may seem like a formidable task at first, but with the right knowledge and techniques, you can become an expert in no time. This guide will walk you through all the aspects of stainless steel welding. We’ll be covering types of stainless steel, equipment setup, pre-cleaning processes, and essential welding techniques. Let’s dive into the key information to help you excel in MIG welding stainless steel.

Understanding the Challenge:

Welding stainless steel poses unique challenges due to its alloy composition. These include iron, chromium, and, in some cases, nickel. The material’s heat retention, thermal expansion, and low heat conductivity make it more demanding to weld compared to mild steel. This is because overheating can lead to carbide precipitation, jeopardising the material’s corrosion resistance and structural integrity.

Identifying Overheating:

Recognizing signs of overheating is crucial. If you overheat the stainless steel, the HAZ and the weld will turn dark purple or black. If the weld is entirely black, it has gone through the carbide precipitation and you should rework it. Straw-coloured, bright yellow, and light blue welds are generally safe, with lighter colours reducing the risk of carbide precipitation and intergranular corrosion.

Equipment Setup:

Proper setup of your MIG welding equipment is fundamental for successful stainless steel welding. Pay attention to shielding gas, polarity, MIG wire selection, and amperage output which we’ll go over in detail below. 

1.Shielding Gas:

You should opt for the “Tri-Mix” gas blend (90% helium, 7.5% argon, and 2.5% carbon dioxide) to achieve optimal bead contour and corrosion resistance. This mixture helps maintain low CO2 levels. Using more CO2 in the mixture can harm the material’s corrosion resistance. 

2. Polarity:

Like almost all MIG welding applications, MIG welding stainless steel requires a direct current electrode positive or DCEP polarity. This means that you should attach the ground clamp to the negative terminal and the MIG gun to the positive terminal. Simply use the MIG welder as if you were welding mild steel; there is no change in polarity.

3. MIG Wire Selection:

You should try to select stainless steel solid MIG wire that matches the base alloy of the stainless steel type being welded. In cases where an exact match is unavailable, opt for a higher stainless alloy MIG wire. You can click here to browse our selection of MIG wire.

4. Amperage Output:

The next step is to determine the correct amperage output through trial and error. You’ll have to consider factors like shielding gas selection, desired penetration, and travel speed. But it’s best to start on a scrap piece with a somewhat lower wire feeding speed and work your way up until you find a setting that works with your travel speed. 

Pre-Cleaning Process:

Thoroughly clean stainless steel surfaces to remove contaminants like paints, oils, and anti-spatter residues. Use dedicated stainless steel brushes or grinding wheels to prevent cross-contamination with mild steel. See, Metal Surface Preparation for Welding Excellence for more information. 

Welding Techniques:

Mastering the art of MIG welding stainless steel involves effective heat and distortion management, proper purging, and attention to travel speed and MIG gun angle.

1. Heat and Distortion Management:

Limiting the weld metal size to the smallest possible size that provides the necessary weld strength is vital. This is because using a lower volume of weld metal reduces the resulting shrinkage stresses and warpage. You also need to ensure a proper and equal joint fit-up. 

You should limit the weld metal size to the smallest possible size that provides the necessary weld strength. Using a lower volume of weld metal reduces the resulting shrinkage stresses and warpage. Additionally, ensure a proper and equal joint fit-up and use fixtures to hold welded parts in place to prevent joint movement while welding. Plus, spacing out tack welds equally over a large joint line may help control distortion when welding thin stainless steel sheets.

2. Purging:

To achieve the highest quality weld on stainless steel, you need to purge the weld. This means that the other side of the welded joint must also be protected from the atmosphere, not just the face of the joint that is protected from the shielding gas from the MIG torch. 

Stainless steel picks up oxygen aggressively when heated, so you must ensure an oxygen-free, shielded environment on both sides of the weld. Otherwise, the weld quality will suffer. To purge a pipe, you need to tape-shut both pipe ends, poke a hole in one of the tapes, and stick a hose with argon gas in the cavity. You don’t need to use a Tri-mix shielding gas for back-side purging because the role of the purging gas is simply to remove the oxygen from the atmosphere. Of course, you can also employ professional pipe purging devices, but this DIY method works well most of the time.

3. Travel Speed and MIG Gun Angle:

You must weld relatively fast to prevent high heat concentration in one spot and avoid carbide precipitation. Finding the sweet spot may take trial and error because if you move the MIG gun too quickly, you may not achieve proper penetration. But going slow will likely result in black, overly fried welds. A good middle ground would be to use a push technique with a stringer bead for optimal results.

4. Post-Flow Shielding Gas Coverage:

Using post-welding shielding gas flow protects the heated metal from oxidising. Under no circumstances should you remove the MIG gun torch from the end of the welding bead immediately after extinguishing the arc. Instead, keep the MIG gun nozzle with post-flow enabled over the end of the weld for about 3 seconds (or more if you use high amperage to weld thick steel). This will prevent weld contamination as it solidifies and cools down.

Final Thoughts

MIG welding stainless steel is a learnable skill that offers numerous benefits. Despite its challenges, practice and dedication will enhance your proficiency. Equip yourself with the knowledge presented here, and with time and experience, you’ll elevate your welding capabilities, creating high-quality projects with stainless steel.