Beatrice Lang
Electric MIG welding stainless steel can be a daunting task, but with practice, it can become as natural as welding mild steel. Stainless steel is renowned for its corrosion resistance and is used where hygiene is essential, but it distorts easily when heated. To prepare for MIG welding, you must first select the appropriate wire for the base metal and ensure that your welding equipment is set up correctly. It is also important to clean the welding joint from contaminants like paints, oils, grease, and other hydrocarbons, as they can introduce carbon into the weld and weaken the joint. When welding, the welding gun should be oriented at between 5 and 15 degrees in the direction of travel, and you must control the heat input to avoid warping the stainless steel.
| Characteristics | Values |
|---|---|
| Stainless steel type | Austenitic stainless steel, 300-series, grades 304 or 316 |
| Wire type | 308L wire for grades up to 316L, ER316LSi wire for 316/316L stainless alloys, ER308LSi wire for 302, 304, 304L, 305, and 308 stainless steels |
| Shielding gas | Less than 5% carbon dioxide content, Argon mixture with 2% oxygen, Tri-mix of 90% helium, 7.5% argon, and 2.5% carbon dioxide |
| Welding technique | Orient welding gun at 5-15 degrees in the direction of travel, use a push technique, pulsed MIG for reduced heat input and spatter |
| Heat control | Critical to avoid distortion and loss of corrosion resistance |
| Pre-welding | Clean joint with a dedicated stainless steel brush to remove contaminants |
| Safety | Local exhaust ventilation (LEV) and personal protective equipment (PPE) like air-fed welding masks |
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What You'll Learn
Choose the right wire
When it comes to choosing the right wire for electric MIG welding on stainless steel, there are several factors to consider. Firstly, it is important to understand the unique advantages offered by different types of MIG wires, such as required speed, size, and strength. Selecting the wrong wire can lead to a failed welding project.
While normal MIG wire is a common option for welding steel, it is not the best choice for stainless steel. Stainless steel is composed of more chromium and less carbon, giving it higher tensile strength, impact resistance, and corrosion resistance than mild wire. Using normal MIG wire on stainless steel will not produce clean and strong joints. Instead, it may result in weld spatter, which can become sharps or burrs on the weld joint, potentially causing injury.
The best type of MIG welding wire for stainless steel is stainless steel wire itself. ER308L wires are among the most common selections as they are versatile and safe for welding stainless steel materials. They are also suitable for welding metals of similar composition, such as 201, 202, 301, 302, 304, 305, and 308 (and L Series). If you are working with a higher grade of stainless steel, you may need to consider an ER 316L electrode. Additionally, the ER309L electrode is used for welding stainless steel to steel, but it produces a rough-looking weld and is challenging to work with.
Another option is to use flux-cored wires, which have specially formulated chemical compounds inside their core. These wires are tubular, with only the outer layer acting as the filler metal. Gas-shielded flux-cored wires offer higher deposition rates and are suitable for welding thick metals. They also work well for out-of-position welding due to the slag that helps hold the molten metal pool against gravity.
It is also important to consider the shielding gas used in conjunction with the welding wire. Shielding gas is critical to MIG welding, and inappropriate or insufficient gas can cause major weld problems. The gas chosen must have less than 5% carbon dioxide content to prevent a decrease in corrosion resistance. Common shielding gas mixtures for MIG welding stainless steel include an argon mixture with 1-2% oxygen and a tri-mix of helium, argon, and CO2.
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Prepare your equipment
Preparing your equipment is the first step in electric MIG welding stainless steel. It can be a daunting task, but with the right tools and setup, you can achieve strong, good-looking welds. Here's a detailed guide to help you prepare your equipment effectively:
Choose the Right Wire and Shielding Gas:
Selecting the appropriate wire for the base metal is crucial. Most stainless steel welds use austenitic stainless steel, typically grades 304 or 316. For these grades, you can use 308L wire, which is suitable for various stainless steel grades up to 316L. Ensure the wire matches the type of stainless steel alloy you are welding. For instance, use ER316LSi wire for 316/316L stainless alloys.
Shielding gas plays a critical role in MIG welding stainless steel. It must have less than 5% carbon dioxide content to prevent weld defects and maintain corrosion resistance. Common shielding gas mixtures include an Argon mix with 2% oxygen and a Tri-mix of 90% helium, 7.5% argon, and 2.5% carbon dioxide. Avoid standard MIG gases for mild steel, such as 75/25 Argon/CO2 or pure CO2, as they have excessive carbon content, leading to poor welds.
Clean the Welding Joint:
Before welding, clean the welding joint thoroughly to remove contaminants like paints, oil, grease, primers, cutting compounds, waxes, pencil marks, soap, and hydrocarbons. Use a dedicated stainless steel brush for cleaning to avoid cross-contamination with mild steel particles, which can negatively impact the welds.
Adjust Welding Machine Settings:
When welding stainless steel, you may need to adjust the settings on your welding machine. Pulsed MIG welding can reduce spatter and heat input, improving weld appearance. Increasing inductance in conventional MIG welding power sources can also help the weld puddle flow more effectively.
Safety Considerations:
Stainless steel welding carries potential health risks, including exposure to Cadmium and Chromium fumes. Ensure you have suitable safety measures in place, such as local exhaust ventilation (LEV) and personal protective equipment (PPE), including air-fed welding masks.
Additional Tips:
- Stainless steel melts more easily and retains heat longer than mild steel, so adjust your welding speed and angle accordingly.
- Use drive rolls and a MIG gun liner dedicated to stainless steel welding.
- Consider using post-flow shielding gas coverage to protect the weld as it solidifies.
- Limit the weld metal size to only what is necessary to reduce shrinkage stresses and avoid distortion.
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Understand stainless steel
Stainless steel is a widely used metal across most sectors due to its durability, corrosion resistance, and aesthetic appeal. It has a high melting point and does not deform or weaken under high temperatures. It is also resistant to bacterial growth, making it a safe choice for food preparation, medical equipment, and transporting corrosive chemicals.
Like many metals, stainless steel comes in various grades with different alloying elements. Austenitic stainless steel, for instance, contains 16-26% chromium and 8-22% nickel, while ferritic steel has a chromium content of 12-18%. Martensitic steel has a chromium content ranging from 11-28%. The base metal composition must match the material welded to each type of steel.
Stainless steel's lower thermal conductivity means heat energy is not quickly dispersed from the welding zone, which is helpful during welding. However, its lower thermal expansion coefficient requires careful joint preparation to avoid gaps or misalignment. It also effectively retains heat, causing it to warp when exposed to high temperatures. Therefore, heat control is essential when welding stainless steel.
Welding stainless steel requires specialised equipment and expertise. Its unique properties demand precise control of parameters such as current, shielding gas, and interpass temperature. The shielding gas must have less than 5% carbon dioxide content to prevent corrosion resistance decrease.
Overall, stainless steel is a strong and durable material, and welding it creates strong joints. Welded stainless steel structures can withstand heavy loads and resist deformation, making them suitable for applications requiring strength and durability.
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Use the right shielding gas
When MIG welding stainless steel, using the right shielding gas is critical. The wrong gas can cause major weld problems, including severe defects under the surface of the weld. The shielding gas you select must have less than 5% carbon dioxide content. Too much carbon in the weld will decrease corrosion resistance.
The two most commonly used shielding gases for MIG welding stainless steel are an Argon mixture with 2% oxygen and a Tri-mix of 90% helium, 7.5% argon, and 2.5% carbon dioxide. A 5% CO2 mix can also be used, but this will make the joint more brittle. Pure CO2 can be used, but it will not shield the joint properly during welding.
It is also a good practice to use post-flow shielding gas coverage. This technique is often used in TIG welding but can be helpful when MIG welding stainless steel. Post-flow helps protect the weld as the weld pool solidifies and shields the weld from atmospheric contamination as it cools.
To avoid compromising corrosion resistance or warping the stainless steel, you need to control the heat input. Stainless steel retains heat and can distort and lose its corrosion resistance if kept at elevated temperatures for too long.
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Clean the welding joint
To clean the welding joint, use a dedicated stainless steel brush to remove contaminants such as paints, oil, grease, primers, cutting compounds, waxes, pencil marks, soap, and other hydrocarbons. These contaminants can introduce carbon into the weld, weaken the joint, and impair corrosion resistance. Avoid using tools that were previously used on mild steel to prevent cross-contamination. Mild steel particles have a higher carbon content, which can negatively impact stainless steel welds.
It is also important to control the heat input to avoid compromising corrosion resistance or warping the stainless steel. When stainless steel is overheated and kept at high temperatures (between 800 and 1400°F), chromium and carbon form chromium carbides. This causes the chrome to migrate and combine with carbon, depleting chromium in certain zones and reducing the stainless steel's corrosion resistance. If the weld turns dark purple or black, it has gone through the carbide precipitation process and should be reworked. Straw-coloured, bright yellow, and light blue welds are safe if everything else is done correctly.
Use drive rolls and a MIG gun liner dedicated to stainless steel welding, rather than equipment used for mild steel. Additionally, use post-flow shielding gas coverage to protect the weld as it solidifies and shield it from atmospheric contamination as it cools.
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Frequently asked questions
Pulsed MIG welding is the best process for electric MIG welding stainless steel as it reduces spatter, improves weld appearance, and reduces heat input.
When electric MIG welding stainless steel, the ideal angle for the welding gun is between 5 and 15 degrees in the direction of travel.
The two most commonly used shielding gases for electric MIG welding stainless steel are an Argon mixture with 2% oxygen and a Tri-mix of 90% helium, 7.5% argon, and 2.5% carbon dioxide.
The best wire for electric MIG welding stainless steel depends on the type of stainless steel alloy being welded. For example, use ER316LSi wire to weld 316/316L stainless alloys, and ER308LSi wire for 302, 304, 304L, 305, and 308 stainless steels.
Electric MIG welding stainless steel can pose severe health risks due to the release of Cadmium and Chromium (VI) fumes. Therefore, suitable safety measures such as local exhaust ventilation (LEV) and personal protective equipment (PPE), including air-fed welding masks, are essential.
