MIG welding is one of the most versatile and popular welding techniques for joining stainless steel. However, it requires some adjustments and precautions to achieve optimal results. In this blog post, we will share some tips and tricks on how to MIG weld stainless steel like a pro.
I. What is MIG Welding?
MIG stands for Metal Inert Gas, which refers to the shielding gas that protects the weld from atmospheric contamination. MIG welding is also known as Gas Metal Arc Welding (GMAW) or wire welding because it uses a continuous wire electrode that is fed through a welding gun and forms an arc with the base metal.
MIG welding is suitable for welding a variety of metals, including stainless steel, carbon steel, aluminum, copper, and more. It offers many advantages, such as:
High productivity and efficiency
Low spatter and slag
Good weld appearance and quality
Easy to learn and operate
Ability to weld in different positions and directions
II. What are the Challenges of MIG Welding Stainless Steel?
Stainless steel is a metal that contains chromium, nickel, and other alloying elements that give it corrosion resistance and strength. However, these elements also make it more difficult to weld than mild steel. Some of the challenges of MIG stainless steel welding are:
Heat sensitivity: Stainless steel has a lower thermal conductivity and a higher coefficient of thermal expansion than mild steel. This means that it heats up and cools down faster, and expands and contracts more during welding. This can cause distortion, warping, cracking, and loss of corrosion resistance if not controlled properly.
Carbon contamination: Stainless steel has a low carbon content, usually less than 0.03%. However, when welding with a carbon steel wire or a shielding gas that contains carbon dioxide (CO2), some of the carbon can transfer to the stainless steel and form carbides. Carbides are hard and brittle compounds that reduce the corrosion resistance and ductility of stainless steel.
Arc instability: Stainless steel has a higher electrical resistance than mild steel, which means that it requires a higher voltage and amperage to maintain a stable arc. However, too high of a voltage or amperage can cause excessive heat input, spatter, and burn-through. Therefore, finding the right balance of welding parameters is crucial for MIG welding stainless steel.
Read How to Weld Stainless Steel Easily: 3 Common Methods Compared.
III. How to MIG Weld Stainless Steel Like a Pro?
To overcome the challenges of MIG welding stainless steel, you need to follow some best practices and tips. Here are some of them:
1) Choose the Right Shielding Gas and Filler Metal
The shielding gas and the filler metal are two important factors that affect the quality and performance of your weld. For MIG welding stainless steel, you need to choose a gas and a wire that are compatible with the type and grade of stainless steel you are welding.
The most common shielding gas for MIG welding stainless steel is a mixture of argon and CO2, usually in a ratio of 75% to 25%. This gas provides good arc stability, penetration, and weld appearance. However, it also introduces some carbon to the weld, which can reduce the corrosion resistance of stainless steel. Therefore, you need to use a filler metal that has a higher alloy content than the base metal to compensate for the carbon dilution.
For example, if you are welding 304 stainless steel, which has 18% chromium and 8% nickel, you can use a 308L wire, which has 20% chromium and 10% nickel. The L stands for low carbon, which means that the wire has less than 0.03% carbon. This helps to prevent carbide formation and improve the corrosion resistance of the weld.
Alternatively, you can use a tri-mix gas, which consists of argon, helium, and CO2, usually in a ratio of 90% to 7.5% to 2.5%. This gas provides better arc stability, penetration, and weld appearance than the argon-CO2 mix, and also reduces carbon dilution. However, it is more expensive and requires a higher flow rate.
Another option is to use a pure argon gas, which provides excellent corrosion resistance and weld appearance but also has some drawbacks. Pure argon has low arc stability, which can cause arc wandering, spatter, and poor penetration. It also requires a higher voltage and amperage, which can increase the heat input and distortion. Therefore, pure argon is only recommended for thin gauge stainless steel or for short-circuit transfer mode.
The filler metal you choose should match the type and grade of stainless steel you are welding, as well as the shielding gas you are using. For example, if you are welding 316 stainless steel, which has 16% chromium, 10% nickel, and 2% molybdenum, you can use a 316L wire, which has the same alloy composition but with a low carbon content. If you are using a tri-mix gas, you can use a 316LSi wire, which has a higher silicon content to improve the wetting and fluidity of the weld pool.
For details, read Which Shielding Gas Should You Use for MIG/MAG Welding?
2) Pay Attention to Preparation
Before you start welding, you need to prepare your workpiece and your welding equipment. This will ensure a smooth and trouble-free welding process. Here are some steps to follow:
Clean the metal: Stainless steel is prone to contamination from dirt, oil, grease, rust, and other impurities. These can affect the weld quality and cause porosity, cracking, and loss of corrosion resistance. Therefore, you need to clean the metal thoroughly before welding, using a degreaser, a wire brush, or a grinder. Make sure to use a stainless steel brush or a grinder with a stainless steel wheel, as using a carbon steel brush or wheel can introduce carbon to the stainless steel and cause carbide formation.
Clamp the metal: Stainless steel tends to warp and distort more than mild steel due to its heat sensitivity. To prevent this, you need to clamp the metal firmly to a flat and rigid surface, such as a welding table or a jig. You can also use tack welds to hold the metal in place, but make sure to remove them after welding, as they can create stress concentrations and cracks.
Set up your welder and gas flow: You need to adjust your welder settings according to the type and thickness of stainless steel you are welding, the shielding gas you are using, and the transfer mode you are using. As a general rule, you need to use a higher voltage and amperage for stainless steel than for mild steel, as stainless steel has a higher electrical resistance. However, you also need to avoid excessive heat input, as it can cause distortion, burn-through, and loss of corrosion resistance. Therefore, you need to find the optimal balance of welding parameters that will produce a stable arc, good penetration, and a smooth weld bead. You can refer to the manufacturer’s manual or a welding chart for some recommended settings, but you may also need to fine-tune them based on your experience and preference. You also need to set your gas flow rate according to the type and size of your gas nozzle, the welding position, and the wind conditions. A typical gas flow rate for MIG welding stainless steel is between 15 to 25 cubic feet per hour (CFH).
3) Watch Your Technique and Speed
The way you hold and move your welding gun can have a significant impact on the quality and appearance of your weld. Here are some tips to follow:
Hold the gun at a 10 to 15-degree angle from the vertical, with the nozzle pointing slightly toward the direction of travel. This will allow the shielding gas to cover the weld pool and the arc, and also help to push the molten metal into the joint.
Keep the stick-out (the distance between the tip of the wire and the base metal) as short as possible, usually between 1/4 to 3/8 inch. This will improve the arc stability, penetration, and weld appearance. However, if you are using a short-circuit transfer mode, you can use a longer stick-out, up to 3/4 inch, to reduce the heat input and spatter.
Maintain a consistent travel speed and a steady hand. You need to move the gun fast enough to avoid overheating the metal, but not too fast to compromise the penetration and the fusion. A good travel speed will produce a uniform and smooth weld bead, with a slight convex shape and a width that is two to three times the diameter of the wire. You can use a weaving or oscillating motion to widen the weld bead and fill the gap, but avoid excessive weaving, as it can cause cold lapping, undercut, and porosity.
Use a push or a drag technique, depending on the transfer mode and the position you are welding. A push technique means that you point the nozzle toward the direction of travel, while a drag technique means that you point the nozzle away from the direction of travel. A push technique is preferred for spray transfer mode, as it provides better gas coverage and a flatter weld bead. A drag technique is preferred for short-circuit transfer mode, as it provides better penetration and a narrower weld bead. For vertical and overhead positions, a drag technique is also recommended, as it prevents the molten metal from sagging or falling.
4) Adjust the Inductance
Inductance is a feature that some MIG welders have, which allows you to control the amount of current that flows through the arc. Inductance affects the smoothness and the spatter of the arc, as well as the shape and the appearance of the weld bead.
A low inductance setting will produce a crisp and fast arc, but also a lot of spatter and a narrow and convex weld bead. A high inductance setting will produce a soft and slow arc, but also less spatter and a wider and flatter weld bead. Therefore, you need to adjust the inductance according to the type and thickness of stainless steel you are welding, the transfer mode you are using, and the weld appearance you want to achieve.
As a general rule, you need to use a higher inductance setting for thicker and harder stainless steel, such as 316 or 410, as it will help to reduce the spatter and improve the fusion. You also need to use a higher inductance setting for spray transfer mode, as it will help to smooth the arc and produce a flatter weld bead. However, you need to use a lower inductance setting for thinner and softer stainless steel, such as 304 or 430, as it will help to increase the penetration and prevent burn-through. You also need to use a lower inductance setting for short-circuit transfer mode, as it will help to create a crisp and fast arc and produce a narrower weld bead.
You can refer to the manufacturer’s manual or a welding chart for some recommended inductance settings, but you may also need to fine-tune them based on your experience and preference. You can test the inductance by welding on a scrap piece of metal and observing the arc, the spatter, and the weld bead. You can increase or decrease the inductance until you find the optimal setting for your welding situation.
5) Post-Weld Cleaning and Finishing
After you finish welding, you need to clean and finish your weld to improve its appearance and performance. Here are some steps to follow:
Remove the slag and spatter: If you used a flux-cored wire or a shielding gas that contains CO2, you may have some slag and spatter on your weld and the surrounding metal. You need to remove them using a chipping hammer, a wire brush, or a grinder. Make sure to use a stainless steel brush or a grinder with a stainless steel wheel, as using a carbon steel brush or wheel can introduce carbon to the stainless steel and cause carbide formation.
Remove the heat tint and oxides: Heat tint is the discoloration that occurs on the surface of stainless steel due to the high temperature of welding. It can range from straw to blue to purple, depending on the temperature and the type of stainless steel. Heat tint and oxides can reduce the corrosion resistance and the aesthetic appeal of stainless steel. Therefore, you need to remove them using a chemical pickling paste, mechanical polishing, or electrochemical cleaning. You can also prevent heat tint and oxides by using a backing gas, such as argon or nitrogen, to shield the back side of the weld from atmospheric contamination.
Apply a protective coating: If you want to enhance the corrosion resistance and the appearance of your weld, you can apply a protective coating, such as a passivation, a painting, or a plating. Passivation is a process that involves immersing the stainless steel in a nitric or citric acid solution, which removes any free iron and forms a thin chromium oxide layer on the surface. Painting is a process that involves applying a primer and a topcoat of paint, which provides a barrier against moisture and chemicals. Plating is a process that involves depositing a thin layer of another metal, such as nickel or chrome, on the surface of stainless steel, which provides a decorative and protective finish.
For details, read How To Clean Stainless Steel Welds After Welding? [Easiest Way].
Conclusion
MIG welding stainless steel is not as difficult as it may seem, as long as you follow some basic guidelines and tips. By choosing the right shielding gas and filler metal, paying attention to preparation, watching your technique and speed, adjusting the inductance, and post-weld cleaning and finishing, you can achieve a professional and high-quality weld on stainless steel. We hope this blog post has helped you to learn how to MIG weld stainless steel like a pro. If you have any questions or comments, please feel free to contact us at: https://www.megmeet-welding.com/en/contacts.
Related articles:
1. Why Does Stainless Steel Rust After Welding?
2. Stainless Steel Welding: What is it, Benefits, & Methods
3. Stainless Steel Weld Cleaning: A Complete Guide to Methods & Techniques
4. TIG Welding Stainless Steel: A Beginners Guide
5. How To MIG Weld Stainless Steel Like a Pro?
