Want To Improve Results When MIG Welding Stainless Steel? | MillerWelds

Want To Improve Results When MIG Welding Stainless Steel?

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From material prep to choosing a shielding gas, learn how to reduce weld defects and lower costs when welding stainless steel.
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Closeup of a stainless steel MIG weld

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Stainless steel is used in welding applications that require long life and corrosion resistance — from fabricating fire pits and smokers to repairing damaged household items. 

Using the MIG welding process for stainless steel projects can provide benefits for efficiency while still producing high-quality welds, but it’s important to keep in mind some best practices. 

Read more to learn some basics for MIG welding stainless steel. 

Stainless steel characteristics 

Stainless steel is corrosion resistant thanks to its added chromium content, which sets it apart from mild steel and other materials. This makes it resistant to rust when it’s welded properly. 

Stainless steel is also more sensitive to heat input than mild steel is. Too much heat can result in burning out the chromium content — called carbide precipitation or sugaring — which causes the material to lose its corrosion resistance. This makes it important to control the heat input and maintain a good travel speed when welding stainless steel. 

Additionally, stainless steel is a more expensive material than mild steel. Following some best practices can help reduce weld defects and distortion of the material and keep costs down when working with stainless steel.

Here are seven tips for optimizing results when MIG welding stainless steel:

1. Choose the right shielding gas and filler metal
You don’t want to put too much carbon into the weld when working with stainless steel because it will affect the material’s ability to resist corrosion, so it’s important to choose a shielding gas containing less than 5% carbon dioxide. A tri-mix gas of argon, helium and carbon dioxide is popular for MIG welding stainless steel. The specific mix of gases depends on what type of welding process you’re using. With a conventional MIG process, use a gas mix with more helium. With a pulsed MIG process, use a mix with more argon. A 98% argon and 2% carbon dioxide mix can also be used for welding stainless steel.

To choose the proper filler metal, first determine which type of stainless steel you’re welding, since stainless comes in several varieties. Many alloys commonly used for stainless steel welding are designed to increase puddle fluidity. Some of the most common stainless steel base metals are 304 and 316. Some of the most common welding wires used with stainless steel are 309 and 316.

2. Pay attention to preparation
When preparing a stainless steel base material for welding, it’s crucial to use a stainless steel wire brush or stainless-dedicated grinding wheel for cleaning or grinding the metal. If you use a grinding wheel on mild steel and then use it on stainless steel, it will contaminate the stainless steel and could introduce weld defects or impurities.

3. Watch your technique and speed
Using a push technique (rather than a drag technique) will provide a better weld bead appearance and wetting of the puddle. Proper travel speed is important to help reduce the heat input into the base metal, so avoid slow travel speeds. Also, be cautious of using a weave motion with stainless steel, due to the amount of heat input that is applied with that technique.

4. Adjust the inductance
A typical stainless steel weld can have what is referred to as a “ropey bead” compared to mild steel. Because of the surface tension of the weld puddle the bead likes to set up faster, which doesn’t allow it to flow to the edges of the weld. If you’re using a conventional MIG welding power source that has adjustable inductance control, increasing the inductance will help the puddle flow a bit more.

5. Consider pulsed MIG
The pulsed MIG process can deliver benefits when welding stainless steel, including reduced spatter, less post-weld cleanup and a nice weld bead appearance. Pulsed MIG spray transfer mode can be a good option to help reduce heat input, which can be especially important on thin materials. It also provides the ability to weld out of position, which can’t be done with conventional CV MIG spray transfer because of the difficulty in controlling the fluid puddle.

6. Swap the drive rolls and liner
When setting up the welding power source, be sure to use drive rolls and a MIG gun liner that are dedicated for stainless steel welding, rather than the same ones that are used to weld mild steel. This helps prevent cross contamination in the stainless steel weld. You can either swap out the drive rolls and liner in the gun or have a separate welding gun that is used only for stainless steel welding. Also, because stainless steel filler metals are a bit harder than other types of filler metal, it can be difficult for V-groove drive rolls to get a good grip on the wire to feed it through the gun. Instead, try using V-knurled drive rolls with stainless steel.

7. Use post-flow gas coverage
It’s a good practice with stainless steel to use post-flow shielding gas coverage. This technique is often used in TIG welding, but it can be helpful when MIG welding stainless steel as well. Post-flow helps ensure the weld is protected as the weld pool solidifies and shields the weld puddle from atmospheric contamination as it cools after welding. For proper post-flow, keep the gun nozzle close to the end of the weld for a half second to three seconds to ensure the shielding gas continues to flow back to the weld puddle as it solidifies. 

Optimize stainless steel welds

MIG welding stainless steel successfully requires paying attention to the heat input and following best practices for choosing the right shielding gas and filler metal. Because stainless steel is so sensitive to heat input, that’s a critical factor in maintaining the corrosion resistant properties of the metal and producing good welds. 
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