MIG Welding stainless to typical steel is troublesome because they are quite different metals. But it becomes harder if you want to use an ordinary wire, such as the ER70S-6. Let’s see the quick answer first.
You will have poor to dangerous results if you weld stainless to mild steel with a typical MIG wire. It will create a bead without sufficient corrosion or heat resistance. The bead will also be somewhat brittle and have lower strength. The final weld may be suitable for quick fixes of low-cost parts but never for high-quality or critical welding.
Why it’s not recommended to weld stainless to mild steel with typical MIG wire
First, let’s see why it’s bad practice to weld those metals with the common cheaper wires and gas.
1. Typical mild-steel wires such as the popular ER70S-6 don’t contain chromium like the stainless part.
When chromium comes in contact with the air’s oxygen, it creates a protective oxide that offers high corrosion resistance. If the stainless part is the 308 grade, it contains 18% chromium.
After welding, both the bead and the heat-affected zone (HAZ) of the stainless part will have a low amount of chromium. The HAZ is the area next to the bead that is affected by the welding process.
As a result, not only the bead but the stainless HAZ will start to corrode sooner or later.
2. Steel wires have a high carbon content, while common stainless steel has a minimal amount of carbon. When you weld, the high carbon content contaminates the stainless HAZ. Then it combines with chromium, forming undesired carbides. All carbides are extremely hard and brittle.
As a result, if the joint comes under high mechanical or thermal stress, it is possible to crack and fall apart.
3. When you use a steel wire, you also use the common C25 or 75/25 gas. Since it contains carbon, it further increases the contamination of the stainless part.
4. Furthermore, if the joint comes in constant contact with an electrolyte such as water, galvanic corrosion (twi-global.com) will take effect. As a result, the bead and the damaged stainless HAZ will corrode at accelerated rates.
For the best results, you should use suitable stainless steel wire and gas. For example, the ER309L wire for dissimilar metals and a tri-mix gas with helium if you have a low-amperage MIG welder.
When can you weld stainless to mild steel with normal MIG wire
However, for many small jobs, you can avoid the cost and the trouble of getting the expensive wire and even more the gas cylinder.
You can use normal wires such as the ER70S-6 and C25 gas if your project is small and simple. Or it’s not necessary to keep the corrosion resistance of the stainless part. Art projects are a typical example.
Ensure you don’t weld a joint that:
- Will take strong or frequent impact.
- Is exposed to extremely high or low temperatures.
- Is exposed to highly corrosive environments.
- Will be used for food storage.
The resulting bead will look narrow, tall, and gray. But it will hold the two components together strong enough for lightweight home or art projects.
Some tips to get better results are:
- Created a symmetrical and tight joint.
- Clean the joint as much as possible.
- If your MIG machine has an inductance setting, set it high.
- Never preheat common stainless steel before welding.
- Attach the working clamp on the stainless part.
- Push the torch for improved gas coverage and lower dilution.
- Tack the joint to prevent the stainless part from warping.
- Point the wire slightly more on the stainless component.
- It would be best to apply an anticorrosion coating to protect your work, including the affected stainless HAZ. It can be in the form of spray or paint. But make sure that it’s a suitable product that can remain on the stainless surface for a long time without flaking off.
Finally, you can read this detailed article I wrote on MIG welding stainless steel to mild steel for further information.