If you’ve ever had to weld stainless steel to mild steel, you know it’s not as straightforward as working with the same type of metal.
The differences in composition, thermal expansion, and corrosion resistance make the process more challenging. But what if you only have a 7018 electrode? Can you use it for welding stainless to mild steel?
I’ve been in this situation before, and I can tell you that while 7018 is a great low-hydrogen electrode for mild steel, it’s not the best choice for joining stainless steel to mild steel.
If you’re in a pinch and need to make it work, there are some things you need to understand about the process, its limitations, and alternative approaches. Let’s break it all down so you can make an informed decision.
Can You Use 7018 to Weld Stainless to Mild Steel?
The short answer is yes, but it’s not ideal. The 7018 electrode is a carbon-steel-based rod designed for structural welding, and it doesn’t contain the right alloying elements needed for proper fusion with stainless steel. When welding stainless to mild steel, you need a filler material that bridges the gap between both metals.
Here’s what happens when you use 7018 on stainless steel:
- The weld may not have good corrosion resistance.
- The weld metal can become brittle due to carbon pickup.
- The joint might lack strength over time, especially in harsh environments.
So while it’s possible to get a weld with 7018, it’s not going to be strong or durable in the long run.
What Happens When You Weld Stainless with 7018?
When you weld stainless steel to mild steel using a 7018 electrode, the following things occur:
Carbon Contamination
Stainless steel has a high chromium content, which gives it corrosion resistance. But when welded with a carbon steel electrode like 7018, the stainless can absorb carbon, making it more susceptible to rust and weakening its corrosion resistance.
Weaker Weld Strength
The weld won’t have the same mechanical properties as a proper stainless-to-mild steel joint. It may crack under stress or fail over time.
Higher Heat Affected Zone (HAZ)
Since stainless and mild steel expand and contract at different rates, welding them with 7018 can create a larger heat-affected zone, leading to distortion and warping.
Better Alternatives for Welding Stainless to Mild Steel
If you want a strong and corrosion-resistant weld, you should use a proper stainless steel electrode or a filler rod designed for dissimilar metals. Here are better options:
| Electrode/Filler | Best For | Why It’s Recommended |
|---|---|---|
| 309L Stick Electrode | SMAW (Stick Welding) | Contains chromium and nickel, allowing good fusion between stainless and mild steel. |
| 309L MIG Wire | MIG Welding | Best for cleaner and more controlled welds, especially for thin materials. |
| 309L TIG Rod | TIG Welding | Produces high-quality, corrosion-resistant welds with minimal contamination. |
| 312 Stainless Electrode | Stick Welding | Has high strength and ductility, making it great for joining dissimilar metals. |
If you’re doing stick welding (SMAW), the 309L electrode is your best bet. It’s designed for joining stainless to mild steel and will give you a much stronger weld than 7018.
Best Practices for Welding Stainless to Mild Steel
If you’re going to attempt welding stainless to mild steel with 7018, or even using the proper 309L rod, here are some key tips to follow:
Clean the Metals Thoroughly
Contamination is one of the biggest issues in stainless steel welding. Remove dirt, oil, and rust before starting. Use a stainless steel brush to clean the stainless side, and a different brush for the mild steel to prevent cross-contamination.
Control Heat Input
Stainless steel retains heat more than mild steel, so if you overheat it, you risk warping, distortion, or even carbide precipitation, which leads to corrosion. Use a lower amperage setting and avoid excessive heat buildup.
Use the Right Welding Technique
- Stick with short weld beads to prevent overheating.
- If using 7018, try to keep the arc stable and avoid excessive weaving to reduce dilution.
- If using 309L, follow standard stainless welding techniques for the best results.
Choose the Right Shielding Gas (For MIG/TIG)
If using MIG or TIG welding, select the correct shielding gas. A mix of Argon with a small percentage of CO2 is commonly used to prevent oxidation.
Prevent Cracking
Since stainless and mild steel expand differently under heat, cracks can form in the weld over time. Preheating the mild steel slightly can help reduce stress in the joint.
When Would You Use 7018 for This Job?
In most cases, 7018 isn’t the best choice. However, there are a few situations where you might use it:
Temporary Repairs: If you just need to tack-weld stainless to mild steel for temporary use, 7018 can hold the metals together.
Low-Stress Applications: If the welded joint won’t be exposed to heavy stress, moisture, or corrosive environments, you might get away with using 7018.
No Alternative Electrodes Available: If you don’t have access to 309L or 312 stainless rods, and you need to make a weld immediately, 7018 might work as a last resort.
Why Using 309L Is the Better Option
If you’re serious about making a strong and long-lasting weld, 309L electrodes or filler rods are the way to go. Here’s why:
- Corrosion Resistance: They contain the right chromium and nickel content to prevent rusting and oxidation.
- Strong Welds: The weld will have good mechanical strength and won’t become brittle over time.
- Ductility: The weld can handle thermal expansion and contraction better than a weld made with 7018.
Conclusion
While 7018 can technically be used to weld stainless to mild steel, it’s far from ideal. It lacks the necessary alloying elements to create a strong, corrosion-resistant joint.
If you’re in a situation where no other option is available, it may work for a temporary fix, but 309L or 312 electrodes are far superior choices.
If you want a weld that lasts and performs well under stress, always go for the right filler metal. In this case, stainless-to-mild steel welding is best done with a dedicated stainless electrode like 309L.
If you have access to the right materials, never settle for 7018 when welding stainless to mild steel—it’s just not worth the risk of failure in the long run.
FAQs
Can I weld stainless steel to mild steel with 7018?
Yes, but it’s not ideal. 7018 is a carbon steel electrode that doesn’t have the right alloying elements to create a strong, corrosion-resistant weld with stainless steel.
What happens if I use 7018 to weld stainless steel?
The weld may become brittle, lack corrosion resistance, and have a higher risk of cracking due to differences in thermal expansion between stainless and mild steel.
What is the best rod for welding stainless to mild steel?
A 309L stainless electrode is the best choice for welding stainless to mild steel because it contains nickel and chromium, which help create a strong and corrosion-resistant joint.
Can I use 312 stainless electrodes for this job?
Yes, 312 stainless electrodes work well for joining dissimilar metals and provide good ductility and strength.
Why does welding stainless to mild steel require special filler metal?
Because stainless and mild steel have different properties, including thermal expansion, corrosion resistance, and melting points. A special filler metal like 309L helps bridge the gap between the two materials.
