Can You Weld Stainless Steel to Mild Steel with Flux Core Wire?

Introduction

Welding is a skill that many people pick up for DIY projects, repairs, or even professional work. If you’re someone who has worked with metal before, or you’re just getting started, you may have come across this common question: Can you weld stainless steel to mild steel with flux core wire?

The short answer is: Yes, you can — but there’s a little more to it than just grabbing your welder and going to town.

In this blog, we’ll explain everything in simple language. You don’t need a technical background to understand it. We’ll walk you through what happens when you try to weld these two different types of metal together, the tools and materials you’ll need, and a few things to watch out for. We’ll also explain what happens to your weld over time — especially if it gets wet or used outdoors.

Let’s dive in.

What’s the Difference Between Stainless Steel and Mild Steel?

Before you start welding, it helps to understand what makes these two metals different.

• Mild steel is the most common type of steel used in things like fences, car parts, tools, and furniture. It’s cheap, strong, and easy to weld. But it can rust if you leave it outside or in water.
  
  
• Stainless steel, on the other hand, is made with extra stuff like chromium to help it resist rust and corrosion. It’s used in kitchens, medical tools, and places where it might get wet or exposed to harsh weather.
  
  

So when you try to weld these two together, you’re dealing with metals that behave differently — especially when it comes to rust, strength, and temperature.

Can You Use Flux Core Wire for Welding These Together?

Yes, you can use flux core wire to weld stainless steel to mild steel. In fact, flux core welding is popular because it doesn’t need gas tanks, and you can use it outside where there might be wind. That’s a big plus if you're working on a farm gate or something in your backyard.

But here’s the thing: not all flux core wires are the same. Some are made for welding mild steel, while others are made for stainless steel. If you're planning to join stainless to mild steel, you should ideally use a flux core wire that’s made for stainless steel.

That way, you’ll get a better bond, and the stainless side of your weld will still resist rust.

Why Welding These Metals Together Is Tricky

Even though it’s possible, welding stainless steel to mild steel isn’t the easiest thing to do.

Here’s why:

1. They expand and shrink at different rates. When you heat metal during welding, it expands. When it cools down, it shrinks. Stainless and mild steel don’t move the same way when they’re heated. This can cause your weld to crack if you're not careful.
   
   
2. Rust protection can break down. Stainless steel usually stays rust-free — but when you weld it to mild steel, that protective magic can wear off in the welded area. You might lose the rust-resisting powers, especially if the piece is going to get wet a lot.
   
   
3. Mild steel might “pollute” the stainless part. During welding, the mild steel can mix into the stainless steel and weaken its ability to resist rust.
   
   
   
   
   
   

Is Stainless Steel Waterproof?

This is a common question, especially when people are welding it for outdoor use. So let’s clear this up:

Is stainless steel water proof?

Well, not exactly. No metal is truly "waterproof." But stainless steel is highly resistant to water damage. It won’t rust easily, which makes people think it’s waterproof. However, if it’s scratched, or if it’s welded poorly, even stainless can start to rust over time.

So, if you’re welding stainless steel to mild steel, and that piece will sit outside in the rain, it’s a smart move to add a protective coating like paint or sealant afterward — especially over the welds.

How to Weld Stainless Steel to Mild Steel with Flux Core Wire – Step-by-Step

You don’t need fancy equipment to do this. Here’s a basic outline of how to do it using flux core wire:

1. Get the Right Flux Core Wire

Use a flux core wire that’s designed for stainless steel. It usually has a name like “309L” or something similar. This type of wire is made to handle the mixing of different metals.

2. Clean Both Surfaces

Before you weld, use a wire brush or grinder to clean the surfaces of the mild steel and stainless steel. This removes rust, dirt, and grease that can mess up your weld.

3. Set Up Your Machine

Use a flux core welder, usually one that runs on 110V or 220V, depending on the thickness of your metal. Set the voltage and wire speed according to the chart on your machine.

4. Tack Weld First

Start by making small tack welds to hold the pieces together. This keeps things from shifting around while you weld the full seam.

5. Weld in Short Sections

Because stainless steel heats up fast, it helps to weld in small bursts. This prevents overheating and cracking. Let it cool between passes if needed.

6. Grind Smooth and Coat the Weld

After you finish welding, grind down any rough spots. If the piece will be outdoors, it’s a good idea to spray some rust-resistant paint or sealant over the weld to protect the mild steel part.

 

What About Long-Term Durability?

This is where people get worried. They want to know: Will my weld last? Will it rust? Will it fall apart?

If you do it right — and you protect the weld — your project can last for years. But you should know:

• The stainless steel side will stay strong and rust-free, unless it was overheated or contaminated during welding.
  
  
• The mild steel side is the weak link. It will rust first, especially around the welds if left unprotected.
  
  

So, always finish your project with a protective coating if it’s going to live outside, near water, or in a damp place.

What About Strength?

Welding stainless steel to mild steel isn’t just about rust — people also worry about strength.

In most light-duty or home projects, the weld will be strong enough, especially if you use the right wire and clean your surfaces well.

But if your project needs to carry a lot of weight — like a trailer hitch or structural frame — then welding these two different metals might not be your best bet. You should talk to a pro or stick to one type of metal for high-stress builds.

Bonus Tip: Try Using a Triangular Bar

If you want to experiment with shapes and structure, using a triangular bar can give your welded joint more strength. The triangle shape naturally spreads out pressure and can help your project hold up better over time.

Whether you’re making a support frame or decorative railing, triangular bars can be a neat addition to your build — and they’re pretty easy to weld to both stainless and mild steel.

Is Stainless Steel Non-Ferrous?

Let’s bust another myth while we’re here.

Some folks wonder: Is stainless steel non-ferrous?

The answer is: It depends.

• Ferrous metals contain iron and are magnetic.
• Non-ferrous metals don’t contain iron and are usually not magnetic (like aluminum, copper, and brass).
  
  

Most stainless steels are ferrous, meaning they do contain iron and can be magnetic — especially the cheaper or lower-grade ones. So no, stainless steel is not fully non-ferrous.

But this matters for welding because mixing ferrous (like mild steel) and stainless steel can still work — as long as you use the right welding wire and techniques.

Final Thoughts

So, can you weld stainless steel to mild steel with flux core wire?

Yes, you absolutely can. But you’ve got to:

• Use the right flux core wire (preferably for stainless steel),
• Clean your metal surfaces,
• Weld carefully to avoid overheating,
• Protect your weld after it’s done — especially if water is involved.

Whether you're fixing a gate, building a shelf, or doing a creative project, welding stainless to mild steel is totally doable for beginners and hobbyists alike. Just take your time and follow these simple steps.

And remember:

• Stainless steel isn’t totally waterproof — so protect your welds if water is a factor.
  
  
• Use strong shapes like a triangular bar to add support.
  
  
• And don’t assume stainless steel is non-ferrous — most of it still has iron.
  
  

Got a project in mind? Give it a try — just make sure your welds are safe, strong, and protected for the long haul.