Welding Chromoly To Mild Steel

We can see tons of welding applications looking around. But if I have to pick fields where welding is widely used, it has to be oil and gas pipelines, construction works, or the automotive industry.

Do you know which metal alloy is common in all those sectors? It’s Chromium-molybdenum alloy, or as you may know it, chromoly.

On the other hand, mild steel is another common metal used for these same projects. So, the question is, can you be welding chromoly to mild steel?

Let’s find out.

What Is The Difference Between Mild Steel And Chromoly?

Before we begin, let’s get to know the two metals we have in our hands. Both metals are very common in different welding jobs, and some critical differences set both materials apart.

Let’s have a glance at the differences.

  • The main difference lies in how both steels are formed. Chromoly is an alloy of chromium and molybdenum. While chromium results in hardenability and corrosion resistance, molybdenum increases the strength and prevents temperature fluctuations .On the other hand, mild steel is an alloy of iron and carbon. The carbon content in mild steel is lower, which increases the melting temperature. As a result, mild steel is more ductile and therefore perfect for fabricating.
  • Regarding raw durability and strength, chromoly is miles ahead of mild steel. In projects where durability is super important, for example, pipeline works and automobile components.
  • The strength alone is insufficient to justify chromoly supremacy as it offers a better weight-to-strength ratio than mild steel. It means, for the same weight, you get a higher tensile strength than mild steel.
  • Chromoly and mild steel have different applications. While chromoly is used for more demanding fields like pipeline welding, automobile parts, and other heavy machineries, mild steel is used for lighter stuff like furniture, decorations, structural steel, etc.
  • Another difference between chromoly and mild steel is the price discrepancy. Chromoly is expensive to manufacture while mild steel is the least expensive type of steel, making it widely available and used.

Can You Weld Chromoly To Mild Steel?

Yes, you can weld chromoly to mild steel. You can use several welding techniques for this job. There are several filler materials you can use, like ER80S-D2 and ER70S-2. Use the filler material that suits your need for tensile strength and flexibility.

How To Weld Chromoly To Mild Steel (Describe Step-By-Step Guide)

Due to the higher price and complexity of the metal, you must be extra careful while welding chromoly to mild steel.

The steps I showed below should ease up the process for you.

  • Step 1: You first need to clear both chromoly and mild steep pieces you have. Any rust, contamination or painting can affect the durability and appearance of the weld.
  • Step 2: Get the torch set up when the materials are ready. Get a torch that’s lightweight so that long hours of chromo don’t put stress on your hand. Also, the torch should have a wide nozzle to reach tight corners.
  • Step 3: After the torch, get the filler material ready. Now, a few filler materials options will be available like ER80S-D2, ER70S-2, and 4130 with different tensile strengths. Here, better tensile strength means lower flexibility.
  • Step 4: When everything mentioned above I ready, get to welding chromoly. Chromoly is known for its heat sensitivity, so it starts with lower heat and smaller tacks. Then, gradually increase the heat and use a few larger tacks to maintain consistency.
  • Step 5: Don’t hurry about cooling the weld after the welding is complete. Let the weld cool down at its own pace. Any outside source of air can make the cooling faster, and it can make the weld brittle.

What Type Of Welder Is Best For Welding Chromoly?

Now that you know you can weld chromoly to mild steel, you might prepare to do it. But do you know what type of welder will be best suited for the job? Well, let’s find out.


MIG welding chromoly with mild steel is doable, but there are a few catches. First, you need to control the temperature strictly; otherwise, the weld will be brittle.

Then you must use the correct wire, like ER80S-D2 or ER70S-2 and avoid 4130 steel.


TIG welder is the best tool if you are planning to weld chromoly with mild steel. There are several benefits of doing it, including it’s the safest, cleanest and most convenient welding method available.


Stick welders might have a reputation for wide usage and high-quality weld, but they are not suitable for chromoly. For thinner metals, TIG and MIG welders are better suited. A stick welder might penetrate the chromoly with higher strength and ruin the welding.


If you happen to have a flux core welder, you can still manage to weld chromoly to mild steel with T-1 flux-cored wires. However, you are less likely to do it since flux welders are best for welding dirty or rusted metals outdoors.

A Few Tips For The Chromoly To The Mild Steel Welding Process

Now that you know everything about welding chromoly to mild steel, here are some bonus tips to help you obtain better results.

  • Use a filler material with lower tensile strength if you need more flexibility.
  • Remember that using the same 4130 steel as the filler will make the weld prone to cracking.
  • As chromoly is usually pretty thin, you don’t need to preheat it. Preheat only if the chromoly sheet is thicker than .12 inches.
  • If your welding job has tight corners, ensure you get a large diameter gas lens.
  • You can use a TIG welding method to weld chromoly with mild steel but go slower than usual.
  • Maintain a short distance between the tungsten and chromoly to get a narrower cone. 


Can you weld chromoly with flux core?

Many people might be unaware of this, but you can weld chromoly with flux core wire. They offer a smooth welding experience with minimum spatter and cleanup. This welding is done with a CO2 alone or a mix of CO2 and Argon gas.

Is chromoly lighter than mild steel?

It’s a common misconception people have is that chromoly is lighter than mild steel. The misconception is often generated by chromoly having a better weight-to-strength ratio. But that doesn’t make chromoly any lighter than mild steel.

Last Words

That’s a wrap for today. As a welder, I know various projects require welding chromoly to mild steel. But due to the lack of proper welding knowledge and the complexity o welding, welders often don’t know whether it’s possible.

Now that you know it’s possible and how to do it, you can easily weld chromoly with mild steel without confusion.


  • James Smith

    This is James Smith, a Hobbyist welder, welding since my 15th birthday. My father was a professional welding mechanic at a garage and used to take me to garage. My love for welding started then and I learnt basics from my father. At my 15th, my father gifted me a stick welder and my journey to become a welder began. Gradually I accustomed to welding and mastered at tig welding.

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