How to Weld Chromoly – A Beginner’s Guide

Welding is a metal fabrication process that is used in numerous manufacturing industries to join different metals together. It fuses two metal surfaces by heating them to the point that they melt and then fuse.

However, not all metals are welded the same way, employ the same techniques, or use the same welding materials. Depending on the application, different combinations of metals that have different strengths have to be used.

Since different metals have different compositions and strengths, they cannot possibly be welded using the same techniques. Metal alloys are used more commonly for various applications as compared to pure metals owing to their strength and durability. One such metal alloy is chromoly.


Chromoly is an alloy that contains the metals chromium and molybdenum, along with some other elements. It is referred to as 4130 steel, which is a code assigned to this alloy by the AISI (American Iron and Steel Institute). This code defines the chemical composition of this steel alloy. 41 indicates that chromoly is a low alloy steel that contains chromium and molybdenum, and 30 indicates 0.30% carbon content. The ratio of chromium in chromoly is usually 0.8 to 1.1%, and molybdenum ranges from 0.15 to 0.25%.

Strength of Chromoly

Chromoly is a strong alloy; much stronger than normal steel (weight for weight). It is often used to make high-end roll cages for racecars, bicycle frames, and fuselages on small aircraft. It has a fairly high strength to weight ratio and an exceptionally high tensile strength. This makes welding chromoly a bit difficult. The welding of chromoly requires a lot more preparation and skill as compared to other alloys.

Welding Chromoly

Although welding chromoly is comparatively different and difficult, following the steps and tricks mentioned below closely will help you weld this alloy like a pro, even if you’re just starting out with using the alloy for your welds! Chromoly can be welded by TIG or MIG welding. This beginner’s guide on how to weld chromoly will efficiently walk you through the process step-by-step to make it simpler for you.

Prepare the Tube

Chromoly is a very thin material. Therefore, any residue that may be left on it after coming from the mill will affect the result of the welding, in terms of appearance and durability. To avoid any problems with the weld in the future, make sure that you prepare the tube well by cleaning it before welding.

It is recommended that you run the tube through a lathe at a slow to medium speed to ensure that the finishing of the chromoly is flawless. This will give you a much cleaner, smoother, and stronger weld.

Fitment and Tacking

Chromoly is a temperature-sensitive metal. When you move on to the next step of welding chromoly, that is, fitment and tracking, you need to make sure that you do not directly expose this metal to heat. Rather, it needs to be done gradually. The arc length should be kept tighter as compared to the usual arc length used for welding other metals.

To keep the chromoly weld in place, you should consider numerous small tacks over 1 to 2 larger tacks as smaller tacks are easier to weld and offer greater consistency.

There should be no gaps at all for the fitment close. In case there are any, you need to close them up. If you fail to do so, a larger heat affected zone will be generated, and your metal will become brittle around that spot.

Chromoly is a hard alloy, and any attempt to make it harder will only increase the possibility of it cracking.

Torch Setup

Choosing the best torch that suits the application of welding is extremely important when welding chromoly. If the welding job is going to last long hours, the weight and mobility of the welding torch are going to affect the ease of use, convenience, and productivity. A high-quality TIG welder should be used if you’re looking to weld this alloy with proper efficiency.

Moreover, having a glass lens over the regular nozzle is also recommended when welding chromoly. A gas lens with a larger diameter will form a larger gas shield that will allow you to hang out the tungsten farther and reach tighter corners with greater ease.

Choosing the Right Filler Materials

There are three options for filler materials when you’re welding chromoly. These include the ER80S-D2, ER70S-2, and 4130 versions of this alloy.

ER80S-D2 and ER70S-2 are the most common choices when it comes to welding chromoly. The ER80S-D2 has a tensile strength of 80,000, which is quite close to that of chromoly itself. This makes it a great option for use as a filler material for chromoly welding.

The tensile strength of ER70S-2 is comparatively lower, that is, 70,000, but offers more flexibility. 4130 chromoly matches the material to be welded, which makes it an option for chromoly welding. However, it cannot be used with Thin Wall Tube.

Strength Under-Match

A filler material with lower strength, like the ER70S-2, can be used as a filler material for chromoly welding when the weld material doesn’t need to match the strength of the chromoly steel material. Such filler materials are associated with benefits such as crack resistance. However, the weld will not match the strength of the chromoly base metal.

Matching Strength

When the chromoly steel structure is in an annealed condition and is not to be subjected to post-welding heating, a filler material of the same strength can be used, such as the ER80S-D2. A metal is annealed to remove any internal stresses to make it tougher. When a filler material with a matching annealed strength is used, the weld is not prone to cracking, and hence, lasts long.

Chemistry Match

In case the chromoly steel has to be hardened post-welding, it is necessary that a filler metal that has the same strength and chemistry should be used, such as 4130 steel. However, one disadvantage is that high carbon levels of 4130 steel make them sensitive to cracking.

The Welding Process

As mentioned earlier, the welding process for chromoly is a bit different as compared to that of other metals. There are certain things that should be taken care of to ensure the weld is strong, smooth, and durable.

  • In the case of chromoly welding, there is no need to preheat the metal if it is less than 0.12 inches thick. If the tubing is thicker, you need to preheat the chromoly tubing to a temperature of 300 to 400 degrees Fahrenheit.
  • When using a TIG welder, welding should be done slower than normal.
  • You should use a lift arc instead of a scratch start when welding chromoly. A scratch start is comparatively more unstable. The case with the lift arc is different. All you have to do is torch the tungsten to your base metal, start the pedal, lift the welder, and start welding chromoly.
  • You should slope in and out gradually as this alloy is quite temperamental. Sloping in and out rapidly will make it brittle.
  • Tungsten should be kept close to the metal to make sure the arc cone is narrower.
  • The weld shouldn’t be cooled rapidly. This can lead to the weld cracking. Therefore, make sure there is no source of breeze or air in the welding room and that drying occurs naturally at its own pace.

Myths about Chromoly

Since chromoly is a newer alloy, there is a significant amount of misinformation about it. Now that we’re discussing chromoly, it is an ideal place to clear some of the most common myths related to chromoly.

Chromoly is a Lightweight Metal Alloy

The reason why this myth is so prominent down to the fact that structures made with chromoly weigh less than the same structures made with other metals. However, if a structure made from chromoly weighs less, it does not imply that it is lighter in weight. Structures made from chromoly weigh less because lesser material is used. Chromoly has excellent strength, due to which thinner gauges are used in building structures, which eventually results in the structures weighing lesser.

It is Necessary to Pre-Heat Chromoly before Welding

It is not always necessary to preheat chromoly before welding, although doing so won’t bring about any harm. For chromoly metal that is not more than 0.12 inches in thickness, it can be welded without preheating at all, whereas, if a thicker metal is used, it is recommended to preheat it at a temperature of about 300 to 400 degrees Fahrenheit.

Related Questions

Can chromoly be MIG-welded?

Yes, chromoly can be MIG-welded.

Is chromoly stronger than steel?

Chromoly is stronger than steel, with greater tensile strength.

What makes chromoly a better option than steel in the manufacturing industry?

Chromoly is a strong alloy. It is possible to use a thinner chromoly gauge tubing to achieve a lighter structure that is much stronger. It is basically the thickness of the metal being used that makes all the difference.

About Pierre Young

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Hey, I'm Pierre Young a qualified AWS Certified Welder. I got into welding in 2009 as a side hustle. Ever since then, I've been doing all kinds of welds - both for business and pleasure. While immersing myself in this wonderful hobby, I've learned from hands-on experience what welding gear works and what doesn't. Welding Headquarters is the site where I share everything I've learned.

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