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How to Weld Chromoly: Tips & Tricks for the Best Results
Many have mastered welding the most common materials, including mild steel, stainless steel, and aluminum. However, now and again, you will encounter less popular metals in your workshop that might give you a headache.
Chromoly, also known as chromoly steel, is a specific type of metal that is often used due to its high strength and low weight. You'll encounter it when trying to weld roll cages, bicycles, or car frames.
Understanding its properties can make or break your project. So, in this article, we gathered helpful tips and tricks on how to weld chromoly like a pro. Without further ado, let's learn more.
TIG Welding Chromoly Tubing
Photo by @ayeenenes on YouTube Shorts
What is Chromoly?
Chromoly is a high-strength steel alloy composed of chromium, molybdenum, carbon, and iron. Besides regular iron and carbon, which are commonly found in mild steels, this type of metal also features chromium (0.8% to 1.1%) and molybdenum (0.15% to 0.25%). These alloying elements give steel improved strength and corrosion resistance, but also the name chromoly.
Chromoly steel is available in sheets, but its most common forms are square or round tubing. In your shop or garage, you will often find it in sporting applications, including racing car frames, roll cages, go-karts, bicycles, and motorcycle frames.
In industrial applications, round or square chromoly tubing has a code designation of 4130. Manufacturers have used chromoly in the aerospace and aircraft industries for years. But why is this metal popular?
4130 Chromoly Tubing
Source: https://www.cn-big.com/ASTM/ASME_standard_alloy_pipes/638.html
Chromoly Properties
The main advantages of chromoly are high strength and low weight. Alloying elements (chromium and molybdenum) increase the strength of the base steel, while carbon content remains relatively low.
Compared to mild steel, a chromoly that is one or two gauges thinner provides the same strength profile. Due to its lower thickness, chromoly is also lighter than mild steel, making it ideal for various car or bicycle frames, as well as aircraft parts.
TIG Welding Chromoly Car Frames
Photo by @d1driven (TikTok)
Compared to stainless steel, chromoly provides similar strength but better flexibility. However, chromoly has a much lower chromium content, ranging from 0.8% to 1.1%, compared to at least 10.5% in stainless steel. As a result, chromoly steel provides much less corrosion resistance than stainless steel.
Following the chromoly patterns, some new, advanced high-strength steels (AHSS) emerged. The automotive industry uses Docol, another high-strength steel with an excellent strength-to-weight ratio. Compared to chromoly, Docol offers the same durability but with enhanced corrosion resistance and improved fatigue life cycles. However, Docol is more expensive than chromoly.
DOCOL R8 Advanced High-Strength Steel
Source: https://competitionmotorsport.com/products/docol-r8-advanced-high-strength-steel
Is Chromoly Hard to Weld?
Thanks to the alloying elements and low carbon content, chromoly is generally easy to weld. Low amounts of carbon, added chromium, and molybdenum improve the weldability, making chromoly a weld-friendly metal type.
In most applications and on thinner chromoly tubing, preheating, post-heat treatment, or back purging are typically not required. You can use standard arc welding methods such as MIG (Gas Metal Arc Welding - GMAW) or TIG (Gas Tungsten Arc Welding - GTAW).
MIG Welding on Chrome Valve Covers
Photo by @montana_garage (TikTok)
Although typically easy to weld, chromoly can be heat-sensitive, especially when working with thin tubing. The potential issues with chromoly welding include:
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Overheating or quick cooling can produce brittle welds. Intermetallic compounds can make welds prone to cracking or failure.
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Insufficient heat can result in a lack of penetration. Excessive heat, especially on thin sections, can cause a burn-through or distortion.
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Uncontrollable heat input can cause warping and bending on chromoly. Lack of uniform heat can compromise the strength and structural integrity of the welds.
Due to possible heat-related issues, most welders recommend TIG welding chromoly. This is just one of several tips for chromoly welding, but we will discuss more of them further in the text.
TIG Welding 4130 Chromoly
Source: https://www.youtube.com/watch?v=8XGOBTOaNpY&t=10s
Chromoly Welding Tips and Tricks
Understanding the properties of the metal you are welding is crucial in preparing and making the right approach. However, that's just the first step in any successful metal welding.
Next, we will cover some practical tips and tricks on chromoly welding. The essential tips include:
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Properly clean and prepare the edges
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Use TIG welding whenever possible
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Use the correct consumables for the job
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Choose the right shielding gas
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Provide the proper gas coverage
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Stress relief thick parts
Let's learn more.
TIG Welding on Chromoly Tubing
Source: https://www.youtube.com/watch?v=IBg84md1Ut0
Cleaning And Preparing Chromoly for Welding
Unlike mild steel, chromoly has a thin layer of mill scale on the surface. Chromoly, especially when TIG welded, requires spotless surfaces. Therefore, properly cleaning and preparing the edges is crucial in producing high-quality welds.
Start by removing any dirt, oil, grease, or dust from the surface. Use an old rag to wipe all oils and lubricants. However, due to mill scale, wiping the surface is often insufficient.
To properly remove the mill scale, grind every joint with a mild abrasive product. Then wipe the area with acetone or a cleaner to make the surfaces squeaky clean. Removing the mill scale on chromoly will improve wetting, allowing you to yield much cleaner and shinier welds.
Grind the joint to properly remove mill scale before TIG welding chromoly.
Source: https://www.youtube.com/watch?v=RiwyjbESOjo
Besides cleaning, chromoly requires a tight joint fitup. Providing a good fit-up allows you to quickly fill the joint. Large gaps take time to fill, increasing the overall heat exposure. As noted above, too much heat can cause burn-through, distortion, or warping. Uniform welds require a uniform gap along the joint, so keep that in mind.
In most applications, thin-walled chromoly tubing (<0.120") won't require typical preheating. Chromoly should be at room temperature, 70ºF or above, before welding. However, some industrial applications with 4130 chromoly require heat treatment to reduce hardness and elongation.
TIG Welding Thin-walled Chromoly Tubing
Photo by @montana_garage (TikTok)
TIG Weld Chromoly Whenever Possible
Although you can successfully TIG and MIG weld chromoly, most welders recommend using TIG (GTAW) welding for this purpose. TIG provides exceptional cleanliness, precision, and heat control, which are crucial factors when welding chromoly.
First of all, TIG welding is a clean process that can yield some of the highest-quality welds. When mastered, GTAW can produce clean and shiny chromoly welds, which is beneficial in applications that require both high quality and aesthetic appeal.
TIG Chromoly Welds
Photo by @rayo_welder7 (TikTok)
However, more importantly, TIG welding offers exceptional accuracy and precise heat control. When TIG welding chromoly, you should maintain a short arc. Reducing the distance between the tungsten electrode and the joint reduces the overall energy. Doing so keeps the heat-affected zone narrow, resulting in a lower risk of distortion or warping. Increasing the arc length increases the voltage and arc energy. High arc energy widens the HAZ, potentially causing defects.
If you MIG weld chromoly, you'll need some adjustments. The best way to MIG weld it is to use adjustable inductance. This feature is common among MIG welders designed for stainless steel, and for chromoly, it improves wetting. Better wetting will improve tie-in, yielding uniform welds without a ropey look. However, the lack of heat control that TIG provides may cause some issues with MIG. Therefore, whenever possible, use TIG for chromoly.
TIG Chromoly Welding
Photo by @mechanically.declined (TikTok)
Filler Metal Type for Chromoly
Choosing the right filler for chromoly welding can be tricky. If you browse the market, you'll find 4130 filler metal, classified for use in chromoly welding. However, this filler has increased hardness and reduced elongation, so it is unsuitable for typical sporting applications (roll cages, car or bicycle frames). Unless you are heat-treating the weld, it is best to avoid using 4130 filler.
Most welders recommend ER80S-D2 filler when welding chromoly. This filler material can produce nearly the same strength as 4130, but with much better weldability and mechanical properties. As an alternative, ER70S-2 and ER70S-6 are also acceptable options, although the weld strength will be slightly lower. Keep in mind that these fillers will typically undermatch the 4130. However, the cross-sectional area and linear inches can compensate for the reduced weld deposit strength.
TIG Welding Chromoly with ER70S-2 1/16” Filler Rod
Source: https://www.youtube.com/watch?v=RiwyjbESOjo&t=448s
Some welders prefer 310 or 312 stainless steel fillers when welding chromoly. These fillers provide much higher strength than 4130, so welds can lack the necessary elongation for flexibility. Additionally, stainless steel fillers are expensive.
When it comes to TIG welding chromoly, ceriated (gray) tungsten is a good choice. Thoriated tungsten is a great alternative; keep the thickness to 1/16 inch to 1/8 inch, depending on the base metal thickness.
YesWelder 2% Ceriated TIG Welding Tungsten Electrode
Filler Metal Thickness
Besides type, you should adjust the thickness of the filler metal based on the thickness of chromoly. Most chromoly tubing for racing or general applications comes in lighter gauges, typically 1/8 inch or thinner. When choosing the thickness of the filler, ensure it is always one size thinner than the base chromoly.
Use filler one size thinner than the base when TIG welding chromoly.
Source: https://www.youtube.com/watch?v=Zu1tHMJCGnI
For example, .049-gauge chromoly requires a .045 filler metal. Larger, 1/16 filler metal requires more energy (heat) to melt. Using more energy can cause burn-through, warping, or distortion. Therefore, be aware of the material's thickness.
Similar rules apply when you weld thick chromoly. Although you are less likely to encounter 3/16 inch or thicker chromoly in home applications, ensure that you adjust the filler thickness to match the metal thickness. Additionally, remember that thick chromoly requires heat treatment.
Preheat before Welding Thick Chromoly
Source: https://www.youtube.com/watch?v=lUiNH7BD5nw
Use The Right Shielding Gas for Chromoly
When TIG welding chromoly, use pure Argon shielding gas. 100% Argon, while necessary with TIG, also runs cooler than some blends (e.g., Argon/Helium). In addition to protecting the weld and tungsten electrode from atmospheric contamination, straight Argon will also reduce the heat exposure. As a result, you will get proper protection without overheating the pieces.
If you decide to MIG weld chromoly, use a C25 mixture (75% Argon, 25% CO2). This blend enhances the stability of Argon in MIG welding and provides slightly better penetration, thereby improving the fusion. The mixture is also your go-to choice for mild steel, so you cannot go too wrong, even with chromoly.
Regardless of the chosen method and shielding gas choice, you should use post-flow when welding chromoly. Post-gas flow keeps the shielding gas running for a few seconds after you finish welding. Although post-flow increases the shielding gas consumption, it protects the weld as it solidifies. Additionally, it cools down the torch and protects the tungsten electrode. Therefore, holding the torch over the end of the weld for a few seconds can have several benefits when welding chromoly.
Use post-flow gas while TIG welding chromoly.
Source: https://www.youtube.com/watch?v=5i7CyKeeMdg
Improve the Shielding Gas Coverage
As noted, chromoly often comes in round or square tubes. When welding tubes, shielding gas tends to roll off the surface, reducing the overall coverage and weld protection. Rolling off can also cause the shielding gas turbulence, which sucks in atmospheric gases, rather than deflecting them from the weld.
To address this issue, use a gas lens instead of a standard collet body when welding chromoly tubes. Gas lens kits are readily available with a TIG torch, providing much wider gas coverage. Additionally, gas lenses produce more stable, laminar flow, which eliminates turbulence. A collet body is sufficient for home applications, but whenever you need good coverage, you should switch to a gas lens.
TIG Welding on Chromoly Tubing with Gas Lens
Source: https://www.youtube.com/watch?v=MEHdS0yQllY
Another advantage of a gas lens is that it allows the tungsten to extend farther out of the cup. Having your electrode positioned further than the cup will help you weld in tighter areas and maintain a lower arc length. As noted above, maintaining a low arc length is crucial for controlling energy and keeping the HAZ narrow.
In most shop and metal fabrication applications, you won't need to back-purge chromoly. However, some fabricators do it to improve the root pass of some welds.
Diagram Showing Back Purging in Welding
Source: https://awswelding.ca/home/f/what-is-back-purging-and-damming
Stress Relieve Thick Chromoly
Thin-walled chromoly tubing usually does not require stress relief. Thin pieces usually won't heat up too much, so there is no risk of brittle welds and cracking. However, welding chromoly pieces thicker than .120" will require both preheat and post-weld stress relief. A standard preheat of 300ºF to 400ºF will yield acceptable results.
To relieve stress on thick chromoly welds, use an oxy/acetylene torch with a neutral flame. Keep a temperature of 1,100°F for tubing applications. Weave the flame torch to avoid creating hot spots on the surface. Proper stress relief will significantly reduce the risk of developing brittle welds and cracking on thick chromoly.
Relieving stress on thick Chromoly welds using a neutral oxy-acetylene flame.
Photo by @kentuckyweldinginstitute (TikTok)
Do not quench chromoly after welding. Rapid cooling can cause cracking and lamellar tearing; therefore, allow welds to cool properly.
Can You Stick or Flux Core Weld Chromoly?
Companies have traditionally relied on Shielded Metal Arc Welding (SMAW, also known as Stick) for chrome-moly tubes and pipes. So, you can stick-weld chromoly, but there are certain limitations.
Stick welding chromoly pipes is often reserved for piping industries and thicker sections. As noted, thicker sections require heat treatment and 4130 fillers to avoid cracking and tearing.
Flux-cored arc welding emerged as a quicker solution for industrial chromoly welding. T1 and T5 self-shielded wires, available in large spools, are more efficient for longer welds. However, they also have some drawbacks, including reduced strength and increased oxygen content.
Therefore, both methods are often used in industrial chromoly welding. But, for your everyday sports projects and applications, you can stick to TIG welding.
Stick Welding Chrome Piping
Source: https://www.youtube.com/shorts/zuqF7cleqT0
Final Thoughts
Chromoly is an exceptional metal that offers high strength and low weight, making it ideal for various applications. Chromoly steel is a frequent part of roll cages, car and bicycle frames, and airplane parts.
Due to the addition of alloying elements, chromoly exhibits excellent weldability. But to successfully weld it, you will need good preparation and heat control.
As long as you keep the surfaces clean and avoid overheating the edges, you can produce remarkable chromoly welds. Although it sounds simple, our essential tips and tricks will help you master chromoly welding.
🧐How to Weld Chromoly: Tips & Tricks for the Best Results FAQ
1. What makes welding Chromoly challenging?
Although Chromoly is generally easy to weld, it’s heat-sensitive—especially in thin tubing. Overheating or uneven heat input can cause brittle welds, cracking, or distortion. That’s why TIG welding is often recommended for better heat control and cleaner results.
2. What filler rod should I use to weld Chromoly?
A reliable go-to choice is ER80S-D2 — it provides strong, clean welds and is easy to work with. You can also use ER70S-2 or ER70S-6 if maximum strength isn’t required. Some welders opt for 310 or 312 stainless steel fillers, but these are more expensive and can produce welds that are too rigid for most Chromoly structures. For TIG welding, use ceriated (gray) or thoriated tungsten, about 1/16” to 1/8” thick, depending on your base metal.
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