Chromoly is a high-strength steel used for applications like roll cages in racing, bicycle frames, motorcycle frames and aircraft parts. With chromoly steel, you can get the high strength demanded by those applications but can use a thinner, lighter material to achieve it.
Compared to mild steel, chromoly can be at least one or two gauges thinner and still deliver the same strength profile.
Read more to get some basic tips for getting the best results when TIG welding chromoly.
Chromoly gets its name from the alloying elements of chromium and molybdenum. These metals have an excellent strength-to-weight ratio. The most common chromoly code designation is 4130, which typically comes in the form of square or round tubing.
The material is available in sheets but its most common form is tubing, like what is used for roll cages on drag cars. Some racing sanctioning bodies require fabricators to use high-strength steels like chromoly for roll cages when the car will exceed a specific speed.
Compared to mild steel, chromoly provides the same or better strength properties in a much thinner, lighter material. When looking at chromoly versus stainless steel, chromoly provides greater flexibility. However, it does not offer the corrosion resistance that stainless does, so chromoly would not be a good choice for headers or applications in extreme heat environments.
There are some newer high-strength, low-alloy steels on the market that compete with
chromoly, including Docol. These materials often have similar strength profiles compared to chromoly but offer better corrosion resistance and fatigue life cycles. They are typically more expensive than chromoly.
Five tips for welding chromoly
1. Pay attention to prep and cleaning
To get the best results when welding chromoly, pay careful attention to prep and cleaning before you start. With some of the newer high-strength, low-alloy steels you can simply wipe off the protective oil and then weld. But with chromoly it’s important to grind every joint with an abrasive product and then wipe the area with an acetone or cleaner. This removes the thin layer of mill scale on the chromoly, and it will result in much cleaner and shinier welds. It also improves your results, since the weld puddle will wet out better and provide a better weld bead. It’s also key to take care with joint fit up when preparing chromoly welds. If there are large gapsand the joints don’t have tight fit ups, you have to spend more time filling those gaps. This adds heat to the weld.
2. Use TIG welding for chromoly
Due to the heat sensitivity of chromoly, TIG is a better process than MIG for welding the
material. With TIG welding, it’s easier to control the heat input by keeping arc lengths closer to the base metal. A shorter arc length (staying closer to the base material with the tungsten) reduces the total arc energy. This helps keep the heat-affected zone narrower. When the arc length is longer, the machine has to produce more voltage to bridge the gap, which increases the power and heat generated. This in turn makes the heat-affected zone larger, causing the zone to be more brittle.
While not recommended for chromoly, if you do MIG weld the material, check to see if your welding power source can be set for different material types. If it can, set the machine for stainless steel. The filler metal used to MIG weld chromoly is harder than the wire used to weld mild steel, but not as hard as the wire used for stainless. Setting the machine for stainless adds inductance control, which will wet the puddle out more and allow it to tie in better at the edges of the weld. Without this step, your weld bead won’t wet out as well, and it may look higher and more ropey.
Use straight argon gas, which runs cooler and doesn’t add as much heat into the weld as a gas blend. For MIG welding, set the power source for stainless steel or increase the inductance setting if that’s an option, but still use a mild steel gas like a 75/25 blend of argon and carbon dioxide.
In addition, be sure to use enough post-flow shielding gas coverage as the puddle cools. Post- flow helps ensure the weld is protected as the weld pool solidifies. It also shields the electrode from atmospheric contamination as it cools after welding. For proper post-flow, hold the torch over the end of the weld for a few seconds until gas flow stops. A good rule of thumb for post- flow gas coverage is one second for every 10 welding amps. For example, if you’re welding at 60 amps, your post-flow should be about six seconds.
4. Select filler metal for material thickness
When welding chromoly, don’t use a filler metal that’s thicker than the base material. Much of the chromoly used for racing or general welding applications is light-gauge material, often 1/8 inch or thinner, so it’s important to choose a filler metal that is thinner than the base material. For example, if you’re welding .049-gauge chromoly — a very common thickness — a 1/16 filler metal will be too thick and require more heat input. Instead, use a .045 filler metal for an .049 base material. If you are welding thicker chromoly, such as materials 3/16 inch or thicker, it may require heat treatment.
It’s typically recommended to weld chromoly with an ER80S-D2 filler metal unless you are heat treating, then use a 4130 filler metal. If you are joining chromoly to mild steel — such as welding a chromoly roll cage to a mild steel vehicle frame —an ER70S2 filler metal is a good option.
It’s not recommended to weld chromoly with a stainless steel filler metal like 312. This filler metal has much higher strength than the 4130 tubing and won’t provide the necessary elongation properties for flexibility of the joint in case of a crash.
5. Use a gas lens
Consumables used in TIG welding include a nozzle and a collet paired with either a gas lens or a collet body. For TIG welding chromoly, choose a gas lens instead of a collet body. A gas lens improves the consistency of the shielding gas coverage and reduces turbulence compared to a collet body because it has several screens inside that produce a more uniform laminar flow. This is especially helpful in applications that use chromoly tubing, since the shielding gas has a tendency to want to roll off of a tube shape. Also, a gas lens allows the tungsten to extend farther out of the cup than a standard collet body does. This lets you get into tighter areas and keep the arc length lower.
Regarding tungsten selection, ceriated (gray) is a good choice for welding chromoly.
Optimize chromoly welds
Chromoly can deliver the high strength and light weight demanded by many applications. Successfully welding chromoly requires paying attention to heat input and following best practices for cleaning the material as well as choosing the right shielding gas and filler metal.