Mild steel welding: find the right filler

Understand the metal characteristics to make the right choice for steel welding.

By Matt Underwood, Hobart October 30, 2018

Mild steels offer numerous benefits—from easy weldability to the ability to bend and mold the material—making them well-suited for a wide range of welding applications. With many mild steel options available, how can an operation determine which filler metal is the right choice for a specific application?

What is mild steel?

Base materials that are considered mild steel typically have little to no alloying elements. Mild steels have a carbon limit of 0.25% and also are referred to as low carbon steels. Two common mild steels used in many fabrication and manufacturing applications are ASTM A36 and AISI 1018.

The characteristics of mild steel differ by material type, but generally, mild steels have lower tensile and yield strengths than low-alloy steels. Tensile strengths are typically 70 kilopound per square inch (ksi) or lower, while yield strengths can be as low as 30 ksi.

Lower tensile and yield strengths mean the material isn’t as hard, so it can be bent or rolled into different shapes. In applications when higher yield and tensile strengths are required, the material can be made into thicker, heavier sections to offset the lower strengths.

In general, mild steels offer very good weldability, and the lack of additional alloying elements in mild steels also makes them a cost-effective option for many welding applications.

Filler metal options

Mild steel filler metals typically are designed to outperform the base materials, with higher tensile and yield strengths.

Standard mild steel filler metals are known to offer good weldability. Mild steel filler metals for wire welding applications are available in solid, metal-cored and flux-cored wire options for manufacturing and fabrication.

Solid wires

Solid wires are versatile for many applications because they can be used at very low or very high amperages. When welding thicker materials with solid wire, it’s recommended to use a spray transfer process; for thinner materials, a short-circuit transfer process works best. Solid wires offer good travel speeds and bead appearance, along with the ability for deep penetration when welding tighter joints or corners due to a narrower penetration profile. Common mild steel solid wire filler metals include American Welding Society (AWS) classifications such as ER70S-3 and ER70S-6.

Solid wires commonly are used in automotive and robotic welding applications. Because solid wires have high column strength and are often copper coated, they are a bit stiffer than other wires and provide good feedability. However, keep in mind that solid wires can have higher heat inputs than metal-cored wires, so the deep, driving arc of solid wires can result in overpenetration or burn-through at times.

Also, be aware that solid wires don’t wet out as well on surfaces with mill scale. Because solid wire has fewer deoxidizers added, it doesn’t handle surface impurities as well as other filler metals can, requiring welders to use slower travel speeds on materials with mill scale or rust.

Metal-cored wires

Nearly any mild steel welding application that uses solid wire can use a metal-cored wire. Where solid wires have a narrower, finger-like penetration profile, metal-cored wires have a broader, wider penetration profile that isn’t as deep. As a result, metal-cored wires wet out and tie into sidewalls better and typically can be used at lower heat inputs while producing the same bead appearance. The wider penetration profile and additional wetting also can make wire placement more forgiving.

Since metal-cored wires have a powdered core they can be formulated with more deoxidizers than solid wires. Because of this, they can handle mill scale and dirty base materials better and with higher travel speeds. Their greater deposition rates result in higher productivity. While metal-cored wires tend to be more expensive than solid wires, the productivity gains can deliver a fast return on investment. Because of these factors, metal-cored wires are often recommended when completing welds 6 in, or longer and for operations that are trying to increase travel speeds. One of the most common metal-cored options for mild steel applications is E70C-6M.

Gas-shielded flux-cored wires

When welding mild steels, two common gas-shielded flux-cored wire options are E70T-1 for flat and horizontal welds and E71T-1 for all-position welds.

Offering similar or higher deposition rates than metal-cored wires, E70T-1 filler metals are well-suited for thick, heavy weldments requiring a lot of weld metal. These wires are also good options for materials with heavy mill scale or rust and can provide good bead appearance and higher travel speeds even on those materials.

Because it can be harder to make large welds that don’t sag with metal-cored and solid wires, the flat and horizontal flux-cored wires are good for these applications because the slag helps hold the weld bead in place as it cools. However, the slag does require more time in post-weld cleanup.

When the application doesn’t allow the weld to be made in a flat or horizontal position, E71T-1 flux-cored wires offer an all-position option. Well-suited for vertical up or overhead welding, these wires deliver higher travel speeds and a much flatter bead profile. Like the E70T-1 filler metals, the all-position wires do generate slag that must be cleaned up after welding. It’s recommended to use a drag technique with wires that generate a slag, and a push technique for wires that don’t generate slag.

Tips for selecting mild steel filler metals

When choosing the right filler metal for mild steel welding applications, there are some additional factors to keep in mind.

The condition of the base material is an important factor when selecting a filler metal. Some A36 and 1018 mild steels come in a hot-rolled or cold-rolled condition. A hot-rolled mild steel can result in mill scale on the material surface, which is more difficult to properly weld. Mild steels that have been cold-rolled have a clean finish but sometimes have a light oil on the surface.

When the base material has a light mill scale or rust, it’s recommended to remove it. If that’s not possible, select a metal- or flux-cored wire, which generally provide better results in these situations. For parts that are cold-rolled or have a light oil on the surface, solid wire filler metals such as ER70S-6 are acceptable.

The joint design to be welded also is a factor. A narrow V-groove weld joint may require deeper penetration into a tighter spot and would be best-suited to a solid wire. All-position flux-cored wires don’t provide the deep, finger-like penetration that solid wires offer and would be better-suited for welding the shallower joint designs.

Mild steel materials provide benefits for easy weldability and flexibility in many welding applications. Understanding the characteristics of the type of mild steel base material, the goals of the welding application and weld joint design—and matching it with an appropriate mild steel filler metal—helps improve productivity, throughput, and weld quality.