Consumables Corner: Selecting the right welding process and filler metal for repair work

Q: We need to weld S690QL material to A572-Gr.50 to repair a tower crane. It has several large weld joints that we’ll weld in the 1G and 2F positions, and we’ll perform all of the work inside our shop. What electrode and weld process would you recommend for us to use to complete this repair job as efficiently as possible?

A: We can certainly provide suggestions, but the answer is, it depends.

Let’s start with selecting your electrode, since that is a fairly straightforward answer.

Between the two base materials, the A572-Gr.50 has lower mechanical properties so the filler metal will be matched to it. As a rule, when welding materials that have different tensile strengths, the consumable you choose should match the material with the lower tensile strength. The preheat temperature you use should match what is required for the material with the higher tensile strength. The A572-Gr.50 has a yield strength (YS) minimum of 50 KSI (50,000 pounds per square inch) and an ultimate tensile (UT) strength minimum of 65 KSI.

The general mechanical properties of A572 do not list or require impact properties, but you can request these as a supplementary requirement. If you know what is required for impacts of the joint design, then you can adjust the electrode you choose based on that information.

To meet the 65-KSI minimum UT, the electrode should come from an E70XX family. Generally speaking, the YS of the electrode will not be more than 15 KSI lower than the UT, but you should always verify this with the manufacturer. This will meet the 65-KSI UT and should meet the 50-KSI YS.

For the welding process, here are several things to consider:

• Welding code
• Skill level and qualification of welders
• Type of welding equipment
• Weld joint consistency, length, width, accessibility, and cleanliness
• Weld shop ventilation

This list can help you narrow down the possibilities, but for the sake of this column let's make a few assumptions and provide you with a couple solutions.

We’ll assume that the tower crane you’re repairing is rather large and the material thickness is greater than ½ inch. If the weld joints are fairly consistent in size, length, and accessibility, then submerged arc welding (SAW) is a good choice. Be aware that many times with groove welds are the complete joint penetration (CJP) type and sometimes have root openings. Therefore, you will need to run a root pass with another process such as gas metal arc welding (GMAW), flux-cored arc welding (FCAW), or shielded metal arc welding (SMAW) and may need to backgouge. Otherwise SAW is a high-quality and high-deposition process that generates very low fume.

If the joints are less accessible or short, then you can choose GMAW (solid or metal-cored wire) or FCAW. If the weld joints are fairly dirty or have contaminants trapped at the root, FCAW is the better choice because the fluxes contained within are designed to deal with impurities. Additionally, many flux-cored wires require 100 percent CO2 or 75 argon/25 percent CO2 shielding gas in which the CO2 helps to remove impurities as well.

While FCAW is less efficient than SAW or GMAW, it is much more user-friendly. A downside of the FCAW process is that it generates more fumes and runs the risk for slag entrapment if you do not use good welding technique.

If the weld joints are reasonably clean, then GMAW may be the best choice. You have two options with this process—solid or metal-cored wire. If you are planning to run a wire diameter larger than 0.052 in., you will probably want to use a water-cooled gun or a higher-amperage (500+ amps) and higher-duty-cycle gun to deal with the amount of radiant heat. Without getting into too many details, this generally has to do with the higher amount of argon in the shielding gas (this could be another topic of discussion on its own).

Metal-cored wire can benefit the process in one of two ways. If you match the welding parameters to produce the same deposition rates, metal-cored wire achieves this with a lower heat input. If you match the welding parameters to produce the same heat input, you will achieve greater deposition. Both processes produce low fume levels, although fume generation largely depends on the cleanliness of the weld joint and base material. Additionally, both processes have a high deposition efficiency, produce very little spatter, and have a lower risk of trapping impurities in the joint.

Last, solid wires are lower in cost because less manufacturing is required to produce them, they are not as susceptible to hydrogen pickup from the atmosphere, and have a longer shelf life than metal-cored or flux-cored wires.

We hope this will simplify your process selection and guide you in making the best choice for this application.