Due to its precise chemical and mechanical properties, low alloy steel has become an increasingly common material in industries ranging from heavy equipment manufacturing and shipbuilding to cross country pipe construction. Compared to conventional mild or carbon steels, low allow steels-through the addition of specific alloys (nickel, chromium and molybdenum, for example)-can increase temperature strength, hardness and corrosion resistance, among other benefits. The addition of alloys such as silicon or manganese further differentiate low alloy steel from other materials by adding greater deoxidizing capabilities.

But like any material, low alloy steels have their own unique welding requirements, especially when they are welded using the submerged arc process, a common choice among many industries that need to weld thicker materials and require faster travel speeds and/or high deposition rates. Not surprisingly, selecting the right filler metal and flux for these applications is critical to achieving the advantages sought by using the submerged arc process in the first place. Following are some of the key details you should know to help you achieve good results on your low alloy submerged arc welding applications.

Why Submerged Arc Welding?
Low alloy steels typical range in strength from 80 to 120 ksi with some grades available as strong as 140 ksi. These materials vary from HY80, 90 and 100 steels used for bridges and off-highway vehicles to HSLA (high-strength low alloy) steels used in structural steel erection and ASTM A514 (or T1) quenched and tempered steels often found in boiler and pressure vessel fabrication. Low alloys steels are also commonly used in thicker sections for many heavy-duty applications, making them good materials to weld with the submerged arc process.

The majority of low alloy steels that companies submerged arc weld range from ½-inch thick or more (some applications can be greater than four inches), and often require multiple passes. Generally, they also require very long, straight and repeatable welds, too, and it is not uncommon for the material to have a variety of different surface conditions. Some are machine, flame or shear cut, have a ground edge and/or are covered with mill scale. In each instance, the submerged arc process is able to pull impurities in these materials away from the weldment during the welding process, while also providing exceptionally high deposition rates. In addition, the submerged arc process (as on many other materials) provides greater travel speeds, better penetration and higher quality welds on low alloy steel applications. In the end, these benefits add up to greater productivity, which is important when welding on any material. Low alloy applications are no exception.

Making the Filler Metal Choice
Selecting low alloy filler metals for submerged arc welding is much the same as selecting one for a semi-automatic or automated application, with the addition of adding an appropriate flux. Before selecting your flux, however, you need to match the chemical and mechanical composition of the submerged arc welding wire to that of your particular low alloy steel. Low alloy submerged arc wires are generally 80-ksi in tensile strength or higher, and like low alloy steel they contain alloying elements such as chromium, nickel, silicon, manganese or molybdenum.

As a general rule, the low alloy submerged arc wire you select should match not only the chemistry of the base metal, but also its strength as closely as possible. It should be a near match in terms of elongation and toughness (Charpy V-Notch) properties, too. As with other applications, overmatching the submerged arc welding wire strength to the low alloy steel base material isn’t typically recommended, as it can lead to cracking. You should only overmatch when a specific joint design indicates it is the best procedure. Similarly, undermatching strengths of the submerged arc welding wire to that of the low alloy steel base material is generally done only when the joint design indicates it can still provide the appropriate strength weld and/or when the specific type of steel being used will be post-weld heat treated in a way that allows for a lower strength weld.

Submerged arc welding wires are available in both solid and composite (metal-cored) versions, with the latter becoming increasingly popular due to its ability to be alloyed more readily. Composite wires often have shorter lead times for manufacturing and delivery for that reason as well, making them viable options if you need to maintain larger inventory for high volume of parts production. They also tend to have higher burn-off and deposition rates, and faster travel speeds, making them a good choice to help improve productivity. Both types of wires are classified under American Welding Society (AWS) A5.23 (Low Alloy Steel Electrodes and Fluxes for Submerged Arc Welding).

Similar to other welding wires, low alloy submerged arc wires have specific classifications to signify their mechanical and chemical properties, but they have the added distinction of including a flux designation. For example, a low alloy submerged arc composite wire with the AWS classification of F8A8-ECMI-M1 indicates the following:

• ‘F’ signifies submerged arc welding flux.

• ‘8′ indicates the wire provides a minimum of 80-ksi tensile strength.

• ‘A’ designates the condition of heat treatment, which here is ‘as welded.’ ‘P’ would signify ‘post-weld heat treated (PWHT).’

• ‘8′ indicates the temperature in Fahrenheit at which the impact of the weld metal meets or exceeds 20 ft-lbs. In this example, it is -80 degrees Fahrenheit.

• ‘EC’ indicates this wire is a composite electrode.

• ‘M1′ refers to the chemical composition of the wire. In this example, the wire is comprised of manganese (.60 – 1.60%), nickel (1.25 -2.00%) and molybdenum (.35%).

• The second reference to ‘M1′ indicates the chemical composition of the weld metal obtained with the flux and the electrode combined.

Based on this information, when selecting the filler metal you need first to match the tensile strength indicated in the wire’s classification with that of your base material, then consider the chemical composition of both the wire and the low alloy steel that you plan to weld. At the same time, you want to determine the type of flux that is compatible with the wire and that can provide the weld properties you need.

Getting to Know Your Flux
As when submerged arc welding any type of material, selecting the right flux for a low alloy steel application is key. As a rule, fluxes for these applications are classified into three main categories: neutral, active and alloyed. (Note there also are other types of flux specially designed for applications, such as pipe welding). Each flux, in combination with a given submerged arc wire, provides distinct properties and is designed to work best on a given type of application.

For instance, neutral flux typically will not change the chemistry of the weld during the welding process and is not affected by fluctuations in the welding parameters (increases in voltage will not increase the consumption of a neutral flux). Because of this feature, neutral flux works well if you are welding on heavy, thick sections of low alloy steel that require multiple welding passes; the ingredients in flux should not compound to alter the chemistry of the final weld.

Unlike neutral flux, active flux depends on the weld parameters, specifically changes in voltage, to produce a weld. In short, increases in voltage cause an increase in the consumption of the flux. Additionally, because of the silicon and manganese contained in this type of flux, active flux generally welds quite well through contaminants like rust and mill scale and it tends to offer faster travel speeds than other types. If you have an application requiring a single pass on a thinner section of materials, active flux would be a good choice. It often has a more easily removable slag than other types of flux.

Finally, alloyed flux, as it’s name implies, works in combination with the submerged arc wire to add specific alloys into the weld. It can be a more cost effective alternative to other types of flux, as it often allows you to purchase a lower cost welding wire that still provides the desired weld properties. Alloyed flux works well if you have applications containing stainless steels, low-alloy chrome-moly steels or if you wish to gain greater strength when welding higher strength low alloy strength steel.

Bringing it All Together
While compatible with many different types of submerged arc wires, each flux provides different properties when combined with a particular wire. Specifically, the combination of wire and flux you choose can affect the deposition rate you achieve, bead profile, travel speeds and slag release. Your selection of wire and flux will chiefly depend on your particular application and the desired mechanical properties, along with the welding system you are using. Joint design also contributes to the choice, as different fluxes release from specific joints more readily than others.

Typically, filler metal manufacturers can advise as to the best combinations for your welding system and application so that you can achieve the desired results. Welding distributors are also a good resource to help make the selection. If, however, you are choosing your own low alloy submerged arc welding wire and flux combination, keep in mind that you will need to do testing to ensure that you are obtaining the correct properties for your application. Whichever way you proceed, the goal is still the same: to gain the productivity improvements brought forth by the submerged arc process through the combination of the most appropriate welding wire and flux for your low alloy steel application.