For welding applications spanning from the energy market to heavy equipment manufacturing and structural steel projects, companies have come to rely on low alloy steels for their mechanical and chemical properties. These metals offer the advantage of increased strength and toughness; the ability to maintain strength and resist creep at high temperature; and a resistance to atmospheric corrosion. These characteristics can make low alloy steels a beneficial and cost effective option for end users.

Low alloy steels gain their mechanical and chemical properties from the addition of alloying elements, including nickel, chromium, molybdenum and manganese. Less frequently, filler metal manufacturers may add vanadium or copper. Each element provides a distinct benefit. Nickel provides toughness, chromium offers creep resistance and high-temperature strength, and manganese offers hardenability and deoxidizing properties. Molybdenum also offers hardenability, in addition to high-temperature strength. Depending on the type of low alloy steel, varying amounts of alloying elements will be present. Common low alloy steels include:

High yield strength steels: HY-80, HY-90 and HY-100 for shipbuilding, bridges and off-highway vehicle production

High strength low alloy (HSLA) steels for cars/trucks, cranes and bridges.

Quench and tempered (Q&T) steels: A514 for structural steel applications, and A517 for pressure vessels. These are sometimes referred to as T-1 steels.

Weathering steels: A242, A588 and A709 grade 50 for building structures, bridges and outdoor sculptures.

Chrome-moly steels: A335, A213 and A387 for petrochemical and power generation applications.

Heat-treatable low-alloy steels: AISI 4130, AISI 4140 and AISI 8630 for shafts, structural tubing and tubes for transportation of pressurized gases.

Typically, low alloy steels offer an ultimate tensile strength of 80,000 psi (80 ksi) or greater. As with any material, they require a filler metal capable of matching their chemistry and providing the desired strength in the final weld. To that end, low alloy filler metals are available in multiple product classifications, making them usable for a variety of low alloy steels. 


Making the filler metal match

As a general rule, the higher strengths provided by low alloy steel make the material less ductile and more prone to cracking after welding. Having a filler metal with low levels of diffusible hydrogen can help minimize cracking in low alloy steels, as can implementing proper preheating procedures.                 

The goal when welding low alloy steel is to match the strength and chemistry of the filler metal to the base material as closely as possible. In some cases, the filler metal may actually have to exceed the metal’s strength if the joint design indicates it is the best procedure. If a welding procedure requires two different types of low alloy steel to be welded together, matching the filler metal to lower strength material can help provide the appropriate ductility to help prevent cracking.

Other factors that influence how to match a low alloy filler metal to a given low alloy steel include:

Material thickness: Some low alloy steels (such as quench and tempered steels) lose strength at thicker dimensions, requiring a lower strength filler metal for the job.

Cyclical loading: A finished part that will be subject to high stress and fatigue will require a filler metal with higher toughness to protect against cracking.

Post weld heat treatment (PWHT): If a welding procedure calls for PWHT, it is important to have a filler metal that can maintain its mechanical properties after heating. Those with added molybdenum are often suitable.

As with any welding application, when in doubt about the proper protocol for matching a low alloy filler metal to a particular low alloy steel, always consult with a trusted filler metal manufacturer or welding distributor.

Understanding the options
Because there are a variety of types and strengths of low alloy steels available in the marketplace and because each has its own intended applications and service conditions, there are also a variety of low alloy filler metals available on the market to meet these needs. Each type of low alloy filler metal contains varying percentages of alloying elements to provide specific properties to the completed weld. Low alloy filler metals are organized into American Welding Society (AWS) specified chemistry classifications, as follows.

Product classification A

These are carbon-molybdenum filler metals. They feature the addition of between 0.40 and 0.65 percent molybdenum to increase the weld’s strength, and maintain that strength at elevated temperatures, even after post-weld heat treatment occurs. “A” products are designed for use in boiler, pressure vessel, and pressure piping applications, and are typically available in shielded metal arc welding (SMAW) electrodes (e.g. E7018-A1 H4R) and gas-shielded flux-cored arc welding (FCAW) wires (e.g. E81T1-A1C).

Product classification B

Filler metals with a “B” product classification contain additions of chromium and molybdenum, and are used in high temperature service applications, such as pressure vessel or boiler welding with chrome-moly steels. The alloy additions range from 1.25 to 10.50 percent chromium and 0.5 to 1.0 percent molybdenum. Commonly used alloys include — B2, B3, B6, B8 and B9 filler metals — in SMAW electrode, and FCAW or GMAW wires. Some contractors are turning to FCAW wires, especially, for the increased productivity they provide (compared to SMAW electrodes), as it can help them become more competitive in the marketplace. See Figure 1 for more on filler metals with a “B” product classifications.

Product classifications C/Ni

For SMAW classifications, a “C” product classification designates that the electrode is nickel-alloyed. An E8018-C3 SMAW electrode is one example. Solid and tubular low alloy wires use “Ni” in the AWS classification to designate nickel alloying, for instance, AWS E81T1-Ni1. Both types of filler metals contain additions of 1 to 4 percent nickel to improve strength and toughness. Applications for these filler metals include earthmoving or mining equipment, and the general fabrication of low alloy steels greater than or equal to 80,000 psi (e.g. HSLA, HY-80 and A514).

Product classification D

SMAW, FCAW and GMAW (including metal-cored wires) with a “D” product classification contain additional manganese and molybdenum, and are designed for welding a wide variety of steels with strengths greater than 80,000 psi. These include materials found in heavy equipment and crane manufacturing, as well as the general fabrication of HSLA, HY-80-100 and A514 low alloy steels. AWS E90C-D2 is an example classification for a metal-cored wire falling into this category.

Product classification G

Low alloy filler metals with a G classification — E81T1-GC or ER90S-G, among others — are a bit tricky. These products do not fit into any of the AWS-defined classifications that cover low alloy filler metals. They must meet tensile requirements specified in the AWS classification, but the alloy requirements are not defined. Instead, the properties are agreed upon between the filler metal manufacturer and purchasers. These products are not allowed to be used in prequalified welding procedures, which makes them more difficult to qualify for certain applications. “G” class filler metals are available for SMAW, FCAW and GMAW (including metal-cored) welding processes.

Product classification K

The K alloy classification is used only for GMAW (metal-cored, exclusively) and FCAW products. These manganese-nickel-molybdenum filler metals are designed to join high strength low alloy (HSLA) steels, as they provide increased strength — up to 120,000 psi minimum tensile strength — and toughness. Fabricators can also use them to join quenched and tempered steels. In some instances, filler metal manufacturers may add chromium to these filler metals. Applications for these “K” filler metals include heavy equipment and crane manufacturing, offshore applications and shipbuilding.

Product classification M

The “M” classification is only used for SMAW electrodes, such as E9018-M H4R. The “M” stands for military or military similar, because these filler metals have similar chemical and mechanical requirements to military classified products. The filler metals are alloyed with combinations of manganese, nickel, chromium and molybdenum (and sometimes others) to increase strength and toughness. They are used for joining HSLA, along with quenched and tempered steels. SMAW electrodes in this category are qualified with minimum tensile strengths as high as 120,000 psi.
Product classification W
The “W” classification is given to filler metals that are used to weld weathering steel. These steels are resistant to atmospheric corrosion and have a unique looking patina or rust. The addition of 0.5 percent copper to these filler metals allows the weld deposit to immediately match this unique surface appearance. Alloying with copper can cause welds to be slightly more crack-sensitive, although the chances for such problems are minimal. As a best practice, always follow proper welding procedures.

What else to consider
As with any welding application, using a low alloy filler metal that provides the appropriate strength, ductility, toughness and crack resistance in the final weld is critical to welding low alloy steels successfully. However, because there are so many varieties of low alloy steel — each with its own unique characteristics — there is simply no “one size fits all” filler metal solution for the job. Always consider the mechanical and chemical properties of the type of low alloy steel being welded and the intended service conditions of the application when selecting a low alloy filler metal. Consult the provided welding procedures, or consider reaching out to the technical support team at a trusted filler metal manufacturer for additional guidance.