What is Low Alloy? An Overview of Materials and Their Filler Metal Matches
Through the addition of particular alloys, low alloy steels possess precise chemical compositions and provide better mechanical properties than many conventional mild or carbon steels. These alloys typically comprise one to five percent of the steel’s content and are added based on their ability to provide a very specific attribute. For example, the addition of molybdenum improves material strength, while nickel adds toughness and chromium increases temperature strength, hardness and corrosion resistance. Manganese and silicon, other common alloying elements provide excellent deoxiding capabilities.
Fortunately, despite the addition of these elements, low alloy steels aren’t necessarily difficult to weld. Still, knowing exactly what type of low alloy steel you have is critical to achieving good weld integrity, as is proper filler metal selection. The following is a brief overview of the materials and available filler metals to help you along the way.
What’s it all for?
The first step in understanding low alloy steel is to know about its common uses—all of which vary greatly across multiple industries. Applications for low alloy steels range from fabricating military vehicles, earth moving and construction equipment and ships to the building of cross country pipelines, pressure vessels and piping, oil drilling platforms and structural steel. There are several common groupings of low alloy steels, beginning with HY 80, HY 90 and HY 100 steels, most of which are used for building ship hulls, submarines, bridges and off highway vehicles. These particular low alloy steels feature nickel, molybdenum and chromium, which add to the material’s weldability, notch toughness, and higher yield strength. When welding these low alloys steels, pre-heat and post-heat treatments are typically not required. Always refer to the welding procedure to determine the necessary requirements.
Another type of low alloy steel, called high-strength low alloy (HSLA) is different than other low alloy grades in that each type has been created to meet specific mechanical requirements rather than a given chemical composition. Applications from warships to structural steel and other such projects known for their strength often employ HSLA steels.
 |
| Knowing exactly what type of low alloy steel you have is critical to selecting the most appropriate filler metal and achieving good weld integrity |
Designed for strength, toughness at low temperatures and ductility, ASTM A514, A517 and T1 steels are quenched and tempered steels used in applications such as heavy equipment manufacturing, as well as boiler and pressure vessel fabrication.
Weathering steel, such as ASTM A242, A588 and A709 Grade 50W are all examples of weathering steels that rely on particular alloys to produce a protective, corrosion-resistant layer. This layer also creates a “weathered” look to the finished steel and was first introduced as Cor-Ten. Weathered steels are popular in artwork, bridges, and as a facing material on buildings to achieve specific aesthetics.
Make your match
As with any base material, the filler metals used to weld low alloy steels (regardless of the specific type) typically match the chemical and mechanical composition of the material. While the filler metal may be indicated in a job’s specifications, it is still important to know how different wires interact with different low alloy base materials. You can then select the right low alloy filler metal by comparing the information you have on the base metal to the AWS specifications of each wire.
As a general rule, low alloy filler metals are classified as featuring 80 ksi tensile strength or higher, and they contain alloying elements such as chromium, nickel or molybdenum. These filler metals are designed to match specific low alloy base materials, their chemical compositions, weld metal strength and application requirements.
To ensure your welding success, filler metals for low alloy steels should match or exceed the base metal’s tensile and yield strengths, as well as its elongation and toughness (Charpy V-Notch) properties. There is not always a perfect “match,” so it is necessary to find the closest one possible—with a few exceptions, of course.
For example, when welding dissimilar low alloy steels it is typically recommended to match filler metals to the lower strength base material. Conversely, to gain a smaller cross-sectional weld, you may overmatch the base material strength. Overmatching occurs when filler metals with a higher strength than the base material are used. This practice is tricky as it can lead to cracking (especially if the strength of the weld metal far exceeds the base metal), as when a low alloy filler metal with a higher chrome-moly content than the base metal is used. You should only overmatch when a specific joint design indicates it is the best procedure. Another factor to take into account when matching low alloy filler metals is the thickness of the low alloy steel you plan to weld. For example, quenched and tempered steels, like A514, have specific tensile, yield and elongation characteristics as long as its thickness remains below 2-1/2-in. Its mechanical properties change if the material is thicker than that. The quench and tempering process is responsible for this change, as thicker material quenches slower and results in lower minimum yield and tensile strengths. The thicker material, therefore, may require lower strength filler metals.
The application itself will also determine your low alloy filler metal choice. For instance, a joint that requires post weld heat treating (PWHT) benefits from a filler metal alloyed with molybdenum to ensure that the material keeps its strength. Such applications include the PWHT of pressure vessels, which helps improve impact or toughness properties and/or reduce any residual stresses in the weld that could cause it to fail prematurely.
Another example is a high-fatigue application such as earthmoving equipment that requires a filler metal with higher toughness. A filler metal alloyed with nickel provides greater resistance to cyclical loading and fatigue in such a situation, while also offering higher strength and better toughness at low temperatures than mild steel.
Get some product class
Like other filler metals, those classified as low alloy have specific AWS classifications. Fig. 1 shows the AWS classifications for low alloy, metal-cored gas-shielded wires in particular, while Fig. 2 shows those for low alloy, flux-cored wires.In both cases, the first space in the classification simply specifies “electrode.”
 |
| Figure 1 |
The second two spaces relate to the tensile strength (x 10 ksi) and the welding position capabilitiy, followed by whether it is a solid (S) or composite (C) wire. The final chemical composition of the weld metal (also known as its product class) is last space. In each of these classifications, the chemical composition matched with the tensile strength directs you to the proper filler metal. The figure given in the chemical composition space of the AWS classification indicates the filler metal’s product class. Each product class in turn caters to specific chemical and mechanical requirements according to the alloy the filler metal contains. These alloys then dictate the overall weldability and usability of the filler metal, the characteristics of the final weld and the application for which it is intended.
 |
| Figure 2 |
For example, low alloy filler metals under the ‘B’ product class (B2, B3, B6 and B8/9) have varying amounts of chrome and molybdenum added to them to increase their corrosion resistance. These filler metals are typically reserved for higher temperature applications. Likewise, low alloy filler metals labeled under the ‘K’ product class (K2, K3 and K4) all have varying amounts of a manganese-nickel-molybdenum blend for higher strength, making them ideal for welding HSLA steels.
Parting thoughts
As in any welding process, education is the key to understanding low alloy steels and the filler metals used to weld them. In fact, arming yourself with this knowledge can mean the difference between substantial mechanical failures and continued welding success. In addition, always carefully consult the welding procedures for your particular application. Finally, remember that contacting a trusted welding distributor or filler metal manufacturer can often clear up any additional questions you may have.