Understanding low-alloy steel

Knowing the type of low-alloy steel you have and matching it with the correct filler metal is critical to achieving weld integrity.

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 1 to 5 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; nickel adds toughness; and chromium increases temperature strength, hardness, and corrosion resistance. Manganese and silicon, other common alloying elements, provide excellent deoxidizing 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.

Common Uses of Low-Alloy Steel

The first step in understanding low-alloy steel is to know about its common uses—all of which vary greatly across many industries. Applications for low-alloy steels range from military vehicles, earthmoving and construction equipment, and ships to the cross-country pipelines, pressure vessels and piping, oil drilling platforms, and structural steel.

Several common groupings of low-alloy steels, beginning with HY 80, HY 90, and HY 100 steels, are used for building ship hulls, submarines, bridges, and off-highway vehicles. These low-alloy steels contain nickel, molybdenum, and chromium, which add to the material's weldability, notch toughness, and yield strength. When welding these low-alloys steels, preheat and postheat treatments typically are not required. Always refer to the welding procedure to determine the requirements.

Another type of low-alloy steel—high-strength, low-alloy (HSLA)—is different from other low-alloy grades in that each type has been created to meet specific mechanical requirements rather than a given chemical composition. HSLA applications include warships, structural steel, and others known for their strength.

Designed for strength, toughness at low temperatures, and ductility, ASTM A514, A517, and T1 steels are quenched and tempered and used in applications such as heavy equipment manufacturing and boiler and pressure vessel fabrication.

Weathering steels such as ASTM A242, A588, and A709 Grade 50W rely on certain alloys to produce a protective, corrosion-resistant layer. This layer also gives 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.

Finding a Filler Metal Match

The filler metals used to weld low-alloy steels (regardless of the specific type) typically match the chemical and mechanical composition of the base metal. 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 rule, low-alloy filler metals are classified by their tensile strength in kilopounds per square inch—80 KSI 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. A perfect match is not always possible, however, 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 the filler metals with the lower-strength base material. Conversely, to gain a smaller cross-sectional weld, you may overmatch the base material strength. Overmatching occurs when the filler metal used has a higher strength than the base material. This practice is tricky as it can lead to cracking (especially if the strength of the weld metal far exceeds the base metal's), such as when a low-alloy filler metal with a higher chrome-moly content than the base metal is used. You should overmatch only 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 less than 21/2 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 postweld 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 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 than mild steel at low temperatures.

Filler Metal Classifications

Like other filler metals, low-alloy filler metals are AWS-classified.

Figure 1 shows the AWS classifications for low-alloy, metal-cored, gas-shielded wires in particular, while Figure 2 shows those for low-alloy, flux-cored wires.

In both cases, the first space in the classification simply specifies "electrode." The next two spaces relate to the tensile strength (x 10 KSI) and the welding position capability, 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 the last space. In each of these classifications, the chemical composition matched with the tensile strength directs you to the proper filler metal.

The letter in the chemical composition space 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.

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 typically are reserved for high-temperature applications. Likewise, low-alloy filler metals in 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.

Figure 3 lists details on other low-alloy filler metal product classes and their included alloys, attributes, and recommended applications. This information should help you to select the proper low-alloy filler metal for your low-alloy steel application.

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 often can clear up any additional questions you may have. n