TAKING A GLANCE AT LOW ALLOY FILLER METALS

As we all anxiously await the upswing in the U.S. manufacturing sectors, we may want to ask ourselves what we can do to prepare for the welding demands that it will bring. One thing is to consider the demands of welding low alloy steel. Applications for low alloy steels range from fabricating construction equipment and ships to the building of cross country pipelines, pressure vessels and structural steel ? all of which stand to increase in prevalence in the coming years.

Low alloy steels have distinct chemical and mechanical compositions created by adding specific alloys, including molybdenum to improve material strength; nickel to add toughness; and chromium to increase temperature strength, hardness and corrosion resistance. Also common is the addition of manganese and silicon, both of which provide excellent de-oxiding capabilities. Manganese also provides additional strength to steel alloys.

Like any welding application, there is no one-size-fits-all technique or equipment for welding low alloy steel. Luckily, knowing how to select the right filler metal, especially, can be a good first step in achieving the best welding performance.

THE BASICS OF LOW ALLOY FILLER METALS
The filler metals used to weld low alloy steels metals typically feature 80 ksi or higher tensile strength. Like other filler metals, these filler metals have distinct AWS classifications that specify their chemical and mechanical properties. Figure 1 shows the AWS classifications for low alloy, metal-cored gas-shielded wires as an example.

The first space in the classification simply specifies that it is an "electrode," and in solid wire, an 'R' follows the 'E' to signify the wire can also be used as a "rod" in the GTAW process. The next two spaces relate to the tensile strength (x 10 ksi), followed by whether it is a solid (S) or composite (C) wire. The last space indicates the final chemical composition of the weld metal (also known as its product class). The alloying elements contained in the filler metal determine its product class and usability.

For example, low alloy filler metals under the 'B' product class (B2, B3, B6 and B8/9) contain chrome and molybdenum to increase their corrosion resistance. These filler metals are typically reserved for higher temperature applications. Low alloy filler metals labeled under the ?K? product class (K2, K3 and K4) contain a manganese-nickel-molybdenum blend for higher strength and would be a good choice for welding high strength, low alloy (HSLA) steels.

Other low alloy filler metals include those with A, D, M, Ni, C and W product classes, each of which has alloys capable of providing distinct weld deposits. See Table 1 for more details.

MAKING THE FILLER METAL SELECTION
Selecting a low alloy filler metal depends on a variety of factors, including the application you are welding, the thickness of the material and whether the materials are similar or dissimilar in strength. Choosing your welding process is the first step in making your filler metal selection. Your available welding equipment, the amount of welding you plan to complete, and even the position in which the welding will take place, will influence this choice.

In the explanation of low alloy filler metal classification above, we have concentrated on solid and metal-cored wires. However, low alloy filler metals are also available in stick electrodes and flux-cored wires. If you will be performing a small amount of welding in the "field," stick electrodes may be the logical choice. However, if you need to perform a large amount of welding, especially in the vertical position, using an all-position flux-cored wire may be a better solution.

If your weldment requires post-weld heat treating (PWHT), you may require a filler metal containing additional molybdenum to help the material keep its strength. Or, for applications that require exceptional toughness due to low temperature service or those that are subject to cyclical loading may require a filler metal alloyed with nickel.

Depending on the thickness of the low alloy steel being welded, different filler metals may also be required. Quenched and tempered low alloys steels (A514, as an example) have specific tensile and yield strength requirements, based on the materials thickness. Thicker material quenches more slowly, resulting in lower minimum yield and tensile strengths. Therefore, they require lower strength low alloy filler metals.

If you need to weld steels that are dissimilar in strength, it is recommended to match filler metals to the lower strength base material. However, you will need to follow the pre-heat and welding interpass recommendations for the higher strength material to ensure sound welds.

Remember, the goal when selecting a low alloy filler metal is the same as for other applications: to obtain the right mechanical and chemical properties in the completed weldment. No single low alloy filler metal is going to fit the bill for all of your applications, so always carefully consult the welding procedures for your particular application, and if in doubt, contact a trusted welding distributor for assistance.

And, as always, be certain to use enough ventilation, local exhaust at the arc or both to keep the individual constituents of the fumes and gases below the OSHA Permissible Exposure Limit (PEL) and the ACGIH Threshold Limit Value (TLV) in the worker's breathing zone and the general area. Read and understand the manufacturer's instructions the precautionary labels on the product, and the product Material Safety Data Sheet (MSDS) for the products you are using. See ANS Standard Z49.1; Safety in Welding and Cutting, which is available for free down load from the American Welding Society at www.aws.org.

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Tim Hensley is the distribution manager at Hobart Brothers, 101 Trade Square East, Troy, OH 45373, 800-424-1543, www.hobartbrothers.com, [email protected]. He is also a columnist for Melting Point, a welding e-newsletter published each quarter by Fabricating & Metalworking.