June 27, 2014
Q: I work for a structural steel fabricator and we are getting ready to start work on a bridge job that specifies A588 material. We have worked on this material in the past using a weathering-grade filler metal. Our engineer has recommended we use a 1 percent nickel electrode. Is this right?
A: Good question! The A588 steel is a weathering-grade material. This high-strength, low-alloy (HSLA) material produces a protective oxide layer that actually improves its corrosion resistance. For example, some bridges look as if they were never painted and appear be rusty. It is a good bet they are made from weathering-grade steel. These steels usually are alloyed with small amounts of copper and could have a slightly higher level of phosphorus as well.
Historically, these steels were developed first, followed by welding electrodes containing small amounts of copper. The main reasons for creating a matching electrode is to match the strength level and allow the weld metal to form the same oxide layer as the base material for a consistent rusty appearance.
In recent years many fabricators have moved away from using filler metals that contain copper (normally weathering-grade materials use 0.30 to 0.75 percent Cu) because of the increased risk of hot cracking problems. Hot cracking can result from lower-melting-temperature elements like phosphorus. The industry has moved toward a safer alternative with filler metals that contain 1 percent nickel. This filler metal normally is used when the material and weld must meet low-temperature impact toughness (ductility), but it also provides some corrosion resistance qualities as well. If proper welding procedures are used, the 1 percent nickel filler generally will present no problems. In some cases, you’ll need to take the joint type, configuration, and weld size into consideration to ensure the weld will form the oxide layer similar to the base metal. This is accomplished by diluting the base material into the weld deposit.
In your case, your engineer will have to consult with your customer to find out if the filler metal it specified is something other than the 1 percent nickel. If the specification calls for a weathering-grade filler material, it is also available and can be used successfully. If your customer specifies a 1 percent nickel, you will need to qualify the filler material and joints you will be using.
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