Consumables Corner: Welding process and fillet size limits

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Q: In our facility we use mostly FCAW and GMAW processes. We do a lot of T-joint welds in the flat and horizontal positions, and a common weld size is 1/4 to 5/16 in. We do have some 3/8- to 1/2-in. weld requirements as well. Some welders can perform the welds in a single pass while others do multiple passes on the larger welds. Some welders claim that is the proper way to weld sizes that are 3/8 in. or larger. Is this true and if so, what are some guidelines?

A: The answer? It depends. There are some considerations for various applications, so let's dive a little deeper into the subject.

The first thing to consider is what, if any, code you are working to. This often will dictate the largest single-pass weld you are allowed to perform in production. The maximum permitted weld size generally depends on the process, welding position, and wire diameter.

Another important consideration is the maximum qualified heat input value. To perform a large, single-pass weld, you must use a slow travel speed which will result in high heat input values. High heat input typically harms mechanical properties in both the weld metal and the base metal’s heat-affected zone (HAZ). Additionally, using slow travel speed on large, single-pass welds can lead to a lack of penetration at the root of the weld joint.

To perform fillet welds in the flat position (1F), arrange the weld joint so the welding torch can be straight up and down. If the wire diameter is 1/16 in. or larger, you can use a single pass for welds up to ½ in. in size as long as you keep the welding arc on the leading edge of the weld puddle and the heat input below the maximum listed value on the welding procedure specification. This is assuming you are working to a welding code.

Welds made in the horizontal position (2F) can be a bit tricky depending on your process, wire diameter, and skill level. Limit smaller wire diameters (less than 0.052 in.) to maximum single-pass weld sizes of ¼ in., and in a few cases 5/16 in. If the required weld size is greater than this, use multiple passes to perform the weld. This should ensure good penetration at the root of the weld and minimize weld quality issues like excessive throat, undercut, and overlap. Since the weld beads will be smaller, the travel speed should be faster with corresponding lower heat input value, which will minimize potential HAZ issues.

For wire diameters of 1/16 in. and larger, you can perform 2F welds successfully up to 3/8 in., which also is the maximum single-pass size permitted by some codes such as in AWS D1.1.

Any fillet weld greater than 3/8 in. in the 2F position, regardless of wire diameter, shall be performed in a multipass progression.

The bottom line: Making fillets larger than 3/8 in. in a single pass is done at the risk of reduced weld quality. If the application emphasizes productivity ahead of quality, then using one pass for 1/2-in. fillets may be perfectly acceptable. However, in cases where weld quality is paramount, always use multiple passes for welds larger than 3/8 in.