Fillet Joints and Terminology

A fillet weld is extremely common in arc welding, and though they are distinct from groove welds some of the terminology crosses over. Fillet welds do not involve any sort of edge preparation or beveling. Instead, fillet welds are the result of depositing a weld at the intersection of two perpendicular surfaces, resulting in a weld bead with a triangular cross-section, as shown above. It is not ideal for a fillet weld to have any sort of root opening, though in some cases, a small opening is tolerable.

Fillet welds are common because they are economical. Without the need for any beveling or special joint preparation, they can vastly reduce production time. There are some limitations.

Because a fillet weld only exists in between two 90o surfaces it cannot be applied to all joints. T joints, lap joints, and open or half-open corner joints can all have fillet welds applied to them. An edge joint and closed corner joint can only be groove welded. They could be applied in butt joints if the two parts had considerably different thicknesses or are overlapped by a 3rd and 4th part creating what is called a splice joint.

The fillet weld has a weld face and weld toes just like groove welds. The location and measurement of these features are just the same. The weld toes are where weld metal intersects base metal, and the weld face is the visible surface of the weld. However, because there is no edge preparation and (ideally) no root opening, joint penetration, and root penetration are not treated separately, and the terms are often used interchangeably. Fillet welds do not necessarily need to have deep penetration to be strong, as long as the penetration extends into the joint root at all it is considered sufficient.

Unlike groove welds, fillet welds are not ever intended to have complete joint penetration. So they are not considered to have a root face or root toes.

Nor, as in the case of groove welds, is joint penetration used to define the size of a fillet weld. Instead, it is a measurement of the weld legs and throat. To understand these terms it is critical to remember that a fillet weld is, roughly, triangular in cross-section. The weld leg is a measurement from the joint root to one or another of the weld toes.

Fillet weld throat is a little more complicated, as the image above shows three different types of throat: Effective, actual, and theoretical throat. We’ll explore the differences between these in the next section, but to put it simply, think of the throat as the shortest measurement from the weld root to the weld face, with the line of this measurement being approximately perpendicular to the weld face.

Fillet Welds and Sizing

The throat, legs, and triangular shape of the fillet weld are crucial to understanding how the weld is sized. The size of a fillet weld is defined as the largest right triangle which can be inscribed in the weld’s cross-section. In simple terms, a fillet weld’s size is the same as the size of the weld legs. A ⅜” inch fillet weld must have both legs measure out at ⅜”. In some cases, a fillet weld will be designed to have different leg sizes, called an unequal leg fillet, but this is not common.

However, the weld throat complicates matters. The actual throat is the actual measurement from the weld root to the weld face, however, just like with groove welds, any convexity at the weld face is not considered an effective part of the weld. If the weld is concave, both the actual throat and effective throat are reduced. This reduces the effective size of the weld, even if the legs are the correct size. Effective throat is a measurement from the weld root, to the face of the theoretical right triangle inscribed on the weld cross-section, so it excludes any convexity at the weld face. Effective throat cannot be measured without some sort of destructive testing on the weld joint, rendering it useless for service. Because of this, the theoretical throat is used to measure fillet welds in production. Theoretical throat is a measurement from the beginning of the joint root (as it was prior to welding) to the face of the theoretical triangle within the welds profile, once again excluding any convexity.

Fillet welds may be single-pass or multi-pass, just as with groove welds. Because the shape of the weld’s triangular cross-section, and throat, are so critical to the effective size the welder must take special care when stacking multiple weld beads to keep the finished weld the correct shape.

Attributions

1. Figure 15.13: Various Parts Of A Fillet Weld As Well As How The Weld Is Sized by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0