What should be Fillet Weld Leg Length?

“When welding critical structures, much attention is paid to the leg of the weld bead. During weld inspection, this parameter is checked separately, and the product can be recognized as unusable and sent for rework, which will entail loss of time and financial costs. Here we will look at what a suture leg is, how to calculate and check it correctly.“

What kind of Welds does the leg have?

“What is a Fillet Weld leg length & size, how to calculate and check it correctly.”

The geometry of the welded joints depends on the way the two sides are joined together. When butt welding plates (on horizontal, vertical, ceiling surfaces), a bead is formed at the joint. To assess the quality of the fillet weld, its height and width are measured, and the depth of penetration is measured by destructive testing (such as fracture test or macro test) or NDT.

The leg length is found only on corner and T-joints and not applicable for butt welds. With a fillet joint, it is quite difficult to determine its height, since there is no plane from which to start off. Therefore, to assess the quality of such joints, the width of the weld, the legs and the depth of penetration are measured (by destructive and non-destructive testing methods).

What Is A Weld Leg Length?

A fillet weld leg is the two sides of a triangle that diverge from a right angle. In fact, the leg here has the same definition as in school geometry. Ideally, such a triangle should be isosceles, then an equal portion of the deposited metal lies on each side. This ensures the best retention of the parts and the strength of the joint.

The concept of weld leg is applicable to all types of welding. A specific leg can be reached by a Stick welding, a semi-automatic torch (MIG/MAG), a non-consumable tungsten rod and an argon welding (TIG). Since there are always two legs in an angle and T-joint (on the vertical and lower surfaces of the workpieces), they are sometimes referred to as Z1 and Z2 when measuring. In a good seam, Z1 and Z2 are equal.

Leg Length Selection

The face of the weld bead itself on the corner, T-Joint or Lap joints can be:

Convex – the weld face protrudes outward, sometimes exceeding the length of the leg itself. Externally, such a weld looks powerful, but the increased amount of weld metal leads to the formation of internal stresses. Because of this, the product is prone to deformation, especially if its walls are 2-3 mm.

Concave – the surface of the weld face is curved inwards and is located below the height of the leg. To get this shape, you need to increase the amperage and guide the electrode or torch faster. On a semi-automatic, it is worth increasing the welding current so that the droplet separation process becomes smoother. This increases the penetration depth and promotes a concave weld surface.
Flat – between the sides of the corner joint, there is an almost flat, oblique plane of the weld surface. This happens less often, but it is still possible. This option is more convenient for mechanical processing of joints – it is not enough to clean the deposited metal and the grinding machine equipment captures the entire surface at once.

In each of the above case, the leg length refers to the length (on the horizontal part of the workpiece) and the height (on the vertical part of the workpiece) of the side of the triangle, starting from the root of the weld. In other words, it is the distance from the edge (outer border) of the weld to the surface of another part.

Fillet Weld Leg Length and Throat Size

Calculation of the Weld leg Length

Since the fillet weld is similar to a triangle in cross-section, you can find out the height and length of its sides using a geometric formula using mathematical skills. The calculation is made according to the formula:

T = S*cos45º

When welding critical structures, much attention is paid to the weld leg of the weld seam. At factories and enterprises, this parameter is checked separately and the product can be recognized as unusable and sent for rework, which will entail loss of time and financial costs. Here we will look at what a suture leg is, how to calculate and check it correctly.

What kind of seams does the leg have?

The geometry of the welded joints depends on the way the two sides are joined together. When butt welding plates (on horizontal, vertical, ceiling surfaces), a bead is formed at the joint. To assess the quality of the seam, its height and width are measured, and the depth of penetration is measured by destructive testing or X-rays. There is no cathet here.

The leg is found only on corner and T-joints. The position of the parts in space does not matter. With a fillet joint, it is quite difficult to determine its height, since there is no plane from which to start off. Therefore, to assess the quality of such joints, the width of the seam, the legs and the depth of penetration are measured (by destructive and non-destructive testing methods).

What Is A Weld Seam Leg

A seam leg is the two sides of a triangle that diverge from a right angle. In fact, the leg here has the same definition as in school geometry. Ideally, such a triangle should be isosceles, then an equal portion of the deposited metal lies on each side. This ensures the best retention of the parts and the strength of the joint.

Leg Selection

The surface of the weld seam itself on the corner joints can be:

Convex – the roller protrudes outward, sometimes exceeding the length of the leg itself. Externally, such a seam looks powerful, but the increased amount of weld metal leads to the formation of internal stresses. Because of this, the product is prone to deformation, especially if its walls are 2-3 mm.
Concave – the surface of the suture is curved inwards and is located below the height of the leg. To get this shape, you need to increase the amperage and guide the electrode or torch faster. On a semi-automatic, it is worth increasing the inductance so that the droplet separation process becomes smoother. This increases the penetration depth and promotes a concave weld surface.

Flat – between the sides of the corner joint, there is an almost flat, oblique plane of the seam surface. This happens less often, but it is still possible. This option is more convenient for mechanical processing of joints – it is not enough to clean the deposited metal and the grinding machine equipment captures the entire surface at once.

In each case, the leg refers to the length (on the horizontal part of the workpiece) and the height (on the vertical part of the workpiece) of the side of the triangle, starting from the root of the suture. In other words, it is the distance from the edge (outer border) of the weld to the surface of another part.

It seems that the more rolls the better, but this is not the case. The large leg of the weld creates stresses in the joint and leads to the inversion of the structure. The heating area of the part increases.

The product can lead a lot. A large leg is always an overconsumption of material (electrodes, welding or filler wire), a delay in time. Creating a high seam requires holding the arc in one place for a long time, which leads to fading of alloying elements and faster corrosion. Therefore, the leg should be calculated correctly for each structure.

Calculation of the Fillet Weld leg

T=S*cos45º

These components are deciphered as follows:

T is the size of the weld leg length that we are trying to calculate
S is the thickness of the plate (in geometry, the hypotenuse of a triangle)

cos45º is the standard value with a factor of 0.7.

Let’s calculate the weld leg in practice. For example, we have a corner joint with a plate thickness of 5 mm. Insert this value into the formula and get 5 * 0.7 = 3.5 mm. This means that the weld leg length is 3.5 mm.

Rule Of Thumb For determining the Fillet Weld Size

Experimentally, the optimal parameters of the welding bead for each metal thickness were determined. If you stick to them, you will get a strong weld connection without overconsumption of filler material. Here is the recommended fillet weld size based on the thickness:

Thickness of workpieces, mm	Minimum leg length, mm
4-5	4
6-10	4-5
11-16	4-6
17-22	5-7
23-32	6-8
4-5	5
6-10	6
11-16	7
17-22	8
23-32	9

As you can see, the size of the leg ranges from 30 to 100% of the thickness of the part – the thinner the workpiece, the closer the size of the leg to its cross-section. When welding parts with different thicknesses, the leg length size is selected according to the larger thickness.

How to Use Fillet Weld Gauges?

For example, if you weld workpieces with a cross-section of 5 and 10 mm with penetration on only one side, the seam leg should be 6 mm (taken as if the two sides are 10 mm thick). If you focus on the thin side, you will get a weak reinforcement on the thick metal and the connection will be unreliable. But in this case, it is important to choose the right amperage and weld with a certain arc technique.

Consequences of Incorrect Leg Calculation

We have already considered the negative consequences of a large weld size. The second common mistake is that the seam leg is too small. In this case, there is little weld metal on the sides, which reduces the strength of the connection.

In case of fracture or vibration, the structure may not be able to withstand the load and the seam will crack. Although the small leg size saves consumables, it is permissible only on non-critical connections.

Another mistake made by welders is an asymmetrical leg. Most often, the bottom flange of the seam is too wide and the upper flange is too short. This happens with the wrong technique or the choice of welding mode, because the molten metal flows down under the influence of gravity. The weld seam looks wide, but only finds a little on the vertical side, so it holds weakly and is not designed for serious loads.