Fillet welds are the most commonly used type of weld in welding fabrication. It does not require special joint preparation, like bevel cutting. A fillet weld joins two surfaces, usually, but not always, at right angles to each other. Fillet welds are used to make lap joints, T-joints or corner joints. The profiles of fillet welds and the associated terms are shown in Figures 3.50 and 3.51. These are equal leg fillet welds.
In Figures 3.50 and 3.51, all the terms are self explanatory. The term “Effective Throat” is the shortest distance measured from the root of the weld to its face, less any reinforcement. Also, it should be noted that the root penetration is only considered as part of the effective throat for fillet welds made by the submerged arc welding process. This is stated in the CSA W59 Standard (Clause 4.3.2.4). In some standards or codes, the root penetration is not considered.
3.18.1
Symbols of Fillet Welds
The composition of welding symbols for fillet welds is governed by a number of explicit rules. For proper application of such welds in welded fabrication these rules require that the following information be shown at designated locations in the welding symbol unless specific general notes covering
standard dimensions of fillet welds appear elsewhere on the drawing. Also, see Examples 2 and 3 given on pages 3-28 and 3-29 for the correct ways of placing symbols to a cruciform joint.
1. The Fillet Weld Symbol
Rule: The vertical side of the triangle representing the weld symbol must always be on the left side from the reader’s viewing position as shown in Figure 3.52.
2. Location of Fillet Weld Size
Rule: The size must be shown for each weld symbol and must always appear to the left of each weld symbol as shown in Figure 3.53.
Arrow Side Other Side
Reader’s Viewing Position
Both Sides Arrow Side SIZE SIZE SIZE SIZE Other Side
Reader’s Viewing Position
Both Sides
Figure 3.52
3) The Length of the Fillet Weld
Rule: The length must be shown for each weld symbol and must always appear on the right side of each weld symbol as shown in Figure 3.54. Absence of a specified length designates a length defined by the side of the joint between two points of abrupt change.
Depending on the specific conditions or requirements for a given application, additional use may be made of the following supplementary symbols:
1. Weld All Around
Symbol represented by a circle placed at the junction of arrow line and reference line (Figure 3.55).
2. Field Weld
Symbol represented by a flag with its direction optional but preferably pointing away from the arrow and placed at the junction of arrow line and reference line (Figure 3.55).
LENGTH
LENGTH LENGTH
LENGTH
Arrow Side Other Side Both Sides
Figure 3.54
3. Contours
Contours may be obtained in either one or two ways:
a) with no application of mechanical means (Figure 3.56)
b) with application of mechanical means (Figure 3.57)
4. References
The designated location for reference is the tail of the welding symbol (Figure 3.58).
Concave Convex Flat
Figure 3.56 Concave by Chipping Convex by Grinding Flat by Machining C G M M Figure 3.57 Reference to Submerged Arc Welding Process Reference to Specific Welding Procedure
Specification or Welding Procedure Data Sheet WG3 SAW Figure 3.58
3.18.2
Size of Fillet Welds – Equal Leg Fillet Welds
The strength of a fillet weld is governed by both the fillet size and the effective throat thickness. The fillet size is the length of the side of the largest triangle that can be inscribed within the weld cross- section as shown in Figures 3.50 and 3.51. For equal leg convex fillet welds, the measured leg size is the fillet size as shown in Figure 3.50. For equal leg concave fillet welds, the fillet size is the side of the inscribed triangle, or the theoretical effective throat multiplied by 1.4 as shown in Figure 3.51.
1) The Specified Size is the size as it appears in the welding symbol and is designated by the letter “S”.
2) The Effective Size is the size that corresponds to the specified size and is designated by the expression “S effective” = “Seff”.
3) The Measured Size is the size established on the basis of measurement and is designated by the term “S measured” = “Sm”.
These sizes for the following three types of fillet welds are as shown in Figure 3.59.
Sm Sm Seff Sm S
S = Seff= Sm
Flat Fillet Concave Fillet Convex Fillet
Seff S Sm am a S = Seff= am 2 S = Sm=Seff S S a
EffectiveThroat MeasuredThroat EffectiveThroat
Figure 3.60 shows a few examples of fillet weld sizes and symbols. The fillet size specified in the design must be the effective size.
or Symbols Desired Welds TEE (T-) JOINT 3/8 3/8 3/8 3/8
It is of interest to note that in some countries the size of the fillet as it appears in the welding symbol may be specifying the size of the throat of the weld rather than the size of the leg (Figure 3.61). The International Standards Organization (ISO) responsible for the formulation of international
standards has recently established its own position on this issue. Sub-committee 7 “Graphical Welding Symbols” of its Technical Committee 44 on Welding (ISO/TC44/SC7) has, in recognition of the
entrenched practices in using one or the other system, officially accepted both, leaving it at the discretion of each country adopting the ISO Standard to opt for the one system it prefers.
However, as a necessary precondition of such compromise, ISO has made it a mandatory requirement that each system be clearly identified by the following means.
It should be understood that the designations “z” and “a” have no other significance except to identify the system. It is also very important for the student to be aware of the difference in interpretation attached to the definition of sizes of fillet welds between the AWS and the ISO concepts, both of which are used internationally. This awareness will be of specific importance to those who, because of involvement with international contracts, are dealing with foreign drawings. In North America, fillet welds are specified by leg dimensions.
(a) s= 10 s= 10 or or z10 a10 z10 a10 a= 10 (Throat) (b) Letter “z” to precede size
to designate LEG size for the desired weld having
Leg Size = S = 10mm
*See preceding figure for effective throats of fillets with different profiles.
The interrelation of both sizes is expressed by:
z = a 2
2 a = z
or
Letter “a” to precede size to designate THROAT size for the desired weld having
Throat Size = 10mm
Figure 3.61: ISO fillet size designation.
3.18.3
Minimum and Maximum Fillet Size
It is advisable to point out that some governing design applications like CSA W59 (Welded Steel Construction) or AWS D1.1 (Structural Welding Code) stipulate minimum fillet sizes as a function of material thickness.
On the other hand, maximum fillet sizes may be set either by considerations of balanced design, that is, by keeping the capacity of the welds reasonably close to that of the parent metal, or by
requirements of good welding practice.
With regard to the latter, specific reference is made to welding against a cut edge where the maximum recommended fillet size for thicknesses over ¼ inch (6mm) is:
3.18.4
Conventional Fillet Sizes
The fillet sizes are usually measured in millimeters (mm) in the metric system or in inches (in) in the imperial system. The smallest dimension is adjusted to the nearest size in mm or in inches, 1/16 of an inch intervals. The common sizes used are shown as follows:
Metric (mm) 3 5 6 8 10 12 14 16 18 20 Imperial (1/16 in) 1/8 3/16 1/4 5/16 3/8 1/2 9/16 5/8 11/16 3/4 1/16 S S where: S = t - 1/16 or: S = t - 2 t (Imperial) (Metric) Figure 3.62
3.18.5
Size of Fillet Welds – Unequal Leg Fillet Welds
For unequal leg convex fillet welds, the effective throat (t), as shown in Figure 3.63, is the shorter leg (a) multiplied by sin θ.
For unequal leg concave fillet welds, the effective throat thickness must be obtained by direct
measurement which is the shortest distance from the root point to the weld surface. Its equivalent leg sizes are the lengths of the sides of the inscribed triangle as shown in Figure 3.64.
The AWS Standard A2.4 does not define the convention of which leg size is specified first. Where it is possible to misinterpret the fillet leg sizes, a sketch defining the leg sizes must be shown on the drawing. In recognition of this shortcoming, AWS A2.4 requires that a sketch of the joint complete with the desired orientation of the fillet weld be shown on the drawing whenever necessary. Figure 3.65 will illustrate this point.
Figure 3.63: Unequal leg fillet (convex).
Consequently, in addition to the basic symbol under (a), an additional sketch showing either fillet weld (b) or (c) – as applicable – must be shown on the drawing. Only then will complete information be provided for an error-free interpretation at the time of welding. Such clarifying sketches may not always be necessary, although they may be considered.
The effective throats of unequal leg fillets are discussed previously.
3.18.6
Intermittent Fillet Welds
There are three types of intermittent fillet welds, although the first one is inherent in the remaining two: 1) basic intermittent fillet welds, applicable to a single line of fillet welds (Example 18)
2) staggered intermittent fillet welds (Example 19) 3) chain intermittent fillet welds (Example 20)
(a)
(c) or
(b)
The Fillet Weld specified in T-joint (a):
could be interpreted to mean:
3/8 1/4
1/4 3/8
1/4 x 3/8
The following sketches will help explain the fundamental concept and the rules governing its application.
EXAMPLE 17
EXAMPLE 18
(Staggered)
Under normal circumstances the general tendency is to keep the length of the increments and the size of the fillets the same on both sides of the joint, if only to simplify fabrication and avoid errors.