Fillet Weld

This article focusses on fillet welds: what is fillet weld, what electrode size should be used, how many passes should be used etc. Some of the terminology associated with fillet welds has been indicated in the above figure. Some information about fillet welds pertaining to ASME Section IX has been included at the latter stages of this article.

The Section IX portion mainly addresses these questions: what does fillet size mean in Section IX? Does it indicate the fillet weld leg size? Or the throat size? How to qualify a welder for making fillet welds on coded jobs? How to qualify a WPS for making fillet welds in accordance with ASME Section IX?

What Is A Fillet Weld?

fillet weld is a triangular weld used to join two surfaces that are at approximately right angles to each other. i.e., lap, tee, and corner joints are normally welded with fillet welds. A fillet weld can be made between two plates, resulting in tee joint, corner joint or lap joints; or it can be made between two pipes as well resulting in a circular weld.

The shape of the weld is generally triangular, and the surface may be of flat, convex or concave shape, depending upon the technique deployed by the welder. Fillet welds of flat shape are sometimes also termed as ‘mitre’ shape.

A fillet weld requires much less joint preparation than a groove weld, and is therefore a cost effective and faster option to join metals when strength of the joint isn’t a critical requirement.

A fillet weld should have a leg length equal to the plate thickness up to 3/8-inch plate. For plate thicknesses 3/8-inch and greater, a minimum of 3/8-inch leg length is required on all welds. A fillet is also one of the types of welds that is used to make socket welds.

Related: Butt Weld Vs Socket Weld: How Are They Different?

Nomenclature Of A Fillet Weld

Any fillet weld comprises of five distinct regions: Toe, Face, Throat, Root, and Leg. Let us see each one of them.

fillet weld terminology
Fillet weld nomenclature

Leg Size Of A Fillet

As shown in the figure above, leg is the spread of the weld metal along the legs of two base metals aligned perpendicular to each other. The fillet size generally specified in codes and standards refers to this leg length. See a small discussion at the end of this article on this.

Root Of The Fillet

This is the deepest part of the weld in the base metals. A properly made fillet weld ensures that good root fusion happens. Good root fusion means complete and equal penetration in both base metals, without leaving any gaps at the corner.

Face Of The Fillet

As shown in ‘3’ in the figure above, this is the portion that is visible to the looker, that lies along the hypotenuse of the right angled triangle.

Toe

This is the farthest portion of the weld along the leg length. Improper amperage results in undercuts in this region. A good fillet ensures that there are no undercuts and the fillet merges smoothly with the base metal at the toe.

Throat

This is the shortest distance from the corner of the joint to the hypotenuse of the right angled triangle. Its dimension can be reckoned as 0.707 times the leg length of the fillet. An informal thumb rule says that the throat must be at least equal to the base metal thickness.

How To Denote Fillet Weld On A Drawing?

Correctly indicating a fillet welding is important so that the welder understands what the designer has intended. Like other welds, the notation consists of a triangle shown over the reference line. Just besides the triangle is written a number that denotes the fillet size. Commonly this is a single number.

However sometimes this number can also be found in the y x z format. This indicates that the fillet is of unequal leg lengths. The number at ‘y’ indicates the leg length of vertical leg, while the number at z indicates leg length of horizontal leg.

The triangle may be placed on the top of the reference line or below it, or a triangle apiece may be placed on both sides. In American drawings, triangle on top of the reference line indicates that the fillet is to be made on opposite side of the arrow line.

If the triangle is shown on the bottom side of the reference line, it indicates that the fillet is to be made on the side of arrow line. Triangle on both sides of the reference line indicates that fillet is to be made on both sides.

In some drawings, this confusion between ‘arrow side’ and ‘other side’ is prevented by use of two lines: one reference line as usual, accompanied by a dashed line just below it. If the triangle is shown hugging the dashed line, it indicates that the fillet weld is required to be made on the opposite side. If the triangle is made on the solid line, it indicates that the fillet is required to be made on the arrow side.

A circle at the intersection of reference line and arrow line indicates that the fillet is to be made all-round.

Sometimes, a flag planted at the intersection of arrow line and reference line can be seen. This indicates that the weld is to be made in field.

Sometimes, a small curve adjacent to the triangle indicates whether the fillet is required to be of convex shape/concave shape/flat shaped.

Sometimes a number is written on the hypotenuse side of the triangle. This indicates the length of the fillet weld. If another number follows this number inside brackets, it indicates intermittent weld, with the number in the brackets indicating the pitch.

Some manufacturer’s also indicate the strength of the fillet just besides the triangle, although this practice isn’t so common.

notating a fillet weld
Notations on a fillet weld drawing

Strength Of A Fillet Weld

The size of the throat is the determinant of the strength of a fillet joint.

Generally, a fillet weld has equal leg lengths. In such case, the face of the fillet lies along a 45° incline to the horizontal, and the throat equals 0.707 times the leg length. For this case, the fillet size can be specified by stating only one dimension, which would indicate the leg size of the fillet.

However, sometimes fillet welds are designed to have unequal leg lengths too. In such case, both leg lengths have to be specified separately. In such case, the face of the fillet does not lie at a 45° incline. The throat dimension does not equal 0.707 times the leg, it has to be calculated separately.

The strength of the fillet weld depends on the throat dimension. Throat is the shortest distance between the root of the fillet and its’ face. The reinforcement has to be ignored while calculating the strength. Root penetration is also ignored in calculation of strength, unless a deep penetration process has been used.

When a deep penetration process is used, the extra penetration can also be considered for calculating the strength. This can permit a reduction in size of the fillet. However, this should be done only extra penetration is ensured throughout the length of the weld, and the work is controlled strictly by welding procedures.

The following figure illustrates normal fillets with equal leg size, fillet welds with unequal fillet size, and fillet welds with deep penetration.

leg lengths of a fillet weld

When it is required to increase the strength of a fillet joint, increasing the throat size is an option. However, it should borne in mind that increasing the throat also incurs increase in weight of the weld metal. For example, if the throat is doubled, the weight of the weld metal needing to be deposited increases by 4 times!

This is illustrated in the figure below. The throat size in the 2nd fillet joint is twice that in the 1st fillet. This doubles the strength of the weld. However, note that the 1st fillet houses one triangle, while the 2nd fillet houses four triangles of the same size! A higher cross-section area of the weld translates into higher volume of weld, and thus higher weight of the weld needing to be deposited.

does increasing throat size increase strength of a fillet weld?

The fillet size for different jobs is not dictated by theoretical design calculations alone though. Many practical considerations go into it as well.  Intermittent fillets are sometimes used when code dictates the requirements, or for practical considerations – instead of design calculations.

Intermittent welds are specified by pitch and length of the fillet besides the fillet size. Specifying intermittent welds with large fillet size should be avoided though. For example, a 3/8 inch (9.5 mm) size fillet, 2 inch (50 mm) long, with a pitch of 4 inch (100 mm) can be replaced with a continuous 3/16 inch (4.8 mm) size fillet weld. The volume of weld metal required to make this continuous weld would be only half as much as the intermittent weld.

See the following paragraph to understand more about an intermittent fillet weld.

What Is An Intermittent Fillet Weld?

An intermittent fillet, as the name indicates, is laid intermittently along the length of the fillet instead of a continuous run. We saw in the figure above how to annotate an intermittent on a welding drawing.

intermittent fillet welds
Intermittent Fillet Weld

Intermittent fillet weld is sometimes resorted to because of two reasons:

  1. Continuous weld is unnecessary. Intermittent fillet will serve the intended purpose.
  2. The base metal is of low thickness, that might warp if subjected to a continuous run.

A staggered intermittent weld on both sides of the vertical part is sometimes deployed, to ‘balance out’ the heat input, in an effort to contain the weld distortion and warpage. In order to indicate a staggered intermittent weld on a drawing, the triangle on the other side of the reference line is placed a little displaced (instead of right below) to the triangle on arrow side.

What Size Of Electrode Should Be Used For Fillet Welds?

As with surface welding, the use of larger wet welding electrodes will result in greater weld metal deposition. However, the larger electrodes tend to produce more porosity (gas voids) in the deposited weld metal.

Also, a larger single pass weld will have a lower toughness and an equivalent size multipass weld; this is the result of the tempering effect that each pass of the multipass weld has on the preceding passes.

For most positional work, a 1/8-inch electrode is recommended. Therefore, the welder will need to make a number of passes, usually 3 to 5, to achieve a 3/8-inch leg length. The number of runs will be determined by position and technique.

The important point is not the number of runs, but obtaining the 3/8-inch leg length. In cases where the metal to be welded is thin and in all overhead work, a 1/8-inch electrode is required. Using the smaller electrode means more passes, but as previously stated, it allows succeeding passes to temper the preceding ones.

Multipass welds using smaller diameter electrodes will actually result in higher quality wet welds with better metallurgical properties.

Related: Multipass Welding: All You Need To Know

How To Qualify A Welder For Fillet Welding?

Different codes specify different rules for qualifying a welder for making fillet welding. We will briefly touch upon the requirements specified by ASME Section IX.

A fillet weld made by a skilled welder shows complete fusion and freedom from cracks at the root and heat affected zone, with minimal concavity/convexity, and with minimal difference in the leg lengths. A suitable welder qualification tests these skills.

As per ASME Section IX, a welder qualified on any groove weld test coupon gets qualified for welding fillet joints on all base material thicknesses, depositing all fillet sizes, and on pipes of all diameters. This is as per QW 452.6. The range is amusingly wide.

However, a welder for fillet welding can be qualified on a fillet weld test coupon as well. This is sometimes the preferred option because of various reasons, which I have outlined in this article. This option requires breaking the test coupon and performing macro examination. The macro examination must meet the requirements of QW 184.

A third alternate is the qualification through production assembly mock-up. I have detailed this option here.

Qualification Of WPS For Fillet Weld

The WPS qualification for welding fillet joint in accordance with Section IX follows a similar trajectory to welder performance qualification. A groove weld test coupon qualifies the PQR for all fillet sizes on all base metal thicknesses and on pipes of all diameters.

The Section IX intends that the WPS qualification for pressure retaining fillet welds must be done through groove weld coupons only. This has been subtly indicated in QW 202.2 (c).

Sometimes the groove weld option is not preferred/not feasible due to many reasons. In such cases, the WPS can be qualified through fillet-weld test coupon as well. This option requires breaking the test coupon and performing macro examination.

What Does Fillet Size Mean? Throat Size Or Leg Length?

Section IX in a number of places uses the term ‘fillet size’ or ‘fillet weld size’. For example, QW 451.3, QW 451.4, QW 452.5, QW 452.6 mention this term. What does this term indicate? Does it mean the throat size of the fillet weld? Or does it indicate the leg length? Is this clarified in Section IX somewhere?

The term fillet size in Section IX signifies the leg length of the fillet weld. This can be inferred from QG 109.2, which lists the definition of “weld size of fillet welds”. The definition has been split into two – one for fillet welds of equal leg lengths, and one for unequal leg lengths.

Moreover, the formats QW 484A and QW 484B for welder and welding operator performance qualifications respectively, list a space for recording “fillet size __ X __”. This gives us an indication that it is the fillet weld leg lengths that need to be recorded (not throat size).

Further, this question got inquired from ASME by someone. The ASME, in Interpretation IX-83-127 (qs2) clarified that the term refers to the leg length of the fillet joint, and not throat. This then, firmly establishes that when Section IX mentions the term “fillet weld size”, it is talking about the leg length (or the fillet weld leg size) of the fillet.

PS: When the fillet weld is of unequal leg lengths, the fillet weld size is said to be the shorter leg length of the largest right triangle that can be inscribed in the fillet weld cross section. This gets understood from ASME’s other codes of construction.

Examination Of A Fillet Weld

Unlike groove welds, volumetric NDE is not a practical option for fillet welds. Hence, methods such as dye-penetrant examination, magnetic particle examination, visual examination etc. are resorted too. These methods of course have their own limitations, and can only examine the weld near surface.

The performance qualification and procedure qualification test coupons in accordance with ASME Section IX are tested through a macro examination of the weld cross section. The macro must reveal complete root fusion and freedom from cracks at the root as well as heat affected zone, for the coupon to pass.

Further the difference in the length of the legs of the fillet must not exceed 3 mm. For performance qualification coupons, the concavity/convexity should not be greater than 1.5 mm.

The performance qualification test coupons are subjected to fracture tests too. In fracture test, the stem of the coupon is loaded such that the root is in tension. When the fracture occurs, the fractured surface must show complete root fusion and freedom from cracks, and sum of lengths of porosity and inclusions must not exceed 10 mm.

The size of a fillet weld can be measured from the sample taken for macro examination. Alternatively, the size can also be measured directly by using a fillet gage. These gages come in a variety of shapes and configurations. Here is an example in the picture below.

fillet gage
Fillet Gage

So this was all about the fillet welds. Please do leave your thoughts in the comments section below. Thanks for reading.

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