It's Just a Fillet Weld
by Robert Schueler
Fillet welds are very basic to most industries, but especially to the world of pressure-containing items. Every code we follow has rules defining fillet welds. They are to be found on every welded nozzle, slip-on flange, socket welded fitting, and lap joint.
Why do we use fillet welds?
The ASME Boiler and Pressure Vessel Code (ASME B&PVC) places considerable importance on fillet welds. They are not something to cover gaps or hide a groove weld. Fillet welds serve a number of important purposes, which when done correctly, greatly improve construction.
In ASME Section I, PW-15.1.2, for example, the fillet weld strength is based on one-half the area subject to shear forces, computed on the average diameter of the weld. In addition, PW-15.2 provides a value of 49% for a fillet weld in shear to be used in establishing the allowable stress value for the weld metal. Even small seal welds used on rolled tubes provide considerable added strength. The size and shape of fillet welds used to attach nozzles provides the additional benefit of controlling local stress
intensification within the nozzle. For the most part, fillet welds are 45° right triangular-shaped welds defined by two legs
measured from the weld root or intersection point. The ASME B&PV codes calculate the weld throat, which is then used to calculate the weld leg, as shown in Example 1.
Example 1:
Typical sample of a code-required fillet weld sizing calculation.(See Section I, Fig. PW-16.1 (d), Section IV, Fig. HW-731(d), and
Section VIII, Div. 1, Fig. UW-16.1(l) ).
Example 2:
Right Triangle (typical fillet weld shape)
In Example 2, both t1 and t2 have solved a 0.25 in. (13 mm) minimum throat dimension.
Solving the minimum weld leg for either t1 or t2 will solve 0.25 • 1.4142 = 0.354 in. or round up to 3/8 in. (10 mm) leg. The weld size information is normally transmitted to the welder in the form of a welding symbol shown in Example 3, which in this case would look like the following:
Example 3:
Once the weld size is determined, the required information is then transmitted to the welder, who is expected to produce the weld of the proper size. This brings us to the main question at hand. How do we know this is done correctly? The simple answer is to measure the weld, but how?
Example 4 shows one type of weld gage that measures fillet weld legs and weld convexity. The following illustrations demonstrate how simple it is to use a weld gage like the one shown in Example 4. The first and second illustrations measure fillet weld legs. The third illustration measures the throat dimension of the weld.
Why take the time to measure the welds?
Isn’t it just good enough to look at the weld? A practiced eye is a good thing; however, should a weld prove to be undersized, this would result in considerable added costs for additional welding and possibly additional postweld heat treatment. In the case of fillet welds, larger is not better. The addition of extra weld metal is both expensive and could lead to the introduction of defects.
Either condition should justify having and using fillet weld gages on a regular basis. They are quick and easy to use and can eliminate problems.
Source: www.nationalboard.org FALL 2012 NATIONAL BOARD BULLETIN
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