The joint is 10 in. long. How long should the fillet weld be if it is expected to be full length? The question is simple, but the answer is a bit more complicated.

Anyone who has ever welded has likely experienced the difficulty associated with terminating a weld at the very end of a joint — in a quality manner. Beyond this workmanship concern, however, we must also consider some design-related issues.

First, let's tackle the easier topic — the workmanship related concerns. The welder, or the programmer of a robotic welder for that matter, must overcome two physical phenomena when terminating a weld at the end of a joint — one is the result of thermal conductivity, the other, the result of magnetic properties.

In the center of the length of a weld, the thermal energy associated with molten weld metal is conducted away from the joint in multiple directions. As the arc approaches the end of the joint, the ends of the material being joined naturally become hotter, as there is no place ahead of the weld into which thermal energy can be conducted.

As the ends of the material become hotter, the rate of thermal conductivity decreases, further restricting the transfer of heat. Thus, the end of the joint rapidly increases in temperature (Figure 1).

Weld craters should be filled to the full weld size. This requires the welder to dwell at the end of the joint, filling the crater while adding yet more thermal energy into the material. Sometimes, undercut results. In other cases, the molten weld metal flows out of the crater, off the end of the joint being welded, and ends up on the shop floor.

Simultaneously, the welder may face another problem: Arc blow. When we use direct current (DC) power, a magnetic field surrounds the electrode. In the center of the joint, this magnetic field is conducted into the surrounding material in a generally uniform manner.

As the arc approaches the end of the joint, the magnetic field becomes constricted, preferring to remain in the metal being welded rather than to be transferred into the surrounding air. The concentrated magnetic field acts on the arc, attracting or repelling it. The arc is displaced, or “blown” away from the weld crater — hence the term “arc blow” (Figure 2).

Starting a weld isn't quite as difficult as terminating the weld, but the same arc blow conditions can exist at the start as well.

To compensate for both the buildup of thermal energy and for arc blow, if given the option, most welders will opt to stop the weld short, terminating it perhaps one fillet weld size short of the end of the joint. Similarly, the weld may be started one weld size from the opposite end (Figure 3).

Thus, if we return to our original question, and if we assume a ¼ in. fillet weld, then for a 10 in. joint, the total length of fillet would be 9½ in.

In the past, some codes required that welds be initiated and terminated one weld size from the end.

That requirement, however, posed another problem: If the welder welded the full length of the joint — even with good weld quality for the whole length of the joint — an inspector might require the ends of the welds to be ground off. The inspector could argue that the code says “fillet welds shall be initiated or terminated not less than one weld size from the end of the joint.”

And then, there is the situation in which no weld length is shown on the drawing — just an arrow with a fillet weld symbol and an arrow pointing to the joint. The weld that terminated one weld size from the end of the joint may be required to “complete the weld”, which for our example, means making a ¼ in. fillet weld on either end of the joint — and to make each segment ¼ in. long. In other words, puddle a bit of weld into either end of the joint.

This practice, of course, causes the metallurgist who is concerned with things like preheat, heat input and cooling rates to shudder.

We've discussed the option of stopping the weld short, or carrying the weld to the end of the part. There is yet a third option — the weld can be wrapped around the end of the part. When an angle is welded to a plate, for example, this option may be called an end return.

The AWS D1.1 Structural Welding Code — Steel currently deals with the issue of weld termination in clause 2.8.3.1. as follows:

General: Fillet weld terminations may extend to the ends or sides of parts or may be stopped short or may have end returns except as limited by the following cases:

Importantly, the next clauses in D1.1 provide exceptions to the general practice, and the exceptions will be our topic next month.

What can we learn from D1.1 on this topic of weld termination? First, we see that any one of the three options — full length, stopping short, using end returns — is generally acceptable, although exceptions exist.

Usually, weld quality can be more consistently achieved by starting and stopping the weld approximately one weld size from the end of the joint. Accordingly, the designer should assume that a full length weld will not actually be the full length of the joint, and determine the fillet weld size accordingly.

In general, the termination shouldn't be a significant focus of the welding inspector, so long as it meets the minimum length.

Next time, we'll look into the exceptions that are listed in D1.1, and we'll find that some welds should be stopped short, or the end return limited. We'll also learn that when there are unacceptable termination options, these need to be clearly communicated to everyone involved in the fabrication process: the engineer, the welder and the inspector.

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Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co., struck his first arc on his grandfather's welder at the age of ten. He is the author of Design of Welded Structures and Design of Weldments, and an internationally recognized expert in the field of weld design. In 1999, Blodgett was named one of the “Top 125 People of the Past 125 Years” by Engineering News Record. Blodgett may be reached at (216) 383-2225.