Anatomy of a Weld – Welds that Pass Inspection

While the goal is to join two pieces of metal, welding often involves a code specifying how the work should be performed.

The basic idea is to run one or more weld “beads” along the joint so the sides fuse together without any gaps in between.

By following the code, a welder ensures a structure or object can withstand any impacts, vibrations, weathering, earthquakes, and the test of time.

As the photo below illustrates, weld metal penetrates into and rises above the joint. Sometimes the final (or only) bead is ground or filed off at the top to leave a flush surface.

More often, however, it’s left in place to furnish extra strength and protection. A bead looks like a series of dimes laid tightly together across the joint.

Of course, if you’re constructing your own barbecue pit, or repairing tools in your shop, your welds won’t be subject to inspection. But if you’re working on steel framing for a bridge, assembling a pressure vessel, or installing a long-distance pipeline, the scrutiny that comes into play is quite formidable.

Organizations like the American Welding Society and American Society for Mechanical Engineers will often have a say about the procedures, materials and type of inspection needed before a weld can be declared acceptable.

There’s a good reason for all the fuss. A weld that fails can cause a structure to collapse. A leaking pipe caused by porosity in a weld can result in contamination or an explosion.

Whether the end result is property damage, environmental consequences, injuries or deaths, an incorrectly performed welding procedure and/or inspection is a big deal for all parties concerned.

That’s why architects and engineers are required by law to incorporate welding and other building codes into their designs and drawings.

Building contractors must factor in the codes and welding procedure specifications (WPS) when they purchase their equipment and consumeables. They must also instruct welders on amperage settings, number of passes, heat treatment and other aspects of their assignments.

Likewise, each welder who works on a job must be certified for the welding process and position (overhead, vertical, etc.).

On a work site, a certified welding inspector (CWI) checks out completed welds and either passes or fails them based on the applicable code and engineering specs.

Not a code-quality weld

Because doing the job right is paramount, most entry-level welders start out on non-critical projects. They assist experienced welders who do the actual welding.

A trainee may be responsible for joint fit-up, which means cleaning, grinding, beveling and tacking the base metals before welding can begin. Welder helpers also retrieve the welding rods and other supplies in advance of the operation.

They load wire spools onto flux-cored and MIG machines, sharpen tungsten electrodes, connect up the leads for various equipment, replace empty compressed gas cylinders, perform fire watch, and clean up the finished welds.

After plenty of observation and practice, students, apprentices or trainees evolve into competent code welders. Success, however, begins with an understanding of weld parameters, defects and terminology.  This tutorial delves into all these areas.

Anatomy of a Weld

For every weld, the “toes”, “face”, “root”, “legs” and “throat” comprise an anatomy that code books, inspectors and instructors refer to constantly.

The diagram below, for instance, depicts a weld as you would look at it from the side.

The straight black lines at the top and to the right represent the work plates.

It may be hard to see the plates at first, but one lies flat, the other perpendicular to it, with the joint at the center.

A fillet weld

Every weld has two legs, one for each work plate, and each extends from the root of the joint to the toe.

The weld toe is where the base metal “ties in” to the weld metal along the weldment face.

In this graphic, the weldment is represented by the blue and shaded areas.

The weld “throat” is the distance between the root and face, which can vary, depending on whether the weld is flush or convex (shown above).

The dotted lines in the diagram show how the weld is measured using the legs, toes and throat.

Here’s a different type of weld:

A groove weld

In this edge view, the two work plates sit side by side, with a butt joint running down the middle.

Notice that the sides are beveled (i.e. diagonally cut) to form a wider pocket to hold weld metal. This is standard practice on plates or pipe thicker than about a quarter inch.

If there were no beveling, it would be difficult to penetrate all the way down to the root from the top of the plates.

Beveling is performed with a saw, torch and/or grinder prior to welding. The angle may be specified in a shop drawing or welding symbol.

Other dimensions for a weld are measured using the length of the legs and the root opening (at the bottom) between the plates

V-shaped joints require more weld metal, so they’re generally completed in multiple passes.

One thing the inspector will look for in any joint is penetration or depth of the weld. The CWI will also check to see that there’s enough root reinforcement and weld metal at the top of the plates (i.e. the throat). D

epending on the WPS, the weld will be either flush with the base metal on either side, or convex. A concave weld (i.e. one that dips downward) is usually unacceptable.

Although it’s not shown in the diagram above, beveling usually includes a “root face” when there’s an opening between the plates. The drawing below shows a face on either side, visible beneath the light blue shaded area.

A V-groove

Root faces may look innocuous, but they serve a purpose.

If the work plates were simply beveled all the way down, their bottom edges would burn out when welded.

A root face provides more surface area to prevent this from happening. Thus, a uniform bead of weld can penetrate down to root without leaving any gaps. 

This drawing, incidentally, doesn’t show the weld metal penetrating through the root face.

While the V-groove “open-root” joint is common in pipe welding, a different fit-up design is more often used in structural welding, as shown above.

A backing plate is welded to the two work plates to catch the weld metal and precludes the need for root faces. So the two plates are beveled completely.

The photo above shows the set-up for a welder qualification test. In order to perform structural welding (with backing plates) in the vertical position, for instance, you would have to perform this weld to the satisfaction of an instructor or inspector in order to be certified.

Notice the wider opening between the two workpieces. When a backing plate is used, the standard spacing is a quarter-inch for plates 1/2 to 1 inch thick. Good fit-up and tacking of the backing plate are crucial to laying down a code-compliant weld.

Next: Types of Beads

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