What is a Socket Weld? (Get the Right Set Back)

I show what a socket weld is, how it works, and explain the right set back and how to get it.

What are Socket Welds?

Socket welds are a type of pipe weld where the pipe is inserted into a socket fitting and a fillet weld is put around the seam. No beveling or open root is needed, making them a good place to start pipe welding.

Socket weld coupling stick welded with 7018 cap on mild steel and a socket weld valve dissimilar TIG weld first pass.

How does a Socket Fitting Work?

The inside diameter of a socket fitting is larger than the outside diameter of the pipe to be welded onto it. Most commonly used with smaller diameter piping, this allows the pipe to be recessed into the fitting before welding.

Socket weld fitting diagram of cross-section showing how pipe is inserted into the fitting and where the fillet weld is added.

The shoulder on the inside of the fitting (as shown above) stops the pipe from going past that point.

What's the Right Set Back for a Socket Weld?

According to ASME (external link), the minimum set back for a socket weld is ¹/₁₆" (1.6mm).

I usually do a ⅛" setback (3.2mm) to ensure a 1/16 gap remains after welding.

But what is set back?

Set back, sometimes called the expansion gap, is a gap left between the bottom of the pipe and the shoulder of the socket fitting before welding.

Set back on socket fitting diagram showing the gap between the shoulder of the fitting and the pipe.

Why is a set back necessary?

Having a set back is the most critical part of doing a socket weld. Without a gap, the bottom of the pipe will compact against the shoulder of the socket fitting as it's welded and contracts.

This puts stress on the whole joint, the weld itself, and can lead to catastrophic failure

Don't forget the set back. Remember, things warp and contract when welded, and the expansion gap is needed to account for this.

How I get the right set back

First, I recess the pipe to the bottom of the fitting onto the shoulder.

Then, I draw a line around the pipe at the top of the fitting to mark the "bottoming out" point.

Finally, I lift the pipe out until there is a 1/8" gap (remember, 1/16" gap is the minimum) between that line and the top of the socket fitting. I tack it there.

This way you can see the line and gap even after the pipe is tacked in place.

Benefits of Socket Pipe Weld vs Butt Pipe Weld (for fitters and welders)

These are two totally different joints. Socket welds are most common on small diameter pipe, usually 2" and under. 1" socket welds are the most common I've encountered. 

This is a list of benefits, not from an engineering perspective, but from the practical standpoint of the welder, fitter, and contractor. 

Benefits of Socket welds:

  • Faster than butt welds
  • No open root (less skill required)
  • Easier to align piping and get correct take-offs (less skill required)
  • No beveling and less prep needed

This is why new pipe welders do a lot of socket welds. But they are also fun and can be a place to showcase your welding skill.

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