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What is a Socket Weld? (Everything You Need To Know)


Socket welding (SW) is used for welding pipes and fittings including reducers, tees and elbows.

Socket weld pipe fittings are used to permanently join pipes that are inserted into a recess in the fitting, flange or valve. Once correctly inserted, fillet type sealing welds are applied to join the pipe to the fitting.

These fittings are commonly known as SW pipe fittings, with SW couplings, SW elbows and SW tees being examples of different types.

Socket welds can be used to change the direction of travel for pipe networks as well as join pipes at different angles and of different diameters.

The high leakage integrity and structural strength of socket welds allow them to be used for a range of piping network applications.


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How Socket Welding Works

Socket welding is named for the fittings into which the pipes are placed for welding. These fittings include recessed sockets into which the pipes are inserted before being welded into place.

When inserting the pipe into the socket it is important to leave a gap between the end of the pipe and the bottom of the socket. This gap prevents a stress failure from occurring when the heat from the welding process causes the pipes to expand against the socket. This gap can be manually measured and marked with a reference line on the pipe, or a permanent fitment tool can be inserted into the fitting to ensure the pipe does not bottom out into the socket. Once placed in position, the pipe is fixed in place by fillet welding where the pipe diameter meets the socket.

Socket welds create sealed, leak-proof, high pressure pipeline configurations for the transporting of liquids or gases. Unlike butt welding, socket welds don’t require any pre-weld machining, although the pipe ends should be clean in order to assure the integrity of the weld.


Socket weld fittings differ according to the pipe diameters and orientation they allow, meaning wide pipes can be joined to narrower ones and a pipe network can change direction or include branches. Socket weld fittings also include different coupling thread arrangements, as shown below:

Full Coupling

This socket connects two pipes together, with threads at both ends to join the pipes. 

Half Coupling

Half couplings are similar to full couplings, but only have threads on one end. With half coupling, the non-threaded end is usually welded or fastened to piping using other means. 

Reducing Coupling

These fittings are used to join two pipes of different diameters, using different width sockets at either end.

Socket Weld Reducer Insert

This reducing fitting also allows for pipes of different diameters to be connected.

Socket Weld Union

Made up of two internal threads either side of an interconnected centrepiece, these three-part fittings use rotation to bring the pipe ends closer together. Because of this, the socket weld union must be screwed up tightly before welding the ends, which will minimise seat warping.

Socket Weld Elbow

Available at90 degree and 45-degree angles, these fittings allow pipe runs to change direction. SW elbows can be either short radius or long radius designs as required.

Socket Weld Tee

A socket weld tee fitting allows a T-junction to be inserted into a pipe network, creating a 90-degree branch off from a straight pipeline. A socket weld reducing tee can be used to create a same T-junction using a smaller diameter pipe as the branch.


Like the threadolet and the weldolet, the sockolet can be used to join a smaller diameter pipe with a larger one. The sockolet includes a socket in the olet (hence the name), which makes welding convenient. Able to create a 90 degree direction change, the sockolet is also able to withstand higher pressures.

Socket Weld Cross

This cross-formation fitting allows for two branches travelling in the same direction to make a 90 degree direction change.

Socket Weld Cap: A socket weld cap is used at the end of a pipe in order to seal it.

Socket Weld Flanges: SW flangeshave a recessed shoulder on the inner diameter which allows the pipe to be fitted correctly. These fittings are typically used for small diameter pipes and can work at higher pressures, offering excellent flow characteristics. There are a number of different SW flange designs, including flat and raised face versions, each with their own pressure ratings.

Advantages of Socket Weld Fittings

Socket weld fittings have a number of inherent advantages for joining pipes, including:

  • The socket easily enables proper alignment, meaning that tack welds are not needed to align the pipes for welding
  • Socket weld fittings have a lower risk of leakage than threaded fittings
  • The socket design means that weld metal does not flow into the pipe bore
  • Socket weld fittings are cheaper to install than butt weld fittings, as they do not require special machining due to the more relaxed dimensional requirements

Disadvantages of Socket Weld Fittings

Because the sockets do not allow complete penetration and must include an expansion gap between the socket shoulder and the pipe, socket weld fittings have areas where substances can build up that are difficult to clean.

These areas mean that socket weld fittings are unsuitable for use in the food industry for hygiene reasons and are also unsuitable for use with radioactive or corrosive substances that could build up and create maintenance or operating issues.

Uses and Applications

As seen above in the ‘disadvantages’ section, there are some applications that are not suitable for socket weld use, but the high pressure ratings associated with socket weld fittings mean that they are ideal for a wide range of other industrial applications.

SW pipe fittings can be used in pipelines to transport flammable, toxic or hazardous chemicals safely due to the lowered risk of leakage compared to other joining techniques.

Creating a leak-free permanent join, SW pipe fittings also allow for excellent flow characteristics. When manufactured to ASME and ASTM standard specifications, these fittings are shown to have met requirements for performance, including tolerances, pressure and temperature ratings, dimensions, materials and markings.

Socket weld pipe fittings are divided according to material type, such as alloy or carbon steel and stainless steel pipes. The different types of fitting lend themselves to different applications, whether couplings, reducers, reducing and regular socket weld tees, elbows, or flanges, with each available in different material types.

Socket Weld Inspection

Visual inspection is typically used to assess socket welds but other non-destructive testing methods can also be used.

Magnetic particle inspection can be used to locate surface and near surface defects in pipe welds, while radiographic inspection can also be used to locate internal voids.


What is the Difference between Socket Welds and Butt Welds?

Socket weld and butt weld fittings are governed by different applicable standards (socket weld fittings are ASME B16.11 and butt weld fittings are ASME B16.9), as there are differences between the two types of weld.

Socket welds involve the insertion of the pipe into a recess at the joint and there is no need to bevel the pipe ends or prepare the materials other than to ensure they are clean.

Butt welds require the fittings to be the same thickness as the pipes so that they can be welded. Butt welded joints also need to be bevelled at the ends.

In addition to these practical differences, there are differences in performance, with butt weld fittings tending to be stronger. For this reason, socket weld fittings tend to be reserved for pipes with relatively small diameters. This difference in strength also means that butt welds are preferred for high temperature and pressure applications or where the weld joint strength does not have to be more than the base metal.

Is a Socket Weld a Fillet Weld?

A socket weld is generally considered to be different from a fillet weld, although there are some similarities between the shapes of the two welds.

How do you fit a Socket Weld?

To fit a socket weld, a pipe is inserted into the fitting’s recess, allowing a gap of at least 1/16” to allow for thermal expansion of the pipe under welding pressure. Once the pipe is in position, it can be welded in place with a series of fillet-type welds. The thermal expansion gap will prevent the weld from cracking should the pipe expand against the fitting.

What is a Socket Weld Flange?

Socket weld flanges tend to be used on smaller diameter, high pressure pipes. The pipes are inserted into the socket end and fillet style welds are applied around the top to hold it in place.

Can you X-Ray Socket Welds?

X-rays can be used to examine socket welds, using radiographic inspection techniques similar to those used in the medical industry.

Can you Radiograph Socket Welds?

Socket welds can be radiographed, although it is not an ideal method for determining the quality of the weld, it will tell you if there is an appropriate root gap.

When to use Socket Weld Fittings

Socket weld fittings are best used for small diameter pipes of NPS 2 or less. They can be used to join small bore pipe sections together using fillet weld type joins to seal the pipes to the fitting.

Why use Socket Weld Fittings?

Socket weld fittings join lines of small diameter pipe used to convey flammable, toxic or expensive materials that cannot be allowed to leak. Socket weld fittings can also be used for steam with a pressure of 300-600 psi. They can create a high strength union with a high flow characteristic and reduced chance of leakage.


Socket welding is used to connect pipes using a range of different fittings, allowing for changes in the direction and diameter of pipe networks as well as creating branches from existing pipelines.

The pipes are inserted into the socket weld fitting, which includes a recess to accept the pipe, making placement easy. Once inserted into the recess, allowing a gap at the bottom of the socket to prevent a potential stress failure, the pipe can be fixed in place with fillet welds around its outer diameter.

Mainly used for smaller sized pipes, socket weld fittings provide leak-proof joins that can withstand high pressures and deliver high-flow characteristics. The fittings are available in a range of styles and materials to suit a range of applications.

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