Plug Welding Complete Guide: Everything you want to know

Plug Weld Meaning

A Plug Weld is type of weld deposited in a round hole in one plate of a welding joint joining that plate to another overlapping plate. Weld is deposited in a hole made in uppermost overlapping plate.

If you weld a Fillet weld in this hole instead of completely filling it, it will not be considered as Plug Weld.

How to decide a Plug Weld Size & its Dimensions?

Plug weld size is equal to the weld metal diameter made in the plug weld hole. The minimum size of a plug weld as per the standard requirements are given in this post later.

The effective load bearing area of a plug weld is directly represented by the nominal hole area. (This opposite to effective area of a filet weld that is equal to throat size multiplied by fillet weld length.)

Plug Weld and Slot Weld

A plug weld and a slot weld appear comparable in design, but a plug weld is made in a round hole while slot weld is welded in a slot elongated in one way.

They are similar in the way weld is deposited: both plug weld hold and slot weld slot is completely filled equal to the base metal thickness.

Plug Weld Symbol & Callout

A Plug weld Symbol & callout specify the

  • Weld size: Placed on the left side of weld symbol/ Callout
  • Plug weld location: shown by arrow line
  • Orientation,
  • Countersink angle (if applicable): placed outside the horizontal side of weld symbol/ Callout.
  • Depth of filling (if specified): Inside the plug weld symbol/ Callout.
  • Pitch/ Spacing (center to center distance dimension in case of multiple plug welds): Placed on the right side of Plug weld symbol/ Callout.
plug-weld-symbols callout
Three examples of Plug welds as:- Plug weld dia: 1/2 inch, Countersink: 45° & Partial Plug weld of 1/2 inch

The size of the plug weld (hole diameter) is always given on the left side of plug weld symbol/ Callout.

Refer below Plug weld symbol & related information as:

  • (5): Number of Plug welds
  • 3: Plug weld pitch

Bottom weld symbol/ callout information

  • ø3/4: Plug weld size (hole diameter)
  • 45°: Countersink size
  • (7): Number of Plug welds
  • 6: Plug weld pitch
plug weld with pitch spacing 1 Plug Welding Complete Guide: Everything you want to know
Multiple plug welds with pitch (spacing) and other dimensions

Plug Weld Joint

A Plug weld joint is one of the basic welds joint out of 5 joints. A plug weld joint is a type of welding where a weld is placed in a pre- drilled hole in one member out of two parallel members.

This type of joint is often used to join two pieces of metal together, such as joining two steel plates.

Plug welds can be used to join both thin and thick materials. When joining thicker materials, multiple passes may be necessary to build up the weld enough to fill the hole.

Plug Weld Size & Strength

AWS AD1.1 and CSA W59 given the minimum plug weld size (Plug hole diameter) required for welds as:

  • Minimum Plug hole diameter (d) = t + 8 mm (as per CSA W59)
  • Minimum Plug hole diameter (d) = t + 5/16 inch (as per AWS D1.1 & CSA W59 in inches)

Here, d is the plug hole diameter, t is the plate thickness having the hole.

Minimum size or hole diameter for plug weld in aluminum parts are given in AWS D1.2, Table 2.1 as 3 times of t for less than 1/8 inch ( 3mm) and 2.5 times t for 1/8 inch and higher thicknesses.

Plug Weld Design for sheet metal and thick steel

A plug weld under stresses shear parallel to its faying surfaces and lead to failure of a plug weld.

The strength of a plug weld is equal to 0.45 times of weld metal (welding filler wire) tensile strength. For example, if a welding wire is having a strength of 100 Ksi, the plug weld shall have a maximum tensile strength of 45 Ksi.

This condition remains active unless the applied shear stress is below the 0.54 times of base metal yield strength.

It is advised to choose a filler weld of equal or higher strength than the base metal for plug welds. Remember: Plug weld & slot weld are prohibited for Fatigue loading welds.

Plug Welding Standards

Plug Welding standards covers the design, inspection & welding requirements for Plug welds. Most used Plug Welding standards/ Code are:

  • AWS D1.1: For steel
  • AWS D1.2: For aluminum
  • AWS D1.3: For sheet metal
  • CSA W59: For steel

Plug Weld Example

A plug weld is a type of spot weld that is used to join two pieces of metal together. The molten metal from the welding electrode is used to fill the gap between the two pieces of metal.

Plug welds are commonly used in the automotive industry to join sheet metal components together. They are also used in the construction industry to join beams and other structural members together.

Plug Weld vs Spot Weld

A plug weld is created by welding in a hole. The hole is drilled into one member that need to be joined with other member as explained earlier, and then the filling the hole by welding. This method is often used when joining sheet metals or thicker pieces of metal together.

Spot welding, on the other hand, doesn’t require a hole. Instead, the welder simply applies pressure and heat to two small spots on the sheet metal. This method is typically used for thinner materials.

There are various spot welding processes available such as resistance spot welding, spot mig welding, spot tig welding, etc.

In terms of welding output, spot welds are faster than plug weld but they are limited only for sheet metal welding applications.

Plug welds can be made on plates when a spot welder is not available to weld two metals.

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