What Is a Fillet Weld?

Quick Answer

A fillet weld is a triangular-cross-section weld that joins two surfaces at approximately 90 degrees. In piping, fillet welds connect socket weld fittings and slip-on flanges to pipes. The weld does not penetrate the full wall thickness; it fills the corner between the pipe OD and the fitting or flange face.

Fillet welds are defined by the American Welding Society (AWS) as welds of approximately triangular cross-section joining two surfaces at right angles in a lap joint, T-joint, or corner joint. In piping, they are the standard connection method for all ASME B16.11 forged fittings with socket weld ends and for slip-on (SO) flanges.

Fillet Weld vs Butt Weld

Feature	Fillet Weld	Butt Weld
Joint geometry	Surfaces at ~90°	Surfaces in same plane (end-to-end)
Penetration	Partial (fills corner)	Full penetration
Joint strength	Lower than butt weld	Equal to base metal
Pipe size	Small bore (NPS 2 and below)	Large bore (NPS 2 and above)
NDE	Visual, MT, PT	RT, UT, MT, PT
Applicable fittings	Socket weld fittings, slip-on flanges	BW fittings, weld neck flanges
Fatigue resistance	Lower (stress concentration at root)	Higher
Standard	ASME B31.3, AWS D1.1	ASME B31.3, ASME IX

The 1/16-Inch Expansion Gap

A critical requirement for socket weld fillet welds is the 1/16-inch (1.6 mm) gap between the pipe end and the bottom of the socket. The pipe is fully inserted into the socket, then pulled back this distance before welding.

This gap serves two purposes:

• Prevents thermal expansion forces from cracking the weld during temperature cycling
• Avoids trapping stresses between the pipe end and socket bottom

ASME B31.3 Figure 328.5.2C specifies this gap. Failure to maintain it is one of the most common fabrication defects in small-bore piping.

Fillet Weld Sizing

The minimum fillet weld leg size for socket weld connections is defined by ASME B31.3:

Pipe NPS	Min Fillet Weld Leg Size
1/2	1.09 x pipe wall thickness
3/4	1.09 x pipe wall thickness
1	1.09 x pipe wall thickness
1-1/2	1.09 x pipe wall thickness
2	1.09 x pipe wall thickness

The general rule per ASME B31.3: the minimum fillet weld leg dimension equals 1.09 times the nominal wall thickness of the pipe (Cx = 1.09 x Tn). For a 1-inch SCH 80 pipe (wall = 3.73 mm), the minimum leg is 4.07 mm.

Where Fillet Welds Are Used in Piping

• Socket weld fittings: Elbows, tees, couplings, caps, unions per ASME B16.11
• Slip-on flanges: Two fillet welds (inside and outside) per ASME B16.5
• Branch connections: Sockolets require a fillet weld at the socket end
• Pipe supports and attachments: Trunnions, shoes, and brackets

Inspection

Fillet welds on socket weld connections are inspected visually and by magnetic particle testing (MT) or liquid penetrant testing (PT). Radiographic testing is not practical on fillet welds due to the joint geometry. This is one reason butt weld connections are preferred for critical, high-pressure, or lethal services.

Read the full guide to pipe fittings