Welding basics for steel fabricators

Common arc welding processes

Weld types and joint types

The joint type describes how the parts meet (T-joint, butt, corner, lap). The weld type describes the weld itself: a fillet weld sits in the corner of a T or lap joint; a butt weld fills a prepared groove and can be complete (CJP) or partial (PJP) penetration. Plug and slot welds fill holes in one plate.

Fillet weld size: leg and throat

A fillet weld is defined either by its leg length z or its throat thickness a. For an equal-leg fillet, a ≈ 0.7 × z (z = a × √2). European drawings usually give a (e.g. a5), American drawings give the leg (e.g. 5/16"). Mixing them up changes the weld volume by a factor of two.

The documents behind a weld

• WPS – welding procedure specification: process, consumables, position, thickness range, preheat, parameters.
• WPQR / PQR – the record of the test that qualified the procedure (e.g. to ISO 15614-1 or AWS D1.1).
• Welder qualification – each welder's certificate for a process, position and thickness range (e.g. ISO 9606-1), kept valid by continuous welding.

Inspection methods

VT visual testing (always first), MT magnetic particle and PT penetrant for surface cracks, UT ultrasonic and RT radiographic testing for internal defects. Acceptance levels come from ISO 5817, AWS D1.1 or AS/NZS 1554.1, as specified.