Weld quality and inspection

Why visual inspection is not enough

A weld can look fine from outside while being defective inside. The typical hidden defects:

• lack of fusion: the weld has not fused with the parent metal,
• cracking: arising during cooling,
• inclusions: foreign matter in the weld,
• porosity: gas bubbles,
• insufficient penetration.

These defects reduce the capacity considerably, and visual inspection does not reveal them.

The tools of inspection

Visual inspection

The base level, for every weld: geometry, surface defects, dimensions.

Non-destructive testing

Procedures for detecting surface cracks and internal defects, carried out by a trained inspector and documented in a report.

The extent of testing — what percentage of welds is tested — depends on the execution class and the type of weld. EN 1090 and execution classes.

What welding requires

A qualified procedure

The welding procedure has to be qualified in advance for the material and the type of weld.

A trained welder

With a valid qualification for that procedure.

A welding plan and record

What, where, by which procedure, by whom.

Suitable conditions

Cleanliness, temperature, protection from wind and moisture. Which is why site welding is difficult.

The designer’s side

The drawings have to give:

• the position, type and size of the welds,
• the execution class,
• the testing requirement, where it departs from the base level of the standard.

Without these the fabricator does not know to what level to work.

The link with the thin-walled structure

Where a thin-walled frame connects to hot-rolled steelwork, the welding is done on the thicker side and the connection itself is bolted. In thin sheet welding is to be avoided for several reasons. Clinching, riveting and welding thin sheet.

The point

Weld quality would be a matter of trust — if there were no testing. So the testing requirement is not a formality but the only evidence.