Excessive Penetration

Excess weld metal protruding through the root of a fusion (butt) weld made from one side only. With pipe welding this type of imperfection may cause effects in the fluid flow that can cause erosion and/or corrosion problems.

Common causes

Penetration becomes excessive when the joint gap is too large, the root faces are too small, the heat input to the joint is too high or a combination of these causes.

Acceptance

The criteria which sets the level of acceptable penetration depends primarily on the application code or specification.

BS 2971 requires that the 'penetration bead shall not exceed 3mm for pipes up to and including 150mm bore or 6mm for pipes over 150mm bore'.

BS 2633 gives specific limits for smaller diameters pipes, eg for pipe size 25-50mm the maximum allowed bore penetration is 2.5mm.

ASME B31.3 bases acceptability on the nominal thickness of the weld, for instance, allowing for a thickness range of 13-25mm up to 4mm of protrusion. However, ASME notes that 'more stringent criteria may be specified in the engineering design'.

BS EN 25817 relates the acceptable protrusion to the width of the under-bead as follows:

Severity of service	Moderate, D	Stringent, B
Limit (up to maximum)	h 1mm + 1.2 b	h 1mm + 0.3 b
Maximum	10 mm	3 mm
Where: h = height of excess & b = width of bead (see Fig.1)

Fig. 1 Excess Penetration

Avoidance

It is important to ensure that joint fit-up is as specified in the welding procedure. If welder technique is the problem then re training is required.