Connect, no.140, January - February 2006, p.6

Welded afterthoughts prompt a closer look

When welded modifications were made to an existing oil and gas structure it left one of TWI's Industrial Members with nagging doubts about the changes the new welds may have made to the original structure.

It asked TWI to examine an original superduplex pipe assembly, on which stainless steel and nickel alloy components had been fitted, and identify any third phase precipitation or softening associated with welds adjacent to the superduplex pipe.

So how did TWI tackle the job?

After detailed visual examination areas of special attention were identified. Three regions close to the additional welds were selected for metallographic sectioning and scrutiny by scanning electron microscope. TWI was particularly looking for fine scale microstructural changes.

The hardness of the base pipe region adjacent to the weld on each section was surveyed using Vickers hardness measurements. This was compared with the hardness of the base material.

TWI was able to tell its client that the surface hardness was higher than expected in two pipe regions, but advised that this was probably a feature of the original pipe, not a feature created by the welding.

No microstructural change was found in these samples so it was deduced that the adjacent welds had made no significant effect. However one of the three sections selected for close scrutiny revealed chromium nitrides in the parent pipe's heat affected zone.

TWI advised that this could lead to locally reduced corrosion resistance.

For more information on corrosion resistant alloys, Industrial Member companies can contact cra@twi.co.uk