Non-destructive testing (NDT) of Duplex welds differs, in some respects, from inspection of carbon steel welds. For the inspection of the latter, various conventional NDT methods are in common use. According to prevailing requirements and circumstances, radiography (RT) and ultrasonic inspection (UT) are used for volumetric inspection. For surface inspection, magnetic particle inspection (MT) as well as, in some cases, dye penetrant testing (PT) are utilised.

Over the last decades, a good combination between these techniques and widely used acceptance criteria for weld defects has evolved. One can say that this is a well-established and accepted situation.

Although radiography (RT) is still the leading method in all-day volumetric weld inspection it is realised ever more that mechanised UT, apart from the fact that it avoids radiation hazards and chemical waste, can lead to a more reliable inspection. This tendency has been even more stimulated by the introduction of the Time Of Flight Diffraction (TOFD) technique, as a powerful alternative for radiography in selected inspections. Currently, various Joint Industry Projects are undertaken to develop dedicated acceptance criteria for weld defects detected with mechanised UT, among which TOFD. It is expected that this will lead to a situation whereby the probability that defects of concern are really detected is enhanced, without increasing overall repair rates.

In the inspection of 'acoustically unfriendly' materials such as welds in austenitic and Duplex steels however, the situation is different. Whereas the typical (coarse and anisotropic) structure of these welds (and sometimes, in Duplex, also the parent metal) does not significantly influence radiography, the influence on ultrasonic and magnetic inspection techniques is significant. Therefore, the inspection of such welds is sometimes restricted. On the other hand, special ultrasonic techniques have been developed over the years that enable full ultrasonic inspection that even meets the requirements of prevailing codes and standards in many cases. Important is, that inspectability is evaluated on a case-by-case basis, using representative welds with known (artificial) defects. Even the use of the TOFD technique, which uses weaker signals and therefore is more vulnerable to metal structure influences, is sometimes possible if adequate measures are taken.

This paper intends to give an overview of the current capabilities of NDT techniques on Duplex steel welds. It will also highlight the measures to be taken to arrive at a reliable inspection, and will elucidate where the inspection differs from carbon steel inspection, not only technically but also from a commercial and a logistical point of view.