Modification of the grain structure of austenitic welds for improved ultrasonic inspectability

Welding is an essential part of the fabrication of austenitic stainless steel components used in industrial plants, such as those designed for nuclear power generation, chemical processing, conventional power generation and, increasingly, for production of renewable energy. The welded austenitic material presents major challenges for ultrasonic inspection due to the grain structure of the weld metal. The typically coarse grain structure, in combination with the elastic anisotropy of the material, leads to increased scattering and affects sound wave propagation in the weld. These effects result in a reduced signal-to-noise ratio, and complicate the interpretation of signals and the localisation of defects by ultrasonic inspection. This paper presents the results of a research project dealing with efforts to influence grain growth in the weld during the welding process, in particular during the solidification process, in order to produce smaller grains. The objective was to achieve improved sound propagation through the weld, so that inspectability can be improved. The welding process was modified by the application of alternating magnetic fields at different frequencies, as well as different temperature cycles and pulsed arc technology. Metallographic sections of the test welds show that modification of the grain structure can be achieved by the use of these techniques. For further optimisation, test blocks for ultrasonic testing were manufactured with testflaws to study sound propagation through the modified weld and to assess the detectability of test flaws. The results of this investigation are of importance in assessing the integrity of highly stressed components in industrial installations, particularly for those components with stringent requirements on safety and quality.