Analysing suspected linear indications in narrow-gap welding of nuclear power plants via radiographic inspection

In nuclear power plants, ensuring the integrity of main coolant line welding is vital. Radiographic testing often shows linear indications in these welds, posing challenges for quality assessment and requiring expensive reworking. To investigate the mechanism of the suspected linear indications observed in radiographic inspections of narrow-gap welding at nuclear power plants, the microstructure, microhardness, and porosity of welds showing such indications were investigated in this study using scanning electron microscopy and atomic probing methods, among others. The microvoid distribution in narrow-gap welds was characterised by porosity measurement. The results showed that the microstructure of the weld consists of an austenite matrix and δ-ferrite, with the latter distributed almost uniformly during welding. The average δ-ferrite content and microhardness were approximately 6.6% and 186 HV0.49N, respectively. The porosity of the welding zone with a suspected linear indication was approximately 0.06%, higher than the 0.03% observed in the zones without such indications. This difference is likely responsible for the linear indications detected in radiographic inspections. However, the results demonstrated that this difference does not affect the welding microhardness. The novelty of this study lies in explaining the role of porosity in causing suspected linear indications in narrow-gap welding in nuclear power plants.