Arc welding process control based on back face thermography:: Application to the manufacturing of nuclear steam generators

The possibility of reducing defects in the arc welding process has attracted research interest, particularly, in the aerospace and nuclear sectors where the resulting weld quality is a major concern and must be assured by costly, time-consuming, non-destructive testing (NDT) procedures. One possible approach is the analysis of a measurand correlated with the formation of defects, from which a control action is derived. Among others, the thermographic analysis of the weld pool and the heat-affected zone have proven to be a useful technique, since the temperature profile of the material being welded has a clear correlation with the process parameters. In this paper, we propose a control system for the submerged-arc welding (SAW) process, based on thermographic imaging of the back face of the joint being welded. In-lab experiments, with simultaneous infrared and a visible imaging, have been performed. Two image analysis techniques are proposed: tracking of the maximum temperature point of the infrared images, and morphological analysis of the visible images. In-lab welding experiments have demonstrated the feasibility of both techniques. They are able to obtain an estimation of the surface temperature and to detect the occurrence of the perforation defect, what has major application for defect detection and reduction in the joining of shell sections of nuclear steam generators.