Development of a flexible robotic welding system for weld overlay cladding of thermoelectrical plants’ boiler tube walls

The walls of the boilers of power plants undergo corrosion and wear during operation. The reduction in thickness is measured periodically to prevent collapse of the tubes and consequent operation stoppages for long periods. The solution that is usually applied to increase the availability of power plants is gas metal arc welding (GMAW) cladding of the walls with metal alloys that are more resistant to wear. Application occurs in two distinct locations: the boiler and the workshop. Small worn areas are coated inside the boiler itself, while in the workshop, panels are coated so that they can either be used in the boiler’s most affected regions or to produce new walls with higher wear resistance. Mechanized welding produces better results than manual welding; however, in current systems, the oscillating movement of the torch is generated by a single axis. This limitation renders the welding process unstable if the aim is to reduce bead reinforcement and the number of weld beads by increasing the amplitude of the torch oscillation. This instability produces high spattering and excess penetration due to the change in the contact tip to work distance (CTWD), since the power of the welding electric circuit is a function of this distance and since the constant changes in CTWD disturb the metal transfer dynamics. This paper shows that it is possible to overcome the technological difficulties for both application sites and to make automatic cladding operations more productive and of a better quality. For this, a dedicated Computer Numeric Control (CNC) robot with four degrees of freedom was developed, as well as the methodology for generation of its trajectory. The cladding obtained by pulsed GMAW with ER309L stainless steel is presented in order to validate the developments.