The aim of this study was the development of repair procedures to minimize the risk of burn-through in the welding of high strength steels (API 5L X70) and ASTM 106 GrB pipes with low residual thickness (3,0 and 3.2 mm). For this purpose, a welding test bench with adjustable water flow and pressure was constructed, that allows the welding of the pipes in longitudinal direction under variable internal fluid pressures (2 and 5 bar) and flow (20, 40 and 80 l/min.).The semi-automatic MIG/MAG (GMAW)welding process was applied, whereby the conditions for minimizing the penetration under the welding techniques pulling or pushing the weld pool were evaluated, as well as the short-circuit or pulsed current metal transfer modes were investigated. The boundary conditions for burn-through were determined associated to specific levels for water flow and pressure. The internal temperature of the pipe wall was determined by an analytical solution for heat transfer in welding. The results showed that for the pulsed current MIG/MAG and pushing the pool technique, maintaining all other welding variables constant, the weld penetration is reduced. Moreover, the welding power levels for the occurrence of burn-through proved to be superior than those reported in the literature for SMAW, indicating MIG/MAG as a robust and suitable process for in-service welding.
Burn-through; In-service welding; MIG/MAG Welding
