Evaluation of burn-through in MIG/MAG welding of high-strength, low thickness pipelines

Abstract

The aim of this study was the development of repair procedures of pipelines with low residual thickness (3.0 and 3.2 mm), manufactured from high-strength steels (API 5L X70) and ASTM 106 GrB pipe, to minimize the risk of burn-through. For this purpose, a weld test bench with adjustable flow and pressure of the fluid was constructed, in order to know internal fluid pressures (2 and 5 bar) and flow (20, 40 and 80 l/min). Using the semiautomatic MIG/MAG (GMAW) process, longitudinal welds were performed on pipes containing water, with later evaluation of the appearance of the least penetration of the weld bead. Thus, two welding techniques were used: pushing and pulling the weld pool, as well as the metallic transfer modes in short circuit or pulsed current. In the study of limiting conditions for the occurrence of perforation, welding parameters based on specific flow and pressure conditions were researched. The internal temperature of the pipe wall was determined by an analytical solution for heat transfer in the welding. The results showed that for the pulsed current MIG/MAG process and the technique of pushing the weld pool, maintaining all other welding variables constant, the weld bead penetration is small. Moreover, the welding power levels for the occurrence of burn-through proved to be greater than those reported in the literature for SMAW, indicating MIG/MAG as a robust and suitable process for in-service welding.