Comparative Analysis of GTAW and SMAW Processes Applied to In-Service Welding of Low Carbon Pipes for Gas Distribution

Abstract

The in-service welding of natural gas distribution pipes is a critical operation. The shield metal arc welding (SMAW) process has low cost and is very simple, but it has limitations in the minimum pipe thickness. Gas tungsten inert arc welding (GTAW) employs more qualified operators and expensive welding machines and accessories. However, it can weld thinner pipes. In this work, both processes were tested and compared for the welding of an API 5L grade B pipe, often used in the distribution lines of Rio de Janeiro State. The results obtained showed that both processes gave satisfactory results. The minimum tensile strength was 430 MPa for both methods, under the standard range of 415 to 760 MPa. The impact toughness was 50 and 48 J for GTAW and SMAW, respectively. The GTAW showed more fine grains than the SMAW process. The hardness values in the SMAW HAZ was around 220 HV, while the GTAW HAZ was slightly lower, at 180 HV. For both processes, the hardness measured in the heat affected zone (HAZ) was superior to the base metal (BM), but much lower than the limit of 350 HV considered critical for hydrogen induced cold cracking.

Key-words:
Mechanical properties; Microstructure; Piping