Effect of Freezing Caused by Fluid during In-service Welding of Pipes with Wall Thickness of 6 and 11 mm

Abstract

This work compares the influence of cooling caused by a circulating fluid and in contact with the inner wall of a pipe, which characterizes the in-service welding. There were evaluated two wall thicknesses of the pipe and two temperatures of the inner fluid. The welding was done to join connections of ASTM A105 steel to an 11 mm and 6 mm thick API 5L Gr X60 steel pipe, using TIG (GTAW) and stick electrode (SMAW) processes. The circulation of the fluid was made with potable water within two temperature ranges, of 6 to 10 °C and 26 to 36 °C. During welding, the inner temperature of the pipe and the cooling rate were monitored. Four welded joints were prepared, evaluated by non-destructive tests (visual inspection, penetrating liquid, magnetic particles and ultrasonic), destructive testing (tensile, bending, impact Charpy V, fracture and hardness), macrographic and micrographic analysis. The results show that, with the adopted procedures, it is not needed the adoption of any special cares regarding the possibility of perforation and leaking of fluid during in service welding. There were evidences that the effect of the fluid freezing inside the pipe was more significant than the pipe wall thickness in order to increase the cooling rate. Still, the temperature of the fluid circulaling the pipe had a secondary effect in the increase of cooling rate and in the reduction of the maximum temperature in the inner wall of the pipe. The freezing caused by the fluid promoted an increase in hardness of both fusion zone and heat affected zone, reduced elongation in the tension test, but still with adequate values. With the results of non destructive and mechanical tests was possible to qualify welding procedures, based on the standards specialized in this process.

Key-words: In-service welding; Freezing; GTAW; SMAW