Influência do gás de purga na formação da raiz do cordão de solda em tubos de aço inoxidável AISI 304

Abstract

Broaden knowledge related to the behavior of the root of stainless steel pipes welded by applying a purge gas, especially the austenitic stainless AISI 304 due to its wide commercial use, may further elevate the quality of welded product. The objective of this study was to evaluate the influence of purging gas in the formation of the root beads obtained in tubular components of austenitic AISI 304 stainless steel. The GMAW (Gas Metal Arc Welding) process with metal transfer and short-circuit STT® technology – (Surface Tension Transfer) was used. The tests were performed in test tubes (TT’ s) with nominal dimensions of 60 mm diameter and wall thickness of 2,7 mm. From the best condition, with root face of joint of 0,5 mm and groove angle of 90º, welds were performed in flat (1G) position. The whole system for purging and free rotation was guaranteed by the TT's development and calibration of equipment for this purpose. In the welding torch shielding, the gas used was argon with 2% O2 at a flow rate of 15 L/min. As purging gases CO2, N2 and Ar commercially pure were used, as well as two commercial mixtures of Ar+25%CO2 and Ar+2%O2, varying the flow rate at 6, 12 and 18 L/min. Both appearance of oxidation and the geometric characteristics of variables assigned to the root section were evaluated. It was also evaluated the microstructure, microhardness and ferrite content in fusion zone, the latter through ferritoscope. The results showed significant differences in relation to the transverse profile of the root in the ferrite content, type and flow rate of the purge gas is possible only notice a slight difference between the face and root of the weld, always with greater reduction in the respective faces of the strands. It was unable to identify significant changes in the microstructure. However, the microhardness tends to reduce the conditions purged with Ar +2%O2 and shows slight improvement in purging with Ar +25%CO2. Regardless of the condition of purging, all sectors of the root side had superficial degradation, resulting from the consequent of pits, the worst results point to the purges where there were presence of more chemical oxidant elements, which the fusion zone purged with CO2 (pure) developed worse signs of pitting potential rated by potentiodynamic test.