High-strength steels such as DOMEX 700 combine high mechanical strength and great ductility. However, when processed by welding their microstructure could present grain growth and deleterious phase formation. Nevertheless, using highpower GMAW-P the effects of electrode and shielding gas composition on the mechanical and microstructure properties of DOMEX 700 welded joints need to be understood. Thus, wire electrodes such as AWS ER 90S-D2 (A1) and AWS ER 120S-G (A2), and shielding gases such as Ar ? 15% CO 2 (G1) and Ar ? 8% CO 2 (G2) were used and the microstructure of welded joints was analyzed through optical and scanning electron microscope (SEM). Mechanical properties of joints was characterized through joint tensile test, impact test from 20 to-40°C, and microhardness in the joint cross-sectional. It is possible to highlight the increase in strength and elongation values with the use of electrode A2, and reduction in impact energy values for specimens welded by gas mixture G2.
Most of pipeline welding still applies manual procedures, which increase production time and is stressful to the welding operator. This happens mainly due to the accurate melt pool control that hand operation enables. It yields high flexibility between material addition and heat source and is therefore adaptable to the welding condition and situation of each moment. This feature is not fully found when mechanized welding with automatic feeding is performed, despite every benefit of welding automation. This renders an optimized parameterization of a complex task. Automatic orbital welding is already a reality, though only applied in large scale in developed countries and/or by few expert companies from developed countries, due to such controllability, repeatability, and robustness difficulties. In this paper, a concept for dynamic wire feeding and respective implementation and analysis are presented. It consists of a low-frequency wire speed oscillation, aiming to decouple wire speed and arc power to a larger extent, which approaches to manual procedure as it guarantees user flexibility, but still keeping the benefits of welding automation. ASTM 139 Grade D tubes were welded under stable processing conditions. The macrographs did not indicate discontinuities such as porosity or lack of fusion, resulting in complete joint penetration. The average welding speed reached was 27.8 cm/min (10.9 in/min), much higher than that found by other authors.
Currently, in the Brazilian Oil and Gas scenario the execution of welding processes for stainless steel consists mostly of manual applications, rendering hard any increase in productivity and repeatability. Therefore, the automation of this step allows several advances such as more reliable results, lower costs and healthier condition for welders. With the aim of developing automation for GTAW welding of these applications, experimental tests were carried out in a 304L stainless steel pipe laid out at 5G position. The samples were orbitally welded by GTAW with dynamic wire feeding process, using a robotic manipulator in two steps of 180º in vertical down progression. The welding parameters and the stable condition obtained are discussed and the weld pool behavior during the root pass was online monitored by HDR (High Dynamic Range) videography. It was possible to reach high welding speed in the root pass, 50 cm/min in a keyhole mode. The wire dynamic movement contributed to reach great process stability and robustness for all joint passes, from root to finishing. The macrographs and x-ray analysis did not indicate discontinuities as porosity nor lack of fusion.
GTAW welding with pulsed current has been misinterpreted in some of the classic literature and scientific articles. General conclusions are presented, stating that its use provides greater penetration compared to the use of constant current and that the simple pulsation of the current promotes beneficial metallurgical effects. Therefore, this manuscript presents a critical analysis of this topic and adopts the terminology of thermal pulsation for the situation where the weld undergoes sensitive effects, regarding grain orientation during solidification. For comparison purposes, an index called the form factor (ratio between the root width and the face width of the weld bead) is adopted. It is shown that the penetration of a welding with pulsed current can be worse than constant current depending on the formulation of the adopted procedure. Moreover, metallurgical effects on solidification, such as grain orientation breakage, only occur when there is adequate concatenation between the pulsation frequency and the welding speed. Finally, a thermal simulation of the process showed that the pulsation frequency limits the welding speed so that there is an overlap of the molten pool in each current pulse, and continuity of the bead is obtained at the root. For frequencies of 1 Hz and 2.5 Hz, the limit welding speed was 3.3 mm/s and 4.1 mm/s, respectively.
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