Soldagem MIG/MAG com alternância de polaridade sincronizada com tecimento transversal (MIG/MAG APSTT) visando automação da soldagem do passe de raiz em tubulações

Abstract

The root pass automated welding in pipelines is associated with difficulty of variable groove gaps and the pipes inner surfaces mismatches (high-lows). The welder overcomes these problems with his ability to control and distribute manually the heat inside of the joint, by increasing or reducing the molten metal volume where necessary. In automatic operation, the most common practice to overcome these difficulties is the use of backing strips, which, in the case of pipes, requires sophisticated and difficult to operate equipment. The objective of this study was to elaborate and evaluate an innovative system (equipment and process) for automatic welding of root passes on pipes without backing. The idea was to look for a optimized distribution of heat and arc pressure on the weld pool, so that more heat and pressure are imposed on the joint sides and less heat and pressure in the center area. This approaches makes possible to achieve high fusion of the joint root (even with a certain mismatch or high-low), but cooling the weld pool when the arc is in the center, to prevent the collapse of the large pool volume, even with a variable groove gap. The proposed technique, called MIG/MAG Welding with Alternating Polarity Synchronized with Transversal Weaving (APSTW), constitutes a transversal weaving of the arc synchronized with alternating GMAW operating modes, Pulsed with higher energy on the sides of the bevel and Negative Polarity (DC-) with lower energy in the weaving transition between the two sides of the bevel, including the central area. A special welding power source and a torch position sensor are required. Initially, parametric conditions for the pulsed and DC- welding were found, so that two different current levels, one for each operating mode, were obtained. A weaving system, which used sensors to indicate whether the torch would be on the groove side or in the groove center, was designed and evaluated. The weaving frequency, amplitude and lateral dwell time effects on the weld pool stability were assessed for different root gaps. The effect of the shielding gas on the melting capacity of the bevel sides was also evaluated to prevent lack of fusion, by replacing a low concentration of O2 by CO2. This system feasibility was assessed by using carbon steel plates with V grooves in the flat position, with varying root gaps and mismatches. At the end, the GMAW technique developed was compared with a controlled short circuit technique using commercial equipment (RMD). The GMAW APSTT technique was able to increase the control of the weld pool at the root, increasing the tolerance for gap variations and mismatches, thus fulfilling its purpose. But it proved to be too sensitive to gap sizes, demanding a different range of parameterization for each weaving amplitude. The use of sensors has been proposed to measure the root gap and indicate the appropriate weaving settings. Therefore, the technique proved to be capable of being used in orbital welding of pipelines with advantages in competition with existing commercial technologies.