Study on notch tensile properties of Magnetically Impelled Arc Butt (MIAB) welded carbon steel tubes

Magnetically Impelled Arc Butt (MIAB) welding is a pressure welding process that uses the circumferential rotating arc to cause uniform heating of the faying surfaces. In this work, notched tensile testing of MIAB welded Carbon steel was carried out to determine the notch sensitivity of Thermo-Mechanically Affected Zones (TMAZ) and to compare the notch tensile property of these zones with the base metal property. In MIAB welding, after sufficient melting of the faying surface, a short pulse of high current is applied to expel the molten metal and impurities from the interface before welding. Insufficient expulsion and formation of Light Band (LB) zone at weld interface resulted in lower Notch Tensile Strength (NTS). Incomplete expulsion with lower upset current at the weld interface contributes to lower Normalized Notch Strength Ratio. Instead higher upset current contributed to higher NTS due to complete expulsion and stronger acicular ferrite formation. Other TMAZs away from the weld interface showed higher notch tensile strength with Notch Strength Ratio (NSR) and Normalized Notch Tensile Strength Ratio (NTSN) greater than unity.

Effect of Upset Current in Magnetically Impelled Arc Butt (MIAB) Welding of Carbon Steel Tubes

Magnetically Impelled Arc Butt (MIAB) welding is an unique forge welding process in which an arc is drawn in the gap between the two tubes to be welded in order to raise them to a high temperature to allow forging to form a solid state weld. This paper presents the investigations carried out on MIAB welding trials of carbon steel tubes with varying upset current. Upset current is the short pulse of high current applied prior to upset. It plays a significant role in expulsion of molten metal and impurity from weld interface. This study aims at studying the effect of upset current on weld properties. Carbon steel tubes of SA-210 Grade A have been chosen with outside diameter of 44 mm and thickness of 4.5 mm. Mechanical and microstructural characterization of MIAB weldments was carried out. Good correlation exists between the mechanical properties/microstructure and upset current. Lower upset current has detrimental effect on weld tensile strength due to incomplete expulsion of decarburized zone.

Magnetic flux distribution modelling of magnetically-impelled arc butt-welding of steel tubes using finite-element analysis

Magnetically-impelled arc butt-welding (MIAB) is a pressure-welding process. In this process, heat is generated prior to forging by an arc created between two clamped and aligned tubes. This arc rapidly rotates along the peripheral edges of the tubes to be welded due to the electromagnetic force resulting from the interaction of the arc current and the magnetic field in the gap. To be precise, the magnetic flux density is the significant parameter that governs the arc rotation and the weld quality. This paper presents a three-dimensional finite-element model to determine the magnetic flux density distribution in the MIAB welding process. The objective of this study is to perform a non-linear electromagnetic analysis using the finite-element package ANSYS, and to explore the interdependence of MIAB welding parameters such as gap size, exciting current in the coil, and coil position from the weld centre, which influence the electromagnetic force generated in the welding process and weld quality. The results of this analysis are verified with the available experimental data for steel tubes (outer diameter 50 mm and thickness 2 mm). The results obtained using finite-element analysis establish that the magnetic flux density distribution in the gap increases with increasing exciting current and decreasing gap size and coil position from the weld centre.

MIAB Welding of Oil and Gas Pipelines

Magnetically impelled arc butt (MIAB) welding is a “single shot” method of joining pipe and tube which is used in highly automated factory production lines in high volume industries such as automotive manufacture. The entire weld over the full joint thickness is made in one single operation, instead of using several passes as in conventional welding. It is believed to be capable of making finished welds in pipe from small diameters of around 75mm (DN75) up to around DN450, and to around 10mm wall thickness. The welding time is around 10 to 15 seconds, and the joint to joint cycle time will be about 1 minute. Under the right circumstances this means that pipelines in this size range could be welded at a rate of up to around 7.5km per day or more, with only a single small welding crew and a substantial reduction in overall cost. Whilst the circumstances that allow construction spreads to take advantage of that potential speed will not exist on every pipeline, there are still major economic and technical advantages to be had from using the process at more moderate rates. The present target thickness limit of 10mm will make it possible to weld Class 900 DN450 pipelines with maximum allowable operating pressures of up to 15 MPa. The use of MIAB welding will enable the entire paradigm of pipeline construction to be changed, and will lead to reductions in construction cost of around 15% or more when the process is first implemented. Larger savings are expected in the longer term.