A Study on Magnetically Impelled ARC Butt MIAB Welding of Carbon Steel Tubes and Low Alloy Steel Tubes

Abstract

ABSTRACT Magnetically Impelled Arc Butt (MIAB) welding is a unique pressure welding process utilized for joining of tubes and pipes. It involves rotation of arc around the tube circumference due to the interaction of arc current and magnetic field at the tube interface. The arc rotation during welding aids in uniform heating of the faying surfaces. MIAB welding involves four stages, viz, arc initiation, arc stabilization, arc rotation and upsetting. Initially, the tubes to be welded are shorted and retracted back to a constant arc gap causing arc initiation. In arc stabilization stage, arc at the faying surfaces begins to rotate slowly along the inner surface of the tubes. During arc rotation stage, arc rotation velocity increases and causes melting of the faying surface. After sufficient melting, an upset pressure is applied to expel the molten metal causing solid state bonding of the plasticized metal at the tube interface. MIAB welding has many beneficial features when compared to other solid state welding and arc welding processes. Some of the benefits include faster welding cycle, no welding distortion, no external shielding and possibility of welding of dissimilar metals. MIAB welding is used for variety of applications such as welding of tubes and pipes, non-circular components, tube to plate assemblies, shaft assemblies and rear axle assemblies. Most of the earlier literature on MIAB welding give emphasis on process principle description and process development. No detailed literature is available on microstructural analysis and weld property analysis of MIAB weldments. Hence an attempt is made to analyze these properties in MIAB welding of carbon steel tubes and low alloy steel tubes. Carbon steel tubes vi and low alloy steel tubes chosen for the study are widely used in economizers and water wall tubes of boiler power plants. Suitability of MIAB welding for these materials will facilitate the possibility of using MIAB welding in production of boiler power plants. In phase I, carbon steel tubes pertaining to SA 210 Gr A with 0.27 wt % C was chosen for MIAB welding. Tubes with outer diameter of 44 mm and thickness of 4.5 mm were used for welding. Five welding parameters, viz, arc initiation current, arc stabilization time, arc rotation current, arc rotation time and upset current were varied at two levels and DoE trials were designed using half factorial method in Minitab software. The welded tubes were characterized using microstructural analysis and mechanical testing. Microstructural analysis includes optical microscopy and SEM analysis. Mechanical testing includes hardness testing, tension testing, notched tension testing and bend testing. In phase II, low alloy steel tubes (T11) containing 0.17 C, 1.33 Cr and 0.55 Mo (wt %) were chosen for MIAB welding. Three welding parameters, viz, arc rotation current, arc rotation time and upset current were varied at two levels and DoE trials are designed using full factorial method. Like carbon steel welding, T11 weldments were also characterized using microstructural analysis and mechanical testing. In carbon steel weldments, microstructural analysis shows the presence of three distinct Thermo-Mechanically Affected Zones (TMAZ) from the weld interface to unaffected parent metal. A distinct white zone known as Light Band (LB) zone is observed at the weld interface for the samples welded using lower upset current (600 A). LB zone contains weak metallurgical structure of predominantly ferrite. This is due to decarburization upon incomplete homogenization and upsetting. Transverse hardness survey vii of samples welded using higher upset current (1000 A) show peak hardness of 222 HV at TMAZ I (weld interface). Higher hardness at TMAZ I is due to strengthening at TMAZ with acicular ferrite and pearlite at the weld interface. However, the samples welded using lower upset current (600 A) show decrease in hardness due to the formation of LB zone at the weld interface. In correlation to hardness test, presence of LB zone at the weld interface deteriorates the weld tensile strength at the weld interface. Samples welded with lower upset current (600 A) and higher arc rotation current (310 A) show failure at the weld interface in transverse tension test. This is due to wider LB zone at the weld interface. Notched tension test was conducted with ‘V’ notch at both the edges of the weld interface. All welded samples show good notch ductility with Notch Strength Ratio (NSR) greater than one. However the samples welded using lower upset current show 3 to 9 % loss in notch strength compared to the base metal. These samples also show poor ductility in bend test. This is due to the incomplete expulsion of molten metal and retention of the impurities in LB zone along the weld interface. Hence from microstructural analysis and mechanical test results, it is evident that upset current plays a significant effect on weld properties of carbon steel tubes. Samples welded with higher upset current (1000 A) show strengthening at the weld interface with higher tensile strength, higher notch tensile properties and good ductility. Samples of MIAB welded T11 steel tubes show four distinct TMAZs from the weld interface to unaffected base metal. Like carbon steel welding, welded T11 tubes also show a distinct LB zone due to loss in carbon and alloying elements at the weld interface. At TMAZ I (weld interface), microstructure contains bainite and ferrite. Ferrite morphology varies with arc rotation current. Samples welded with lower arc rotation current (270 A) show acicular ferrite and bainite with voids at the weld interface. However, viii samples welded with higher arc rotation current (290 A) show bainite and polygonal ferrite with defect free weld interface. TMAZ II contains lath like upper bainite. At TMAZ III, microstructure contains granular bainite. Fine grained ferrite and pearlite is observed at TMAZ IV. In transverse hardness survey, TMAZ II shows peak hardness of 325 HV. All TMAZs show higher hardness than the base metal hardness. Transverse tension test of the welded specimens show failure at the weld interface in the samples welded using lower arc rotation current (270 A). The presence of voids due to insufficient melting deteriorates the weld tensile strength at the interface. However, samples welded using higher rotation current show failure at the base metal due to defect free weld interface. Among the failed samples, samples welded using longer arc rotation time and lower upset current show higher weld tensile strength. This is due to less void formation as the result of large plasticized metal with minimal metal expulsion at the weld interface. In notched tension test, all welded samples display notch ductility with NSR value greater than one. However, samples welded using lower arc rotation current show 20 to 40 % loss in notch strength due to insufficient melting. These samples also show poor ductility causing cracking at the weld interface in bend test. From test results, it is evident that the arc rotation current plays a significant role in formation of defect free weld interface in MIAB welding of T11 tubes. Samples welded using higher arc rotation current (290 A) show strengthening at the weld interface due to defect free structure containing bainite and ferrite. These samples show higher tensile strength, higher notch tensile properties and good ductility.