TWI Industrial Member Report Summary 76/2011
By K I Johnson, A W Carter, W O Dinsdale, P Threadgill and J Wright
Magnetically impelled arc butt (MIAB) welding has been developed for the welding of thin wall (less than 5mm) steel tubing. It is particularly attractive to the mass production industries because it is a fast process, is relatively clean, requires low power and material consumption and is suitable for noncircular components. An evaluation of a commercially available MIAB machine has been conducted when welding 51mm diameter, 3mm wall mild steel tubing. Solid phase welds can be made in air using a 6sec dual heat welding schedule which gives a defect occurrence on bend testing of less than 5%. Metal consumption is less than when flash or friction welding and the internal weld flash satisfies the specification for welded boiler tubes. Such welds could be made over a range of ~+/-10% welding current, of ~+/-30% force, of ~+/-25% heat duration, of initial welding gap of 1.5 to 2mm and of tube end misalignment of up to 1.0mm. Welds showing high defect levels and evidence of cast structures at the weld line were obtained at low heat input and forging levels.
Good quality MIAB welds with a single heat schedule were more difficult to obtain in air at shorter weld times of 4, 3 and 2sec. A 2sec welding condition was however achieved which gave solid phase welds and a defect level of less than 5% after bend testing by using CO2 shielding. Detailed study of the effect of current and forge force on weld quality, however, showed that these welds were generally slightly inferior to those obtained using the dual heat 6sec schedule in air.
Rotating bending resonance fatigue tests were conducted on tubes welded at a 7sec dual heat schedule in air, and were compared to fatigue test results obtained previously on manual metal arc (MMA), flash and friction welded mild steel tubes. The fatigue strength of the MIAB tubes was similar to those made by MMA and friction but inferior to those made by flash. One possible reason for this is the shape of the bore protrusion profile.