"Narrow-Gap C02-Welding in the Vertical Position"

Abstract

MIG welding using Narrow-Gap Welding technique ( NGW) is a well known process in which the gap width, depending on plate thickness, varies between 7 to 30 mm and is mostly applied in the flat position. In tl1is work, a narrow-gap technique has been developed for welding mild steel plates in

which reduced gap widili between 3 to 5 mm has been used. Such narrow widili was made possible by keeping the contact tube outside the gap and making use of a long stick-out. Welded plate iliickness had been limited up to 12 mm, which is the most widely used in the welded structures. Moreover, carbon dioxide was used as a shielding gas, which is tl1e most economic compared to other gases normally used. Welding has been performed in the vertical-up position in one single pass. Welding experiments had been conducted using eiilier direct or pulsed ctment under different frequencies and off­ time ratios of pulse. The filling wire was either solid or flux cored. The results show that ilie above described .process could be used sucessfully to weld

steel plates in the vertical position without weaving motion. With respect to the mechanical properties, the narrower gap of 3 mm achieved a better toughness and strengili. On the other hand, welding with 5 mm gap gave more stable process. Metallographic studies showed that the narrower the gap width the finer is ilie weld metal microstructure. The proposed combination IS recommended as a most economical process for fabricating welded structures such as tanks in which ilie plate thickness rarely exceeds 16 nun.