Plasma Arc Welding for Thick Plate (Report 2)

Bead Formation in Welding With the Use of Back-up Bars

Abstract

In this report, the application of back-up bars is studied for experiment to prevent drop of molten metal in order to obtain a sound root bead in the plasma arc welding of thick plate.
As a result, it is found possible to overcome a series of arcing, blow holes and poor appearance of the underbead produced when a normal back-up bar is used, by improvements such as gas shielding, edge preparation of root gap in I groove joint and setting of two back-up bars. Thus, sound root beads were obtained.
The results of the experiments are summarized as follows:
1) Although the plasma arc is considered more rigid than normal open arc, its column somewhat changes its shape during welding due to magnetic force, depending on the distance from edge of the test piece. The arc is faced forward up to 10cm from weld-start edge, then it changes to the opposite direction. Sound bead can be obtained when the arc is faced backward.
2) It is confirmed that the optimum value of the flow rate of argon shielding gas is 50l/min, that of the hole diameter of the shielding gas passage in nozzle cover is 15-20 mm and that of the root gap of I groovej oint is 5-8mm.
3) It is also found that the optimum value of the gap between two back-up bars is 1.0-2.0mm and that of the clearance between the back-up bars and the base metal is 0.5-2.0mm to produce sound beads.
4) Although the bead is affected by various welding factors, such as welding current I, plasma gas flow rate Q_p, welding speed ν, arc constricting wall length of nozzle l₁ and arc length l_a, in general a good bead obtained in this experiment has shown inherent characteristics in its cross section which has near-hyperbolic fusion lines.