TWI Industrial Member Report Summary 866/2007
By V Kumar
The keyhole plasma welding technique offers the potential advantages of deep penetration and high speed welding. As for the Tungsten Inert Gas (TIG) process, the arc is formed between a non-consumable electrode and the work piece. However, in plasma welding the electrode is positioned within the body of the torch and the emerging plasma is constricted by a fine copper nozzle so that it becomes very directional and columnar. By increasing the plasma current and gas flow rate, the plasma jet can be forced to penetrate the material forming a keyhole. During welding the keyhole progressively cuts through the material, whilst the molten metal flows around the keyhole to form the weld bead. Keyhole plasma welding can be applied to a range of materials including carbon steels, stainless steels, nickel alloys, titanium alloys, etc. in a range of material thickness up to 18mm, depending on the material type.
The present project focussed on developing orbital welding procedures for butt joints in carbon steel and stainless steel pipes. For pipe wall thickness exceeding 3mm, butt welding using the conventional TIG process requires the use of a bevel joint preparation together with the deposition of several weld passes using filler wire. The keyhole plasma process offers the prospect of using a square edge joint preparation and completion of the weld in a single pass. Whilst circumferential welding is well established for the flat position (or rotated pipe position), suitable procedures have not been established for the horizontal and orbital positions (ie, PC and K-L0 positions, respectively). In the past, major limitations of the keyhole plasma process for these applications were related to control of the keyhole in all welding positions and closure of the keyhole without defects. The recent developments in plasma welding equipment are considered to offer more precise control of the keyhole parameters, which should facilitate both orbital pipe welding and keyhole closure.
The programme of work was initiated with a review of literature published over the last 20 years. Subsequently, welding trials were carried out to develop orbital keyhole plasma welding procedures for carbon steel and stainless steel pipes in the horizontal and orbital positions (PC and K-L0 positions, respectively).
To develop plasma keyhole welding techniques for the PC and the K-L0 position welding of small diameter austenitic stainless steel and carbon steel pipes with wall thickness in the range 3-5mm.