Lucas Process Pipe and Tube Welding

A Guide to Welding Process Options, Techniques, Equipment, NDT and Codes of Practice
1. Auflage 1991
ISBN: 978-1-84569-888-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 160 Seiten

Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies

The welding of tubes is an essential requirement in the fabrication of components in many industries. The original idea for this book came from a seminar organized by The Welding Institute which attracted over 100 specialists concerned with design, fabrication, production and quality assurance and yielded a number of valuable papers. 'Process Pipe and Tube Welding' contains some of these papers together with additional chapters to provide comprehensive coverage of all aspects of tube welding from initial design considerations through production to final inspection. In the first three chapters the authors outline the process and equipment options available for both manual and mechanized welding. This is essential for design and production planning when faced with the choice of competing processes such as MMA, MIG, TIG or plasma, helping engineers make the right choice for particular applications and ensuring the most cost effective welding techniques are employed. Five further chapters are devoted to the application of tube welding in the aero-engine, ship building, power generation, petrochemical and chemical plant industries with numerous details on processes, materials, techniques and equipment. The welding parameters and production data provided by the authors are a valuable source of information and will help engineers to overcome problems in production.This title includes Process options and manual techniques for welding pipework fabrications; Mechanised arc welding process options for pipework fabrications; Process techniques and equipment for mechanised TIG welding of tubes; Welding pipes for aero-engines; TIG welding pipework for ships; Automatic tube welding in boiler fabrication; TIG and MIG welding developments for fabrication of plant for the chemical, petrochemical, and offshore oil and gas industries; Fabrication of aluminium process pipework; A fabrication system for site mechanical construction; Qualification of welding procedures for the chemical process industry; Non-destructive examination of welds in small diameter pipes.

Autoren/Hrsg.

Lucas, W.

Leseproben

Chapter 1 Process options and manual techniques for welding pipework fabrications
K R SPILLER Publisher Summary
Joining pipework with any of these processes is regarded as a specialized operation, with the most important requirement relating to the quality and profile of the penetration bead. The successful achievement of this critical application is one of the most exacting tasks the welder encounters, particularly when root runs are made on unbacked, unrotated butt joints. Where high quality root runs are required, TIG welding, with or without the use of fusible inserts, is usually preferred. Where the quality becomes less stringent, MMA with basic, rutile and cellulosic electrodes or the MIG process finds acceptance. Whichever process is used, the production of controlled penetration beads on an unrotated pipe is possible, provided the correct application of specific techniques is carried out. This chapter discusses, in turn, the relative merits of the manual techniques of each process. Of the methods available for joining pipe, a principal technique is manual welding using the manual metal arc (MMA), tungsten inert gas (TIG), and metal inert gas (MIG, MAG or CO2) processes. Joining pipework with any of these processes is regarded as a specialised operation, with the most important requirement relating to the quality and profile of the penetration bead. The successful achievement of this critical application is one of the most exacting tasks the welder encounters, particularly when root runs are made on unbacked, unrotated butt joints. Where high quality root runs are required, TIG welding, with or without the use of fusible inserts, is usually preferred. Where the quality becomes less stringent, MMA with basic, rutile and cellulosic electrodes, or the MIG process find acceptance. Whichever process is used, the production of controlled penetration beads on an unrotated pipe is possible, provided the correct application of specific techniques is carried out. This chapter discusses, in turn, the relative merits of the manual techniques of each process. Physical and geometrical aspects of pipewelding
Welding a pipe butt joint, especially when its axis is in the fixed position, i.e. vertical, horizontal, or 45° inclined (classified as 2G, 5G, and 6G respectively, Fig. 1.1), is one of the most difficult challenges to the welder, particularly when making the root run in the overhead quadrant, which will henceforth be referred to as the ‘critical area’. It will be appreciated that on larger pipe diameters, for example, 300mm OD, a greater degree of tolerance will be present than when pipes of smaller diameters, i.e. 100mm, are welded.
1.1 Classification of non-rotatable pipewelding positions: a) 2G: pipe vertical; b) 5G: pipe horizontal; c) 6G: pipe 45° inclined. The techniques used to make the all-important root run in pipe butt joints are given in Table 1.1. Table 1.1 Techniques for making root runs on unbacked pipe butt joints Welding process Manual welding technique MMA 1 Vertical-up: rutile electrodes   2 Vertical-up: basic electrodes   3 Vertical-down: cellulosic electrodes   4 Vertical-down: basic electrodes TIG 1 Vertical-up: filler rod additions   2 Vertical-up: fusible inserts   3 Vertical-up: fusing the root. No fusible inserts or filler rod MIG or CO2 1 Vertical-down: solid wire dip transfer   2 Vertical-up: solid wire dip transfer The limitations imposed on the maximum permissible protrusion of the penetration bead into the bore of the pipe are given in Table 1.2. While these acceptance limits are laid down, any concavity should merge smoothly into the adjacent surfaces and should be of a continuous and uniform profile around the internal circumference of the welded joint. The mismatch of the internal bore will also affect the penetration bead formation and profile. Many codes give allowable tolerances on the amount of bore mismatch which can be permitted, but this should never be taken as a licence to tolerate the limits and every effort should be exercised to keep the bore mismatch to a minimum. Table 1.2 Limits of bore protrusion and root concavity Nominal bore of pipe Maximum penetration in bore, mm Less than 12mm 0.8 12mm up to but not including 50mm 1.5 50mm up to but not including 100mm 2.5 100mm and larger 3.0 Root concavity. 1. The bore surface of the joint is of smooth contour 2. The depth of root concavity is no greater than 10% of the pipe wall thickness or 1.2mm, whichever is the smaller 3. The thickness of the weld is not less than the pipe wall thickness Manual metal arc (MMA) welding
The MMA process may be used on a wide range of materials and pipe sizes and is one of the most commonly practised for welding steel pipe. Although it is capable of being used to produce high integrity joints, consistent control of the penetration bead profile is difficult to achieve and can vary from being concave to a typical 3mm convex protrusion. The design of the joint preparation and the fit-up obtained make a significant contribution towards the achievement of controlled penetration beads. A representative sample of joint designs as given in various piping codes is reproduced in Fig. 1.2. Two techniques are available for making the root run: vertical-up and vertical-down welding. The choice of method is not affected by the diameter of the pipe, but depends primarily upon the wall thickness and type of material being welded.
1.2 Typical joint preparations for MMA welding of process pipework. Pipe walls: a) Up to and including 20mm; b) Over 20mm; c) Up to and including 12.7mm, suitable for vertical-down welding. When a vertical-up procedure is used it is customary to start welding at the 6 o’clock position and weld upwards until the 12 o’clock position is reached. This method is preferred for thick-walled pipe where the walls act as a ‘heat sink’ by dissipating the heat more rapidly from the weld area. The rapid cooling rates can cause metallurgical changes within the heat affected zone (HAZ) which could prove detrimental. To overcome this the cooling rate must be reduced, and this is accomplished by decreasing the rate of welding and depositing a heavier weld bead. When vertical-down welding is used, the weld is started at the top side of the pipe (12 o’clock position) and is then continued downwards to the 6 o’clock position. Vertical-down welding is primarily used to weld relatively thin-walled steel pipe since it allows fast travel speeds. On heavier walled pipe it is possible to use the vertical-down procedure with larger diameter electrodes and current values, but in doing this the welder will experience considerable difficulty in maintaining control over the weld pool. The weld pool increases in size which results in the pool over-running the arc and so flooding the joint. Moreover, because of gravitational forces, the increased weld pool and associated slag coverage cushions the arc and can cause defects such as pinholes, lack of root fusion and cold lapping. Formation of root penetration around the pipe
When making the root run on a fixed 5G position pipe using the conventional vertical-up welding, the welder has to maintain correct electrode angles and execute small changes in welding technique to produce the desired penetration bead. In the critical area, the welder will rely on the position of the electrode tip and combined arc force to give the weld pool some support. As the weld is progressed to the vertical quadrant, the weld pool tends to sag downwards under the arc and tries to flow into the bore of the pipe at a much faster rate than when welding in the critical area, and the penetration bead becomes markedly convex. In the inclined and flat quadrant, the weld pool becomes even more fluid and the penetration bead can become excessively convex and, in more severe cases, burnthrough can occur. Irrespective of which type of electrode is used, achieving a controlled penetration bead will be highly dependent upon the formation of a keyhole. The keyhole or pear-shaped enlargement is formed by the root faces burning away, which causes a localised enlargement of the root gap just ahead of the arc. Ideally penetration is obtained when the keyhole is between one and one and one-third times the electrode diameter (core) being used. If the keyhole becomes enlarged, excess penetration, burnthrough or internal undercut will develop. Conversely, if the keyhole...