The application of magnetic arc oscillation to mechanised orbital TIG welding - Part 1

TWI Bulletin, July/August 1989

David Harvey, MA, is a Senior Research Engineer in the Arc Welding Department.

A current research programme in the Arc Welding Department is looking at the feasibility of applying magnetically controlled arc oscillation to the orbital TIG welding of a range of materials. ^[1] Benefits include the completion of welds in fewer passes, less overall weld upset and the suitability of narrow-gap joint geometries for weld preparations.

Orbital welding equipment

The increasing demands in terms of productivity and quality, particularly from the power generation and chemical industries, have prompted the use of mechanised orbital TIG welding equipment for the fabrication of process pipe in a range of materials including austenitic stainless steels, ferritic steels and cupro-nickel alloys.

The first fixed head orbital TIG welding equipment was developed in the early 1960s for the autogenous welding of thin-walled tubing. The addition of wire feed, the precision available from thyristor and transistor controlled power supplies and, subsequently, pulsed techniques have promoted the application of mechanised orbital welding to more difficult fixed pipe positions. The use of micro-electronics has also enabled multi-level programming, welding head oscillation with end dwells, synchronised pulsed welding current, pulsed wire feed, pulsed travel and automatic arc voltage control (AVC).

Orbital welding equipments containing all of these features are referred to as full function systems. ^[2] Basic function welding systems, although incorporating pulsed welding current facilities, are restricted to stringer passes. When using the latter systems, the benefits from mechanisation in terms of process repeatability and joint integrity are thus tempered with the requirement for more welding passes. Indeed, it has been reported that orbital welding systems without a weaving facility are unable to compete with manual welding if three or more passes are required. ^[3]

The limitations of a basic function welding system were clearly demonstrated in a recent programme of work at The Welding Institute which involved the development of procedures for 60mm OD X 5.7mm wall, type 304 stainless steel pipe. When completed manually, this joint required a root pass and four weaved filler passes ( Fig.1). Completion of the same weld using the basic function system required a root pass, seven stringer bead filler passes and a cosmetic capping pass ( Fig.2).

Furthermore, a bore constriction of only 3mm is permitted in specific standards for this size of pipe. ^[4] Welds completed with weaved passes, either manually or mechanically, were found to comply, but the stringer bead welds exhibited bore constriction between 0.5-1.0mm above the specified limit. Not only would this reduce fluid flow but was also a potential source of corrosion and/or erosion in the root region.

The difference in cost between a basic function system and a more advanced system with a mechanically oscillated welding head is sufficiently great that the development of magnetic arc oscillation would permit a significant upgrading of a basic system for the cost of a control unit and an electromagnet.

This article describes the application of magnetic arc oscillation to orbital TIG welding using a modified basic function system. The initial development trials were carried out on 60mm OD X 5.7mm wall, type 304 austenitic stainless steel pipe.

Magnetically controlled arc oscillation

Magnetic arc blow, which has been a problem in DC arc welding processes for many years, results from the residual magnetism in ferromagnetic materials, electric currents associated with the arc welding system or unrelated currents giving rise to large external fields. ^[5] Magnetic fields are classified with reference to arc welding as shown in Fig.3a. The direction of the arc deflection can be predicted by Fleming's left hand (or motor) rule ( Fig.3b).

The only report of magnetic arc oscillation being applied to mechanised orbital TIG welding in the 5G (or fixed horizontal) position was the fabrication of 12.7mm thick, 216mm OD carbon steel and type 304 stainless steel nuclear power plant pipe work in Japan. ^[6] This reduced the number of passes required with both 'V' preparations and 'narrowgap' preparations, and improved sidewall fusion. The arc was oscillated in synchronisation with both the pulsed current and the pulsed wire feed. However, no details were given of either the pulse parameters or the magnetic field parameters used in this work.

Experimental programme

Magnetic arc oscillation was assessed on a typical basic function thyristor controlled transformer/rectifier welding system, with a rated output of 200A at a 60% duty cycle ( Fig.4). The power source comprises a single level programme with HF striking, pulsed welding current, wire feed and preheating. A remote wire feed unit supplies wire to the horseshoe type welding head with 'clamp-on' feet. The welding head is limited to stringer bead passes.

Initial trials indicated that the commercially available electromagnet, supplied with the arc oscillation unit, was too heavy for the welding head and the torch arm fell away from the pipe in the overhead position. A much smaller and lighter electromagnet, compatible with the magnetic arc control unit, was built at The Welding Institute ( Fig.5). This electromagnet was of a simple design; a shellac-coated copper wire coil wound around an insulated aluminium bobbin surrounding a soft iron core ( Fig.6). It was mounted on a bracket attached to the welding head between the torch arm and torch head ( Fig.7).

Two further minor modifications were made to the welding head:

1. The existing wire guide, made from copper, was replaced with one made from soft iron, in order to concentrate the magnetic field in the vicinity of the arc ( Fig.7).
2. A spring was added to the torch arm to counterbalance the weight of the electromagnet ( Fig.8).

Materials

Two casts of 60mm OD X 5.7mm wall, type 304 pipe were selected for the welding trials. Root passes were completed with 2.4mm type 308L, shape A, EB inserts. Type 308L, 0.8mm diameter wire was used for the filler passes, Table 1.

Table 1 Chemical analyses of pipe material, filler wire and consumable inserts*

Argon-1%H ₂ and commercial welding grade argon (99.95% min) were used as the shielding gases. Standard 2.4mm diameter, 2% thoriated tungsten electrodes were used during the welding trials. A vertex angle of 60° was found to be satisfactory for use with the argon-l%H ₂ shielding gas. This was reduced to 28° with the argon shielding gas in order to maximise arc stability.

Root pass procedure

An autogenous root pass procedure using a pulsed welding current was developed for a 'J' preparation with an EB insert using the basic function system. Initially the procedure was developed for use with the argon-1%H ₂ shielding gas, but a change to argon (which was preferred for the filler passes) required an increase in the peak welding current of 2A and a smaller increase in the background welding current to maintain it at 30% of the peak value; otherwise the welding parameters remained the same, Table 2. This procedure was successfully transferred to the 'narrow-gap' joint preparation, where the root face and insert have the same dimensions.

Table 2 Optimised root pass parameters for 60mm OD x 5.7mm wall (2in NS Schedule 80), Type 304 stainless steel pipe in the 5G position

Filler pass procedures

There are three standard orbital TIG welding process variants available to complete butt welds in pipes:

1. Manually;
2. Mechanised with a stringer bead technique;
3. Mechanised with mechanical welding head oscillation.

Welding procedures employing magnetic arc oscillation were developed for two joint geometries - a conventional 'J' preparation and a 'narrow-gap' preparation ( Fig.9).

For this joint size with a 'J' preparation, manual welding requires four filler passes ( Fig.1), the stringer bead technique eight ( Fig.2) and the mechanical welding head oscillation process four passes ( Fig.10).

Magnetically controlled arc oscillation, 'J' preparation

Four passes were required to fill this joint with a 'J' preparation with magnetic arc oscillation, Table 3 ( Fig.11). It was found that control of the weld pool was possible using a continuous welding current. The end dwells were used to wash the weld pool into the sidewalls. The electrode to workpiece distance was set at 2mm and manually adjusted during each pass in order to maintain the shortest, stable arc length without stubbing into the molten pool.

Table 3 Optimised parameters for the filler passes, using magnetically controlled arc oscillation, J preparation. Weld MW6

The magnetic field strength, required to weave across the joint as it widened towards the top, resulted in the outer regions of the arc flaring and melting the top edge of the joint ( Fig.12), and the torch shroud. Apart from the damage to the shroud, it was found necessary to use a much wider final filler pass to cap the weld than would have been required if the top edge of the joint had not been melted. The 'arc-flaring' was reduced by changing the shielding gas from the hydrogen containing gas to argon. However, the use of argon shielding gas encouraged crystallisation around the electrode tip, necessitating regrinding between each pass but this was minimised by reducing the electrode vertex angle - eventually down to 28°.

Magnetically controlled arc oscillation, 'narrow-gap' preparation

Despite meeting the requirements of BS 4780: Part 1, 1981, welds completed in three passes were rejected on the grounds that the porosity levels were above those generally accepted for mechanised TIG welding. It was considered that pores were being trapped as the result of the increased wire feed chilling the weld pool. This problem is usually overcome by increasing the welding current, but this could not be done without weld pool flooding. Therefore, it was necessary to use four filler passes, but at least the same magnetic and welding parameters could be used for the first three filler passes, Table 4 (Fig.13).

Table 4 Optimised parameters for the filler passes, using magnetically controlled arc oscillation, narrow gap preparation. Weld MW12.

Bore constriction

Welding procedures involving a manually, mechanically or magnetically controlled weaving process, were found to comply with the standard ^[4] ( Fig.1, 10, 11 and 13), whilst the stringer bead procedure produced constriction and protrusion in excess of the permitted values ( Fig.2). The minimum constriction and protrusion values were achieved with the 'narrow-gap' preparation completed with the magnetic weaving procedure.

Discussion

Welding process aspects

In developing procedures using magnetically controlled arc oscillation, parameters typical of the mechanical oscillation procedure, in particular welding current, travel speed and welding head oscillation frequency, were used as a starting point. As with mechanical welding head oscillation it was possible to use a continuous ( i.e. non-pulsed) welding current. Differences in the welding parameters between the two weaving techniques were found to be small. This is not surprising since the orbital TIG welding process is governed by the need to control the weld pool behaviour.

From the process point of view, the main advantage of magnetically controlled arc oscillation over mechanical welding head oscillation is that the electrode remains in the centre of the joint. This eliminates the very real risk associated with mechanical oscillation of striking the sidewall on either side of the joint.

One main problem encountered during procedure development was with the shielding gas. It is common practice to use a hydrogen-containing, argon-based shielding gas for the TIG welding of type 304 stainless steel in order to improve the penetration characteristics and the cleanliness of the weld bead. There is the further benefit of reduced electrode degradation. However, the argon-1%H ₂ gas was susceptible to flaring with the result that the top edge of the joint could be undercut. The problem was overcome by using a hydrogen-free, commercial welding grade argon shielding gas. Removing the hydrogen from the shielding gas created no penetration problems, but it did encourage tungsten recrystallisation ('whiskers') around the electrode tip.

Whilst a comprehensive study of the influence of each welding and magnetic parameter on the magnetic arc oscillation technique was outside the scope of this study, several general observations were made:

1. Completion of the first two filler passes was possible with a wide range of magnetic parameters. A large deflection could be tolerated as long as the arc was not deflected over the top edge of the joint. A range of oscillation frequencies was acceptable if each end dwell blended in with the previous end dwell and the sidewall.
2. As with all mechanised orbital TIG welding there was less tolerance to changes in the welding current, where the need to produce a fluid weld pool is tempered by the problems associated with too large a weld pool when welding vertically down or in the overhead position.
3. Tighter control of the magnetic parameters (particularly amplitude) is required for the later filler passes in order to minimise the width of the final capping pass.
4. Magnetic arc oscillation was tolerant to misguided wire feeding. This is a common problem resulting from the cast and/or helix put on the wire during the manufacturing process. The weld pool formed on one side of the joint is sufficiently fluid to be washed across its entire width by the arc.
5. Demagnetisation of the equipment was not found to be necessary since the magnetism resulting from magnetic arc oscillation was sufficiently strong to override any external residual field.
6. The magnetic arc oscillation process was more tolerant to the electrode position in the joint than the standard stringer bead technique. Occasionally the bead was deposited on one side of the joint when the electrode was too close to the sidewall ( Fig.14), but this could be corrected on the next pass. It should be noted that the position of the electrode in the joint is manually adjusted, thus the occasional misplacement is inevitable.

Operator skill

In deriving suitable TIG welding parameters, a considerable degree of skill is required. In particular, a knowledge and understanding of weld pool behaviour and characteristics is essential. Although the addition of a magnetic arc oscillation control unit involves an extra operation during procedural development, it does eliminate the need to develop the pulsed welding parameters required for the stringer bead procedure. It is no more difficult to program or modify the magnetic parameters with the arc oscillation control unit than it is to program a mechanised welding head with mechanical weave.

Bore constriction

Bore constriction results from the contraction of each welding pass on solidification, and is therefore largely dependent on the number of welding passes used to fill a joint and the amount of molten metal involved with each pass. Hence, it is not surprising that the use of a weaving procedure, be it manual, mechanical or magnetic, should result in less constriction than a stringer bead welding procedure, which in this case needed seven filler passes plus a cosmetic pass.

Joint preparation

A combination of factors - control of root penetration, control of root fusion concavity and joint volume - have led to a 'J' preparation being adopted for mechanised TIG welding of type 304 stainless steel pipes. This joint preparation can also be used for manual welding. Despite meeting the root penetration and root fusion concavity requirements, the 'narrow-gap' preparation has not been favoured for either manual welding, where access to the joint is difficult, or for mechanised welding, where in the event of an unsatisfactory welding pass, recovery is again difficult because of the narrowness of the joint. However, a 'narrow-gap' preparation is more appropriate to magnetically controlled arc oscillation, since the electrode is not weaved across the joint in order to achieve sidewall fusion, and there is less chance of the electrode striking the wall with subsequent contamination of the electrode and the weld pool.

Whilst the increase in joint completion rate is not significant for pipes of this wall thickness (5.7mm), it is to be expected that savings could be made on thicker walled pipes.

Economic assessment

The main economic advantage from upgrading a basic function system with magnetically controlled arc oscillation, in preference to purchasing equipment with mechanised welding head oscillation, is the small additional cost involved. The minimum cost of a mechanised orbital TIG system with welding head oscillation is about £35 000, more than twice that of a basic function system. It should also be noted that it is not possible to modify an existing basic function orbital welding system to one with mechanical welding head oscillation. This makes the addition of magnetically controlled arc oscillation for under £2 000 very attractive. Since the magnetically controlled system has no moving parts associated with the arc oscillation, it could be expected that maintenance of this equipment would cost considerably less than that of the mechanically oscillated welding head.

The labour, power and consumables costs for both the mechanically and magnetically weaved processes are broadly similar. These costs are greater for the manual welding process as a result of lower welding speeds and a lower duty cycle, and for the stringer bead technique as a result of the greater number of welding operations and hence welding time required.

Conclusions

The following conclusions were drawn from the initial study on 60mm OD X 5.7mm wall, type 304 stainless steel pipe:

1. The application of magnetic arc oscillation to orbital TIG welding provides a practical, low cost alternative to mechanical welding head oscillation.
2. The electromagnet can be fitted to the welding head without increasing its overall size, thus making it suitable for use in areas of restricted access.
3. The magnetic arc oscillation technique reduced the joint completion time to half that required for the stringer bead procedure.
4. Magnetically controlled arc oscillation is particularly suited to 'narrow gap' pipe end preparations. As the electrode does not cross the joint, the use of a 'narrow-gap' preparation does not involve the risk of striking the sidewall.

References