G Verhaeghe, S Fisher (BNFL), W M Thomas, R L Jones and P Woollin
Presented at International Conference on 'Exploiting advances in arc welding technology', Cambridge, UK, 30-31 March 1998
Fig.1 Principle of Moving Contact Arc Welding
Moving Contact Arc Welding (MCAW) is an easy-to-use method for welding, weld overlay and repair. The process, being developed at TWI, is suitable for either manual or mechanised operation and has potential for use in situations with restricted access or where remote operation is needed. The technique (see Fig.1) uses low cost portable equipment and is simple to operate  . Compared with conventional MMA, improved productivity is made possible by the use of extended consumable lengths, which allow continuity of welding and elimination of stop/starts.
The MCAW technique differs from the traditional Firecracker (Elin-Hafergut) and lying consumable welding processes because it involves a sliding or rolling electrical contact supplying current to a lying consumable. The key to the process is a shaped, flux-covered electrode, referred to as the Ridgeback™ consumable. Its crucial feature is the exposed metal ridge that protrudes above the flux. Electrical contact between the welding tool and the consumable is kept at a relatively short distance from the welding arc. This restricts the resistive heating of the electrode, enabling the use of almost infinite lengths of consumable in one operation. The salient features of the process are illustrated in Fig.2.
Fig.2 Basic principle of MCAW using a Ridgeback™ consumable
The process works as follows: the current supply is made with a sliding or rolling tool to the consumable via a narrow ridge which is part of the consumable core as shown in Fig.2. Initially an arc is struck, using a fuse or fine wire wool at the end of the electrode to ionise the consumable/substrate arc gap. The flux underneath the consumable core ensures electrical insulation between base material and the Ridgeback™ consumable and maintains a controlled arc length throughout the welding operation. The arc length can be changed by altering the thickness of the flux covering or by changing the shape of the metal core. The arc burns along the consumable electrode leaving a weld deposited onto the workpiece, as illustrated in Fig.4.
This paper describes the initial development work carried out by TWI and BNFL, in co-operation with Metrode Products Limited. Suitable welding consumables have been developed and welding trials have been carried out to demonstrate the feasibility of the process. Future work is currently underway at TWI to improve process control and welding productivity and to tailor the technique to meet the requirements of specific applications. A TWI Group Sponsored Project has been proposed to develop the process further.
The Ridgeback™ consumable
The use of lying electrodes for weld metal deposition is not a new concept. Since the development of the Elin-Hafergut process, commonly known as the firecracker process, a number of attempts to improve the technique have been made [2-7] .
Essentially, the advantages of the lying electrode technique can be seen in those application areas with limited access. In addition, its predictable arc gap and its inherent low skill requirement make the process user-friendly and readily mechanised. The use of firecracker welding for butt, fillet, groove and multipass welds has been reported to achieve acceptable welds  .
However, the firecracker type of lying electrode has limitations, one of which is the length of consumable. As for the conventional MMA process, the length of the consumable which can be used in a single welding operation is restricted because of the resistive heating during welding. Attempts to overcome this length limitation have met with only limited practical success [2,5,6] . In contrast, the concept of a moving contact provides a more elegant solution to the welding of comparatively long consumable lengths. However, it was not until the consumable profile was significantly changed to the Ridgeback™ type geometry, that acceptable welds could be achieved without the use of additional granulated flux or gas shielding  . The additional amount of flux on either side of the thin metal contact ridge on the upper surface of the consumable, provides the necessary slag protection to prevent atmospheric contamination and facilitates metal transfer to the underlying weld pool  .
Scope of work
The consumable electrodes used in the experimental programme were produced from 6mm diameter BS970 grade 316L stainless steel. To suit the Ridgeback™ geometry these bars were machined nominally to the profile shown in Fig.2. The substrate material comprised BS970 grade 304L stainless steel. The flux composition of the Ridgeback™ consumables was chosen to suit the required weld metal chemistry.
The welding tool supplies the welding current to the consumable at a distance of approximately 50mm from the arc end. The welding tool, depicted in Fig.3a, resembles a bicycle handle bar and consisted of a rectangular copper block mounted upon an insulated steel bar. To minimise friction between the welding tool and the electrode, a copper wheel can also be used instead of the skid, as is illustrated in Fig.3b.
Both an AC transformer and a DC power source were used for welding. Welding was carried out manually, mainly in the flat position. However, welding in position was also possible providing the right choice of process conditions, flux composition, core material and consumable geometry was selected. For the purpose of this paper, only bead-on plate welds have been reported.
Fig.3a The MCAW skid tool
Fig.3b The MCAW wheel tool
The welding performance was assessed in the form of arc stability, level of spatter, ease of slag-removal, weld bead profile, etc. The completed MCAW welds were subject to a visual inspection, a metallurgical examination, a Vickers hardness survey and a simple bend test. Metallographic sections were taken transverse to the weld and prepared to a 1µm finish, followed by electrolytic etching in an aqueous 20% H 2SO 4 solution with 0.1g/1 NH 4CNS. A Vickers hardness survey was also performed using a 5kg indenting weight, to EN288 Pt3 specification whereby weld metal, heat-affected zone (HAZ) and fusion line were examined closely.
Acceptable weld deposits were achieved, providing both satisfactory mechanical and metallurgical properties as far as these were determined. The typical visual appearance of a deposit produced with a Ridgeback™ consumable is shown in Fig.4, which displays a well-defined weld surface ripple and is generally similar in appearance to those weld deposits produced by conventional MMA electrodes.
Fig.4 Example of surface appearance of MCA weld made with Ridgeback™ consumable, BS970 grade 316L deposit onto BS970 grade 304L substrate
Fig.5 Transverse section of a typical MCA weld
The macro photograph in Fig.5 illustrates modest penetration of weld metal into the base steel, which is comparable with that achievable with a MMA electrode of a similar size. Bend tests confirmed the soundness of the weld deposit. The welding conditions used to produce these acceptable Ridgeback™ weld deposits in the flat position ranged between the following:
A number of comparative weld trials were also made using lying electrodes manufactured to a configuration conforming to that previously reported in the literature  . For this purpose, conventional MMA electrodes were used with the upper layer of the flux removed to provide electrical contact for the current supply. Trials with these partially exposed round core consumables proved unsatisfactory, resulting in a very unstable arc, which frequently extinguished, and a high level of spatter. The final weld deposit profile was not uniform. In contrast, the weld performance of the Ridgeback™ type consumables was much better.
Fig.6 Example of partly consumed electrodes, Ridgeback™ electrode on the left-hand side, round wire (prior art) consumable on the right-hand side.
The additional amount of flux on either side of the thin metal (core) ridge not only provided the necessary protection against atmospheric contamination, but also deflected the arc downwards and minimised the level of spatter. Figure 6 clearly shows that the two electrode types had been consumed in a different manner. During welding it can be seen that the 'prior art' electrode arc burns in a more exposed manner resulting in a very unstable arc which is much more susceptible to atmospheric contamination. For this type of consumable, the use of additional flux or gas shielding has been found necessary for satisfactory operations  .
The Ridgeback™ type consumables were coated with several fluxes to determine the optimum composition for welding in the flat position on the stainless steel substrate. It should be noted that different flux compositions of the Ridgeback™ consumable will be required for welding in position and to suit a particular welding application, much in the same way conventional electrodes are produced. With the optimum flux, the Ridgeback™ consumable provided a stable arc, smooth metal transfer with good weld metal deposition characteristics. The slag was self-removable and the overall result was a weld bead with a very smooth surface appearance.
As indicated by previous research  , the point of connection of the welding current return supply influences the arc transfer characteristics and the formation of the weld bead. The MCAW arc control was aided by positioning the work return supply up stream of the arc, i.e. welding to be carried out in the direction away from the current return supply. In this respect the inherent magnetic fields created by the welding current tend to deflect the arc directly towards the substrate, thus minimising the amount of weld metal lost through spatter.
During the MCAW trials it was evident it was convenient, although not essential, to add a further electrical connection from the electrode supply to the tip of the core wire opposite to the arcing end. In this way, unintentional loss of contact of the welding tool with the electrode during welding, did not result in momentary arc interruptions.
Evaluation of weld deposits
Microstructurally, the MCAW deposit is predominantly single-phase austenite, with a cored dendritic structure, and a HAZ area with no significant change in structure when compared to the base steel. Visual assessment indicated little, if any, delta ferrite in the bulk of the weld metal, but an increased volume fraction of delta ferrite (visually assessed as 3-4%) within approx. 50µm of the fusion line. For further development using 316L stainless steel core wire, the chemical composition of the consumable should be designed to give higher levels of weld metal delta ferrite, in order to reduce the potential susceptibility to solidification cracking.
Results of the hardness survey, given in Table 1, show that the parent material was harder than both the weld metal and HAZ, which would suggest a small degree of cold work of the parent material prior to welding. The region with an increased volume fraction of delta ferrite gave 153 HV 5, i.e. very similar to the bulk weld metal.
Table 1: Results of Vickers hardness survey
|Region of weld||Vickers hardness, HV 5|
|Parent 304L Stainless Steel
|Weld Metal 316L Stainless Steel
The microstructural characteristics and hardness properties observed are generally typical of arc welds in austenitic stainless steels deposited by any of the common processes for a given heat input.
Benefits and potential applications
The MCAW is simple to operate and eliminates the need to use skilled welders as required for conventional manual processes. For this reason, reproducible and defect free weld deposits should be achievable, whilst at least matching the metallurgical properties of other arc processes.
At the same time, the technique will offer a more convenient weld overlay process. Large areas can be coated in one pass, simplifying the operation and reducing the number of interfaces in the coating. The consumable can be shaped to suit the substrate geometry, resulting in accurate and optimal placement of the coating material.
Currently, applications such as welding under water or in radioactive environments require sophisticated welding equipment. These are often expensive and time consuming to operate. MCAW is seen as a way of reducing down times and set-up times when welding in such restricted situations. The practicability of restricted access conditions has already been demonstrated using a local habitat chamber as shown in Fig.7 for the welding under water under dry conditions.
Fig.7 MCAW local habitat for dry under water welding
Fig.8 Restricted access welding
Internal welding of pipes or tubes also present access difficulties which can be overcome using the MCAW technique (see Fig.8).
Investigations at TWI have confirmed the feasibility of the Moving Contact Arc Welding process for bead-on plate welding in the flat position using Ridgeback™ electrodes. Trials have shown that stable weld deposition conditions can be achieved and sound weld deposit can be produced. Metallurgical examination of Ridgeback™ weld deposits showed a structure consistent with similar stainless steel composition deposits produced by conventional MMA welding.
The authors would like to thank Mr. P Temple-Smith, Mrs. W Martin, Mr. J C Carr, Mr. G H Dixon and Mr. C Wing for their technical and practical support.
The assistance of Metrode Products Limited in the design and the manufacture of the MCAW electrodes is also acknowledged.
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In: 'Exploiting Advances in Arc Welding Technology', Proceedings, International Conference, Cambridge, UK, 30-31 Mar.1998. Published by Abington Publishing, Abington, Cambridge CB1 6AH, UK 1999. (ISBN 1-85573-416-8).