New developments in advanced welding
Optimising productivity
Most of the guidelines that apply to increasing productivity for other welding processes apply equally when the consumable is a tubular wire - fillets are more effective than butt welds, downhand welding is faster than positional welding, roots made on a backing are faster than open roots and so on. The wide range of tubular wires available makes it easier to optimise productivity in any situation.
For downhand welding, large diameter rutile wires can be used to give deposition rates approaching 20kg/h, or similar rates can be achieved using the twin arc process, where the wires are connected in parallel so only one power source is needed. As described above, tandem welding offers greater versatility at some increase in equipment cost. Where downhand welding is not possible, welding downhill with a metal-cored wire can deposit 5.5kg/h or welding uphill with a rutile wire, 4 kg/h. The latter are figures that would be difficult to match with any other process. Because tubular wires, even of the flux-cored type, produce less slag than MMA electrodes, it is possible to use narrower joint preparations without running the risk of trapping slag: so where, for example, a joint with a 60° included angle might be used with MMA electrodes, 50° might be used with rutile flux-cored wire and 40° with a metal-cored wire. In mechanised pipe welding, metal-cored wire has been used downhill in preparations as narrow as 6° and was found to fill the joint with 20% fewer runs than a solid wire.
By choosing a suitable rutile wire, large standing fillet welds can be made in a single pass or, with metal-cored wires, a fillet of several runs can be made without deslagging. Basic and metal-cored wires can be used on an open weld root with no backing, but as is the case when unbacked roots are used with other processes, this must be done with a low current to avoid burning off the joint edges and the process is therefore slow. However, rutile wires perform very well with a ceramic backing, which allows relatively high currents to be used and is highly productive.
Tubular wires lend themselves particularly well to mechanisation and robotisation. In the first place, these move the operator out of the weld zone with its heat, radiation and fume, allowing welding conditions to be used routinely that would be prohibitively uncomfortable for a welder. Secondly, only mechanised processes can take advantage of the very high welding speeds of which tubular wires are capable. In addition, with wire regularly available in packs of 300 kg or more, the duty cycle can be increased to a high level.
2.4 Process control and quality
Tubular wires are capable of operation over a much wider range of conditions than many other processes. For example, a basic or metal-cored wire might be used at less than 100 A in positional welding with dip transfer, or at over 300A in downhand welding. At these extremes, users might need to consider the implications of a low or high heat input, while at some intermediate currents and voltages, globular transfer might threaten poor bead appearance, lack of fusion and increased spatter. It is therefore important that despite the apparent ease with which untrained operators can pick up a torch and start welding with tubular wire, proper training is given in setting up the equipment and selecting appropriate parameters.
None of the problems that can occur with tubular wires are unique to the process and welding engineers will be familiar with their causes. However, because the parameters may change by a large factor with few visible signs that anything is different, it is more important than with other processes to be able to monitor parameters in high integrity joints. Especially when welding high strength steels, the properties of both the weld metal and the heat - affected zone are strongly affected by heat input. Equipment is now widely available to keep a continuous record of all welding operations and this is becoming standard in critical applications.
Gas metal-arc welding with solid wire has failed to gain acceptance in some areas because a perfect weld surface can conceal serious lack-of-fusion defects, which may be difficult to detect by radiography. Even with solid wire, the problem is being overcome with improved power sources and the increasing use of ultrasonic testing, but tubular wires offer a further reduction in susceptibility. Their better arc profile, especially when welding with argon - rich gases, and wettability have allowed them access to applications hitherto closed to gas-shielded welding. In vertical-up joints, the ability of rutile flux-cored wires to operate at twice the current of solid wires has led to their near monopoly of positional welding in offshore fabrication. However, in horizontal-vertical butt joints, where often only the upper plate is bevelled, positional rutile wires may not be best because their stiff slag is not needed to support the pool and may become trapped: in this case, a basic wire is usually preferred.
It has been claimed with some justification that there are now no ‘no-go areas’ for tubular wire. Even the power generation and pressure vessel industries, formerly seen as bastions of conservatism, have adopted them. The sector using the greatest quantity of tubular wires is shipbuilding, where the large amount of mechanised fillet welding to be done is ideal for the tubular wire process. The ability to weld over prefabrication primer with metal-cored or low-slag rutile wires is also important here.
Offshore construction is another high tonnage user of tubular wires, since many of the joints have to be welded in position and no other process can do this so productively. In covered yards, nickel-containing rutile wires are most productive, but for final assembly where conditions may be windy, self-shielded wires are often used.
Manufacturers of earth-moving equipment pioneered the use of flux-cored wires in the USA and introduced it into their European factories in the 1960s. This is a highly competitive and innovative sector where there has been heavy investment recently in laser welding, but tubular wires are still in a leading position. The use of metal-cored wires with robots using through - arc sensing for guidance will quite possibly be a productive technology in the future.
Flux-cored wires were used on German submarines during World War II9 and later this was one of the first examples of their use on high strength steels. Crane jibs and offshore jack-up rigs were other examples where steel yield strengths up to 690 MPa were welded. The potential of the process for pipeline welding has yet to be fully exploited, but as X80 pipelines with 550MPa yield strength have started to become more widespread, the use of tubular wires to weld them is increasing.
Improved quality control by all manufacturers over the last 20 years has meant that problems in using tubular wire have been much reduced.
2.8.1 Arc instability and feeding problems
Welders occasionally report what they describe as arc instability and snatching or sticking of the wire in the torch. The difficulty is to know whether electrical or mechanical problems came first: poor electrical contact can cause arc
stability and an increase in feeding force as the wire momentarily welds
itself to the tip, while feeding problems caused by a kink in the wire can result in similar voltage fluctuations and instability. Consumable manufacturers use high speed, multi-channel recorders to reveal the order of events, but the welder cannot always be expected to know which came first. The user is advised to check the equipment to see that the conduit and contact tip are of the right size and in good condition and that there is no build-up in them of debris from the wire surface. Feed rolls should not be over-tightened, as this can lead to powder escaping from the wire seam and causing clogging. The wire should be free from kinks and its curvature should not be excessive. There should be a small amount of lubricant on the wire surface. This may appear to vary from manufacturer to manufacturer and if there is any doubt about what is the correct amount for a given wire, the supplier should be consulted.
Although porosity tended to be a fact of life in tubular wire welding in the 1960s, the lowering of hydrogen levels since then has made it normally easy to avoid. Hydrogen is the commonest cause of porosity in steel weld metals, arising in the past from excessive drawing lubricant on the wire surface or from hygroscopic materials in the core, but today more often from contamination of the plate surface by paint, grease or rust. The wires most susceptible to this are the all-positional rutile types, because of their stiff slags - basic wires, with their fluid slags, and metal-cored wires, with almost no slag, are much more resistant. More oxidising shielding gases, especially pure CO2, help to prevent hydrogen absorption by the droplet and so reduce porosity.
In the past, tubular and solid wires with increased deoxidant levels were often used when welding on oxidised plates. However, they were not widely used in Europe, where the argument has been that if the oxide surface contains rust, the hydrogen in this is at least as likely to be a cause of porosity. Oxygen is more likely to cause porosity when it is entrained from the atmosphere together with nitrogen. Manufacturers recommend what gas flow rate to use for a given current and nozzle diameter. Problems can still occur either when welding in windy or draughty conditions, or if the gas shroud becomes clogged with spatter. Self-shielded wires rely on preheating of the core components as the wire moves from the contact tip to the arc to provide instant shielding and are prone to porosity if the stickout is too short.
Related to porosity is the appearance of gas trails on the surface of the weld. These happen when gas is trapped at the interface between the weld metal and the slag as they solidify. If the slag is relatively stiff, the gas cannot escape through it and moves horizontally to create a characteristic ‘worm trail’ on the weld surface. In a marginally more acceptable version, the gas does not move so far but leaves flat areas a few millimetres across on the weld surface. Both these types of defect are most likely to occur if a wire designed for all-positional welding is used at high current and with a low stickout in the downhand position.
