Advantages and disadvantages
Welding processes are sometimes classified according to their power density. Those described in this book cover a wide range, with tubular wire welding somewhere in the middle. At the lower end of the scale, electroslag welding may run at less than 0.1 W/mm3. Low power density processes such as this do not require accurate machining or alignment of the joint edges and do not subject the weld to rapid heating and cooling cycles that could cause problems with hardness. On the other hand, energy is being lost through conduction almost as fast as it is being applied to the weld, so the processes are thermally inefficient.
At the other end of the scale, lasers can concentrate power in a very small volume of material at 50kW/mm3 or more. Joint edges must be machined and the welding system must be capable of very accurate tracking so that it does not miss the joint. Rapid heating and cooling rates lead to the formation of hard martensite in the softest mild steel and the high welding speeds demand high purity base materials if hot cracking is to be avoided. The melting efficiency of power beams may be high, but until now most of this benefit has been lost because of the inefficiency with which the beams themselves were generated. One of the significant advantages of the high - energy density processes, however, is their ability to form a keyhole when welding onto a closed joint preparation, so allowing efficient welding from one side without backing.
Arc welding falls mid-way between these extremes, with power densities typically of the order of a few W/mm3, and welding with tubular wires spans a useful range of the power density spectrum. This makes arc welding the most versatile of all the welding processes and tubular wires perhaps the most versatile of all consumables. They can be used semi-automatically with inexpensive equipment, as when small reels of self-shielded wire are used by hobbyists as an easy way of welding thin sheet. With more capital investment, shipyards can use them on tandem mechanised equipment at speeds which would need ultra-low sulphur and phosphorus levels for crack-free laser welding.
While most processes can be faster if joint preparations are accurate and consistent, this is not always possible, for example on the closing joints of large structures. In offshore fabrication, it is sometimes necessary to weld a brace to a chord with no internal access, a job which is not made easier by the acute angle between them. Here, self-shielded wires have been used to achieve excellent root profiles on the back of these difficult joints.
Another type of closing joint, however, shows the limitation of tubular wire welding in not being able to create a reliable keyhole. When the ends of two sections of pipeline meet and are to be joined, it is not possible to use an internal clamp and the fitup may be less than ideal. Cellulose electrodes can produce a root keyhole because hydrogen ions in their fierce arc give them good penetration and allow them to succeed in variable joint preparations. They are much faster than the self-shielded wires that would be a possible alternative. Cellulosic electrodes have therefore been widely used for the roots of pipeline tie-ins in steel grades up to X80 (550MPa yield strength), although flux-cored wire is preferred for the filling and capping runs. But pipe welding procedures are well-proven to avoid hydrogen cracking: cellulose electrodes would not be an option for the offshore application described above where the section sizes are greater and the steels more hardenable.
Perhaps the greatest challenge for tubular wires remains solid wires, whose single but important advantage is consumable cost. This is not the same as the cost of the weld and there are many cases where a more expensive consumable reduces the cost of the job, but it is true that modern power sources can narrow the performance gap between solid and tubular wires. Nevertheless, the fact that the market share held by tubular wires is increasing throughout the world shows that the benefits of this versatile and productive process are far from exhausted.
Much of the information presented here is to be found in expanded form in the author’s book Tubular Wire Welding,10 while a companion book, Selfshielded Arc Welding11 by T. Boniszewski, deals in more detail with that aspect of the process. More recently, the ‘Flux Cored Arc Welding Handbook12 has been published. As with all welding processes, welding manufacturers are the primary source of information and several publish excellent handbooks and pamphlets describing tubular wire welding. They will also have technical specialists who can help with specific problems.
Research institutes such as TWI in the UK, the Edison Welding Institute in the USA and the Institut de Soudure in France are a key source of information for their members and are always working to solve the most intractable problems of the industry. Finally, all welders and welding engineers should be aware of the benefits of belonging to a professional body such as the Welding and Joining Society or the American Welding Society, whose members between them will have encountered and overcome just about every difficulty that an individual practitioner is likely to encounter.