Welding

Re-engineering the welding process

Welding has become a high-technology process that needs sophisticated procedures, strong technical backup and a new way of thinking, as E&T discovers.

The common weld is a much maligned beast. Despite the layman's preconception that welding is a rough, unsubtle industrial process, it is actually a complex procedure with many parameters affecting the result.

Shipbuilding is the classic welding industry, and the cost of welding plays a crucial role in decision-making. While many variables affect this cost, including equipment, consumables, and energy, the most significant one is labour, which can represent 80per cent of the overall manufacturing cost. And of course getting it wrong introduces distortion, which reduces product quality and increases the cost of component assembly.

In addition, advances in naval architecture such as the use of thinner-section, higher-yield steels have put a premium on high-quality welding. Fatigue has become a major issue, so new procedures have been developed to ensure appropriate weldment material and geometric properties. There has also been a move towards advanced containment systems for liquified natural gas (LNG) vessels, which require greater attention to weldment quality than does the hull itself.

In order to maximise output and minimise risk, BMT Fleet Technology developed a value chain to support the physical welding operation. The importance of high-quality welders must not be overlooked, but they should also be supported by welding engineers and welding technologists. The welding engineers' role is to take a broad view of the project, including the proposed or available welding processes and technology. Welding engineers tend towards naval architecture or structural systems.

The welding technologists then bridge the gap between engineers and welders, often translating theory into practice. They understand the theory behind welding, the equipment, and they are themselves accomplished welders. They can optimise the technology of the system and can offer training to suit by operating at the production level.

The parameters to be considered in any given job range from key design features where a structural connection needs to be made or a combination of materials joined, to selection, storage and use of consumables. Economic factors such as productivity must also be reviewed, along with the design of procedures, quality control issues and regulatory compliance. There might also be project-specific issues, such as the need to preserve cladding or material properties during welding.

The wellbeing of the welder is always a consideration and, where options exist, the welder's position and electrode type should be selected to ensure a comfortable working environment that aids productivity.

The value chain can also be extended to include the manufacturers of consumables such as welding rods and wire. Welding procedures often refer to a classification of rod or wire. However there are often a variety of options within any given classification. By working closely with consumables manufacturers it is possible to identify which consumables are best for different applications and help the fabricator to select the best consumables for the job. This relationship, and the feedback it delivers, can also drive improvements in quality and technology.

Simulation

Rather than rely only on empirical methods, BMT Fleet Technology has developed numerical modelling and computer simulations to analyse the welding process or the application of a weld. This makes it possible to assess the suitability or outcome of a particular welding scenario very quickly and with far less trial and error than traditional methods. The simulation also provides a better understanding of the mechanics, physics and metallurgy.

One example where computer simulation can pay dividends is analysing the effects that the environment has on the weld. Any moisture, grease or paint local to the weld can drive hydrogen into the molten metal, embrittling the cooled steel. To make matters worse, the hydrogen is mobile, in a process called effusion, and is attracted to high-stress locations where embrittlement increases the risk of cold cracking. The rate at which the hydrogen moves is temperature-dependent, so in colder weather there will be a longer delay before a crack can be identified by manual inspection. Simulation minimises the hydrogen cracking risk, the delay time for cracking and the time for final inspection.

The holistic approach paid dividends in BMT's work with the US National Ship Research for Production (NSRP) programme, funded by the US Navy and shipbuilding industry. The aim was to improve the integrity of welding in both naval and commercial shipbuilding, improve productivity and reduce construction costs by establishing an optimal electrode/flux combination, along with single-sided submerged arc welding (SAW) procedures.

A combination of welding technologies was investigated, and, through manipulation of the variable balance AC waveform and the use of metal cored electrodes, weld metal deposition rates were significantly increased. In addition, heat transfer was lower compared to conventional DC or AC welding, giving better heat-affected zone impact toughness. With the use of variable waveforms, arc blow - magnetic deflection of the weld - was virtually eliminated resulting in lower defect rates.

Laser welding

BMT has also been involved with the burgeoning area of automated laser welding systems. In terms of speed and efficiency it is far superior to traditional methods, with outputs measured in metres per minute rather than centimetres. Combined with an automated inspection system, shipyards could expect higher quality, more consistent welds that are produced faster.

However, the vast improvement can only be realised after some much-needed refinement to the system. One of the major issues is the need to maintain the speed and productivity in a range of environments. In welding pipelines, exactly the same circumferential weld is completed over and over, but in a shipyard there are a larger variety of weld types and sizes, joining different shapes and sizes of material. Any increase in set-up time can easily negate the benefits of welding faster, so a flexible cost-effective solution is required.

Optical tracking systems have successfully been used, but can often add the complexity of robotics that require large initial capital expenditure. Some of the European yards have looked at this option already but it has not been widely adopted.

In summary, the commercial benefits of higher welding quality and productivity are clear to see, but the change in mind-set and culture required to take a more holistic view might not be as clear-cut or as palatable. So, while the technology, skills and expertise needed to maximise the outputs from the welding value chain are all in place, the challenge now is to successfully deploy them.

Aaron Dinovitzer is president and executive engineer at BMT Fleet Technology

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