Huntingdon Fusion Techniques HFT®”s USA Partner has recently helped solve a major environmental problem in a remote area of Oregon. Construction of a new access to the Willamette River was necessary, as part of a plan to replenish a salmon hatchery1 but this necessitated removal of part of a 10-inch (254 mm) pipeline that was causing an obstruction.
The pipeline had been isolated and abandoned previously and filled with water that had probably become polluted. Simply cutting the pipeline would release over 1 million 300 thousand gallons (5,000 m3) of contaminated water into surrounding land lying within a sensitive, environmentally protected area. A decision was made to use liquid nitrogen to create ice plugs and isolate the small section of pipe causing the obstruction. The pipe could then be cut, releasing only limited contaminated water and this could be contained and removed from the site.
Pits were excavated on either side of the access exposing the pipe and the anti- corrosion coating was removed. Freezing commenced in the early morning in record high temperatures combined with little to no shade in the area.
The reactive metals by classification are zirconium, titanium and beryllium. We also include here tantalum and columbium (niobium), being from the refractory class and which also present similar challenges to the welding engineer.
Aerospace, automotive, medical and military industries are increasingly using all these materials. They have many technological attractions being durable, low density, bio-compatible and offering high corrosion resistance but they are expensive. Welding procedures need to be carefully developed and stringently applied to avoid expensive waste, rework or risk of service failure.
Successful fusion joining techniques have evolved1 since the alloys were first used in engineering applications. The majority of metallurgical problems, even considering dissimilar metal welding, have been resolved and filler materials are readily available.
The planned surge in new electricity power generation plant and refits across the world over the next two decades will provide outstanding opportunities for the fabrication sector. Recent innovative developments in welding equipment will support the drive towards the production of consistently better quality joints, many of which are in the safety critical class.
Over 300 nuclear reactors have been proposed of which 136 will be in China, 24 in the USA and 23 in Russia1. India’s massively delayed nuclear power programme will see a resurrection after Électricité de France (EDF), the world’s biggest electricity company, agreed build six nuclear plants in the country. The Indian Jaitapur project is expected to become the world’s biggest nuclear contract and one of the world’s largest nuclear sites. The 10,000 MW project will have six reactors of 1650 MW each.
Effective weld purging is only achieved by making sure that oxygen is displaced from the purge zone prior to and during welding. Any residual oxygen can cause significant loss of corrosion resistance and a reduction in joint strength. It is therefore essential to seal the pipe either side of the joint and maintain this seal throughout the process. The residual level of oxygen in the purge zone needs to be consistent with the welding procedure so continuously monitoring to ensure compliance is crucial.
1 Choose a dependable sealing material
The cheapest is seldom the best so examine the options available.
Zirconium and its principal alloy zircaloy possess physical properties unmatched by most other metallic materials. The combination of mechanical strength, corrosion resistance and their high temperature stability make them attractive for use in sectors as diverse as biochemical, nuclear, aerospace and petrochemicals.
More specifically, zircalloy is used in the manufacture of pressure vessels and heat exchangers. The alloy has excellent resistance to most organic and inorganic acids, salt solutions, strong alkalis, and some molten salts and these properties makes it suitable for use in pumps where strength coupled with corrosion resistance is mandatory. Zirconium alloys are biocompatible, and therefore can be used for body implants: a Zr-2.5Nb alloy is used in knee and hip implants.
By far the most significant applications however are in nuclear power plant. Zirconium alloys are widely used in the manufacture of fuel rods especially in pressurised water reactors 1.
Figure 1. Zirconium alloy welded with effective inert gas protection showing no discolouration.
The tungsten arc welding concept, originally introduced as a practical tool in 1950, is now established as the most versatile technique for producing fusion welds to the highest quality standards.
A temperature of around 4,000ºC is generated in the arc during welding and the role played by the electrode is therefore crucial. It must have a high melting point and it must be non-consumable: tungsten quickly established itself as the most suitable material.
As the knowledge of arc characteristics increased however it became clear that the use of pure tungsten presented some limitations on process development, particularly arc starting, stability and electrode wear.
Early research showed that the addition of thoria resulted in overall improvements in performance and from this work a range of tungsten electrodes containing oxide additions or ‘dopants’ were introduced progressively.
Racing car recreated using advanced welding technology
With the progressive development of racing cars has come a need to embrace fusion welding as an essential part of the manufacturing process.
Whilst dramatic improvements in engine design have made a significant contribution to track performance, reduction in weight and aerodynamic refinements have also been important. Safety conventions need to be continuously revised to protect drivers in the event of accidents.
Welding has played an increasingly important role during production of body parts. Reduction in weight has been achieved by using slender suspension and steering components and replacing steel with lower density titanium.
Fabrication of titanium alloys however requires skills orders of magnitude greater than steel: they are difficult to form and challenging to weld.
Part 4 Eliminating Oxygen from the Purge Gas and the use of Monitoring Equipment
Even using specialised weld purging equipment does not guarantee defect free welds. Control of the oxygen content of the purge gas is crucial to success.
In this final part of the series the significance of maintaining a low level of oxygen in the purge gas is considered. Several factors will determine what oxygen content can be tolerated in order to prevent oxidation, the most crucial being the material being welded. Sensitive alloys such as titanium may require oxygen to be limited to 50 ppm1 whilst some stainless steels will tolerate 150 ppm without noticeable surface discolouration.
The first, and crucial, step and an aspect often overlooked is the need to provide effective sealing around the weld zone. Poor sealing invariably allows air to enter the weld zone and thus defeat the objective of providing a low oxygen environment.
Part 3 Welding Enclosures and Trailing Shields
In Parts 1 and 2 the author outlined the problems in selecting the optimum gas for use during weld purging and then analysed the options available to engineers when choosing a purging technique for tube and pipe welding.
This article examines protective enclosures and trailing shields as potential solutions to the need to prevent contamination during welding.
Traditionally, permanent metal enclosures have been used when welding materials that are sensitive to contamination from atmospheric gases. Such enclosures may be evacuated using vacuum pumps or purged with inert gas.
Part 1 The Importance of Gas Control on Weld Quality
Some form of gas protection is necessary during welding of many metals, but in particular stainless steels and titanium alloys, to maintain their physical properties and to prevent reduction in corrosion resistance. Which is the best gas and which is the best technique to use when weld purging are principle questions facing engineers.
Dr Fletcher sets out in this 4 part series, to cover all related topics including gas selection and purging equipment. He has researched published material on the subject of protective gases from a very wide range of sources. Typically this has included steel manufacturer, major users (ref.2), consumables and equipment suppliers (ref.3) independent authorities (ref.4) and many welding engineers (ref.5,6,7).
NEW COST EFFECTIVE WELD PURGING
It took a long time for engineers to recognise that the use of inert gas purging during the welding of tubes and pipes could not only improve overall weld quality but could save both time and money.
Purging using designs based on advanced technology offers dramatic reductions in weld defects, significant savings on welding time and elimination of post weld cleaning operations. All this simply by effectively protecting the rear and the topside weld beads from contamination, and especially oxidation, using inert gas coverage.
Purge systems have evolved rapidly during the last decade as advances in materials and control equipment have been incorporated.