New superalloys still need careful purging during welding.
Significant developments have been made recently and have resulted in the introduction of new nickel alloys that offer major improvements in mechanical properties.
Not least is Inconel 740H (Ref 1), an alloy offering enhanced resistance to coal ash and therefore of considerable interest to fossil fuel fired boiler manufacturers.
Whilst these new materials help to expand the use of nickel-based alloys in areas where mechanical properties and corrosion resistance at elevated temperatures are mandatory, the need to maintain strict control during fusion welding remains, in order to preserve these characteristics.
The marine industry in general has been slow to embrace the 3D printing concept. The use of continuous liquid metal deposition under computer numerical control has created opportunities to produce complex shapes such as forgings and castings whilst avoiding the need for expensive tooling and the time delays in fabricating moulds.
Notwithstanding this slow start, development work at Delft Technical University in 2017 has led to the production of the world’s first metal deposited marine propeller.
The majority of published documents on 3-D printing have been restricted to high precision applications, particularly in the medical sector.
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 2 Pipe and Tube Weld Purging
In Part 1 Dr Fletcher set out to show many inconsistencies in the approach to weld purging, especially in the choice of gases used.
His conclusions were as follows:
Much more definitive work is now essential if fabricators are able to proceed with confidence in consistently producing welded joints capable of meeting the demanding standards imposed by service conditions. In the meantime the precautionary note from the American Welding Society offers the best advice with a conclusion that the shielding gas needs to be matched to the metal composition.