Stainless steels and titanium alloys are widely used in space rocket manufacture. They offer resistance to corrosion and mechanical properties are good at elevated temperatures. Extensive published information is available on both materials but some of the problems associated with fusion welding have only been recognised relatively recently.
The temperature cycles experienced during welding affect the weld and heat affected zone properties of these materials in different ways.
Stainless steels
The most widely used are the austenitic materials of which the 300 series constitute the largest subgroup. Of these, EN 1.4301 (AISI 304), is favoured for rocket body manufacture. It is readily available, can be formed easily and offers good general corrosion resistance coupled with stability at higher temperatures.
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The oil, gas, water, food and beverage, aerospace, power generation, construction and pharmaceutical industries all fabricate many thousands of metres of tubes and pipes every year and all joints need to be tested for leak tightness before release for use. This is particularly so in the nuclear sector where potential release of toxic compounds presents a health hazard. It’s also a significant requirement in the aerospace industry where leaks could endanger life.
Hydrostatic testing is the safest and most common method employed for testing pipes and pressure vessels and this is normally undertaken using water. Pneumatic testing using compressed inert gas or air may be used, but only under carefully controlled conditions. Failure during testing with water releases only nominal energy because water is almost incompressible. Escape of gas during pneumatic procedures can be dangerous because it can result in the sudden release of very large amounts of energy.
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Die Metalllegierungen, die heutzutage verwenden werden, wurden durch jahrzehntelange Forschung weiterentwickelt und viele stellen den Höhepunkt der Leistung in Bezug auf Festigkeit und Korrosionsbeständigkeit dar. Ohne diese Materialien wären die bedeutenden Fortschritte in den Bereichen Kernenergie, Medizin, Pharmazie, Stromerzeugung und Petrochemie nicht möglich gewesen.
Ein entscheidender Durchbruch ereignete sich 1912 in Sheffield, als sich herausstellte, dass Chrom-Eisen-Legierungen korrosionsbeständig sind. Seitdem haben wir die Einführung von niedriglegierten warmfesten Stählen, Nickelbasislegierungen mit erhöhten Temperatureigenschaften und in jüngster Zeit die Entwicklung leichter Titanlegierungen mit hohem Festigkeits-Gewichts-Verhältnis erlebt.
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Historical use of some metallic alloys such as titanium was limited by the cost and problems associated with processing, not least welding. However, a recognition of the high strength to weight ratio and corrosion resistance now continues to spearhead their use in the aerospace and sports car sectors and in the nuclear and process industries. In the aerospace industry alone titanium content on wide bodied aircraft has increased in the last five years by over 22%.
Whilst these alloys offer significant advantages over alternative materials, especially in reducing weight and increasing corrosion resistance, fabrication using fusion welding needs a specialised approach to avoid the introduction of contamination and reduction of mechanical strength that can lead to failure in service.
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Fusion cladding provides protection for metallic components by depositing a layer of material, normally using an arc welding process, to provide a corrosion- or erosion-resistant surface.
The technique is well-established and has been in use for decades on applications as diverse as turbine blades, high performance steam valves, bearing surfaces and sub-sea components.
A wide range of surfacing metals is in use and these include copper, nickel and cobalt based alloys and many stainless steels. Cost savings are impressive: fully cladding a carbon steel component with nickel 625, as opposed to producing it in solid alloy, can reduce costs by as much as 60%.
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Characteristics of Measuring Weld Purge Results from a Distance.
Pipework users across the entire industrial spectrum occasionally face the problem of having to repair or replace pipe sections or change in-line components such as valves and instrumentation.
The conventional approach to these problems involves isolating and emptying the appropriate section. This might necessitate draining a large volume of product and could also cause serious interruption to production.
An attractive and economical solution is to freeze the pipe contents upstream of the repair or component replacement zone.
Alternatively, freeze both sides of the repair site and simply drain the material between the freezes.
For many years, plumbers have used canned spray systems but their potential is limited to around 50 mm (2”) pipe diameter.
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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.
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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.
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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.
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There is a popular misconception that powder based additive manufacture is superior to the wire alternative. This impression has been created largely through aggressive marketing and by the technical press preferring the more glamourous powder method used for creation of body implants.
Whilst it has to be conceded that the relatively delicate powder deposition process is excellent for producing small components, often requiring no further machining, in terms of speed and cost-effectiveness the wire option wins hands-down.
Wire and Arc Additive Manufacture (WAAM), is performed by laying down progressive beads of metal under computer numerical control to create a shape. The alternative version uses a laser or electron beam as the heat source in conjunction with metal powder, Direct Metal Laser (or Electron Beam) Sintering (DMLS or DMEBS).
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Since 3D printing was introduced over 30 years ago there have been a number of significant developments. Various melting techniques have been used to achieve this aim including electron beams and lasers. but one being most actively pursued currently is Wire and Arc Additive Manufacture (WAAM) using a GTAW (TIG) power source. To be specific, additive manufacturing is not the same as 3D printing!
The recent use of fusion welding as a deposition source has opened up wide ranging possibilities in manufacturing. The process is one in which metal is deposited layer-by-layer under computer control to form a three dimensional shape.
No longer is it necessary to keep an inventory of high value generic stock: parts can be customised and manufactured on demand. A recognition that components can be fabricated using WAAM technology has spawned a new industry to exploit a wide range of exciting opportunities.
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