POLYSOUDE
PRODUCTS & EQUIPMENT
Figure 5: Column and boom equipment for overlay welding with additional cross-slides
Figure 4: Location and accessibility of the surfaces to be clad inside the valve body
easy and rapid, proven experts in metallurgy and welding technique are needed to accompany the projects successfully, and last but not least is it up to skilled welders to ensure the expected quality of the coatings. An important subcontractor for overlay welding tasks is the British company Arc Energy Resources (AER). Concerning material, form and size of the workpieces the company covers a wide variety of possibilities. Among their smallest applications is the cladding of the inside of bores with a diameter of only 20 mm (Fig. 2). Bigger components are generally limited to a weight of 15 t, the loading capacity of their hoisting devices. In 2016 the company faced a big
challenge: two 30” valve bodies had to be clad. The scope of order included to cover all wetted
20 IMT December 2017
surfaces with a corrosion resistant coating of two layers of 2.4856 (Inconel 625) by overlay welding, followed by corresponding machining operations, pressure tests and non-destructive testing. The first rough casting arrived
with a total mass of 27 t, which far exceeded the lifting capacity of the installed indoor crane of the workshop. A hired mobile crane with a sufficient capacity could not be operated inside the existing building, so a new hall of lightweight construction had to be erected nearby (Fig. 3). A further complication arose from the extremely short delivery time which had been imposed by the customer. The specified quality of the coatings could only be guaranteed if TIG overlay welding would be applied, but the available welding installations
were designed for the conventional TIG process, which is characterized by low melting rates and poor productivity. Hence, the equipment they already had was not suitable to finish the work on time.
State-of-the-art TIG overlay welding The French company Polysoude, a renowned manufacturer of state-of-the-art equipment for mechanized and automated welding, has recently developed a process with the goal to increase the productivity of TIG overlay welding operations to an economically interesting level. The new TIGer technology
(Tungsten Inert Gas electrical reinforcement) is based on an especially designed bi- cathodic welding torch with two tungsten electrodes in tandem configuration. These electrodes are supplied separately with welding current from two independent power sources, but due to their particular layout
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the resulting single arc provides distinct advantages for overlay welding purposes. Compared to the conventional TIG process at the same welding current intensity the pressure which prevails in the arc is significantly lower. Thus, elevated welding currents and faster welding speeds can be reached, whereas the welding process remains stable and less penetration occurs. At the same time, the specific heat input of the weld is considerably reduced. A further improvement of
productivity comes from the hot wire technique. The energy to melt down the filler material must be generated by the arc, so the feeding speed of filler wire is limited. In the hot wire technique however a particular hot wire current is supplied by a dedicated power source, so the filler wire arrives pre-heated at the molten pool of the weld and absorbs less energy. As the calculations showed,
with the new technique it would be possible for AER to meet the
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