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July 23, 2018

ESAB, a world leader in welding and cutting technologies, has supplied a total of six submerged arc welding systems including ARISTO 1000 ACDC INVERTER to FAI Officine Di Carvico S.p.a. for fabricating high-pressure spools that are used in hydrocarbon flow lines. FAI specializes in the production of high-pressure vessels, pipeline components, sea water intake screening systems, reactors, and shell and tube heat exchangers. In addition to producing new fabrications, the company also undertakes repairs. Given the nature of its work, FAI requires reliable welding systems that achieve high productivity and high quality, yet are sufficiently versatile to cope with a wide variety of base materials.

FAI Officine Di Carvico S.p.a. was established in 1961 and has continually expanded and invested to maintain its position as a market leader in its field. Quality has always been very important to FAI and today the company holds ASME stamps S, U and U2. Currently the facility at Carvico, Italy benefits from a 14,000 m2 factory plus there is a further 40,000 m2 outside. Components up to 300 tonnes can be handled, with a maximum diameter 6.5 m and length of 60 m.

The specialist nature of the projects completed by FAI means that a variety of metals are used; these range from carbon steel and low-alloy steel, though most types of stainless steel (austenitic, ferritic, super-austenitic, duplex and super-duplex), to nickel and nickel alloys (Monel, Inconel and Hastelloy), copper and copper alloys, and titanium. This extensive capability, and the fact that some materials are fabricated whereas others are clad or weld-overlaid, calls for a broad range of welding processes and consumables. FAI Officine Di Carvico S.p.a. has been using ESAB welding equipment and consumables since the early 1970s and one example of the type of system that plays a crucial role in FAI's production is the ESAB Aristo 1000, six of which are used by FAI for submerged arc welding (SAW).

ESAB's Aristo 1000 AC/DC SAW power source benefits from a number of features that help FAI to maximize productivity without compromising quality. For example, Bead Profile Modelling™ provides adjustable AC settings for precise control of penetration profile and depth, dilution, arc stability and weld appearance for the optimum productivity and quality of each weld. In addition, the machine can switch between AC and DC 'on the fly' to eliminate the stop/start routine that is normally required and which can result in downtime and weld defects. CableBoost™ is a patent-pending technology that ensures the performance of the power source is unaffected, even when long welding cables are used. For applications requiring higher welding currents, two Aristo 1000 AC/DC units can be connected in synchronized parallel mode.

FAI uses the ESAB Aristo 1000 power sources in conjunction with column-and-boom manipulators, normally with roller beds or positioners as well, for fabricating high-pressure spools from F22 modified alloy steel. For these spools, which are destined for use in hydrocarbon flow lines, the diameters range from 2 to 40 inches (50 mm to 1 m) and the wall thickness from 20 to 100 mm. Typically the welding wire diameter is 2.5 mm and the heat input is maintained at a low level specified by FAI, namely 0.86 to 1.54 kJ/mm for diameters of 6 to 8 inches (CTOD @-40 °C are not required)  and 0.96 to 1.35 kJ/mm for 10 inches and upwards (in these case CTOD @-40 °C are required). With a welding current of 310 to 335 A and AC in order to minimize the undesirable arc distortion due to residual magnetism from the machined pipes as well as increase the toughness properties, voltage of 31 to 33 V, welding speeds of 40 to 65 cm/minute are achieved. Furthermore, the Aristo 1000 reliably strikes a perfect arc every time, even in the worst-case conditions thanks to the stability of the square AC current comparable to a DC current.

The welding procedure employed for fabricating the spools is to apply a root weld and support runs using gas tungsten arc welding (GTAW, also known as tungsten inert gas welding or Tig welding), then filling with SAW. Joints are preheated to 180 to 200 °C with infrared panels or electrical resistance, followed by dehydrogenation heat treatment (DHT) at 300 °C with electrical resistance heating, and post-weld heat treatment is carried out at 690 °C in a gas-fired furnace. All welds are checked with Zetec Topaz TOFD (time-of-flight diffraction) and phased array ultrasonic testing (PAUT) systems.

Claudio Perego, the Managing Director of FAI Officine Di Carvico S.p.a., comments: "The ultrasonic scanning inspection method is more than severe, in fact planar indications is not acceptable; we need only think, for example, that a stringent workmanship like DNV OS-F101:2013 table D-6 permit a planar indications of 25 mm or 12.5 mm depending on echo amplitude, so we thought we might have difficulties achieving welds of a sufficiently high quality. However, the ESAB Aristo 1000 is very flexible, and the support provided by Vago Saldatura was superb, so the welds meet all of our requirements in terms of quality and productivity."

Aristo 1000 in operation