The metal flexible hoses of the Gnali Bocia catalog
The flexible hoses have solved many design problems because they can be extended, by design, up to twice their original length. Inside the Gnali Bocia catalog there are different types of flexible hoses: among these, metal flexibile hoses have a particular importance. They are made of stainless steel of the AISI 316 family with low carbon content, which makes them easily weldable during assembly.
There are two main categories of metal flexible hoses that differ both in their applications and in their constitution and are:
- Flexible stainless steel hoses with polymer sheath coating: it must not be structural, so it must not contribute to the resistance during the tests of compliance with the standards. Furthermore, it must be made of a material that does not suffer degradation and that corrodes the external surface of the hose over time. This version of tubes, which is suitable for any type of domestic connection, has been designed to comply with the EN14800 standard and the protection, as mentioned, is designed to limit the abrasion effect of the metal material.
- Stainless steel flexible hoses without coating in polymer sheath: these tubes were made following the UNI CIG 7129 standard, initially developed for pipes suitable for transporting fossil fuels, but remained in place for the conduction of fluids. The tube is corrugated and made of stainless steel like the previous one, but the absence of the polymer sheath does not make it suitable for transporting gases.
The weldability of metal flexible hoses
As previously stated, metal flexible hoses are made of stainless low carbon content steel to facilitate the weldability of the fittings. The process normally used is TIG welding in a controlled atmosphere which allows to obtain a clean joint, free of slag and inclusions: in fact, these defects could compromise the correct functioning of the system.
The TIG welding technique
"TIG welding, an acronym for" Tungsten Inert Gas ", is an arc welding process with an infusible electrode, under inert gas protection, which can be performed with or without filler metal".
This welding technology, which requires specialized operators, dates back to the Second World War where it was used to replace the rivets of airplanes allowing their lightening. The process consists of a torch in which the tungsten electrode is inserted (the same infusible material used for the filaments of the bulbs) around which the inert gas flows by means of a nozzle made of ceramic material. The torch is moved along the joint and the electrode is kept at a constant distance of a few millimeters in order to ensure the homogeneity of the cord. An important clarification is that you should never let the electrode come into contact with the metal bath: the risk is that an attachment section is created between the two components and that the welding operation will be blocked.
Qualities and defects of this type of welding
Like all techniques, TIG welding also has advantages and defects that lead to evaluations and comparisons with other types of processes.
The main advantages are to be found in the penetration and the estate, which are excellent, as is the aesthetics. The process can be completely automated and is therefore often used for welded tubes. It can be performed in any position and can make both continuous and spot welds. It is advisable to do it indoors because even a little wind can remove the shielding gas. The problems are related to slowness and treatable thicknesses: it is not possible to weld thicknesses higher than 2-3 mm for steels but, in general, thicknesses of more than 5-6 mm are never welded. For these reasons, the technique is often used to make the first pass while the subsequent ones are implemented with high productivity procedures. A further problem to consider is given by the tungsten inclusions that can give problems of fragility.
The advantages listed show how this welding technique is suitable for metal flexible hoses, which have low thicknesses and require a strong seal.
The main components of TIG welding
Electrode: can be made of different materials:
- Pure tungsten: it has high thermoelectric power which guarantees arc stability. From the economic point of view it is the least expensive. The main problem is connected with fragility.
- Thoriated tungsten: it has a rapid ignition and ability to carry high currents as well as good arc stability.
- Tungsten with zirconium: for welding aluminum, magnesium and its alloys with medium-low currents.
- Cerium: high emission of electrons, good penetration and resistance to wear.
They are generally used in direct current and with direct polarity (positive pole on the piece). In case we want to weld light metals we use the inverse polarity which also allows a better arc stability: however, the higher currents required tend to replace the CC with reverse polarity with the CA.
- Shielding gas: the main purpose is to replace air in order to avoid contamination with harmful agents present in the atmosphere. Usually, argon, helium, argon-helium mixtures and argon-hydrogen mixtures are used. With the argon the arc is fairly stable but the bath is less hot: it is used for thin thickness welds. Furthermore, it is less expensive than helium and this is an important discriminant in the choice. Helium develops more heat and, if used on materials with high conductivity, it allows a higher welding speed. The main problem is related to its lightness: being lighter than air, it must be used in larger quantities than argon and this raises the cost. Argon-helium mixtures are a cross between the two pure gases.
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