At our plant, in-line welding is carried out with a completely automatic LASER and TIG (Tungsten Inert Gas) method, depending on tube wall thickness according to the commonly used production norms.
Welding is carried out after cold forming by fusing the two strip edges thus limiting the thermally altered area and protecting the inner area with an appropriate shielding gas. LASER and TIG welding methods give high reliability of the weld area making the product suitable for any kind of application.
See picture n° 1 : Laser welding – heat affected area of around 1mm (x50)
See picture n° 2 : TIG welding – heat affected area of around 4,5 mm(x50)
Welding method is carried out for the production of tubes having structural purposes as well as for the production of car exhaust-systems. For these applications, the HF method is preferred because of its advantages in terms cost effective productivity. On the contrary, the small weld seam obtained with High Frequency (see picture n° 3 – x50), cannot always grant the optimum in terms of workability, withstanding of pressure and corrosion resistance due to the lack of fusion of strip edge and the oxide formation on the weld edges.
Picture n°. 3 – x50
Bright annealing is carried out in a furnace full of Hydrogen (H2) at temperatures ranging between 1040 ° C and 1100° C and is followed by a rapid cooling. The Hydrogen is NOT an oxidising agent and therefore no surface oxidation is created and pickling is no longer required after the bright annealing.
The main advantage of this system, besides a bright and even surface that eases further processing of the tubes, is the improved corrosion resistance of the material. Such treatment, carried out at the ﬁ nal stage of the production process, ensures the complete solution of the possible carbides precipitated at the grain border, thus obtaining an austenitic matrix free of defects. This makes it possible to avoid the dangerous phenomena of intergranular corrosion.
The austenitic structure obtained through off-line bright annealing, is homogeneous with regular grain size (dimension varies from 6 to 8 ASTM); the consequence is an improvement of stainless steel tensile properties, in particular traction and elongation, with an increase of plasticity and a decrease of residual stress. This is a a material characteristic very well appreciated by all end users who are making further manipulations on tubes such as bending and forming.
Picture n°. 4: where the dentritic zone material has been totally replaced by the austenitic
Welded tubes can be supplied in not annealed condition. This product undergoes the same production process with the exception of the heat treatment. The tubes are instead submitted to a pickling chemical treatment. The pickling bath is composed of Sulphuric and Fluoridric acids.
This process can eliminate, both on the outside and inside surface, and on the ends, any trace of ferrous contamination and also any possible oxides which may be on the metal surface as a result of mechanical working (welding rolls, abrasive belts, cutting equipment) and welding.
Picture n°5: x50 not annealed tube
On the marketplace brushed tubes are available. The brushing is made on the external surface only, to avoid the chemical treatment of pickling. .
These products, however, have a lower corrosion resistance than pickled tubes if submitted to the same environmental attack. This is due both to the deposits on the metal surface contaminated during the production process and to the greater surface roughness which can easily retain oxides and traces of ferrous contamination. The abrasive belts in itself can leave material that could originate corrosion. Brushed tubes, because of their ﬁnishing, need a more frequent periodical maintenance compared to pickled tubes.
It must be underlined that brushing is exclusively external and therefore cannot remove any contamination existing on the inside surface