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