Important technical terms
The eccentricity is the measure for the deviation of the tube axis of the outside diameter from that of the inner diameter. Eccentricity results from the wall thickness deviation.
Eddy current test
In the eddy current test, three coils surround the test unit: the primary coil and two secondary coils. In the surrounded part of the test tube, the primary coil generates a magnetic alternating field, which induces voltage in the secondary coils. In case the two secondary coils are connected against each other, the value of the resulting alternating voltage becomes zero, when a completely homogeneous tube is passed. Should the tube, however, be defective, there will be difference voltage in the secondary coils, which can be displayed and recorded.
This type of lettering is executed by means of electrolytic oxidization and can be applied for nearly each metal. The lettering is permanent, abrasion-resistant, heat- and acid-resistant. The lettering image is clearly readable and has, in contrast to laser or engraving methods, no negative influence on the surface structure. The lettering of the metal surface is executed by means of an aqueous electrolytic solution and an electrolytic current through a template. The lettering duration is approx. 3 sec. at a voltage of about 12 - 25 V (direct / alternating current). The method can be applied manually by means of an electric stamp or automatically in a marking machine.
This term designates a tube surface, which due to its appearance is comparable to elephant skin. The tube surface is very rough and has deep, irregularly distributed scars and/or grooves. The formation of such a surface can have various causes, among others badly formable preliminary material, defective glass lubrication, but also too long heating in the rotary hearth furnace.
Elevated temperature steel
Elevated temperature steel is austenitic steel with a very low carbon content, at least 13 % Cr, high Ni contents of always more than 10 % (up to 34 %). At temperatures of over about 550 °C up to about 600 °C, this steel has good stability properties, even under long-term mechanical stress. High temperature steel is predominantly utilized in the construction of power plants, where the normal steel grades (DIN 17175 and 17177) are not sufficient.
Tubes with beveled ends as well as seamless stainless steel tubes with special internal cleaning are protected from damages and/or against contamination during transportation/storage with an end protection (plastic caps).
In the preliminary material reception test, a slice of approx. 10 mm is cut off of every bar delivered and pickled in a pickling solution to verify center line flaws and cracks, which result in increased rejects in tube production.
For expanding, a larger expanding mandrel is pressed into the smaller center bore of the billet, whereat the displaced material of the expanded bore results in a lengthening of the billet and thus improves the yield (instead of chips when boring out the final billet diameter).
In case corrosion particles reach the surface of non-corrosive steel - during storage, transportation or processing - these can "infect" the stainless steel and initiate corrosion.
Is a hot-forming procedure, which extrudes special sections or tubes from solid or pre-punched billets. The glowing billet is pressed through the die taper, over the mandrel, by the ram. Thus the die determines the outside diameter and the mandrel the inner diameter of the finished tube. Glass powder is utilized for lubrication. Surface and dimensional accuracy can be further improved by means of consecutive drawing and/or pilger rolling.