TPT September 2012

Article

The dual annealing system is designed to be used with either round coils or linear inductors with field concentrators matched to the weld seam. An energy source with a common cooling water system for induction full body and seam annealing enables cost-effective system design with a small footprint and optimal adaptation to individual annealing requirements in the temperature range extending from 500°C for recrystallisation annealing to 1,200°C for diffusion annealing. Annealing processes in an inert gas atmosphere Inline tube annealing processes are performed in a nitrogen atmosphere to prevent scale formation, while bright annealing requires a reducing hydrogen/argon atmosphere. In the manufacture of brake pipes, the tubing is sized to a final diameter of 8.0mm, 6.35mm or 4.75mm after HF welding at a rate of approximately 100m/min. The sized tubes are induction annealed in a nitrogen atmosphere at temperatures above AC 3 and subsequently chilled to below 100°C in double- walled, water-cooled chilling tubes. Inline bright annealing of tubes is normally used for tubes made from austenitic stainless steels, duplex steels, nickel alloys and titanium due to the limited welding rate of TIG and laser welding systems (up to to 20m/min). The range of wall thicknesses is 0.5mm to 6.0mm, especially for tube diameters from 5.0mm to 114mm. For economic reasons, induction annealing is no longer used for larger tube diameters due to high gas consumption. Inline bright annealing is integrated in continuous tube production processes and, as an independent unit, can be retrofitted in existing tube production lines. Bright annealing creates a corrosion resistant seam while retaining the bright surface of the tube. The process requires a bright tube at the inlet of the induction zone. If necessary, a washing unit with a tank containing a suitable, heated solution must be used. This removes dirt, oil and grease from the tube surface as well as dries it. It is essential that the tube transport system keeps the tube in equilibrium throughout the entire annealing process, ie the same compressive and tensile forces must be present when the tube is fed in to the heating zone and when it is removed from the annealing and cooling zone, in order to prevent tube compression, tapering or tearing at annealing temperatures between 1,100°C and 1,300°C. In the inert gas chamber, the metallic bright tube passes through controlled induction heating and soaking zones. Heating to the required annealing temperature takes place in the chambers with the exclusion of oxygen and a reducing atmosphere. Argon, hydrogen or a mixture of these gases can be used as inert gas. Hydrogen gas is burned off upon leaving the chamber. Argon, an inert gas used for welding, is used inside

Weld on a longitudinally welded pipe with subsequent double- annealing (microsection etched in order to show the ferrite line)

DNSA mill: Turks head In inductive double annealing with water quenching after the first annealing stage, fine carbide precipitates in a fine crystalline matrix produce the desired favourable combination of strength and toughness. Hardness values above 300HV can be achieved after normalisation. After the double annealing process, the hardness between the inside and outside surfaces in the seam region is approximately 180 to 200HV, which corresponds to the hardness of the parent metal. No yield strength loss occurs in thermo-mechanically treated steels after induction double annealing of the seam region, unlike the situation with furnace annealing of tubes. Repeating the alpha-gamma transformation process (alpha- gamma transformation starts at approximately 723°C and ends at approximately 785°C) in multi-stage annealing with water quenching greatly improves the quality of the seam. In order to achieve the toughness properties of the parent metal in the weld area, the heated seam region must be water-quenched at higher than the critical cooling rate after annealing and then reheated above AC 3 . The demand for higher notch impact strength can be met in practice only with the fine grain structure achieved by double annealing with intermediate cooling. There is increasing demand for weld seam and full body annealing to achieve homogeneous material properties. The newly launched “Dual annealing with cooling” system combines induction full body annealing with EloTube TM and seam annealing with EloSeam TM . Modern internal scraper systems with coolant return are an essential element here. Even with tubes under 30mm in diameter, it is now possible to reliably achieve a uniformly annealed structure, both in the weld seam and over the tube cross-section, with full body heating of tubes without internal liquids. Full body heating is used for tubes with outside diameters up to approximately 40mm, while induction seam annealing is used for larger diameters to save energy. EloMat TM converters, preferably with L-LC technology, are used as the energy source for both annealing processes. Dual annealing with cooling section

150

www.read-tpt.com

September 2012