Production

The hot-finished tubes are produced by means of the Erhardt push bench process. High-grade square billets that have been separated into blocks by billet shears or a carbide-tipped saw are the input material for the process. The blocks are preheated in a rotary plate furnace and in the following rotary hearth furnace they reach the optimal working temperature for the given grade of steel. After removal from the furnace, the blocks are sized and descaled, then formed to fill out the shell in a punching press. The shell is then bloomed approx. 80 % in a downstream piercing press.

Rolling out the tube blanks is performed at the same heat in a push bench with an impact force of 138 t over a 15.5 m long mandrel bar in up to 16 roll passes. The resulting tube blank is slightly expanded between rolls in a detaching mill and the mandrel bar is then withdrawn in a two-roll mandrel extractor. The base and back end of the tube blank is then sawn off on the following hot saws. The tube blanks are then heated in a walking-beam furnace by inductive heating to the rolling temperature which is optimal for the given steel quality.

In a 6-stand sizing mill and connected 28-stand stretch reducing mill, the tube blanks are rolled out using state-of-the-art rolling technology to finished tube dimensions of 21.3 mm up to 139.7 mm outer diameter. The entire rolling process is computer-controlled and computer-monitored to ensure the highest possible process precision and product quality. The hot-finishing stage of the tube is followed by finishing lines that straighten all of the tubes, check them for surface defects and leak-tightness by non-destructive means, divide them into supply lengths and package them as required for a particular order.

Both plain-end "black" tubes and threaded tubes then move to the shipping storage area where they can be threaded by a modern thread-cutting machine and/or provided with a coupling. Rohrwerk Maxhütte is Europe’s only tube manufacturer that offers tubes galvanized in its own facility.

Cold-finished tubes are fabricated to very strict size tolerances and have excellent surface properties. Rohrwerk Maxhütte uses two processes of cold forming to produce these tubes– cold pilger rolling and cold drawing. Prior to the cold forming process, the tubes undergo an appropriate heat treatment (including carburization if requested). Afterwards, the tube blanks are pretreated in a fully automated hydrochloric acid pickling facility with attached bonderising. After cold forming, the finished tubes undergo a heat treatment, if requested. Then they are straightened in finishing lines, undergo non-destructive testing, are packaged according to purchase order and released for shipping.

Cold pilgering

Cold pilgering involves rolling out the tube blanks over a fixed mandrel between two roll stands which are calibrated to gradually reduce the cross-section over a predefined segment of the rolling circumference down to the diameter of the finished tube. During this rolling process, the tube blanks are reduced in outer diameter and wall thickness through forward and backwards movement of the roll stand within the cold pilger rolling machine.

Cold drawing

The tube blanks are swaged in preparation for the cold-drawing process, then a mandrel is inserted into the swaged tube. The gripper of the drawing bench grips the swaged end of the tubes and draws it with a maximum drawing force of 50 to through the drawing die. The drawing die determines the outer diameter of the finished tube, while the inner diameter is determined by the mandrel selected.

Heat treatment

Depending on requirements, the hot-finished and cold-finished tubes undergo a heat treatment. With our continuous furnaces, we are able to meet customer requirements in terms of microstructure, mechanical/technical properties and surface quality.