Mandrel is made of the most advanced hot working tool steel
The most important quality requirement of mandrel is long service life. In steel tube mill, mandrel must be able to withstand continuous alternating stress and high thermal stress under special rolling force. In the rolling process, the surface temperature of mandrel reaches more than 800℃, which is the special high performance requirement of mandrel products. Only special hot work tool steels can guarantee long service life in these harsh environments. The type of material used to make the mandrel depends on the process requirements such as tensile strength, quenched and tempered structure, grain size and purity. These properties will vary according to the rolling process in which the mandrel is operated.
2 From scrap to mandrel
The mandrel production process begins with scrap smelting. Scrap steel is melted in a 130T eAF at the company’s Witten steelmaking plant in Germany, followed by secondary refining and alloying elements. A certain amount of alloying elements Cr, Ni, Mo, W, V and Co should be added to hot work tool steels to obtain the properties of the materials for different purposes. After secondary refining, molten steel is either cast into ingots or cast into billets. The vertical caster is a feature of Deutsche Edelstahlwerke. Billets and ingots are then rolled at Witten or forged into bars at Krefeld.
The forging plant has one of the longest forging machines (RF 70 type) in the world. Ingots are processed by rolling or forging process, depending on the customer’s technical requirements and the length and diameter of the mandrel required. In all cases, the mandrel is hardened and tempered in a continuous furnace to ensure the required microstructure. From there, the material is sent to the Vitten plant for peeling and straightening.
3 From mandrel to seamless steel tube
It is very important to ensure the long life of the mandrel without defects. If the mandrel cracks in the rolling process, it will cause damage to the pipe jacking machine, and the repair work is time-consuming and labor-consuming. Thermal cracks located on the surface of the mandlet, although they may not be visible to the naked eye, will cause internal defects in the steel pipe, and such quality problems are unacceptable to customers in the automotive, energy and engineering industries.
The Zeithain hot rolled pipe plant in Saxony, Germany, has long produced seamless steel tubes and has been a customer of Deutsche Edelstahlwerke for more than 20 years, purchasing mandrel of approximately 17m in length made from Thermodur 2740. Zeithain steel Pipe Works requires complete fining of mandrel with non-metallic coating. Deutsche Edelstahlwerke machined a groove 100mm in diameter and 140mm in length on the front of the mandrel. This groove is critical to the pipe jacking process. For top tubes, the blanks used to process seamless tubes are heated to 1150℃~1300℃ in an annular furnace. Billet is continuous casting round billet, length 400mm~1900mm, diameter 180 or 220mm. The billet is heated and sent to a three-high cross pipe mill for perforation. At this point, the billet is rolled and preformed with a special internal tool (i.e., the top) to form a specific inner diameter and wall thickness.
Next, into the rolling machine, mandrel began to play a role. The mandrel is preheated to about 450℃ and inserted into the hollow capillary. The mill then fixes the hollow billet through the previously mentioned groove at the mandrel tip. Thus, the mandrel of high quality manufacturing and processing is how important.
The seamless tube then begins the main forming process in the jacking machine. The mandrel is inserted into the wool tube, and then a set of rolls with 60° offset each other are rolled. In this way, the hollow tube blank is processed to a predetermined outer diameter and a specific wall thickness, forming a blank tube. On the stripper, the mandrel is released and released from the pipe. This process is easy due to the coating of the mandrel with a non-metallic layer mentioned earlier. Special separators and lubricants are applied to the surface before the mandrel is re-entered into the process.
