TPT May 2016

AR T I C L E

SMS Group

Furthermore the press has extensive ancillary and auxiliary facilities for billet and die manipulation. The billets are mechanically pre-drilled as standard. The pilot bore improves the eccentricity of the wall of the pierced billet after expanding (Figure 3). The piercing process without pilot bore is performed after brief upsetting with the upsetting ram (Figure 4). Displacement piercing thereby transitions into rise piercing. This method is less accurate and is therefore used only for large inside diameters. In order to avoid

Figure 2: Heating concept

excessively large discards, the underside of the billet is closed off by a closing cylinder until shortly before the mandrel pierces the base. Due to the lack of the pilot bore, the material losses are generally lower. After expanding and/or piercing, the billet cools at different rates and has an uneven temperature profile before entering the reheating station (Figure 5). The temperature profile of the billet is marked by a significant temperature drop between the billet ends and the billet centre. A further aspect is the temperature difference between the billet surface and the billet core. This uneven temperature distribution can be more than 200 K.

In the subsequent billet preparation section, the peeled bars are sawn to length, a pilot bore is drilled in the end and the face of the billet is machined to produce an outer radius and inner cone at the bore. After billet preparation, the machined billets are degreased on the inside (bore) and outside before they are transported to the extrusion press line on pallets. The extrusion press line consists essentially of a preheating station, the billet lubrication unit, a vertical piercing or expanding press, a reheating station, the billet lubrication unit, the horizontal extrusion press and the run-out section with downline water or air cooling of the extruded tubes (Figure 1). Billet heating to the required temperature of 1,100-1,150°C is performed either in rotary-hearth furnaces with reducing atmosphere or in induction furnaces, or alternatively in a combination of the two with a gas-fired furnace as pre-heater up to around 700°C and final induction heating in vertical coils (Figure 2). The slower heating in rotary-hearth furnaces from the SMS group with the associated more uniform through-heating has a homogenising effect on the microstructure. The cost benefit of the gas energy is particularly remarkable with infrequent alloy changes and three-shift operation. The possibility of a rapid temperature change for small batch sizes or where the line is not operated in three shifts favours induction heating. Induction furnaces can be expensive to operate, but they heat up quickly and can control the temperature very precisely, an aspect of great importance with the narrow temperature ranges within which stainless steels are extruded. A combination of rotary-hearth furnace and vertical induction heating is therefore a very flexible solution, particularly suitable for high- alloy materials. The induction furnaces come from IAS GmbH, a member of the SMS group. After heating to approx. 1,150°C, the billets are transferred to the vertical expanding and piercing press where the billets are expanded to a defined inside diameter or pierced directly. Before piercing, the billets are lubricated on the inside and outside with glass powder. The SiO 2 -based glass power melts on the hot billet surface and forms a thin lubricating film with good sliding properties. The lubricating film also has a heat insulating effect and protects the billet from secondary scaling. The piercing press from the SMS group is of vertical design with shifting container. It has an innovative cylinder construction that permits upsetting and piercing with the same cylinder pair.

Figure 3: Expanding process

Figure 4: Piercing with prior upsetting

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M AY 2016