TPT March 2014

Technology News

Benefits of induction heat treatment for tube and bar By Guido Opezzo and Dirk M Schibisch of SMS Elotherm

INDUCTION quench and temper (Q&T) has now become firmly established practice, particularly in applications that require precise, fast, flexible and repeatable heat treatment of tubular and bar material. A process in successful use for heat treatment of bar material for many years, induction quench and temper has recently been extended to cover tubular products. The new processes developed include technologies for pipes with upset ends, creating special demands on the induction heating processes used. Induction quench and temper was initially used only in the production of automotive components such as stabilisers, which for the cold-bending process require a consistent hardness and microstructure along the workpiece, properties that could not be achieved with conventional combustion furnaces. After the induction quench and temper of bar steel had gradually prevailed in the industry, the process was also developed for tube material such as seamless tubes and inductively longitudinally welded tubes. Compared with the conventional processes using combustion furnaces and supported also by the booming market of OCTG products (oil country tubular goods) for the exploration of oil and gas, the results were so convincing that the induction quench and temper process for tube material has within the last two years prevailed completely. Compared with conventional methods using furnaces with fossil firing systems, the induction quench and temper of long products is beneficial to owners in several respects. The main differences that lead to sustained savings in production are outlined briefly in the following. Current demands for a reduction of stocked material and the large number of different materials lead to a flexible just-in-time production strategy in heat-treatment shops, with small batch sizes of just 30 to 50 tons. In many cases this means that batches with different material dimensions and grades need to be heat-treated at dif- ferent quench-and-temper temperatures several times a day. The limits of conventional combustion furnaces are

Shaping roller made of GP4M hardened to 58-60 HRC

rated for a maximum throughput of 13 tons/hour and can process workpieces with outside diameters from 3 to 9", is capable of achieving these 13 tons/hour with 3" tubes or with 9" tubes. This is possible because induction generates the heat directly in the workpiece and does not need to be supplied from outside. Long material passes through the induction coils at the speed adjusted to the respective diameter and is thus heated to or soaked at the material-specific austenitisation or temper temperature within a few seconds. It is only by induction quench and temper that owners are able to always fully utilise their equipment, as material can be produced at a maximum throughput, irrespective of the material dimensions. Still, if smaller batches need to be run because of small production lots, a matured converter technology as used by the Elotherm iZone™ system also allows certain coil sections to be cut off. This ensures that only the minimum amount of energy needed to heat a defined amount of material is always consumed, which will have a lasting positive effect not only on the direct reduction of production costs, but also on the reduction of CO 2 emissions.

quickly reached here, as these furnaces are unable to respond flexibly enough to varying batch sizes, and temperatures can be adjusted only very slowly. Especially where flexible production lots are concerned, the benefits of the induction technology become clearly recognisable. For a change of the batch, the whole induction line can be run empty within a few minutes and set up again for the next batch with a minimum work effort. To do so, the owner has at his disposal high-speed tool changing systems to ensure that operation can be changed over to energy-efficient heating coils with a suitable diameter within a few minutes. Another benefit of induction in this context is the rapid and easy identification of heat treatment recipes for new materials or component geometries. This allows test production runs or quality tests to be performed at a minimum cost for customers who, in turn, will thus gain a competitive edge – a classical win-win situation for all parties involved. Whereas conventionally heated chamber or continuous furnaces always require the entire furnace space to be heated to the necessary temperature, thus achieving a good efficiency only with fully loaded furnace, an induction quench and temper line will respond flexibly to different batch sizes and dimensions. An induction quench and temper line which, for example, is

SMS Elotherm GmbH 
– Germany Website: www.sms-elotherm.com

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