TPT July 2015

Article

SMS Elotherm GmbH

Elotherm TemperLine™ – benefits of induction heat treatment for tube and bar material By Dirk Schibisch of SMS Elotherm

Excellent homogeneity of material hardness

Very high flexibility 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 metric tons. In many cases this means that batches with different material dimensions and grades need to be heat-treated at different quench-and-temper temperatures several times a day. 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 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. Induction quench and temper (Q&T) has 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, mostly for OCTG (oil country tubular goods), including technologies for pipes with upset ends.

A characteristic feature of an induction quenched and tempered workpiece is its very homogeneous microstructure and hardness. Actually, these results are not exclusively due to the induction principle alone, but mainly to combining the process technology with a matched mechanical and electrical design of the induction line. In the TemperLine™ induction quench and temper system by SMS Elotherm, the material to be treated sequentially passes through the whole process of austenitisation, tempering and, if necessary, soaking. In the process, each incremental cross section of the long product is subjected to identical heating and quenching conditions, as the passage speed is constant throughout the whole process. This then results in the correspondingly high homogeneity referred to the hardness and microstructure of the material. In conventionally fired units, the tempering process takes place within a temperature range of 400 to 600°C for a long period of time, to achieve a reduction of the hardness or, respectively, tensile strength via the microstructural transformation after soaking. However, an undesired side effect is the so-called “temper brittleness”, which significantly reduces the notch impact strength. This effect arises through the precipitation and recrystallisation of carbides at the grain boundaries during the transformation of the microstructure, especially in nickel-chrome steels that are susceptible to such an effect. This is in contrast to the induction quench and temper process in which the critical temperature range between 400 and 600°C is passed so quickly that the described temper brittleness with precipitation of carbides cannot arise. The material retains its good strength characteristics and notch impact strength without becoming brittle. High strength and notch impact strength

Design of a typical Elotherm TemperLine™ Q & T induction line for long products

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J uly 2015