TPT July 2015

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88 July 2015
Article SMS Elotherm GmbH
Elotherm TemperLine™ –
benefits of induction heat
treatment for tube and
bar material
By Dirk Schibisch of SMS Elotherm
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.
Excellent homogeneity of
material hardness
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.
High strength and notch
impact strength
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.
Design of a typical Elotherm TemperLine™ Q & T induction line for
long products