TPT September 2013

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Technology News
58 September 2013
Tube producers benefit from
quenching and tempering
INDUCTIVE quenching and tempering
of steel bars has gradually become
established in the industry and SMS
Elotherm has further developed this
process for tube products such as
seamless tubes due to the booming
market in the OCTG product area for oil
and gas exploration.
The present requests for a reduction
of stock and a large variety of materials
result in a flexible manufacturing
strategy in the Q&T shops with small
batch sizes. In many cases, this means
that batches of different material
dimensions and qualities have to be
heat-treated at different quenching and
tempering temperatures. For a batch
change the entire induction plant can be
emptied within a few minutes and newly
set up for the next batch with a minimum
of effort.
An inductively heat-treated work
piece is characterised by a highly
homogeneous microstructure and
hardness. The evaluation of the
Rockwell hardness over the entire
cross-section and along the entire long
product normally reveals a variance of
less than 1 HRC.
In conventionally fired plants the
tempering process takes place over
a long period in a temperature range
between 400 and 600°C to make
sure that a reduction of hardness or
tensile strength via microstructure
transformation after soaking is reached.
An undesired side effect is, however,
the so-called “tempering brittleness”,
reducing the notch impact toughness to
a large extent. This effect results from
the precipitation and recrystallisation of
carbides on the grain boundaries during
transformation of the microstructure, in
particular from nickel-chromium steels
susceptible to it.
In contrast to this is the inductive
quench and temper process where the
critical temperature range between 400
and 600°C is passed so quickly that
the tempering brittleness described
above cannot even arise. A precipitation
of carbides does not take place and
the material retains its good strength
properties and notch impact toughness
without becoming brittle.
Long dwell times at high temperatures,
above all in the austenite area of the
material to be treated, normally cause an
undesired decarburisation of the surface
layer. This negative effect weakens
the work piece within the subsurface
resulting in an inhomogeneous hardness
over the entire material cross-section.
For inductive quenching and tempering
such decarburisation does not take place
since the material is retaining a high
temperature only for a very short period.
A result of the properly synchronised
inductive Q&T process is long products
whose straightness is better than the
standard requirement according to API
guidelines. The reasons for that are as
diverse as complex and can be found
above all in the work piece transport
as well as in the process control from
heating and quenching or cooling.
After many heat treatment facilities for
steel bars and tubes have been installed
with induction technology, particularly
in the last few years when the owners
were able to realise the production cost
savings described, they are now looking
for possibilities of transferring the
advantages of induction to even more
complex components.
Currently, Elotherm has developed an
innovative induction solution allowing
seamless tubes to be heated with upset
ends so homogeneously that the desired
material properties are achieved. Since
2012, this process has been in use at a
world market leader for OCTG products
in its new plant in Saudi Arabia for the
production of tubes with upset ends and
strongly stressed jacket tubes for oil
exploration.
The exploration of some oil and gas
deposits requires jacket tubes for lining
the actual borehole, which have to be
highly stress-resistant. For example,
particularly deep boreholes, low ambient
temperatures or saltwater require the
use of jacket tubes with especially
high demands on strength or leakage
protection. Such jacket tubes typically
have outer diameters from 5 to 10¾"
and are upset at the end to diameters
up to 16".
A special challenge for the induction
process is a significant difference
in weight between the straight tube
body and the upset ends which may
be up to 65 per cent according to the
relevant API specification 5CT. To reach
a uniform austenitising temperature
and homogeneous hardness and
microstructure conditions after tempering,
additional heating of the ends is required.
For this reason, inductive preheat
coils are provided upstream of the actual
austenitising system. The position of
the upset ends is precisely sensed via
an integrated position control system.
This ensures that in fact only the upset,
ie thicker tube areas, are heated in
the preheat units and the remaining
tube body almost stays at ambient
temperature.
SMS Elothern GmbH – Germany
Website:
Quenching and tempering
tube products