TPT May 2016

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72 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
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.
Figure 2: Heating concept
Figure 3: Expanding process
Figure 4: Piercing with prior upsetting
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 SiO2-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.