Improving Performance - Billet Casting Improving ... - Pyrotek

Improving Performance - Billet Casting
Jonathan Prebble,
Manager of
Aluminium Process
Technology
Pyrotek has
the expertise,
technology,
experience
and the
global
resources to
maximize the
performance
of your
billet casting
systems.
Page 2
LATEST BiLLET CASTing
TECHnOLOgY in THE MODERn
CASTHOUSE
improving Performance For The
Customer
This year, Aluminium international
Today (AiT) published a series of Pyrotek
articles on the processes, techniques and
equipment used in Furnace Building and
Repair, Furnace Melt Treatment, Degassing,
Filtration, Slab and Continuous Sheet
Casting.
We have detailed the metal process path from
the furnace to the casting process in order to
demonstrate clearly why each of the various
stages is necessary to achieve a final, desired
metal quality designed to meet or exceed
the end-customer’s expectations. This article
is the last one in the series, addressing the
casting of Billet in a Vertical Direct Chill
(VDC) Pit.
The in-line processing work which has
been already undertaken (please refer to the
previous articles in this AIT series) upstream
of the casting pit has been required to
provide the billet with the correct levels of
metal cleanliness, chemistry and hydrogen
content. Billet casting covers a range of
alloys in many diameters for extrusion or
forging applications. 1xxx, 3xxx, 6xxx, and
7xxx alloys are all cast into billet form for
applications that require them to be either
extruded to shape or cut to blanks and
forged. It is worth remembering at this point
that the 6xxx series alloys are capable of
being solution heat-treated as well as being
quenched and aged to different tempers after
extrusion. This necessitates homogenising
furnaces in the casthouse area and aging
ovens in the extrusion plant. Furthermore,
ultrasonic non-destructive testing is usually
carried out on all cast billet to ensure
that there are no internal cracks or major
defects present. Billet casting needs to be
done correctly, requiring high investment
in equipment and skill, in order to earn the
premium it commands in the market.
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iMPROVing PERFORMAnCE in THE BiLLET
CASTing PROCESS
In general, most aluminium casthouses
would deal with casting 6xxx series.The other
alloy series are used for more specialised or
exotic products for aerospace, automotive,
transportation, etc. However, all billet
casting requires the same basic approach
to metal distribution and solidification,
best performed in a VDC Pit. These pits are
becoming larger and deeper to allow for
more, longer cast billets because this reduces
process scrap and production downtime
between casts. The new casting challenges
are due to operating VDC billet tables on a
vast scale: more than 100 strands of billet per
cast is increasingly commonplace!
The upstream metal treatment, which we have
focused on previously, has been designed to
reduce inclusions, alkali metals and hydrogen
– all or some of which can cause problems
during the subsequent extrusion or forging
processes. The conditions in the actual
billet casting mould, however, can prove to
be just as troublesome during downstream
processing if the casting parameters are
not well inside the “envelope”, which will
produce acceptable finished quality. Here
we are talking about effects such as grain-size,
segregation, shell zone, cracking, surface
defects including drag marks, tears, bleeds,
etc., and diameter tolerances. These factors
generally result from the conditions inside
the actual billet casting mould - where no
amount of upstream processing can assist.
A good understanding of the billet casting
technology being used is vitally important
as this permits the right methods to be
selected for the best possible productivity
and performance. Most casting recipes
are a compromise: a balance between the
best repeatable quality achievable for a
particular casting technology and practical,
safe, dependable casting which can be relied
upon to produce good product consistently.
In this article, we will first review the
various types of billet casting technology in
general use today. We will also mention the
importance of casting lubricant selection
and how this can both improve cast surface