Graphite-Based Technology in Continuous Casting of Re-Draw Rod for by etssetcf


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									Copper Wire Rod Process

Technology in Continuous
Casting of Re-Draw Rod
for Multi-Wire Drawing
by Sir Michael Nairn - Chairman
Rautomead International Limited - Scotland, UK

Background                                  prevents these from dissolving in the       formation of oxides along the grain
                                            copper matrix and adversely affecting       boundaries which, as mentioned earlier,
Prior to the 1970s, most copper rod was     conductivity and annealability of the       is often a cause of wire breaks later on.
produced by a batch process involving       rod and drawn wire. Conversely,
casting of wire bars, hot rolling,          however, the presence of occluded           Unlike Cu-ETP, which is cast as a billet
pickling and butt welding to form           oxides in the copper wire rod, while        and hot rolled, a modern oxygen-free
longer lengths. In the thirty subsequent    solving one problem, can lead directly      copper plant is generally designed to
years, this traditional process has been    to another, as oxides tend to form hard     cast directly at 8mm diameter, as the
progressively superseded by continuous      particles and lead to wire breaks in fine   accepted input size for wire breakdown
billet casting and hot rolling by the       wire drawing.                               throughout the world. In terms of
familiar Contirod, Properzi and                                                         performance in subsequent wire
Southwire processes, as the accepted        Oxygen-free                                 drawing, it is significant that rolling is
means to produce electrolytic tough         high conductivity - Cu-OF                   eliminated as a process stage.
pitch copper (Cu-ETP) rod. With good
process control and careful grading of      Cu-OF and its sister material Cu-OFE        Why oxygen-free ?
coils during the production run, the best   have minimum copper contents of
quality Cu-ETP rod has conductivity         99.95% and 99.99% respectively.             The question is often asked “Why
characteristics in excess of 101% IACS      Alongside the advancing technology of       oxygen-free copper ?” “What are the
and is capable of being drawn to            Cu-ETP rod production, other                benefits ?” “Who needs it ?” The facts
superfine wire with an acceptable rod-      companies        developed     processes    are that by comparison to Cu-ETP, the
break performance. Characteristically,      for production of Cu-OF rod.                market for Cu-OF is relatively small,
these are however big plants in the         Characteristically, these types of plant    but it is growing. Cu-OF will serve any
output range 50,000 to 200,000 tonnes       are smaller than the well-known             of the purposes fulfilled by Cu-ETP, but
per year, which must be run close to        Cu-ETP plants, with outputs in the          has some additional properties of
capacity to be economically viable.         range 2,000 to 30,000 tonnes per year,      importance in particular applications.
                                            implying a significantly lower
Electrolytic tough pitch - Cu-ETP           investment. As the name of the material     The absence of occluded oxides at the
                                            suggests, there is little or almost no      grain boundaries in Cu-OF results in a
Cu-ETP has a minimum copper content         oxygen present in the redraw rod. There     more ductile material. This can be
of 99.90%. Oxygen is intentionally          is thus little or no oxygen to react with   important in such applications as in
alloyed with the copper in production of    impurities which may be in the copper       aerospace,       automotive        wiring
Cu-ETP and is controlled to around          and to avoid these becoming dissolved       harnesses, robot arms and other similar
200-400ppm. The oxygen acts as a            in the matrix of the copper. Production     applications, where the copper
scavenger for dissolved hydrogen and        of oxygen-free copper is thus more          conductor core of a wire or cable is
sulphur and will react also with most       demanding of the quality of the in-going    subjected to repeated flexing. The same
other impurities, to form insoluble         cathode feedstock used in the process.      characteristic of Cu-OF produces less
oxides at the grain boundaries. This        Conversely, there will be little or no      “noise” and is thus of increasing interest

                                                      EuroWire - November 2001
                                                                                                           Copper Wire Rod Process

in such applications as high quality                      The plants are smaller and more               In reality, a wire break can be the result
sound recording systems and in                            versatile than the characteristic large       of a combination of the two sources.
headphones for both military and                          Cu-ETP plants. Alloyed coppers can be         Detailed studies into wire breaks
civilian use. The elimination of the risk                 produced, such as Cu-Ag, which is             occurring in Cu-ETP in drawing to
of hydrogen embrittlement in copper                       often used in commutator section of           0.05mm have been carried out recently
through the use of Cu-OF is an                            electric motors on account of its higher      in USA, involving examination and
important characteristic in welding                       softening temperatures; Cu-Sn, Cu-Cd          categorisation of over 2,500 wire break
applications. Creep resistance of Cu-OF                   and Cu-Mg as trolley wire alloys.             samples[1].
is also superior, which favours its use in                Changeover is relatively simple and
such applications as trolley wire.                        downtime minimised. Rods of different         This work established that over 90% of
Another important market trend which                      sizes can be made simultaneously.             all wire breaks were particle failures
is favouring Cu-OF is the explosive                       Investment is much less than for a            and that within this total, over 50%
growth of the market for electronic                       typical casting and hot rolling plant to      were ferrous inclusions and over 30%
devices       and    the      progressive                 produce Cu-ETP.                               refractory inclusions. The dominant
miniaturisation        of      electronic                                                               wire failure gauge occurs at around
components. This has resulted in a                        Wire breaks in drawing                        0.1mm (38 AWG) and a defect particle
demand for ever finer wire gauges,                                                                      size of 0.05mm seemed to be the
where now wires of 0.050mm diameter                       More and more attention is now being          statistical mean causing this. Using
are not unusual. Demand is increasing                     given to the causes of wire breaks in         scanning electron microscopy, the
for wires of 0.030mm diameter and less.                   drawing, as with miniaturisation of           principal elements causing wire breaks
Fine wires are now being regularly                        electronic components, demand for             in descending order were found to be:
drawn on multi-wire machines with up                      finer and finer wires increases. The use      • Iron: .......................................25.6%
to 32 strands. Wire break performance                     of multiwire drawing machines,                • H-13 tool steel: ......................11.0%
is critical to production efficiency. The                 processing up to 32 strands                   • Silicon, aluminium: .................9.5%
purity of the metal and the absence of                    simultaneously greatly increases the          • Off-centre hollow: ...................8.3%
oxide particles in the structure become                   efficiency of the fine wire drawing           • Silicon:.....................................7.6%
matters of serious concern. Here too,                     process, but only so long as wire breaks      • Slag:............................................1%
Cu-OF has distinct advantages over Cu-                    can be routinely avoided. Conversely,
ETP. In production, the cast material                     the whole machine must be stopped and         Discussing these in sequence
exits the casting die and cooler                          rethreaded when a wire break occurs.
assembly at around 80°C, well below                                                                     iron occurs in many continuous casting
the surface oxidation temperature. Cu-                    It is common practice to divide wire          plants, as well as in wire drawing
OF thus has a very thin layer of surface                  breaks into two basic sources, from rod       machines, making it difficult to identify
oxides, significantly less than Cu-ETP.                   manufacturing and from wire drawing.          the source. However, contact with
The product thus lends itself to                          It is not always a simple matter to           ferrous materials in the Rautomead RS
production of fine magnet wire and as a                   identify the source, but it is important to   system is limited to the profiled
feedstock for continuous extrusion.                       study the process sequence as a whole.        withdrawal rolls and coiler rolls, both of
                                                                                                        which are made in specially hardened
w   Model RS 3000/8/8 copper rod casting machine - 6,000t./year
                                                                                                        steel and very unlikely to contaminate
                                                                                                        the surface of the copper rod. No hot or
                                                                                                        cold rolling occurs and the rod is coiled
                                                                                                        on wooden pallets. Wire drawing is also
                                                                                                        a source of iron contamination, though
                                                                                                        in superfine wire drawing equipment,
                                                                                                        capstans are now often made in ceramic
                                                                                                        materials and guide pulleys and contact
                                                                                                        sheaves in plastic.

                                                                                                        H-13 tool steel can generally be traced
                                                                                                        back to rod mill rolls and guides in the
                                                                                                        Cu-ETP plant and is likely to have been
                                                                                                        introduced in the hot rolling process
                                                                                                        stage. No rolling takes place in the
                                                                                                        Rautomead RS process for production
                                                                                                        of 8mm redraw rod, so that this source
                                                                                                        of wire breaks is eliminated.

                                                                                                        Silicon & aluminium are typical
                                                                                                        refractory materials used in fabrication
                                                                                                        of conventional induction furnace
                                                                                                        linings and hot metal launders. It is a
                                                                                                        unique advantage of the Rautomead RS
                                                                                                        graphite furnace technology that
                                                                                                        graphite takes the place of fritted

                                                                     EuroWire - November 2001
Copper Wire Rod Process

alumina ceramic furnace linings and is       A synthetic graphite is used,               die is very important in continuous casting
thus substantially reduced as a possible     manufactured from carbon based              and is the most significant property
source of this major cause of wire breaks.   materials, rather than the natural mined    governing heat transfer.
More details of the graphite material and    substance. Synthetic graphite is superior
its properties are given below.              to the naturally occurring material,        The grade of graphite commonly used
                                             which can suffer from mechanical            in the fabrication of dies has a thermal
Off-centre hollows is a condition in         weakness associated with impurities         conductivity value in the range
Cu-ETP production, where a small             and ash content. Under non-oxidising        100-200W/m K.
particle of refractory is introduced into    conditions, synthetic graphite is the
the molten metal flow, floating just         highest temperature-stable elemental        Reaction with liquid copper
below the surface as the metal               solid known. This is related to its high
solidifies. A skin of copper is created      binding energy and its heat of              Copper (together with tin, gold, silver
over the defect, which then ruptures in      sublimation. In manufacture, it involves    and lead) is virtually inert relative to
drawing. Again, the graphite furnace         powder processing technology on a           graphite at the temperatures necessary
technology obviates this risk.               large scale. Carbonaceous fillers are       for continuous casting.
                                             bonded with carbon-yielding binders,
Silicon and slag can collect at the          cured at around 1,500°C and sintered        Investigation has shown that the
surface of the melt in a Cu-ETP plant        together at temperatures of the order of    solubility in wt. % C, is about 0.0001 at
and wash into the cast. The principal        3,000°C. In the process, the temperature    1,100°C, 0.00015 at 1,300°C, 0.0005 at
components are copper oxides, with the       of the baked carbon is gradually raised.    1,500°C, and 0.003 at 1,700°C.[2] As
inclusion of silicon, aluminium and          Graphitisation       commences         at   carbon does not diffuse through solid
iron. Again, in the Rautomead RS             approximately 2,200°C. This process         copper, solubility is exceedingly
process, the transfer from the melting       slowly transforms the baked carbon, a       small.[3] Claims that the copper
chamber through to the casting chamber       very hard, abrasive material with low       processed in the Rautomead system can
of the crucible is through the base, so      thermal and electrical properties, into     somehow become contaminated by
that this risk does not arise.               graphite, an allotropic modification of     carbon can thus be easily dismissed.
                                             carbon, crystallising as hexagonal
The Rautomead                                platelets. This material possesses          Graphite crucibles
RS Upwards Vertical Process                  excellent lubricity and high thermal and
                                             electrical conductivity.                    Crucibles are machined from solid
Rautomead        has    been     building                                                blocks of vibration moulded graphite. In
continuous casting machines based on         The bulk filler material is carbon,         the case of integrated plants for the one-
the use of graphite furnace technology       carefully milled to a specific particle     step conversion of copper cathode
since the late 1970s. The technology was     size and combined with an appropriate       plates to Cu-OF redraw rod, these
adapted to production of Cu-OF in the        volume of binder. The binder used in the    crucibles are of a twin-chamber design,
early 1990s, when the company saw an         graphite industry is coal tar pitch, the    where the cathodes are lowered and
opportunity to improve on processes          product of destructive distillation of      melted in a melt chamber, with a bottom
available at that time and particularly to   coking coal. The filler material is         port to a separate casting chamber. The
use the naturally reducing characteristics   calcinated petroleum coke. The precise      significant mass represented by the
of graphite to best advantage.               type and volume of filler, particle size    heated crucible itself, forms part of the
                                             and quantity of binder all have a marked    potential energy of the furnace design
The process uses a single furnace in         influence on the density and final          and contributes significantly to furnace
which to melt, hold and cast the copper.     physical properties of the product. The     temperature stability.
This contrasts with other systems, which     aggregate mix is ground to a specific
almost invariably use ceramic refractory     particle size and either moulded or
lined, induction-heated furnaces, often      extruded into “green” blocks. Vibration-
positioned in tandem, one for melting        moulded blocks are now preferred for
and the second for casting, with the         large crucible manufacture, being more
copper being poured from one to              isotropic than extruded materials. High-
another.                                     quality die grade graphite is made by
                                             isostatic pressing.
Characteristics which favour the use of
graphite include the purity of this          The tensile strength and elastic modulus    v   Graphite crucible
material as elemental graphitised            of graphite increase with increasing
carbon, its machinability, its thermal       temperature, up to 2,400°C. Graphite        The flow of molten copper is thus
conductivity, its naturally reducing         exhibits very high thermal shock            necessarily first-in-first-out and the
function, whereby oxygen present in the      resistance - orders of magnitude higher     design incorporates a graphite filter bed
molten metal will react with the carbon      than most high-temperature ceramic          at the base of the casting chamber to
and be eliminated from the melt and its      refractories. High-strength graphite        ensure that the de-oxygenating process
excellent high temperature stability and     with a bulk density of around               is complete, before the metal enters the
strength. Graphite is not wetted by          1.8gm/cm3 has an open porosity of around    casting dies at the top of the casting
copper in the molten state.                  8%. Thermal conductivity in the casting     chamber.

                                                       EuroWire - November 2001
                                                                                               Copper Wire Rod Process

                                                                                           picked up and fed under control to the
                                                                                           melting chamber of the graphite crucible.
                                                                                           The cathode passes through a layer of
                                                                                           graphite flake, which acts as a hot metal
                                                                                           cover and protects the surface of the
                                                                                           molten copper from oxidation.

                                                                                           The liquid copper sees only graphite
                                                                                           contact surfaces. Crucible design is such
                                                                                           that the copper must follow a first-in-
                                                                                           first-out path, downwards to the transfer
                                                                                           port and then upwards through a graphite
                                                                                           filter arrangement to the casting dies. The
                                                                                           casting dies themselves are immersed
                                                                                           through the layer of graphite flake cover
                                                                                           in the copper, where the metallostatic
                                                                                           pressure created is used to channel the
                                                                                           liquid copper into the die and cooler
                                                                                           arrangement. LME grade A cathodes
                                                                                           entering the process may be expected to
                                                                                           contain about 60-80ppm oxygen. This is
                                                                                           reduced to 2-3ppm in the process. No hot
                                                                                           metal transfers between furnaces take
v   Furnace cut-away elevation drawing                                                     place, thus avoiding the risk of hydrogen
                                                                                           pick-up at that stage of the process.
                                             Moisture vapour entering such a gas
                                             generator from the atmosphere,                Furnace heating is by electric resistance
                                             particularly in areas of high humidity, is    by means of a chain of graphite heating
                                             a major potential source of hydrogen in       elements positioned adjacent to the wall
                                             the melt and of consequent gas porosity       of the graphite crucible, with the heat
                                             in the rod. With no hot metal transfers       being transferred to the copper by
                                             taking place in the Rautomead system,         radiation and convection. The This
                                             this source of possible hydrogen gas          results in a still metal bath and an ideal
                                             inclusion is effectively eliminated.          condition from which to cast.

                                             As furnaces have become larger,
                                             Rautomead has recently carried out
                                             extensive finite element analysis work in
                                             finalizing the design of graphite crucibles
                                             in association with the University of
                                             Strathclyde, Glasgow. This work has
                                             enabled performance at temperature to be
                                             carefully studied and any points of high
                                             stress in the design to be eliminated.

v   Finite element analysis crucible study   Rautomead process design

A ceramic one-piece liner is fitted on       A pick-and-place mechanism is used to
top of the crucible to protect this from     lift the cathodes and to transfer them to a
erosion through exposure to atmosphere       tilting table, which turns them to the
and from physical abrasion. A layer of       vertical position, from where they are
high purity graphite flake is used to
protect the surface of the molten metal      w   Cathode feed
                                                                                           v   Heating element chain
and an inert gas atmosphere is
maintained inside the furnace to protect
the graphite hot-working parts from                                                        Casting is through water-cooled graphite
erosion.                                                                                   casting inserts and backer inserts.
                                                                                           Withdrawal is via two pairs of profiled
In other systems utilising separate                                                        nip rolls, using pneumatic pressure and
induction melting and holding furnaces,                                                    rapid pulse indexing drive. Separate
a charcoal-based carbon-monoxide gas                                                       drives are provided for either side of the
generator is often used to protect the                                                     machine, making it easy to cast different
molten metal from oxidation, as it is                                                      sizes of rod simultaneously and selecting
poured from one furnace to another.                                                        the casting speed best suited to each size.

                                                         EuroWire - November 2001
Copper Wire Rod Process

Rod surface temperature at the cooler
assembly exit point is less than 100°C.
The rods are then fed over nylon
support rollers and down to rod coilers
via a dancing arm speed control station.

The whole process is PLC controlled
and is designed to monitor and manage
all the major process variables without
the need for human interference.

From this brief description, it will be
clear that in the Rautomead process,
there is no exposure to alumina or other
types of frit ceramic furnace linings and
no pouring of liquid metal from one
furnace to another. On the other hand, a
completely still metal bath is created
from which to cast oxygen-free copper.
                                            v   Temperature trace

                                            and heated to approximately 200°C             developed in the early 1990s, the target
                                            before being fed to the furnace.              casting speed was 3.0m/min, to give
                                                                                          80kg/per strand per hour. This balanced
                                            The effect of the cathode preheater is to     with a melting capability of around
                                            dry off any surface moisture,                 700kg/per hour for the machine in an
                                            condensation      or    surface-trapped       eight strand configuration.
                                            electrolyte and thus to ensure that
                                            hydrogen from those possible sources is       Recently, improvements in rod cooler
                                            eliminated. It avoids metal splashing at      design and in mechanical withdrawal
                                            the feed point and contributes to the         have permitted increases in casting
                                            overall melting burden of the furnace         speed to 5m per minute. An eight strand
                                            itself.                                       machine today is now rated at 500KVA
                                                                                          and is capable of over 1,000kg per hour.
                                            Improved temperature control
                                                                                          Feedstock and impurities
                                            Maintenance of stable production
                                            parameters is one of the most important       This paper would be incomplete
v   Rod withdrawal                          criteria for production of good and           without a section on the selection of
                                            consistent quality copper redraw rod.         feedstock. Except in the removal of
Cathode preheater                                                                         oxygen, the Rautomead Cu-OF process
                                            Among these parameters, none is more          is not a refining one. Thus, the quality
A recent innovation in some of              important than stable control of              and cleanliness of the feedstock used is
Rautomead’s copper rod casting              temperature at the point of casting. In       of critical importance in production of
machines has been to incorporate a          the Rautomead process, operating              good quality copper redraw rod.
cathode preheater. This is part of the      temperature is measured by two
cathode feed system and operates on the     thermocouples, one of which is                The specification set by Rautomead calls
principle of an electric toaster, whereby   immersed in the molten metal, close to        for the use of LME grade A cathode (Cu-
each cathode is lowered into an             the casting dies and the other in the         CATH-1). Impurities generally enter the
electrically powered preheat furnace        furnace, in close proximity to                production chain at source. Cu-CATH-1
                                                           the crucible wall. Using       specification is, however, relatively
w   Line sketch of cathode preheater                       a cascade temperature          broad and permits the presence many of
                                                           control arrangement, this      the elements which can be deleterious in
                                                           enables a tolerance of         wire drawing. Maximum impurities
                                                           ±3°C to be automatically       permitted are 0.0065%, although most
                                                           maintained at the casting      cathodes produced have significantly
                                                           dies.                          lower impurities at around 0.002%.
                                                                                          Therefore, it is not sufficiently precise
                                                               High-speed rod             for the user to specify grade A cathode,
                                                               casting                    as the feedstock. Rather, a more detailed
                                                                                          appraisal of both nominal composition
                                                               When the RS upwards        and of manufacturing consistency must
                                                               vertical casting process   be made in the selection of suitable
                                                               for 8mm Cu-OF rod was      cathode feedstock.

                                                        EuroWire - November 2001
                                                                                         Copper Wire Rod Process

Composition - cathode                      cathode should be
and wire rod                               treated with caution.
                                           There is no substitute
Within the total allowable level,          for taking rod samples
individual impurities or groups of         after casting. This takes
impurities may also be close to the        into account any mixing
maximum permitted individual or group      of cathode during the
levels (see Table 1).                      process.

Composition is influenced by the           Hydrogen levels in
mining source of the copper ore and by     molten copper are easily
the technology of the refinery where the   raised where oxygen
cathode is produced. The most critical     levels are low. The
impurities in relation to wire drawing,    presence of hydrogen
graded as to their influence on wire       may create voids in the
breaks are as follows:                     matrix structure, which
• Severe effect: Bi Te Se (cause grain     may close down or may
    boundary cracks)                       propagate during wire
• Deleterious: Pb As Sb S (cause grain     drawing. It is important
    boundary cracks)                       that dry cathode is
• Low effect: Cr Fe Sn P Si Ag (affect     charged into any of the
    annealability)                         production process. A
                                           further     source     of
Hydrogen also accentuates the              hydrogen may be from
detrimental effect of other impurities     the          electrolytic
which may be present at grain              deposition process used
boundaries. Care must be taken to          in cathode production.
eliminate as many sources of hydrogen      Electrolyte can become
ion from the process as possible.          trapped in the cathode
Cathode selection and handling, furnace    and enter the melt. The
atmosphere, as well as melt surface        electrolyte     contains
protection, must all be carefully          thiourea and glue, both v Table 1
regulated to minimise hydrogen pick-up.    of which are high in
                                           hydrogen ions. Cathodes should be           benefits of this rod production
Procedures for testing cathode for         selected which have been shown to have      technology will play an increasingly
impurities during production vary          low levels of trapped electrolyte and       important role in association with the
between cathode producers. Some take       should be stored ready for use in a dry     adoption by wire and cable
20mm diameter samples either from          environment. Care must be taken to          manufacturers of multi-wire drawing
each or from a proportion of cathodes.     avoid surface condensation on cathodes      machines and enable them to obtain the
The samples are melted and analysed.       exposed to changes in atmospheric           best efficiency from these machines. s
However, chemical composition is           temperature and humidity before use.
recognised as not necessarily consistent                                               Acknowledgements
over the full area of a cathode. A true    Summary
representative analysis requires a full                                                1] Masoud Garshasb and Timothy J Day,
                                                                                          “Quality Enhancement of Ultrafine Magnet
cathode to be melted. As this is rarely    It can thus be shown that both in the          Wire utilizing Failure Analysis”, Conference
done, quoted chemical compositions of      selection of materials and in the process      Proceedings of the Wire Association
                                                          design, the Rautomead           International’s Wire & Cable Technical
                                                          RS upwards vertical             Symposium, 2001, pp 220-226.
                                                          casting            process   2] M. B. Bever and C. F. Floe, Trans AIME, 166,
                                                                                          1946, 128-141
                                                          effectively      prevents       Solubility of Carbon in Copper
                                                          many of the common           3] W. Baukloh and F. Springorum, Z. anorg.
                                                          rod       manufacturing-        Chem., 230, 1937, 315-320
                                                          related sources of wire         Solubility of Carbon in Copper
                                                          breaks and offers users
                                                          real advantages in
                                                          processing to fine and
                                                          superfine wire. It does
                                                          not however eliminate
                                                          the need for careful               Rautomead International Ltd
                                                                                                       PO Box 100
                                                          selection of feedstock
                                                                                            Dundee, Scotland DD1 9QY - UK
                                                          cathode and similar                  Fax: Int’l +44 1382 622941
                                                          detailed attention to all          E-mail:
                                                          stage of the wire                  Website:
                                                          drawing process. The

                                                     EuroWire - November 2001

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