Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BIMETALLIC PLATE
This invention relates to a process, and to moulding apparatus, for
the production of composite metal articles comprising bimetallic plate.
Numerous prior art proposals for producing composite metal articles are
discussed in U.S. patent 4953612 to Sare et al (corresponding to WO 85/00308).
Those proposals suffer from various disadvantages or limitations, at least
some of
which are overcome by the teaching of USP 4953612. The teaching of USP
4953612 is well suited for the manufacture of a range of composite metal
articies
io comprising a cast component bonded to a substrate component. However, the
teaching is less well suited for the production of a composite metal article
comprising bimetallic plate, in particular plate which is relatively thin
and/or has a
relatively large surface area. Thus, the teaching of USP 4953612 can encounter
difficulties, such as uneven bonding, in the production of bimetallic plate in
large
sizes, such as about 300x300mm and greater, with a thickness of less than
about
30mm and a thickness ratio of about 1:1 or less for cast metal to substrate.
The present invention seeks to provide a process and moulding apparatus
which enables production of relatively thin bimetallic plate. However, while
enabling such production, the invention also can be adapted for use in the
production of thicker plate. In each case, the invention also enables
production of
plate in relatively large sizes, such as up to and in excess of 1800x1000mm,
while
indications are that plate at least up to 3000x1500mm is able to be produced.
In the process of the present invention a plate (hereinafter referred to as a
"substrate"), which is formed of a first metal, has a component (hereinafter
referred to as "cladding") of a second metal cast against it to form
bimetallic plate. The
first metal for the substrate may be titanium, nickel or cobalt, a ferrous
alloy or a
titanium-, nickel- or cobalt- base alloy. The second metal for the cladding
may
be copper, nickel or cobalt, a ferrous alloy or a copper-, nickel- or cobalt-
base
alloy. While not necessarily the case, the first and second metals usually
3o are compositionally different. However, where the first and second metals
are the
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same or similar, in being closely related compositionally, this can be to
achieve a
difference in properties based on microstructure, such as due to the substrate
being hot- or cold-worked and the cladding having an as cast microstructure.
As in USP 4953612, the surface of the substrate against which molten
alloy is to be cast to form the cladding needs to be rendered substantially
oxide-
free. Also, the substrate is preheated and is protected against oxidation by a
suitable coating. The coating may be formed from flux which is applied over
the
substrate surface, and melted to form a protective film during preheating.
However, other protective coatings can be used, such as a deposit of a
suitable
to metal formed for example by electroless or electrolytic plating of nickel
or another
metal, or a non-metallic coating such as of colloidal graphite containing a
silicate
binder. Depending on the protective coating use, it is either displaced by or
alloyed with the alloy cast to form the cladding, facilitating wetting of the
substrate
surface by the cast alloy.
Also as in USP 4953612, the molten alloy to form the cladding is poured at
a superheated temperature to facilitate the attainment, with preheating of the
substrate, of an overall heat energy balance to achieve a diffusion bonding
between the cladding and the substrate. The diffusion bond preferably is
obtained substantially in the absence of fusion of the substrate surface
against
which the cladding is cast.
In the production of bimetallic plate, it can be very difficult to achieve a
sufficient heat energy balance for good bonding between the cladding and
substrate. This is particularly the case where the plate is large in area,
and/or
relatively thin and/or has a relatively low thickness ratio of cladding to
substrate.
Under these conditions, it is found that loss of heat energy to the mould
becomes
a significant factor preventing the attainment of such energy balance, with
this
loss being from both the preheated substrate and from the molten alloy as it
flows
over the substrate. This loss can be exacerbated by delays between preheating
the substrate and pouring the molten alloy to provide the cladding and/or by
an
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unduly long period during which the molten alloy is poured. Also, it is found
that
loss of uniformity of heat energy balance, with resultant non-uniformity of
bonding, can result from uncontrolled or irregular flow of molten alloy over
the
substrate, such as to give rise to an unduly long flow path and/or a reducing
flow
rate for the alloy.
We have found that substantially improved bimetallic plate can be
produced by controlled casting of molten alloy to provide the cladding. In the
process of the invention, the cast alloy is caused to flow across the surface
of the
substrate along a controlled melt front which is advanced in a manner which,
t0 having regard to the temperature to which the substrate is preheated and
the
superheat temperature of the molten alloy, provides over substantially the
entire
surface of the substrate a heat energy balance within limits sufficient for
achieving
a diffusion bond between the cladding and substrate.
While not necessarily the case, the bimetallic plate may be square or other
rectangular form. For ease of further description, a rectangular substrate and
resultant plate is assumed in the following. Also for ease of description,
directions
across the substrate are designated as iongitudinal, for the direction in
which the
melt front advances, and lateral for the direction in which the melt front
extends
transversely with respect to its direction of advance. However, while the
substrate and resultant plate may have a longitudinal extent which is greater
than
its lateral extent, the converse may apply or the longitudinal and lateral
extents
may be substantially equal. Additionally, while the longitudinal direction of
melt
front advance can be substantially between longitudinally opposite edges of
the
substrate, longitudinal melt advance can be over part of the longitudinal
extent of
the substrate. Moreover, the lateral extent of the melt front and, hence, the
width
of cladding in that direction, may be over substantially the full lateral
extent of the
substrate or over a part of that extent.
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In the process of the present invention, a controlled melt front is advanced
in a manner providing required heat energy balance for bonding by at least one
of
the following features:
(a) causing the molten alloy to enter a mould cavity, in which the
substrate is positioned, through a laterally disposed series of gates
providing
communication between a runner and the mould cavity, whereby the molten alloy
forms a laterally extending melt front, and
(b) causing the melt front to advance longitudinally over the substrate
at a rate which is substantially uniform across the lateral extent of the melt
front.
The process of the invention preferably utilises each of features (a) and
(b).
Thus, according to the present invention, there is provided a process for
the production of a composite metal article comprising bimetallic plate,
wherein a
plate (hereinafter referred to as a "substrate") formed of a first metal is
preheated
and, with the preheated substrate positioned in a mould cavity of a mould with
a
major surface of the substrate facing upwardly and to fill a portion of the
depth of
the cavity, a component (hereinafter referred to as "cladding") of a second
metal
is cast against said major surface of the substrate to form with the substrate
said
bimetallic plate; prior to the cladding being cast, said major surface is
rendered
substantially oxide-free and is protected against oxidation by a suitable
coating;
the cladding is cast by a melt, of a composition required for the cladding,
being
poured at a superheated temperature whereby, with the preheating of the
substrate, an overall heat energy balance is achieved between the substrate
and
the cladding and causes a diffusion bond to be achieved between the major
surface of the substrate and the cladding; and wherein attainment of the
required
heat energy balance is facilitated by causing the melt to enter the mould
cavity
through a series of gates which provide communication between at least one
runner and the mould cavity, with the series of gates disposed laterally with
respect to flow of the melt therethrough whereby the melt forms a laterally
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extending melt front, and by causing the melt front to advance away from the
gates, over the substrate surface, at a rate which is substantially uniform
across
the lateral extent of the front.
The invention also provides moulding apparatus, for use in producing
bimetallic plate having a plate (hereinafter referred to as a "substrate")
formed of
a first metal and a component (hereinafter referred to as the "cladding") of a
second metal cast against the substrate, wherein the apparatus includes a
mould
having a drag section and a cope section; the drag section defining part of a
mould cavity formed by the mould in which the substrate is positionable with a
major surface of the substrate facing upwardly; the cope section defines part
of
the mould cavity whereby, with the mould in a closed position, a melt of a
composition required for the cladding is able to be poured so as to fill the
mould
cavity above the substrate for forming the cladding; the mould sections define
a
series of gates which provide communication between at least one runner and
the
mould cavity; the series of gates is disposed laterally with respect to flow
of the
melt therethrough whereby a laterally extending melt front is able to form;
and
wherein the arrangement is such that the metal front is able to advance away
from the gates, over the substrate surface, at a rate which is substantially
uniform
across the lateral extent of the front whereby, with preheating of the
substrate and
superheating of the melt to achieve a suitable heat energy balance between the
substrate and the cladding, a diffusion bond is able to be achieved between
the
major surface of the substrate and the cladding.
To enable attainment of feature (a), moulding apparatus according to the
invention includes a mould defining a mould cavity in which a substrate is
positionable, and in which molten alloy is able to be cast against an upper
surface
of the substrate. The mould defines at least one feed sprue by which molten
metal is receivable, with the feed sprue communicating with at least one
lateral
runner by which molten metal passes from the feed sprue to each gate of the
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series. At least where the cladding is to extend from a transverse edge of the
upper surface of the substrate which is adjacent to the series of gates, the
mould
cavity may have a galley portion at which the gates communicate with the
cavity.
In a casting operation with a mould providing for feature (a) molten metal
flows into the mould cavity via each gate with streams of molten metal from
successive gates merging to generate a molten metal melt front which passes
longitudinally over the upper surface of the substrate. Where the mould cavity
has a galley portion, the merging of streams preferably occurs in the galley
portion before the melt front reaches the substrate.
To enable attainment of feature (b), the lateral runner may be configured
substantially to equalise metal pressure at each gate of the series. For this
purpose, the runner can decrease in cross-section after each successive gate
in
a direction extending laterally away from the feed sprue, such as by the
runner
having stepwise reductions in its depth. Additionally, or alternatively,
attainment
of feature (b) can be facilitated by the mould being configured so that the
substrate, when positioned in the mould cavity, has its upper surface inclined
upwardly from the feed sprue, i.e. inclined upwardly in the direction of melt
front
advance. Thus, across its lateral extent, the melt front is constrained to a
substantially uniform advance, under the influence of gravity.
While it usually is preferred for the substrate to have its upper surface
substantially horizontal or inclined upwardly from the feed sprue, there can
be
benefit in having the surface slightly inclined downwardly from the sprue.
That is,
the upper surface may be inclined downwardly in the direction of melt front
advance. The downward inclination has the benefit of increasing the flow
velocity
of the metal. The extent to which the inclination is possible is dependent
upon
melt viscosity, and the magnitude of the inclination needs=to be limited so as
to
ensure that a substantially uniform rate of melt front advance is maintained
across
the lateral extent of the front.
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Sand moulds have been found to be well suited for use in the present
invention, although permanent moulds can be used. The mould is designed to
separate in two main sections, namely a drag section and a cope section. The
drag and the cope sections preferably are contained in steel mould support
frames by which the mould sections can be clamped together, such as
hydraulically. The drag section has a cavity in which the substrate is
positionable
and which forms at least part of the mould cavity. The drag section may have a
sprue well into which molten alloy is received from the feed sprue, while it
also
may have at least one lateral runner: The cope section has the bottom part of
the
feed sprue, while it may have a cavity which forms part of the mould cavity
and in
which the cladding is cast. The cope section also may have the lateral series
of
gates and remote from the feed sprue bottom part and the gates, the cope
section may have a lateral cavity for receiving excess cladding alloy.
The mould sections preferably are able to be clamped together with a
clamping force which, in combination with the mould design, ensures adequate
mould sealing is able to be achieved. Thus, recourse to sealing aids provided
between opposed or mating surfaces of the mould sections can be avoided, with
a saving in time between preheating the substrate and closing the mould in
preparation for casting cladding alloy.
In one suitable arrangement, the draft and cope sections of the mould are
made, in their respective support frames, from a moulding sand and a binder,
such as a sodium silicate binder. A silica sand is suitable, although other
moulding sands such as olivine or zircon sands can be used. To reduce erosion
by molten alloy, critical areas of the runner and gating system may be moulded
from bonded sand, such as silicate bonded sand selected from otivine, zircon
or
chromite sand or, if moulded from silica sand, those areas can be protected by
refractory mould paint. Also, to improve the surface finish of the cast
cladding,
the mould cavity surface of the cope section may be coated with a refractory
mould paint. The support frame for each section may be constructed from fully
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welded mild steel channel sections, preferably with the drag section frarne
including a steel bar passing underneath the sprue well to support the sand
against the force of poured molten alloy.
In the mould of that arrangement, the dimensions of the cavity in the drag
section, particularly in the lateral and longitudinal directions, are
sufficient to allow
for thermal expansion of the substrate. However, when the substrate is
positioned in that cavity, its upper surface preferably is flush with an
opposed,
peripheral, upper surface of the drag section by which the latter is engaged
by a
peripheral, lower surface of the cope section. The cope section, when clamped
to
the drag section, preferably acts to provide a clamping action on margins of
the
substrate, such as detailed later herein.
As indicated, the substrate is preheated prior to the casting of cladding
alloy. It is highly desirable that there be minimum delay between the
completion
of preheating and the commencement of casting, while preheating the substrate
after it is positioned in the drag section cavity is the most practical
option. In
practice, it is not possible to completely uniformly preheat the substrate
and, as a
result, the substrate deforms or buckles, usually by a central region bowing
upwardly but with some lifting at edges also being likely. Casting of cladding
alloy
with the substrate in this form exacerbates deformation or buckling and
further
makes difficult the production of useful bimetallic plate. Also, the
deformation or
buckling can be such as to make difficult the attainment of feature (b)
detailed
above. Thus, the deformation or buckling of the substrate therefore needs to
be
minimised or obviated.
Threaded metal studs welded to the lower surface of the substrate and
restrained by nuts tightened against the drag mould frame can be used to
offset
or prevent deformation or buckling of the substrate. The deformation or
buckling
alternatively can be offset by utilising the force by which the drag and cope
sections of the mould are clamped together, so as to generate compressive
loads
acting to press the substrate to an approximately flat condition. In one
suitable
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procedure for this, a series of laterally spaced, longitudinally extending
metal
strips are tack-welded to the upper surface of the substrate, thus forming
longitudinal channels on the substrate along which the cast alloy is able to
flow.
In still another suitable procedure, a plurality of metal chapiets are tack-
welded to
the upper surface of the substrate in a suitably disposed array. The metal
strips,
which are dimensioned to form channels of a depth corresponding substantially
to
the required cladding thickness, may be of a similar composition to the cast
alloy
and become incorporated therein as part of the cladding. The chaplets, which
have a thickness corresponding substantially to the required cladding
thickness,
also may be of similar composition and become incorporated in the cladding.
On closing the mould and clamping the drag and cope sections together,
the clamping force causes the cope section to engage the strips or chapiets
with
generated compressive forces thereby acting to force the substrate down
against
the drag section. The substrate can be forced into a somewhat flat condition,
but
with minor bowing between successive strips or chaplets. The compressive
forces are such that the substrate is able to be retained substantially in
that
condition during casting of the cladding.
The use of longitudinal strips or of chaplets in a central region of the
substrate, to achieve such somewhat flattened condition, results in edges of
the
substrate being urged downwardly in the drag section cavity. Due to this,
molten
alloy for forming the cladding can be substantially prevented from flowing
under
the substrate. However, it can be beneficial to positively hold down the
substrate
at longitudinal side edges. For this latter purpose, a respective longitudinal
refractory bar, for each of those edges of the substrate, may be moulded into
the
cope section of the mould at a location at which it engages and holds down an
edge of the substrate when the drag and cope sections are clamped together.
Alternatively, where the sand of the cope section has sufficient strength, it
can
overlap and hold longitudinal edges of the substrate when the drag and cope
sections are clamped together.
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Where the mould sections abut at opposed peripheral surfaces as they are
clamped together, the area of contact is sufficient to enable the sand of the
mould
sections to withstand the clamping force. Also, an area of cope sand directly
over
each lateral edge of the substrate, such as by 25 to 30mm, can withstand
compressive forces exerted on it by the bending forces generated in the
substrate
edges due to thermai stresses. However, at longitudinal strips or at chaplets
used to flatten the substrate, the compressive forces per unit area can reach
a
level at which damage to the sand of the cope section can occur. To avoid
this,
the cope section can include ceramic pins, ceramic-tipped metal pins,
longitudinal
1o refractory bars or the like which transfer the compressive forces to the
strips or
chapiets. The pins, bars or the like may be fixed to or engaged with the
support
frame of the cope section, such that the compressive forces are transferred
from
the cope section support frame, to the substrate, via the pins, bars or the
like and
via the strips or chaplets.
Immediately adjacent to the gates, there can be difficulty in holding down
the adjacent lateral edge of the substrate. Consequently, there is a risk of
that
edge of the substrate lifting during casting, and molten metal penetrating
under
the substrate. This risk is high due to thermal gradients from the upper to
the
lower surface of the substrate, caused by the superheated molten metal and its
fast flow rate and the resulting bending forces in the substrate. However, if
chaplets are used to hold down the lateral edge of the substrate adjacent to
the
gates they are likely to be dissolved rapidly by the fast flowing molten metal
unless they are of a sufficient size and/or placed outside the direct metal
stream
emanating from the gates. A similar situation can occur if, rather than use of
chaplets, longitudinal metal strips are used to hold down the substrate unless
the
strips are positioned out of direct alignment with any of the gates so that
little or
no turbulence is created in the metal flow and there is little chance of the
strips
dissolving too quickly. Accordingly, an alternative way is desirable to offset
__ ,___
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deformation or buckling of the substrate resulting in lifting of its lateral
edge
adjacent to the gates.
One suitable way in which to restrain lifting of the lateral edge of the
substrate is to bend the substrate so as to cause the lateral edge to be
forced
down onto the drag mould sand. Another suitable way to restrain the lateral
edge
is to weld a strip of steel to the underside of substrate along that edge. A
suitable
strip, such as of mild steel, may for example be about 25x6mm in cross-section
and welded on edge for a substrate of about 10mm thick. The strip is
accommodated in a correspondingly positioned lateral groove in the drag
section
at which the depth of the drag section cavity is increased. During casting,
location of the strip in that groove prevents penetration of molten alloy
beneath
the edge of the substrate.
For use in the present invention, there may be a casting station providing
solid support for the drag section of the mould, means for convenient
manipulation of a preheat furnace, and means for accurate placement and
clamping of the cope section in relation to the drag section on completion of
a
preheat cycle for a substrate. At the casting station, there may be a support
structure mounted on a solid support surface, with the drag section resting on
or
secured to the support structure by its frame. Adjacent to the support
structure,
there is means for pouring molten alloy for casting the cladding. This may be
a
ladle into which the alloy is received from a nearby furnace. However, it is
preferred that the furnace is adjacent to the support structure and is adapted
for
pouring the molten alloy into the mould. The furnace may for example be an
induction tilt furnace.
The cope section of the mould may be supported or mounted so as to be
able to be raised from and lowered to a position in which it is able to be
clamped
to the drag section, as required. This movement of the cope section may be by
any suitable device, such as by an overhead hoist, extendible hydraulic
actuators
or the like. The frame of the cope section preferably is provided with rollers
which
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ride on posts of the support structure and thereby guide the cope section in
its
movement.
In its raised position, the cope section may be spaced above the drag
section sufficiently to enable the preheat furnace to be positioned
therebetween. The support structure may include horizontally disposed rails
along which a carriage, which forms part of or supports the preheat furnace,
is
able to travel between a retracted position, and an advanced position in which
the preheated furnace is above the drag section.
The preheat furnace can take a variety of forms, which as a gas
burning preheater, an induction preheater or an electric element preheater.
For trials with 10mm thick substrates about 1950mm long and 1050mm wide,
one form of suitable preheat furnace and a downwardly open stainless steel
shell with 125mm thick low heat capacity insulation to the internal top and
side
surfaces, and helical nichrome alloy wire elements supported by ceramic
tubes. This furnace was connected to a threephase 415V control box and
had a maximum power output of 150kW.
In accordance with an aspect of the present invention, there is provided
a process for the production of composite bimetallic plate, wherein the
process comprises the steps of:
(a) causing a major surface of a substrate plate formed of a first
metal to become oxide-free;
(b) providing a coating over said oxide-free major surface whereby
said major surface is protected against oxidation;
(c) preheating the substrate plate;
(d) positioning the substrate plate in a mold cavity of a mold with
said major surface facing upwardly and horizontally to thereby
fill a lower portion of the depth of the mold cavity;
(e) securing the substrate plate in the mold cavity; and
(f) casting a cladding of a second metal over said major surface of
the substrate plate to form, with the substrate plate, said
bimetallic plate wherein said cladding is cast by pouring, at a
superheated temperature, a melt of the second metal for flow of
the melt into the mold cavity to fill an upper portion of the depth
of the mold cavity,
wherein the securing step (e) secures the substrate plate whereby the
substrate plate is restrained against buckling during the casting
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step (f), and wherein the preheating of the substrate plate in
step (c) and the superheated temperature of step (f) achieve an
overall heat energy balance between the first and second metals
whereby a diffusion bond substantially free of fusion of the major
surface of the substrate plate is achieved therebetween on
solidification of the melt;
and wherein the process further comprises the steps of:
(g) causing the melt poured in step (f):
(i) to flow in at least one elongate runner which extends
along a first edge of the substrate plate, and
(ii) to enter the mold cavity through a series of gates
providing communication between the runner and the
mold cavity along said first edge of the substrate plate,
whereby the melt is at the same pressure at each gate and on
entering the mold cavity forms a laterally extending melt
front along said first edge of the substrate plate; and
(h) causing the melt to fill the upper portion of the mold by
said melt front advancing over said major surface away
from said first edge at a rate which is uniform across the
lateral extent of the melt front, whereby attainment of the
required heat energy balance is facilitated.
In accordance with another aspect of the present invention, there is
provided a molding apparatus for use in producing composite bimetallic plate,
comprising:
a mold having a drag section and a cope section which together
define a mold cavity having a form corresponding to the bimetallic plate
to be produced therein;
at least one elongate runner defined by the mold and extending
along a first end of the mold cavity; and
a series of laterally spaced gates which are defined by the drag
and cope sections of the mold and which provide communication
between the at least one runner and the mold cavity at said first end;
wherein a lower portion of the mold cavity is defined by the drag
section of the mold and has a flat, horizontal support surface which
extends between said first end and a second end of the mold cavity
remote from the first end, and on which a substrate metal plate is
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positionable whereby a major surface of the plate faces upwardly and
is horizontal; and
wherein the apparatus further comprises means for securing a
substrate positioned on said support surface and thereby restraining the
substrate plate against buckling during the casting of cladding thereon.
In order that the invention may more readily be understood, description
now is directed to the accompanying drawings, in which:
Figure 1 is a schematic side elevation of a casting installation used in
trials in accordance with the present invention;
Figure 2 is a top plan view of the installation of Figure 1;
Figure 3 s a part end elevation/sectional view of the installation of
Figure 1;
Figure 4 is a side elevation of an alternative component of the
installation of Figure 1;
Figure 5 is a plan view of the alternative component of Figure 4;
Figure 6 is a plan view of a drag mould frame of the installation of
Figure 1;
Figure 7 is a side elevation of the frame of Figure 6;
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Figure 8 is an end elevation of the frame of Figure 6;
Figures 9 to 11 are similar to Figures 6 to 8 but show a cope frame;
Figure 12 is a schematic plan view of a general form of mould for the
installation of Figure 1;
Figure 13 is an end elevation of the mould of Figure 12;
Figure 14 is a sectional view taken on line A-A of Figure 12;
Figure15 is a schematic end representation of the runner and gate system
of the mould of the installation of Figure 1;
Figure 16 is a schematic plan representation of the system of Figure 15;
Figure 17 corresponds to Figure 12, but shows detail of a mould used in
trials with the installation of Figure 1;
Figure 18 is a sectional view on line X-X of Figure 17; and
Figure 19 is a sectional view on line Y-Y of Figure 17.
With reference to Figure 1, the casting installation 10 has a support
structure 12 formed of welded steel members and bolted in a concrete base 14.
At a casting station 16, structure 12 has secured therein the drag section 18
of a
mould 19. Above station 16, structure 12 also is engaged by the cope section
20
of the mould 19, while adjacent structure 12 at station 16 installation 10
inciudes
a melt furnace 22. Drag section 18 rests on structure 12 at a fixed location.
However, the cope section 20 is supported by the chain system (not shown) of
an
overhead crane (also not shown), such that cope section 20 can be moved
between the elevated position shown in Figure 1, and a lower position in which
it
can be clamped to drag section 18 to close the mould 19 for a casting
operation.
in its movement, cope section 20 is guided by being provided with rollers (not
shown) which run on guide rails sections of posts (also not shown) of
structure
12.
Installation 10 also includes a preheat furnace 24 which is adjustably
mounted on support structure 12. For this mounting, structure 12 has a
laterai(y
spaced pair of longitudinal rails 12b which extend from each side of drag
section
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18, beyond the latter in a direction away from melt furnace 22. The
preheat furnace 24 is mounted on a carriage 28 by means of hydraulic
actuators 29, with carriage 28 having rollers 30 by which it runs on rails
12b, such that furnace 24 is movable from the retracted position shown in
solid line in Figure 1 to a position shown in broken outline in Figure 1 in
which it is between mould sections 18 and 20, closely positioned over
drag section 18 (assuming that cope section 20 is in its elevated position).
As shown most clearly in Figure 3, the preheat furnace 24 has a
housing 24a in the form of an inverted trough which therefore is
downwardly open. The housing preferably is of stainless steel and has
lateral and longitudinal extents greater than that of the substrate S.
The interior surfaces of housing 24a are lined with low heat capacity
insulation 24b, while a longitudinal array of laterally extending
resistance heating elements 24c is mounted in housing 24a. The
elements 24c may, for example, comprise helical nichrome alloy wires
supported on ceramic tubes and adapted to be heated by power from a
suitable electric power source (not shown).
The mould has respective sand mould parts 18a and 20a, of the
drag and cope sections 18 and 20, as shown in Figures 12 to 14. The
parts 18a and 20a are formed in a welded steel drag support frame 18b
(see Figures 6 to 8) and welded steel cope support frame 20b (see
Figures 9 to 11), respectively. As seen most clearly in Figures 12 to 14,
the drag mould part 18a has a large rectanguiar cavity 34 in which a
substrate S is positionable. Cavity 34 has a depth corresponding to
the substrate thickness, and longitudinal and lateral dimensions sufficient
to accommodate the substrate S and provide a clearance 36 allowing for
thermal expansion of substrate S.
At the end nearer to furnace 22, and adjacent to an end of cavity 34,
drag mould part 18a has a sprue well 38 and, to each side of well 38, a
respective lateral runner 40 (shown also in Figures 15 and 16). At the
same end of cope mould part 20a, there is a bottom feed sprue part
42, with an enlarged upper end 62, which is vertically aligned with sprue
well 38 and, to each side of sprue 42, there are four gates 44. Part
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20a also has a large rectangular cavity 46 which has a depth which may be
similar to that of cavity 34, depending on the required cladding thickness for
substrate S. However, cavity 46 is of less lateral width than cavity 34 and,
at its
end nearer to furnace 22, cavity 46 extends beyond 'cavity 34 form a galley
portion and to achieve communication with each gate 44. At the other end of
cavity 46, part 20a has an enlarged overflow damping cavity 47 which is over
the
end of substrate S.
The drag section. 18 of the mould is mounted or rests on. support structure
12 such that its upper surface and, hence, substrate S is at a small angle to
the
horizontal. Specifically, as is evident in Figure 1 the arrangement is such
that
substrate S is inclined upwardly from its end adjacent to furnace 22 to its
remote
end at an angle of a few degrees, such as up to about 5 , for example, about
30.
The cope section 20 may be similarly inclined or, altematively, it may be
substantially horizontal but adjustable when lowered onto section 18 so as to
become similarly inclined, thereby facilitating closing of the mould. Also,
the
actuators 29 which support furnace 24 above carriage 28 are able to hold
fumace
24 at an angle to the horizontal such that furnace 24 is substantially
parallel to
substrate S, while actuators 29 can enable variation in the height of furnace
24
above carriage28, as may be required, such as to lower furnace 24 to a
required
spacing above substrate S.
As indicated above, the sand mould parts 18a and 20a, of drag and cope
sections 18 and 20 of mould 19, are formed on respective welded steel frames
18b and 20b. As shown in Figures 6 to 8, frame 18b has a lower series of
lateraliy spaced, longitudinally extending C-section channels 48a having their
webs uppermost. On the channels 48a, frame 18b has an upper series of
longitudinally spaced, laterally extending C-section channels 48b which also
have
their webs uppermost. Around the rectangular grid formed by channels 48a and
48b, frame 18b has a rectangular perimeter provided by C-section channels 48c.
The channels are securely welded together at junctions therebetween, while the
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upper flange of each channel 48c has openings formed therein, at intervals
along
its length.
As shown in Figures 9 to 11, cope frame 20b is somewhat similar to drag
frame 18b, with upper channels 49a and lower channels 49b corresponding to
channels 48a and 48b, respectively and peripheral channels 49c corresponding
to channels 48c.
As indicated, drag and cope sections 18 and 20 need to be strongly
clamped together on closing the mould, to seal the interface between sections
18
and 20 against molten alloy leakage, while clamping needs to be achieved
quickly
to minimise neat loss. For this, clamping devices of a number of forms can be
used. However, the preferred form is that of device 70 shown in Figure 9, with
there being a respective device 70 at each of a number of locations around the
periphery of the mould. Each device is mounted on a respective bracket 71
welded at intervals along each channel 49c of frame 20b. Each device 70
comprises a hydraulic swing clamp, such as type SU(UR)S 201 available under
the trade mark ENERPAC, providing about 18.8kN clamping force at about
35MPa oil pressure. These devices have a cylinder body 72 mounted on the
support frame 20b of cope section 20, and a depending piston rod 74 extending
from body 72. Hydraulic pressure lines (not shown) supply oil to body 72 to
enable rod 74 to be extended and retracted relative to body 72. Engagement
between rod 74 and its body 72 is such that rod 74 rotates in one or other
direction as it is extended or retracted.
Below each device 70, the support frame 18b of drag section 20 has a
respective one of the above-mentioned openings (not shown) cut-out from the
upper flange of a respective channel 48c . The size of each opening is such
that,
as cope section 20 is lowered onto drag section 18 with rod 74 extended, the
rod
74 and an eccentric collar 75 secured on rod 74 passes through the opening .
The rod 74 then is able to be retracted and, in simultaneously rotating, its
collar
75 is engaged below the flange from which opening is cut-out. Thus, the drag
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and cope sections 18 and 20 are able to be strongly clamped together, under
the-
simultaneous action of several devices 70.
When the mould is closed, it is required that parts 18 and 20 be clamped
together, to achieve a seal between opposed surfaces around cavities 34 and 46
which substantially prevents the leakage of molten metal therebetween. The
clamping preferably is able to achieve this by sand-to-sand surface contact
between mould sections 18 and 20, without the need for application of a
sealing
aid.
With mould section 20 raised, substrate S is positioned in cavity 34. Prior
to this, at least the upper surface of substrate S is treated, to remove all
oxide.
This may, for example, be by sand, grit or shot blasting, use of a wheel or
belt
abrader or by pickling. When the cleaned substrate S has been positioned in
cavity 34, its upper surface is protected by a flux coating, such as provided
by flux
comprising a flux powder, a liquid flux or a flux powder in a liquid
suspension.
The flux is to substantially prevent re-oxidation of substrate S and, if
required,
other means detailed herein can be used instead of flux. The preheat furnace
24
then is moved along rails 12b to its position over drag section 18 for heating
of
substrate S to a sufficient preheat temperature.
The preheat furnace 24, as will be appreciated, is to apply heat energy to
raise the temperature of the substrate S to a level sufficient, in combination
with
superheating of the molten alloy in melt furnace 22, to achieve required
bonding
with cast cladding alloy. While furnace 24 preferably is an electric element
heater
such as described above, it could be a gas heating or induction furnace
Before detailing a cycle for casting cladding, it will be appreciated that
preheating of substrate S by furnace 24, such as to about 750 C, will result
in
thermal stresses in substrate S and its resultant deformation. Also, casting
molten alloy onto substrate S, by pouring alloy into a mould cavity comprising
cavities 34 and 46, increases the thermal stresses and deformation. In the
arrangement as generally described to this stage, the deformation would
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substantially preclude the production of a useful bimetallic product. A number
of
further features need to be utilised, in combination with the inclination of
the drag
section 18 and substrate S, and the disposition of runners 40 and gates 44, in
order to produce such product.
As shown, the base 40a of each runner 40 is stepped upwardly after each
gate 44, such that the cross-section of each runner 40 decreases laterally of
sprue well 38. Particularly under the pouring conditions detailed below, the
form
of each runner is such that substantially the same pressure and flow-rate of
molten metal passes to and through each gate 44. The resultant separate
1o streams of molten metal passing through gates 44 very quickly form into a
single
stream and tend not to give rise to non-uniform longitudinal flow of molten
metal
along substrate S. Avoidance of such non-uniform flow also is facilitated by
the
inclination of substrate S, since the flow of molten metal along the substrate
is
against the action of gravity. Rather, there is generated a melt front which
preferably is substantially uniform laterally of substrate S and which moves
substantially in that form iongitudinally along substrate S.
To offset the effect of thermal stresses at the lateral edge of substrate S
nearer to furnace 22, a steel strip 50, such as about 25x6mm in cross-section,
is
welded on edge across the lower surface of substrate S, at that edge. A
corresponding lateral channel 52 is formed in drag mould part 18a, at the
corresponding end of cavity 34 such that, with substrate S positioned in
cavity 34,
strip 50 is neatly accommodated in channel 52. Deformation of substrate S
immediately adjacent gates 44 is substantially prevented by the provision of
strip
50 with leakage of molten alloy under substrate S at that edge substantially
being
prevented. Leakage is further restrained by provision of a ceramic fibre seal
or
the like in channel 52, below strip 50. Also, a layer of ceramic fibre paper
may be
provided in cavity 34 below substantially the full area of substrate S if the
preheat
furnace capacity is low, as such insulation under the substrate can assist in
reducing the time required for preheating substrate S.
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As will be appreciated, the provision of strip 50 is but one suitable
arrangement for preventing deformation or buckling of substrate S at its
lateral
edge nearer to furnace 22. As detailed above, alternatives for achieving that
end
include the use of chapiets or longitudinal strips on the upper surface of
substrate
S, or threaded metal studs welded to the underside of substrate S.
Alternatively,
use can be made of appropriate mould design enabling the lateral edge of
substrate S to be forced onto the drag mould by the sand of the cope mould.
As indicated above, cavity 46 in cope mould part 20a is of lesser lateral
extent than cavity 34 in drag mould part 18a. The extent of this difference is
greater than thermal expansion clearance 36 and, as a consequence,
longitudinal
margins S' of substrate S are engaged by overlapping areas of cope mould part
20a when the mould is closed. At least for a major part of this overiap, part
20a
may be provided with a refractory ceramic insert strip 54. The arrangement of
the
strips 54 is such that with the drag and cope sections clamped together, each
strip 54 is forced downwardly on a respective substrate margin S'. The force
necessary for closing the mould to seal against leakage of molten metal is
sufficient to cause strips 54 to hold margins S' substantially flat and
thereby
prevent significant leakage of molten metal under substrate S via those
margins.
However, ceramic strips 54 need not be provided, as their function can be
obtained with cope sand overlapping margins S' where the strength of the cope
sand is sufficient to hold margins S' substantially flat.
Controlling deformation of substrate S so as to prevent leakage of molten
metai under its edges is important in achieving production of a useful
bimetallic
plate. However, a good degree of uniformity of thickness for the cladding also
is
important, particularly in the central region of the substrate where upward
bowing
of the substrate often is severe. To at least reduce such deformation of the
central region, suitable spacing means of a suitable alloy are provided over
the
upper surface of the substrate, and retained such as by tack welding. In the
arrangement shown, the means comprises an array of circular chaplets or discs
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56 each having a thickness corresponding to that required for the cladding. On
clamping the drag and cope sections together, compressive forces on discs 56
act to press substrate down into cavity 34 so that the substrate assumes a
somewhat flat condition. Upward bowing of substrate S can still occur between
successive discs 56, but this is relatively minor and its extent can be
controlled by
the spacing between discs 56. As shown, discs 56 can be used over the central
region of substrate S, as well as along its lateral edge remote from furnace
22.
For forming cast cladding on preheated substrate S, to produce a
bimetallic plate, molten alloy at a suitable superheated temperature is poured
from furnace 22 into the mould, to fill cavity 46. It is highly desirable that
cavity 46
be filled quickly. This is to ensure an overall heat energy balance, resulting
from
preheating substrate S and the superheating of the molten alloy, is maintained
at
a suitable level until filling of cavity 46 has been completed, to thereby
obtain
required bonding between the cladding and substrate S over substantially the
entire interface therebetween. To enable rapid filling,of cavity 46, a pouring
basin
is mounted on cope section 20.
In Figure 1, there is shown a pouring basin 58 used for initial trials in
producing bimetallic plate of about 600x600mm with a substrate and cladding
thickness each of 10mm. Basin 58 is mounted in relation to cope section 20 by
means of an upper feed sprue part 59 which provides communication between
the interior of basin 58 and bottom feed sprue part 42 of cope section 20.
Basin
58 and upper sprue part 59 are raised and lowered with cope section 20. With
section 20 lowered onto and clamped to drag section 18, basin 58 is positioned
for receiving molten alloy from melt furnace 22, as the latter is titled
forwardly, i.e.
over basin 58.
Operation with basin 58 and sprue part 59 generally is satisfactory for
producing bimetallic plate up to about 600x600mm in size. However, for such
piate, it was found desirable to adopt an arrangement as shown in Figures 4
and
5, with that arrangement being necessary for plate of larger sizes. The
arrangement of Figures 4 and 5 includes a pouring basin 58' and in upper feed
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sprue part 59'. The important differences between basin 58' and sprue part 59'-
of
Figures 4 and 5 and basin 58 and part 59 of Figure 1 are:
(i) a reduction in the height of part 59' and a corresponding increase in
the height and internal volume of basin 58';
(ii) the more central location of the outlet of basin 58' to sprue part 59';
and
(iii) the provision of a top on basin 58', such that with furnace 22 tilted to
pour molten alloy into basin 58', the latter is substantially closed around
the spout
of furnace 22.
As a consequence of these differences, it is possible to essentially dump
into basin 58' substantially the full quantity of molten alloy required for
the cast
cladding for a bimetallic plate of a suitable size. Also, molten alloy is able
to flow
from basin into mould 19, via sprue part 59', at a higher flow rate due in
large part
to the more direct through-flow possible with basin 58'. Thus, a melt front of
molten alloy formed on the substrate S in the mould 19 is able to advance
across
substrate S at a higher rate, enabling completion of casting within a period
of time
in which a heat energy balance consistent with uniform bonding can be
maintained.
As will be appreciated, dumping of molten alloy into basin 58' enabies a
melt front to be quickiy generated in mould 19. Also, the melt front is able
to
commence quickly to advance across substrate S. Thus, minimum time and,
hence, minimum heat energy, is lost between commencing pouring and initiating
a suitable flow of molten alloy across substrate S. This benefit combines with
other factors enabled by installation 10, in that, after preheating substrate
S by
furnace 24, the latter can be retracted quickly along rails 12b, and cope
section
20 then is able to be lowered and clamped to drag section with minimum delay.
Thus, from completion of preheating through to completion of casting, loss of
heat
energy is able to be minimised.
As shown in Figures 4 and 5, the pouring basin 58' is of rectangular block
form. It has an outer shell 60 of steel plate and an internal refractorv liner
61. in
CA 02328306 2007-05-03
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its lower half, the internal surfaces of liner 61 converge to an outlet which
leads to
sprue part 59, basin 58' having an interior somewhat similar to a hopper of
rectangular section.
The furnace 22 is an induction furnace for melting cladding alloy, and is
tiltable to enable its molten alloy charge to be poured into basin 58'. In
use, the
molten charge is dumped into basin 58' such that the pressure head of molten
alloy held therein provides a steady, but strong, driving force for filling
cavity 46. In
the case of the Figure 1 arrangement, basin 58 has an open top 64 of elongate
rectangular form to define a chamber 66 which is between sprue part 59 and
io furnace 22 and is separated from sprue part 59 by a lateral ridge 68. The
melt is
poured, rather than dumped, and enters basin 58 at its chamber 66, while ridge
68 acts to prevent undue turbulence in the melt as it flows to fill sprue
parts 59
and 42 and as its level rises above ridge 68 in basin 58.
In Figures 17 to 19, there is shown detail of a mould 119 used in trials, with
the installation of Figure 1, producing bimetallic plate of 1800x1000x10mm on
10mm, i.e. plate 1800x1000mm in area having 10mm of cast cladding bonded to a
10mm thick substrate. In Figures 17 to 19, components corresponding to those
of
Figures 12 to 14 have the same reference numerals plus 100. However,
description is essentially limited to matters by which mould 119 differs from
mould
19 of Figures 12 to 14.
Mould 119 has a drag section part 118a and a cope section part 120a of
bonded sand. While not shown in Figures 17 to 19, each part 118a and 120a is
formed in a respective steel support frame as shown in Figures 6 to 8 in the
case of
part 11 8a and Figures 9 to 11 in the case of part 120a.
The cavity 134 in mould part 118a has a lateral dimension of about
1120mm which is about 20mm greater than the initial lateral dimension of
substrate S, to leave an expansion clearance 136 at each side of substrate S
of
about 10mm. Similarly, while substrate S has an initial longitudinal extent of
about 1950mm, that of cavity 134 is about 1970mm so that a clearance 136 of
CA 02328306 2007-05-03
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about 20mm is provided at the end of substrate S remote from furnace 22
(Figure 1) and bottom feed sprue part 142. Again, parts 118a and 120a are
clamped together to achieve a seal by sand to sand contact therebetween. For
this, and to prevent substrate S from lifting at its edges, the lateral width
of cavity
146 of cope part 120a is about 1050mm, so that respective side margins S'
of substrate S, which initially are of about 25mm wide, are held down by
overlapping surface areas 141 of cope part 120a. Also, rather than
provide chaplets along the end of substrate S remote from furnace 22, an
end margin S" of substrate S is similarly held down by an overlapping
io surface area of cope part 120a. Margin S", also initially about 25mm wide,
results from the longitudinal extent of cavity 146 being about 1925mm,
compared with about 1950 for the initial extent of substrate S (and,
allowing for end clearance 136, compared with a longitudinal extent of
about 1970mm for cavity 134 in drag part 118a.)
As seen in Figures 17 to 18, there are two gates 144 to each side of
sprue 142 by which molten alloy is able to flow from each runner 140.
Again, each runner 140 is progressively reduced in depth after each
gate 144 so as to substantially equalise the melt pressure and flow rate
through each gate 144.
At the end of mould 119 remote from furnace 22, cope part 120a
again defines an overflow damping cavity 147 which is over the corresponding
end of substrate S. However, a comparison of Figures 14 and 19 shows a
difference between respective moulds 19 and 119. In mould 19, cavity 47 is
positioned such that it straddles the end edge of substrate S. In contrast, in
mould 119, cavity 147 is above substrate S and is spaced from that edge by
margin S". In Figure 14, cavity 47 is shown simply as a downwardly open
lateral channel in cope part 120a, although venting through part 20a is
desirable. In Figure 19, cavity 147 again is shown as a downwardly
open, lateral channel, such as about 115x115mm in the sectional view
of Figure 19, although cavity 147 opens through cope part 120a by provision
of three vents 147a along its length.
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As indicated, mould 119 holds substrate S down at two margins S' and at a
further margin S". As also shown, the lateral edge of substrate S adjacent to
furnace 22 and sprue 142 is provided with a lateral strip 150 which is located
in a
lateral channel 152 formed in drag part 118a. While not shown, means need to
be provided to prevent deformation of substrate S inwardly of its edges, and
such
means can comprise alloy strips or chaplets as detailed above.
Trials have been conducted with an installation as in Figure 1, using a
mould as in Figures 17 to 19 which incorporated a support frame as in Figures
6
to 8 and a support frame as in Figures 9 to 11. In these trials, the mould was
1o arranged so that it was inclined upwardly from furnace 22 at an angle of
about 3 .
The substrates, each comprising 10mm thick wrought 250 grade, low carbon steel
plate initially, were 1050mm wide and 1950mm long. The alloy used for forming
the cladding, to a thickness of 10mm on each substrate, was a 15/3 Cr-Mo high
chromium white iron of near eutectic composition, suited for forming a wear-
resistant overlay material.
The substrates were prepared by grit blasting the top surface of each, that
is the surface with which the cladding was to be bonded. The blasted surface
of
each substrate, substantially free of oxide, then was painted with a
suspension of
a commercial copper and brass flux available from CIGWELD, to protect the
substrate from oxidation during preheating and to promote formation of a
diffusion
bond. Also, the bottom surface of each substrate was painted with a zirconia-
based mould wash to prevent bonding between the substrate and any cast alloy
penetrating underneath the substrate.
Before the substrates were subjected to blast cleaning, a 25x6mm steel
strip was welded on edge to the bottom surface of each substrate, across its
front
edge, i.e. the lateral edge to be nearer to furnace 22. This was to reduce the
risk
molten alloy penetration below the substrates during casting. Also, buckling
control means were provided over the upper surface of each substrate. In the
case of a first series of substrates, the control means comprised three 10x3mm
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steel strips tack-welded on to the upper surface of each substrate, to form
four
distinct longitudinal channels of the same lateral width, along which cast
molten
alloy could flow. In a second series of substrates, such strips were not used;
rather, the control means comprised for each substrate 24 discs of high
chromium
white cast iron chaplets, 25mm in diameter and 10mm thick, which were spot
welded to the substrate in a uniform array. In each case, the control means
was
to ensure buckling of the substrate was restrained and such that it could not
disturb the flow of molten alloy to an extent such that all of it would run
over one
area of the substrate without wetting another area. -
For each trial, about 260 kg of hypereutectic high chromium white cast iron
was melted in the induction tilt furnace and heated to between 1600 C and
1650 C. This represents a superheat of about 350 C. The melt composition was
adjusted as appropriate during the melting cycle and a final spectro sample
was
taken just prior to casting.
During the melting procedure a substrate was positioned in the mould drag
section and preheated to a temperature of about 750 C. At this preheat
temperature the flux is liquid, wets the substrate and greatly reduces
oxidation,
although the time that the substrate remains at that temperature before
casting
the white cast iron should be kept to a minimum. Since it is a physical
impossibility to have a completely uniform temperature throughout the
substrate
during preheat, with the edges being cooler than the centre of the substrate
and
the top surface being hotter than the bottorn, the substrate will bow up and
buckle somewhat. Therefore, the substrate is allowed to soak for about ten
minutes after the preheat temperature has been reached, which allows the
temperature to equalise somewhat and bowing is reduced. The preheat cycle is
timed such that when the substrate is fully preheated, the liquid metal is at
the
correct superheat temperature and available for casting.
On completion of preheating, the preheat furnace is switched off, lifted and
moved out of the way. The mould is closed by lowering the cope and
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hydrauiically clamping the mould sections. The liquid metal is then
immediately
poured and caused to flow over the substrate. The whole operation of preheat
furnace removal, mould closure and pouring needs to be relatively quick to
minimise heat loss. The operation desirably takes less than one and a half
minutes, such that the temperature drop in both the preheated substrate and in
the melt are quite small. Pouring of the 260 kg of metal is done in only a few
seconds to ensure a fast flow rate of the liquid metal over the substrate
surface.
After casting, the mould is left clamped for about 30 minutes to allow
sufficient solidification in the runner and overflow cavities. The cope is
then lifted
1o off and the casting is allowed to cool further. When cold, the bimetallic
plate is
removed from the mould, the gates and the excess metal at the back of the
plate
are cut off and the plate cleaned. Also, as the cladding does not extend over
margins of the substrate by which the substrate is clamped between the mould
sections, such margins also are cut-off to provide a bimetallic plate which is
1800x1000mm in area and which has a thickness of 10mm of cladding of white
iron on 10mm thick substrate steel.
In forming the mould cope section for initial trials, fast-response type R
and bare-tip type K thermocouples were installed in the cope mould so that
they
extended through the sand into the overlay cavity. The type R thermocouples
were used to measure the cast metal temperature above the substrate after
casting and the function of the type K thermocouples is to measure the flow
speed and flow pattern of the cast metal. During the course of the
experimental
program it was found that the response time of the type R thermocouples was
almost identical to that of the type K thermocouples and only type R
thermocouples were used after that.
The bimetallic plate produced by the trials was found to be of excellent
quality. While some plates were found to be slightly curved on cooling, this
curvature was such that it couid be removed. The white iron cladding was found
to be substantially defect free and to have a good degree of uniformity in its
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thickness. Also, the cladding was found to have achieved a sound diffusion
bond
with the substrate characterised by a narrow bond zone exhibiting
substantially
no evidence of fusion of the substrate. Also, the control means were similariy
incorporated in the cladding layer.
The trials indicate that to produce large bimetallic plate of good quality, it
is
necessary that:
(a) To achieve good bonding everywhere, in the case of providing
cladding of high chromium white cast iron on a steel substrate, the
temperature in the melt should not be allowed to drop below about
1400 C at any position in the mould as it flows over the substrate,
with the substrate at a suitable preheat temperature.
(b) The cast metal must flow substantially evenly over the whole of the
substrate surface.
(c) To avoid the use of excessively high superheat temperatures in the
melt, pouring must be fast.
(d) Preheat furnace removal and mould clamping has to be done very
quickly to minimise heat loss from the preheated substrate and from
the melt.
(e) To save time, mould sealing must be achieved without the use of
external sealing aids.
Finally, it is to be understood that various alterations, modifications and/or
additions may be introduced into the constructions and arrangements of parts
previously described without departing from the spirit or ambit of the
invention.
