Casting system

Abstract

Provided is a system for casting molten metals. The system includes a mould comprising a casting cavity having an inlet, and a bore between an upper surface of the mould and the inlet. The system further includes a shroud comprising a funnel and a hollow shaft, wherein the funnel is located outside of the mould, adjacent the upper surface, and the hollow shaft is received within the bore and is moveable therein. A lifting mechanism is located on the upper surface of the mould, the lifting mechanism being operable to lift the funnel of the shroud away from the upper surface for bringing the shroud into engagement with a ladle nozzle. Also provided is a method for casting molten metals using the system.

Claims

1. A system for casting molten metals comprising: a mould comprising a casting cavity having an inlet, and a bore between an upper surface of the mould and the inlet; a shroud comprising a funnel and a hollow shaft, wherein the funnel is located outside of the mould, adjacent the upper surface, and the hollow shaft is received within the bore and is moveable therein; and a lifting mechanism located on the upper surface of the mould, the lifting mechanism being operable to lift the funnel of the shroud away from the upper surface for bringing the shroud into engagement with a ladle nozzle.

2. The system of claim 1, further comprising a rotating mechanism for rotating the shroud relative to the mould.

3. The system of claim 1, wherein the lifting mechanism is further operable to rotate the shroud relative to the mould.

4. The system of claim 3, wherein the lifting mechanism enables rotation of the shroud to be effected independently of lifting of the shroud.

5. The system of claim 1, wherein the lifting mechanism comprises a first part which is mounted on the surface of the mould, and a second part which supports the funnel of the shroud, wherein the second part is moveable relative to the first part.

6. The system of claim 5, wherein the position of the first part is fixed relative to the mould and wherein the second part is moveable between a first position, in which the shaft is substantially received within the bore of the mould, and a second position, in which a portion of the shaft is lifted out of the bore.

7. The system of claim 5, wherein the position of the first part is moveable relative to the mould, the first part being moveable between a first position, in which the shaft is substantially received within the bore of the mould, and a second position, in which a portion of the shaft is lifted out of the bore.

8. The system of claim 7, wherein the lifting mechanism comprises a third part disposed between the first part and the surface of the mould.

9. The system of claim 1, wherein the lifting mechanism comprises a mechanical, hydraulic or pneumatic actuator or a motor.

10. The system of claim 1, wherein the lifting mechanism comprises a cylindrical cam.

11. The system of claim 10, wherein the lifting mechanism comprises concentric inner and outer collars, and wherein one of the inner and outer collars supports the funnel of the shroud and has a follower which rests on a ramped surface of the other of the inner and outer collars which is mounted on the upper surface of the mould, such that relative rotation of the inner and outer collars causes linear motion of the shroud, and optionally wherein multiple ramped surfaces are provided, each ramped surface extending over a portion of the inner or outer collar in a circumferential direction.

12. The system of claim 11, wherein the shroud is seated on the inner collar, the inner collar having a follower which rests on a ramped surface of the outer collar.

13. The system of claim 11, wherein the lifting mechanism further comprises a handle for effecting relative rotation of the inner and outer collars, optionally wherein the handle is attached to or constitutes the follower.

14. The system of claim 1, further comprising one or more filters located between the bore and the inlet of the casting cavity.

15. The system of claim 14, wherein the one or more filters are located within a housing which is connected to the bore and which receives an end of the shroud, optionally wherein the system further comprises a running system between the housing and the inlet of the casting cavity.

16. The system of claim 15, wherein the housing contains an impact pad, and optionally wherein at least one outlet is provided in the shaft adjacent to the end of the shroud, and wherein the impact pad comprises at least one pillar having a surface which abuts the shaft, such that the shroud is rotatable between a position in which the pillar is aligned with the outlet so as to close the outlet and prevent metal flow therethrough, and a position in which the outlet is at least partially open.

17. A method of casting molten metals comprising the steps of: providing a system according to claim 1; positioning a bottom pour ladle containing molten metal over the mould such that a nozzle in a base of the ladle is located substantially vertically above the funnel of the shroud; operating the lifting mechanism so as to lift the funnel of the shroud away from the upper surface of the mould to bring the shroud into engagement with the nozzle; opening the nozzle, thereby allowing molten metal to flow from the ladle into the shroud; closing the nozzle to stop the flow of molten metal; and operating the lifting mechanism so as to lower the funnel of the shroud towards the upper surface of the mould to disengage the shroud from the nozzle.

18. The method of claim 17, wherein operating the lifting mechanism so as to lift the funnel of the shroud also effects rotation of the shroud relative to the mould.

19. The method of claim 17, wherein operating the lifting mechanism lifts the funnel of the shroud without rotating the shroud relative to the mould, and wherein the method further comprises the step of, after opening the nozzle, rotating the shroud.

Description

(1) Embodiments of the invention will now be described with reference to the accompanying drawings in which:

(2) FIG. 1 is a perspective view of a casting system in accordance with the first aspect of the invention, wherein the funnel of the shroud is not engaged with the ladle nozzle;

(3) FIG. 2 is an exploded perspective view of the housing of FIG. 1 showing the individual components;

(4) FIG. 3a and FIG. 3b are perspective views of the two components comprising the lifting mechanism of FIG. 1;

(5) FIG. 4 is a perspective view of the casting system of FIG. 1, wherein the funnel of the shroud is engaged with the ladle nozzle;

(6) FIG. 5a and FIG. 5b are cross-sectional views showing the connection between the ladle nozzle and the shroud of FIG. 1 in a situation where there is displacement between the mould and the ladle;

(7) FIG. 6 is a perspective view of a third component of a three-part lifting mechanism in an alternative embodiment of the invention;

(8) FIG. 7a is a cross-sectional view of a three-part lifting mechanism and a funnel of a shroud supported thereby in an alternative embodiment of the invention;

(9) FIG. 7b and FIG. 7c are perspective views of the lifting mechanism of FIG. 7a in different stages of rotation. FIG. 7b shows the mechanism prior to rotation, while FIG. 7c shows the mechanism after rotation;

(10) FIG. 8a is a perspective view of an end of a shroud in accordance with an embodiment of the invention;

(11) FIG. 8b is a perspective view of an impact pad for use with the shroud of FIG. 8a; and FIG. 8c and FIG. 8d are perspective views of the shroud of FIG. 8a assembled with a housing and the impact pad of FIG. 8b, showing the transition between the partially open (FIG. 8c) and fully open (FIG. 8d) positions.

(12) With reference to FIG. 1, an embodiment of a casting system 10 according to the invention comprises a mould 12 in which is formed a casting cavity 14. The mould is comprised of an upper part 12a and a lower part 12b joined horizontally at a parting line 13. The casting cavity 14 is bottom-fed via two inlets 16. Molten metal is supplied to the casting cavity 14 through a shroud 20, which prevents re-oxidation of the metal by protecting it from the atmosphere. The shroud 20 comprises a funnel 22 into which molten metal is poured, and an elongate hollow shaft 24 which feeds the metal into the casting cavity 14. The funnel 22 is located outside of the mould 12, so that it can engage with a nozzle 26 of a ladle (not shown) when in use. The shaft 24 of the shroud 20 is received within a bore 30 formed in an upper surface 32 of the mould 12, and extends substantially perpendicularly thereto. The bore 30 is sized to receive the shroud 20 such there is substantially no gap therebetween while still allowing linear movement of the shroud 20. In fluid communication with the casting cavity 14 is an open feeder sleeve 15, which extends between the casting cavity 14 and the upper surface 32 of the mould 12.

(13) The bore 30 extends between the upper surface 32 and a housing 34, located within the mould 12. The housing 34 is cuboid in shape, comprising four prism shaped sections fixed together and having an upper wall 36, a lower wall 38 and four side walls 40. The housing 34 may be made from a suitable refractory material, such as fused silica. The shaft 24 of the shroud 20 passes through an opening 42 in the upper wall 36, such that an end 44 of the shroud 20, opposite the funnel 22, is received within the housing 34. Two of the four side walls 40 have an outlet 46 therein. A filter (not shown) may be located adjacent each of the outlets 46, such that molten metal passes through the filters when it exits the housing 34.

(14) A running system 48 comprising a pair of conduits 50 extends sidewardly from the filter housing 34, with one conduit 50 leading from each of the outlets 46. The conduits 50 bend upwardly to join the inlets 16 of the casting cavity 14, each conduit 50 feeding into a separate one of the inlets 16. Thus, a flow path for molten metal is provided downwardly through the funnel 22 and shaft 24 of the shroud 20, into the filter housing 34, through the filters and out of the filter housing 34 through the outlets 46, through the conduits 50 and upwardly into the casting cavity 14.

(15) As stated above, the shroud 20 is linearly moveable within the bore 30 so that it can be lifted upwardly for engagement with the ladle nozzle 26. The shroud 20 is lifted by a lifting mechanism 52 which is located on the upper surface 32 of the mould 12.

(16) FIG. 2 shows the four individual segments 35a, 35b that fit together to form the housing 34. Two of the segments 35a have an outlet 46 in the sidewall 40, while the other two 35b have no outlet. Each of the segments 35a, 35b has a triangular base 39, a sidewall 40 and a triangular shaped roof 37, the roof having a quarter circle (i.e. 90°) cut out 43. When the pieces are fitted together, the four roof segments create the upper wall 36 of the housing and the cut outs 43 create the circular opening 42 through which the shaft 24 of the shroud 20 passes. Similarly, the four triangular bases 39 fit together to create the lower wall 38 of the housing. Two of the housing segments 35a have integrated into an internal surface 40a of the sidewall 40 a raised profiled frame 45 that holds in place a ceramic foam filter 47, such that the centre of the filter 47 is positioned over the outlet 46 in the sidewall 40. In use, the segments 35a, 3b are fixed together by fastening and tightening a metal band (not shown) around the four sidewalls 40 of the housing 34.

(17) With further reference to FIGS. 3a and 3b, the lifting mechanism 52 comprises an inner collar 54 which sits concentrically within an outer collar 56. The inner collar 54 comprises an annular seat 58 surrounded by a circular rim 60. In use, the funnel 22 of the shroud 20 is supported on the annular seat 58, with the shaft 24 of the shroud 20 passing through a central hole 62 in the seat 58. On an exterior surface 63 of the circular rim 60 two pegs 64 are provided and, spaced apart from the pegs 64, a handle 66.

(18) The outer collar 56 comprises a cylindrical wall 68 surrounded by an annular base 70. The base 70 is mounted on the upper surface of the mould 12, in use. During preparation of the mould 12 the outer collar 56 is placed in position and is held in place when the moulding sand cures and hardens. Portions of an upper end 69 of the cylindrical wall 68 are cut away so as to provide three ramped or spiral surfaces 72. In the embodiment shown, each spiral surface 72 extends around approximately 120° of the circumference of the cylindrical wall 68.

(19) When the lifting mechanism 52 is assembled, the pegs 64 and handle 66 of the inner collar 54 rests on the spiral surfaces 72 of the outer collar 56. It can be seen that as the inner collar 54 is rotated using the handle 66, the pegs 64 and handle 66 travel along the spiral surfaces 72, causing the inner collar 54, and thus the shroud 20 supported by the inner collar 54, to be lifted upwardly. The inner and outer collars 54, 56 thus function as a cylindrical cam, with the pegs 64 and handle 66 constituting followers.

(20) In FIG. 1, the inner collar 54 of the lifting mechanism 52 is in a first position, with the pegs and handle at a lowest point on the spiral surfaces 72. In this position, the shroud 20 is lowered such that the shaft 24 extends almost to the bottom of the housing 34, and the funnel 22 is not engaged with the ladle nozzle 26. It can be seen that rotation of the inner collar 54 anticlockwise through an angle of approximately 90° causes the pegs and handle to travel upwardly along the spiral surfaces 72 of the outer collar 56, thereby moving the lifting mechanism 52 to a second position as shown in FIG. 4. In the second position, the inner collar 54 and the funnel 22 seated therein are lifted upwardly, away from the upper surface 32 of the mould 12, and the funnel 22 is brought into engagement with the ladle nozzle 26. The end 44 of the shroud 20, opposite the funnel 22, is lifted away from the lower wall 38 of the housing 34 but remains within the housing 34. It will be therefore be appreciated that the angle through which the inner collar 54 must be rotated will depend on the extent of vertical movement of the inner collar 54 and shroud 20 that is required to bring the funnel 22 into engagement with the nozzle 26, which may vary according to the height of the mould 12 and the positioning of the ladle. The lifting mechanism 52 may be retained in the second position manually during pouring by an operator holding the handle 66 to prevent it from travelling downwardly along the spiral surface 72. However, it will be appreciated that in some embodiments a lock may be provided to hold the lifting mechanism 52 in the second position.

(21) With reference to FIGS. 5a and 5b, perfect alignment between the ladle and the mould 12 may not always be achieved such that there is vertical displacement. In the embodiment shown in FIG. 5a, the longitudinal axis L.sub.1 of the ladle nozzle 26 is offset from the longitudinal axis (L.sub.2) of the shroud 20 by 5°. As shown more clearly in FIG. 5b, a tip 74 of the ladle nozzle 26 is part-spherical, or flat top domed, in shape. The funnel 22 of the shroud 20 has an interior surface 76 that is also part-spherical in shape, with a flat bottom 78 and a curved side 80. The interior surface 76 of the funnel 22 is lined with a gasket 82. The part-spherical shape of the nozzle 26, funnel 22 and gasket 82 ensures that the connection is hermetically sealed even if displacement between the ladle and the mould 12 occurs.

(22) In the embodiments of the invention described above, rotation of the inner collar 54 of the lifting mechanism 52 relative to the outer collar 56 (which remains fixed relative to the mould 12) effects lifting of the shroud 20 for engagement with the ladle nozzle 26, with simultaneous rotation of the shroud 20. However, in alternative embodiments, the shroud may be lifted by rotation of the outer collar such that the inner collar and shroud are not rotated during lifting. To facilitate this, a third component may be provided to give a three-part lifting mechanism. FIG. 6 shows a third component or mounting ring 90 which comprises an annular base 92 surrounded by a circular rim 94. In the embodiment shown, an upper surface 96 of the circular rim 94 has a series of holes 98 therein that extend downwardly though the full height of the rim 94. The holes 98 receive nails or metal pins 99 to fix the mounting ring 90 to a surface 112 of the mould. In use, the inner and outer collars of the lifting mechanism fit concentrically into and on top of the mounting ring 90.

(23) With reference to FIG. 7a, a three-component lifting mechanism 152 comprises an inner collar 154 which sits concentrically within an outer collar 156. In turn, the outer collar sits concentrically within the circular rim 194 of a mounting ring 190 that is fixed to the upper surface of a mould (not shown). Thus, unlike the embodiment of FIG. 3, the outer collar 156 is not fixed to the upper surface of the mould but is rotatable relative to it and also relative to the mounting ring 190.

(24) FIG. 7b shows the lifting mechanism 152 prior to rotation. Clockwise rotation of the outer collar 156 results in vertical movement of the inner collar 154 without rotation of the inner collar 154, to the position shown in FIG. 7c. Thus, a shroud supported by the inner collar 154 is simply lifted, without rotation of the shroud. Subsequent rotation of both of the inner 154 and outer 156 collars together would then effect rotation of the shroud. It will also be appreciated that the three-part lifting mechanism may be operated in the same way as the two-part lifting mechanism of FIG. 3, i.e. anti-clockwise rotation of the inner collar 154 with simultaneous lifting of the inner collar 154 and shroud supported therein.

(25) FIG. 8a shows a lower end 144 (i.e. opposite the nozzle) of a shroud 120 that may be used in conjunction with the lifting mechanism 152 of FIG. 7. The bore of the shroud 120 is closed by a base 122 having a central opening 126 therein. Four horizontal outlets 128 are provided in the shaft 124 of the shroud 120, adjacent to the base 122. The base 122 is shaped, having four petal-shaped indentations 130 which radiate from the central opening 126 to the periphery where the base 122 meets the shaft 124.

(26) FIG. 8b shows an impact pad 132 for use with the shroud 120 of FIG. 8a. The impact pad 132 comprises a substantially square block 134 having an upper surface 136. The upper surface 136 has a central region 138 which is complementary in shape to the shape of the base 122 of the shroud 120. Four pillars 140 extend vertically upwardly from the upper surface 136, one pillar 140 in each corner of the impact pad 132. The pillars 140 are substantially triangular in cross-section, the apex of each triangle being approximately aligned with the corners of the square block 134. An inward-facing surface 142 of each pillar is slightly curved, the degree of curvature being selected to match that of the shaft 124 of the shroud 120. The height and spacing of the pillars 140 and the width of their inwardly-facing surfaces 142 are selected so that the pillars 140 can completely cover the horizontal outlets 128 of the shroud 120 in the assembled system. The shaping of the base 122 of the shroud 120 and the central region 138 of the upper surface 136 of the impact pad 132 facilitates correct alignment between the horizontal outlets 128 and the pillars 140. It will be appreciated that the fit between the shroud 120 and the impact pad 132 must be such that the pillars 140 are able to prevent the flow of metal through the horizontal outlets 128 when the pillars 140 and the outlets 128 are aligned (both when the shroud is lowered and raised), but that the shroud 120 can still be rotated relative to the impact pad 132.

(27) FIG. 8c and FIG. 8d show the lower end of the shroud 120 assembled with the impact pad 132 and two segments of the filter housing 146 (for simplicity the other two segments are not shown). One segment is shown with a filter 147 in position; the other is shown without a filter so that the housing outlet 148 can be seen, although it will be appreciated that a filter may be present in use. Depicted is the transition from an intermediate partially open position (FIG. 8c) to a fully open position (FIG. 8d) upon operation of the lifting mechanism (not shown).

(28) With reference to FIGS. 7 and 8, in use, prior to lifting of the shroud 120 by the lifting mechanism 152, the base 122 of the shroud 120 mates with the complementary central region 138 of the upper surface 136 of the impact pad 132, such that the central opening 126 in the base 122 is closed. The horizontal outlets 128 in the shaft 124 are also aligned with, and closed by, the pillars 140, thereby ensuring that metal flow from the shroud 120 is prevented.

(29) Upon rotation of the outer collar 156 of the lifting mechanism 152, the inner collar 154 and the shroud 120 supported therein are lifted upwardly. Accordingly, the base 122 of the shroud 120 is no longer in contact with the upper surface 136 of the impact pad 132, thereby enabling metal flow through the central opening 126. However, since no rotation of the shroud 120 has occurred, the horizontal outlets 128 remain closed by the pillars 140. On casting, the stopper in the bottom pour ladle is opened and metal flows through the nozzle, into the shroud 120. The metal exits the shroud 120 via the central outlet 126 in the base 122, flows through the gap between the shroud 120 and the impact pad 132, primes the filters (not shown) present in the filter housing and then begins to flow into a running system (not shown).

(30) The inner 154 and outer 156 collars are then rotated together relative to the mounting ring 190 and the mould. This rotates the shroud 120 without changing its vertical position relative to the mould (or the ladle nozzle). Prior to rotation the shroud 120 is in a closed position in which the horizontal outlets 128 are blocked by the pillars 140 of the impact pad. Upon rotation of the shroud 120, the horizontal outlets 128 are moved out of alignment with the pillars 140 and opened partially (FIG. 8c) and then fully (FIG. 8d), thereby steadily increasing the flow of metal into the filter housing 146, the running system and the casting cavity within the mould.

(31) The use of a lifting mechanism in which rotation of the shroud can be effected independently of lifting, together with the provision of horizontal outlets in the shroud which can be opened and closed by rotation of the shroud relative to the impact pad, gives the advantage of greater control of metal flow. Initially, when the mould cavity is empty and there is no back pressure, a low flow rate can be used by opening only the central outlet in the base of the shroud. The flow rate can then be increased as the metal level in the mould cavity rises by opening the horizontal outlets. This retains and controls the metal pressure within the whole system throughout pouring. In addition, controlling the flow when the metal first enters the filter housing reduces the impact and metal pressure on the filters and hence reduces the potential for filter breakages and turbulence behind the filters. The present invention enables these advantages to be achieved while keeping the shroud pressurised, which is typically done by keeping the ladle nozzle fully open.

EXAMPLES

(32) Testing was conducted in a European steel foundry making large steel castings for construction industry vehicles.

Comparative Example 1

(33) Conventionally poured steel castings having a cast weight of 750 kg were bottom fed via three equally sized tangential ingates equally spaced around the circumference of casting cavity and connected by three runners to the base of the downsprue. Three open exothermic risers (feeders) were positioned above and in direct fluid communication with the top of the casting cavity. The horizontally parted moulds were made from acid set furan resin bonded reclaimed chromite sand, and purged with argon prior to casting. The castings were poured from a conventional bottom pour ladle placed above the mould so that the nozzle was less than 300 mm above the surface of the mould, positioned above the pouring cup and downsprue of the mould. Liquid metal was poured from the bottom pour ladle at a pouring temperature of 1555° C.

Example 1

(34) The running system of Comparative Example 1 was modified to accommodate a fused silica shroud having dimensions 1250 mm length, 80 mm external diameter and 40 mm internal (bore) diameter. The funnel of the shroud was placed inside a lifting mechanism in accordance with FIG. 3, fitted to the top of the mould. At the base of the downsprue a triangular prism shaped fused silica housing was located, having three side walls. Each of these walls had an outlet with a zirconia based 10 ppi foam filter, 100 mm×100 mm×25 mm, manufactured and sold by Foseco under the STELEX Zr brand, located adjacent to the outlet. The outlets were connected to the bottom of the casting cavity in a similar manner to the ingates of Comparative Example 1. The mould was purged with argon and the shroud raised up using the lifting mechanism so that the funnel of the shroud engaged with an iso-pressed clay graphite nozzle, sold by Foseco under the VAPEX trade name, attached to the base of bottom pour ladle. The funnel of the shroud and the end of the nozzle were sealed by a graphitised gasket. Liquid metal was poured from the bottom pour ladle at a pouring temperature of 1555° C. The pouring time was 28 seconds from the stopper in the ladle opening till closing.

(35) Castings produced by the system of Example 1 were seen to be considerably cleaner than those produced by the system of Comparative Example 1, such that it was possible to conduct the first magnetic inspection after just shot blasting and before any heat treatment and grinding. Magnetic particle inspection (MPI) of the Example 1 casting surface showed it to be significantly cleaner than the Comparative Example even after any heat treatment and grinding. Furthermore, steel castings have to undertake a series of welding cycles to remove any inclusions and surface defects detected by magnetic inspection before being shipped to the end user customer. For the Comparative casting produced by conventional pouring, typically the casting had to undergo at least 5 welding cycles. In contrast, the casting produced by the casting system of the invention (Example 1) only required a single welding cycle of a few spot defects before quenching and DC magnetic control before being ready for shipment, this equating to a reduction in welding time of over 30 hours (per casting) giving the foundry considerable cost savings and significantly reduced delivery time to the end user customer.