GB2255737A - A casting process for producing nodular iron castings - Google Patents

Abstract

Additives containing magnesium are mixed with molten flake iron in a ladle (40) to produce a nodular iron melt which is delivered to moulds to form the castings. The additives comprise a magnesium additive (55) of magnesium ferro-silicon alloy introduced first to the ladle (40) and overlaid with a protective cap (56) of ferro-silicon additive. The additives (55, 56) are placed in an additives compartment (51) formed on one side of a dam (50) in the ladle (40). Flake iron melt is delivered to a melt compartment (52) on the other side of the dam (50) eventually overflowing the dam (50) into the additives compartment (51) to react with the additives (55, 56). The arrangement of the dam (50) and the ferro-silicon cap (56) allows a more controlled mixing of the flake iron melt and magnesium additive. <IMAGE>

Description

A casting Process This invention relates to a casting process for the production of nodular iron castings.

It is known in a casting process to make ductile or nodular iron from a molten flake iron which would otherwise solidify as grey iron by treating the molten flake iron with additives.

The mechanical and physical properties of the casting depend on the shape and distribution of free carbon in the iron on solidification. In grey iron for example, the free carbon precipitates in the form of flakes, whereas in nodular iron it precipitates in the form of microscopic spheroids or nodules of graphite. This nodular iron structure results in a casting of high strength and appreciable ductivity. The nodular iron structure can be achieved by treating molten flake iron with a prescribed amount of additive containing magnesium, the resulting material characterised by the amount of additive retained on solidification. The magnesium employed may be in elemental or alloy form. Flake iron is prepared in a furnace and treatment with the nodularising magnesium additive is carried out between the furnace and the casting cavity.This may occur in a pouring ladle between the furnace and the mould, in a pouring base and immediately before entry into the mould or even further downstream in the mould itself.

Where the magnesium additive is mixed with the flake iron outside the mould a number of difficulties arise. For example a volatile and explosive reaction occurs between the flake iron melt and magnesium in the presence of oxygen, a reaction which can be hazardous if it occurs in the proximity of an exposed foundry operator. A difficulty also arises in ensuring good inter-mixing between the magnesium additive and the flake iron melt.

The in-mould methods employ at least one chamber for retaining the nodularising additive, located in the mould downstream of the pouring basin and mould spew, wherein the inoculation occurs. These methods present their own difficulties and disadvantages. The chamber occupies space in the mould which could otherwise be used for casting and any metal which solidifies therein is scrapped. In addition it is difficult to verify whether the reaction has taken place in the chamber or in the mould itself because the results of the inoculation process are not visible until the casting has been removed from the mould. Failure to inoculate will produce a grey rather than nodular iron casting.

It is an object of the present invention to overcome these problems and provide an improved process for the production of nodular iron castings.

According to the invention there is provided a process for the production of nodular iron castings, comprising the steps: (a) preparing moulds; (b) delivering the moulds to a pouring station; (c) preparing a flake iron melt in a furnace; (d) delivering a quantity of a magnesium additive in the form of magnesium ferro-silicon alloy to a ladle; (e) covering the magnesium additive in the ladle with a cap of ferro-silicon additive; (f) pouring the flake iron melt into the ladle and mixing the flake iron melt with the additives in the ladle to form a nodular iron melt; (g) pouring the nodular iron melt into moulds at the pouring station; (h) cooling the moulds on an elongate cooling conveyor to solidify the castings; and (i) removing the castings from the moulds.

In a particularly preferred embodiment of the invention the pouring of the flake iron melt is controlled such that the flake iron melt is poured into the ladle remote from the additives and is allowed to gradually envelop the additives as the flake iron melt fills the ladle.

Ideally the process includes delivering the additives to an additives compartment in the ladle and pouring the flake iron melt into an adjacent melt compartment in the ladle, and overflowing the flake iron melt into the additives compartment when a preset quantity of flake iron melt is delivered to the ladle.

In a further embodiment the process includes the step of regulating the thickness of the ferro-silicon cap and the speed of pouring of the flake iron melt such that at least two thirds of the ladle is filled with melt before the melt ruptures the cap and mixes with the magnesium additive.

In a particularly preferred embodiment the magnesium additive contains 4 - 5 % magnesium, 45 - 47 % silicon and 44 - 46 % iron by weight, and the cap of ferro-silicon additive contains 74 - 75 % silicon and 23 - 24 % iron by weight.

In a further embodiment each mould is a two-part mould comprising a pair of mould boxes mounted one above the other, each mould box having a rectangular sidewall and being open at the top and bottom, the mould being prepared by mounting each mould box on a bolster, positioning a pattern within each box, pouring sand into each box, compacting the sand within each mould box, stripping each mould box from the pattern and mounting one mould box above the other to form the mould.

Ideally the moulds and mould boxes are transported around a closed loop conveying system in carrying out the process, and after removing the castings from each mould, the process includes the further steps of delivering the mould to a knockout unit which knocks out any remaining sand from the mould, splitting the mould boxes, inverting one of the mould boxes so that both mould boxes are similarly oriented on the conveying system and delivering the empty mould boxes to the moulding machine for re-use.

In another aspect of the invention there is provided a ladle for use in the process, the ladle having a base with an upstanding sidewall with a spout at a rim of the sidewall and a removable cover on top of the sidewall, the cover having inlet openings for flake iron melt and the additives, an upstanding dam being provided on the base extending partially up the sidewall to form separate compartments, namely an additives compartment and a melt compartment, for reception of the additives and the flake iron melt.

Preferably the cover has a lug which locates in the spout to position the cover on the sidewall locating the additives inlet opening in the cover above the additives compartment and locating the flake iron melt inlet opening above the melt compartment.

The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic illustration of a casting process according to the invention; Fig. 2 is a plan view showing portion of a casting line for carrying out the process; Fig. 3 is an elevational view of the casting line; Fig. 4 is a perspective view of a ladle used in the process; Fig. 5 is a sectional elevational view of the ladle, shown in use; Fig. 6 is another sectional elevational view of the ladle, shown in use; Fig. 7 is a sectional elevational view of the ladle, in use pouring molten metal into a mould; and Fig. 8 is a perspective view of moulding boxes used in the process.

Referring to the drawings the casting process according to the invention will now be described. Moulds are prepared by delivering high pressure moulding boxes 10 to a moulding machine 12. Each mould box 10 has a rectangular sidewall 11 and is open at its top and bottom. Within the moulding machine 12 each mould box 10 is mounted on a bolster and a pattern positioned within each mould box 10. Sand is delivered to each mould box 10 and compressed about the pattern to form the required mould cavity in each mould box 10. Then the mould box is stripped from the pattern and discharged from the moulding machine 12.

Mould boxes 10 discharged from the moulding machine 12 are inverted in a rollover unit 14 and delivered to a sprue cutter 16. Core setting may then be carried out if required. The mould boxes 10 then pass through a second rollover unit 18 which inverts every second mould box 10. At a closing station 20 each inverted mould box 10 is raised and the next mould box 10 is delivered beneath it and then the two mould boxes 10 are stacked, the combined pair of mould boxes 10 thus forming the mould. Each mould is then delivered to a pouring station 22.

At the pouring station 22 molten nodular iron is poured from a ladle into each mould and then the moulds are delivered along cooling conveyors at 24 to allow the molten metal to cool and form the required casting in each mould.

Downstream of the cooling conveyors 24 the castings are punched out of each mould at a punch-out unit 26. The mould boxes 10 are then delivered to a knock-out unit 28 which knocks out any remaining sand in each mould box 10. The punch-out unit 26 is positioned over the knock-out unit 28.

The mould is punched out in the punch-out unit 26 and the sand falls onto the knock-out unit 28 with the casting in the sand.

Downstream of the combined punch-out unit 26 and knock-out unit 28 the empty mould box is delivered to a box splitting station 30. The box splitting station 30 separates each pair of mould boxes 10 which are then delivered back to the moulding machine 12 to carry out the process again. A rollover unit 32 downstream of the box splitting station 30 inverts one of each pair of mould boxes 10 so that the mould boxes 10 are similarly oriented for delivery to the moulding machine 12. Sand retrieved from the knock-out unit 28 is collected in a sand unit 34 and after re-conditioning is delivered to the moulding machine 12 for re-use.

At the pouring station 22 a ladle 40 is provided for pouring the molten nodular iron into each mould. The ladle 40 has a generally cylindrical body 41 comprising a base 42 with an upstanding cylindrical sidewall 43. A pouring spout 44 is provided at an upper end of the sidewall 43. A removable cover 45 sits on top of the sidewall 43 closing the ladle 40, the cover 45 having a lug 46 which locates with the spout 46 to correctly position the cover 45 on the sidewall 43. The cover 45 has an additives inlet 47 closable by a plug 48. The cover 45 also has a flake iron melt inlet 49.

Upstanding from the base 42 is a dam 50 forming an additives compartment 51 and a melt compartment 52 in the bottom of the ladle 40. The dam 50 bisects the base 42 and projects above the base 42 a distance of approximately 1X9 the height of the sidewall 43. It will be noted that the additives inlet 47 in the cover 45 is directly above the additives compartment 51, and similarly the melt inlet 49 is directly above the melt compartment 52.

To prepare the nodular iron for the castings a quantity of magnesium additive 55 comprising magnesium ferro-silicon alloy is delivered through the additives inlet 47 into the compartment 51. Then a ferro-silicon mixture is introduced through the additives inlet 47 to form a cap 56 over the magnesium additive 55. Examples of the additives which are in granular form are listed in the tables below; Magnesium Additive Item wt. % Si 46.30 Fe 44.72 Mg 4.80 RE 1.58 Ca 1.76 Al 0.84 Ferro-silicon Additive Item wt. % Si 74.50 Fe 24.24 Al 0.96 Ca 0.18 Ti 0.10 C 0.02 Molten flake iron is then poured into the ladle 40 through the inlet 49 falling to one side of the dam 50 into the melt compartment 52 and eventually cascading over the dam 50 into the additives compartment 51 and over the additives 55, 56 within the additives compartment 51.By the time the flake iron has penetrated through the cap 56 to the magnesium additive 55 the molten flake iron will have filled approximately 2/3 to W of the ladle 40. Then the magnesium in the magnesium additive 55 is released and mixes in a controlled manner with the molten flake iron. When mixing is completed then the cover 45 is removed from the ladle 40 and the nodular iron melt can be poured into the moulds.

It will be appreciated that by forming a cap 56 over the magnesium additive 55 this enables controlled mixing of the magnesium additive with the flake iron to be achieved. The cap 56 allows a delay to control the reaction between the magnesium additive and flake iron which is less volatile than a reaction in which the magnesium additive is mixed directly with the flake iron. It will also be appreciated that the arrangement of the ladle having a cover is such that any vigorous reaction between the magnesium additive and the flake iron is shielded within the ladle thus promoting greater safety for operators controlling the ladle.

The invention is not limited to the embodiment hereinbefore described which may be varied in both construction and detail.

Claims (12)

1. A process for the production of nodular iron castings, comprising the steps: (a) preparing moulds; (b) delivering the moulds to a pouring station; (c) preparing a flake iron melt in a furnace; (d) delivering a quantity of magnesium additive in the form of magnesium ferro-silicon alloy to a ladle; (e) covering the magnesium additive in the ladle with a cap of ferro-silicon additive; (f) pouring flake iron melt into the ladle and mixing the flake iron melt and additives in the ladle to form a nodular iron melt; (g) pouring the nodular iron melt into moulds at the pouring station; (h) cooling the moulds on an elongate cooling conveyor to solidify the castings; and (i) removing the castings from the moulds.

2. A process as claimed in claim 1 wherein the pouring of the flake iron melt is controlled such that the flake iron melt is poured into the ladle remote from the additives and is allowed to gradually envelop the additives as the flake iron melt fills the ladle.

3. A process as claimed in claim 2 including the step of delivering the additives to an additive compartment in the ladle and pouring the flake iron melt into an adjacent melt compartment in the ladle, the flake iron melt overflowing the melt compartment into the additives compartment when a preset quantity of flake iron melt is delivered to the ladle.

4. A process as claimed in any preceding claim including the step of regulating the thickness of the ferro-silicon cap and the speed of pouring the flake iron melt such that at least two-thirds of the ladle is filled with melt before the melt ruptures the cap and mixes with the magnesium additive.

5. A process as claimed in any preceding claim wherein the magnesium additive contains 4 - 5 % magnesium, 45 - 47 % silicon and 44 - 46 % iron by weight, and the ferro silicon cap contains 74 - 75 % silicon and 23 - 25 % iron by weight.

6. A process as claimed in any preceding claim wherein each mould is a two-part mould comprising a pair of mould boxes mounted one above the other, each mould box having a rectangular sidewall and being open at the top and bottom, the mould being prepared by mounting each box on a bolster, positioning a pattern within each box, pouring sand into each box, compacting the sand within each box, stripping each box from the pattern and mounting one box above the other to form the mould.

7. A process as claimed in claim 6 wherein the moulds and mould boxes are transported around a closed loop conveying system in carrying out the process, and after removing the castings from each mould, the process includes the further steps of delivering the mould to a knock-out unit which knocks out any remaining sand from the mould, splitting the mould by separating the mould boxes, inverting one of the mould boxes so that both mould boxes are similarly oriented on the conveying system, and delivering the empty mould boxes to the moulding machine for re-use.

8. A process substantially as hereinbefore described with reference to the accompanying drawings.

9. Nodular iron castings whenever produced by the process of any of claims 1 to 8.

10. A ladle for use in the process as claimed in any of claims 1 to 8 wherein the ladle has a base with an upstanding sidewall with a spout at a rim of the sidewall and a cover on top of the sidewall, the cover having inlet openings for flake iron melt and the additives, an upstanding dam being provided on the base extending partially up the sidewall to form separate compartments namely an additives compartment and a melt compartment, for reception of the additives and the flake iron melt.

11. A ladle as claimed in claim 10 wherein the cover has a lug which locates with the spout to position the cover on the sidewall locating the additives inlet opening in the cover above the additives compartment and locating the flake iron melt inlet opening above the melt compartment.

12. A ladle substantially as hereinbefore described with reference to Figs. 4 to 7 of the drawings.