Nodularising method

Abstract: A method for the production of nodular iron is disclosed in which a nodulariser is introduced into the molten metal to be treated in at least two stages. In particular exemplified embodiments the nodularising agent is introduced in a first stage prior to entry of the molten metal into the mould cavity and, in a second stage, the nodularising agent is contained in the mould cavity or in a chamber associated therewith prior to introduction of the molten metal or the agent is added to the metal stream during its passage into the mould cavity.

Description: The present invention relates to a method for the production of nodular iron. In recent years there has, within the iron foundry industry, been an increase in the use of relatively large molten metal holding and pouring systems. Such systems provide a continuous supply of molten metal to the moulding plant and assist in increasing productivity, reducing costs and improving the consistency of the cast product. The use of these holding and pouring furnaces is satisfactory when used for holding and pouring flake graphite cast iron but problems arise in the case of nodular cast iron. Holding nodular cast iron which, may contain, for example, about 0.03% to 0.08% of magnesium leads to the following difficulties. A build up of slag and reaction products can occur especially in the zone adjacent to the filling and pouring spouts and, when fitted, in the inductor channel. A gradual loss of the magnesium and hence nodularity arises, thus necessitating the addition of further quantities of magnesium-containing alloys. These difficulties can be largely overcome by introducing the magnesium into the metal after it has left the holding/pouring furnace as is the case with the process described in U.K. Patent No. 1278265 (known as the INMOLD process). For certain applications, the INMOLD process, however, suffers from several disadvantages, namely: Relatively large alloy reaction chambers need to be incorporated in the gating system in order to house the nodularising alloy in quantities ranging, for example, from 0.8% to 1.3% of the casting spray weight which results in a reduction in casting yield of some 5% to 7%. The relatively large quantity of nodulariser present in the reaction chamber increases the risk of inclusions in the casting thus necessitating the use of elaborately designed gating systems which also result in a lowering of casting yield and corresponding increases in cost. The casting pouring rates need to be carefully controlled to within relatively narrow pre-determined times so as to ensure consistency of nodularisation. A common problem associated with such a nodularisation method is described in the discussion of the prior art in U.K. Patent No: 1278265 and is that of fading. Fading occurs when after the total amount of nodulariser has been added, some of the nodulariser is oxidised over time causing the iron to revert to its original flake form. U.K. Patent No: 127 8265 refers to the further addition of more magnesium during pouring to restore magnesium loss through fading. However, restoring fading in this manner leads to increased costs of manufacturing and the use of excessive quantities of nodularising agents which can result in the production of dirty castings through the formation of oxides and silicates. FR-A-2 150 329 also describes a process for producing magnesium-treated spheroidal graphite cast iron. The method involves a first pretreatment stage whereby magnesium is added to molten cast iron, followed by heating of the molten cast iron in a furnace to a temperature of at least 1480°C. The cast iron is heated at this temperature until sufficient magnesium has been removed so that the treated iron no longer forms spheroidal graphite. This pretreatment is then followed by the main magnesium treatment which is also carried out in the acid-lined treatment ladle using the pour-on method in an amount just sufficient to form spheroidal graphite. The purpose of the pretreatment stage is to desulphurise and deoxidise the bath and furthermore, the reheating allows the satisfactory cleaning of the base iron. The present invention therefore is directed to a method for the production of nodular graphite cast iron in which a required total amount of nodularising agent is introduced into the molten metal to be treated in at least two stages. The method comprises at least one treatment stage which takes place prior to the introduction of the molten metal into the mould cavity. In a first treatment stage a portion of the total amount of nodularising agent is added, as specified in claim 1, to result in molten metal in which only a part of the graphite is in the form of nodules. The method further comprises at least one further treatment stage in which the remainder of the total amount of nodularising agent is added. According to the method of the present invention, as the molten metal undergoes the final treatment stage, at least a portion of the graphite is in the form of nodules. Hence, as the molten metal undergoes the final treatment stage, the molten metal is partially nodularised. Preferably the nodularising agent is introduced, in a first stage, to the molten metal prior to its entry into the mould cavity and, in a second stage, to the metal within the mould cavity. In the second stage, the nodularising agent may be contained in the mould cavity itself or in a chamber associated therewith, prior to introduction of the metal or it may be added to the metal stream during its passage into the mould cavity. The method of introducing the nodularising agent at each stage may be any convenient known method of introducing nodularising agents into molten grey cast iron. For example, any known process, as described in the British Foundryman, F1527, January 1986 pages 18 et seq, in the article by G.E. Else and H.T. Dixon, could be employed. In a particular embodiment of the invention the nodularising agent is introduced, in a first stage, by the IMCONOD process as described in European Patent No. 0086549 and, in a second stage, by the INMOLD process as described in British Patent No. 1278265. In the IMCONOD system the nodularising agent is placed in a container supported in the base of a covered jacket or funnel and which has an aperture at the top of its peripheral walls, and optionally one or more other apertures in its peripheral walls remote from the funnel exit. The grey molten metal is poured from via a pouring bush over the nodularising agent held in the container wherein the metal reacts with the nodularising agent and the metal containing the agent is allowed to flow through the apertures in the container into an enclosed space around the additive containers subsequently allowing the molten metal to flow into a pouring ladle arranged beneath the additive container. The molten metal may flow in a zig-zag path and during the process contact of the molten metal with the atmosphere is restricted by means of the covered jacket or funnel housing the container. In the present invention the partially nodularised molten metal is subsequently processed, in the second nodularisation step, by a conventional technique such as the INMOLD process. In the INMOLD process, the moten metal to be treated is introduced into the mould cavity by way of a conventional pouring system which additionally includes one or more intermediate chambers containing a nodularising agent in an amount sufficient to complete conversion of the graphite to nodular or spheroidal form. The intermediate chamber may, for example, be connected directly to a pouring bush by means of which the mould is filled, or it may be connected directly to a sprue used in filling the mould, or to a feeding riser, runner or ingate of the mould or it may be connected to a whirlgate in the gating system. In another embodiment the nodularising agent is introduced, in a first stage, by the "Pourover" or "Overpour" system, a well known technique discussed in the article by Else et al above, and, in, a second stage, by the INMOLD process. The nodularising agent may be any known agent, including magnesium-containing and non-magnesium-containing nodularising agents as known in the art. The proportion of the nodularising agent which is introduced in the first stage is a matter of choice for the Foundryman and will depend, inter alia, on the particular treatment methods employed, the amount of active component e.g. magnesium contained in the nodulariser, the temperature and sulphur levels of the molten metal. A typical range of proportion would be 20%-80%, preferably 60%-70%, referring to a nodulariser containing 4%-6% magnesium. It should be understood however, that the proportion of nodularising agent introduced in the first stage must be sufficient to ensure that a significant degree of partial nodularisation takes place in the first stage. The term "partial nodularisation" used herein describes a structure consisting of graphite nodules and/or malformed nodules and/or compacted graphite flakes. The process according to the invention consists of adding the nodulariser in at least two distinct steps, as compared to conventional methods in which the total amount of the magnesium alloy is added to the metal during one operation. The first step of this process is one of partial nodularisation to a significant degree in contradistinction to pre-conditioning where little or no nodularisation takes place because the composition and quantity of additives used for pre-conditioning merely influence nucleation and response to nodularisation. The first step in the processing cycle consists of adding a part of the nodulariser to the molten metal held in either a ladle or furnace in order to effect partial nodularisation. This metal could optionally be held for an extended time in the partially nodularised condition by adding at intervals further small additions of nodulariser. Alternatively the final stage in the processing cycle could proceed without the holding stage. The final stage consists of complete nodularisation of the metal by the further addition of nodulariser/inoculant to the metal stream either within the mould itself, for example by the INMOLD process or added into the metal stream during entry of the metal into the mould cavity. The invention is illustrated by the accompanying drawings wherein Figures 1 and 2 are schematic representations of two embodiments respectively of an apparatus suitable for carrying out the process according to the invention. Figure 1 represents a combined pour-over and Inmold system. The nodularising agent is introduced, in a first stage, in the alloy chamber of a ladle 1. Molten metal from a furnace is poured over the nodularising agent contained in the alloy chamber of the ladle 1. In a second stage the partially nodularised molten metal is introduced via the ladle 1 into an Inmold system. Nodularising agent is introduced into a chamber adjoining or associated with the mould cavity. Figure 2 represents a combined Imconod and Inmold system. In this embodiment, in the first stage of the process, the nodularising agent is contained within an additive container housed within a jacket or funnel 2. Molten metal from a furnace enters the Imconod system via a pouring bush 3 and the partially nodularised molten metal is collected in a ladle 1 and subsequently enters an Inmold system as in the embodiment illustrated in Figure 1. The invention is illustrated by the following Examples which compare the multiple stage process according to the present invention with conventional processes. Examples comparing the multiple nodularisation process according to the invention with conventional processes. Foundry A:- Conventional nodularising process: IMCONOD (European Patent NO. 0086549) Nodularising/Inoculant addition (Typical) Metal Treatment Wt. (Nominal) 1000 kg Nodulariser Addition (into IMCONOD unit) (Alloy containing 3.5% - 4.2% Mg, 45-50% Si, 0.5-1.25% Ca, 0.5-1.0% Al, and 1.4-1.6% CeMM (Cerium mischmetal) 22 kg 2.2% (Alloy containing 4.5% - 5.0% Mg, 42-47% Si, 1.25-1.75% Ca, 0.5-1.0% Al and 1.5-2.0% CeMM) 18 kg 1.8% Inoculant addition (in Mould) (Typical inoculant composition: 76.0% Si, 3.50%- 5.0% Al, 0.45%-1.0% Ca, Balance Fe). 0.10%-0.15% Final metal composition (Typical) % :- T.C. Si. Mn. S Mg. 3.70 2.8 0.3 0.01 0.03 Foundry A:- Conventional nodularising process INMOLD (British Patent No. 1278265) Final metal composition (Typical) %:- T.C Si Mn S Mg. 3.6 2.60 0.4 0.01 0.04 Foundry A:- Multiple nodularisation process: Combined IMCONOD/INMOLD Metal composition after step 1 % :- T.C Si Mn S Mg 3.74 2.24 0.4 0.012 0.018 Final metal composition % :- T.C. Si Mn S Mg ND 2.69 0.4 0.014 0.031 ND = Not determined Structure/Mechanical Properties The structure and mechanical properties of the nodular cast iron produced by the multiple stage process were measured using samples taken from two flywheel castings. These samples revealed that the material has a structure typical of nodular cast iron consisting of well formed graphite nodules in a predominantly ferritic matrix and the mechanical properties were found to be exceptionally good, being significantly better than the minimum specified. Mechanical Properties Minimum Specified Obtained Tensile 500N/mm 2 601 N/mm 2 592 N/mm 2 Elongation 7% 15 15 Hardness 250 max. 197 197 Foundry B:- Conventional Nodularising Process 'Pour Over' Final metal composition (Typical) %:- T.C. Si Mn S Mg 3.60 2.40 0.5 to 0.40 0.014 0.050 Foundry B:- Conventional Nodularising Process: INMOLD Final metal composition (Typical) %:- T.C Si Mn S Mg 3.50 2.40 0.05 to 0.40 0.014 0.05 Foundry B:- Multiple nodularisation process: 'Pour Over'/INMOLD Nodularising additions/INMOLD reaction chamber dimensions Processing Step 1 Pour Over Metal Treatment Wt. 400 kg Nodulariser Addition (Alloy containing 4.5% - 5.0% Mg. 42-47% Si, 1.25-1.75% Ca, 0.5- 1.0% Al and 1.5-2.0% CeMM) 3.2 kg 0.8% Metal composition after Step 1 %:- T.C. Si Mn S Mg 3.79 2.09 0.06 0.014 ND ND = Not determined Final metal composition %:- T.C Si Mn S Mg ND ND ND ND 0.029 ND = Not Determined Structure/Mechanical Properties As in the case with the example from Foundry A, the structure and mechanical properties were measured using samples taken from the castings with the following excellent results for a ferritic nodular cast iron. Structure Mechanical Properties Well formed graphite nodules in ferritic matrix. Tensile N/mm 2 0.2% Proof Stress N/mm 2 Elongation % 412 257 30 We have found that the process according to the invention affords several advantages over prior art techniques. Introduction of the nodulariser in two or more stages, for example by the combined IMCONOD/INMOLD process exemplified (Foundry A) offers the following significant advantages when: a) compared with IMCONOD Considerable cost savings resulting from the use of less nodulariser, i.e. 1.3% compared to (1. 8-2.2%) without the need for an addition of some 0.15% of mould inoculant. b) compared with INMOLD The combined process, whilst using similar amounts of nodulariser when compared to INMOLD, i.e. 1.27% cf 1.37%, offers a definite cost advantage in that the size of the INMOLD reaction chamber is considerably reduced, i.e. from 87 mm. X 63 mm. X 74 mm. high to 45 mm. X 45 mm. X 64mm high. Apart from the obvious reduction in weight leading to an increase in casting yield, the significant reduction in area means that less space is taken up on the pattern plate thus minimising one of the disadvantages of the process. In addition to the cost saving referred to, the use of relatively small INMOLD reaction chambers and low quantities of alloy in the mould has surprisingly resulted in noticeable improvements in casting cleanliness when compared with the conventional INMOLD process. It should also be noted that quality of the nodular cast iron produced by the combined process method has been exceptionally high. As also exemplified (Foundry B) we have shown advantages of the present process: a) Compared with Pour Over Significant cost savings resulting from the use of less nodulariser, i.e. 1.15% compared to 2.3%. b) Compared to INMOLD The same advantages as reported by Foundry A. In the case of Foundry B the alloy reaction chamber was reduced from 75 mm. X 75mm. X 80 mm. high to 45 mm. X 45mm. X 60 mm. high. The division of the nodularising addition is also advantageous in the holding of metal containing magnesium after the first stage of the nodularisation process. An important advantage of the process according to the invention is that it permits the use of a reaction chamber of much simpler design and of an extremely small surface area. The use of a reaction chamber with a small surface area offers practical and economic advantages to the user in that a relatively small amount of space is taken up on the pattern plate. We would also expect the volume of the reaction chamber to be less when using the present process thus resulting in another important practical advantage. We have found that the small surface area of the reaction chamber and hence the relatively small area of alloy exposed to the metal stream, leads to significant improvements in casting cleanliness. With the combined nodular process, problems due to magnesium variations are anticipated and the process leads to improved consistency as compared to conventional techniques.

Claims: A method for the production of nodular graphite cast iron in which a required total amount of nodularising agent is introduced into the molten metal to be treated in at least two stages, which comprises:- (1) at least one treatment stage which takes place prior to the introduction of the molten metal into the mould cavity and wherein in a first treatment stage a portion of the total amount of nodularising agent is added to result in molten metal in which only a part of the graphite is in the form of nodules, and (2) at least one further treatment stage in which the remainder of the total amount of nodularising agent is added. characterised in that as the molten metal undergoes treatment stage (2) at least a portion of the graphite is in the form of nodules, wherein between 20 to 80% of a nodulariser comprising 4 to 6% magnesium is added during the first treatment stage. A method according to claim 1, which further ccmprises a holding stage which takes place after stage (1) and wherein further additions of nodularising agent can be made. A method according to any of claims 1 and 2, wherein the nodularising agent is introduced in said first treatment stage, to the molten metal prior to its entry and, in the second stage, to the molten metal within the mould cavity. A method according to claim 3, wherein, in the second stage, the nodularising agent may be contained in the mould cavity or in a chamber associated therewith prior to the introduction of the molten metal. A method according to claim 3, wherein, in the second stage, the nodularising agent is added to the molten metal stream during its passage in to the mould cavity. A method according to any of claims 1 and 2, wherein in the first stage, the nodularising agent is held in a container which is housed within a covered jacket or funnel to restrict contact of the molten metal being treated with the atmosphere, said container having an aperture at the top of its peripheral walls and optionally one or more other apertures in its peripheral walls, wherein molten metal is poured over the nodularising agent held within the container from via a pouring brush which opens directly or indirectly into said container, and the molten metal containing the nodularising agent flows through said aperatures and into a pouring ladle and in the second stage, the metal containing the nodularising agent is introduced into the mould cavity by way of a conventional pouring system which additionally includes one or more intermediate chambers containing the nodularising agent in an amount sufficient to complete the conversion of the graphite to nodular or spheroidal form. A method according to any of claims 1 and 2 wherein, in the first stage, the nodularising agent is introduced into the molten metal by the Pourover system and, in the second stage, the metal containing the nodularising agent is introduced into the mould cavity by way of a conventional pouring system which additionally includes one or more intermediate chambers containing the nodularising agent in an amount sufficient to complete the conversion of the graphite to nodular or spheroidal form. A method according to any of claims 1 and 2 wherein, the nodularising agent is introduced at the first stage into the molten metal contained in a holding/pouring furnace and in the second stage during the passage of the molten metal into the mould cavity. A method according to any of claims 1 and 2 wherein, in the first stage, the nodularising agent is introduced into the molten metal in a holding/pouring furnace and in the second stage, the metal containing the nodularising agent is introduced into the mould cavity by way of a conventional pouring system which additionally includes one or more intermediate chambers containing the nodularising agent in an amount sufficient to complete the conversion of the graphite to nodular or spheroidal form.

Inventors: CANNING HENRY CECIL, MANNION GEOFFREY, CANNING, HENRY CECIL, MANNION, GEOFFREY
Applicants: INT MEEHANITE METAL CO, THE INTERNATIONAL MEEHANITE METAL COMPANY LIMITED
Application year: 1991
Main classification: C21C1
IPC codes: C22C33, C21C1