Method of casting compacted graphite iron by inoculation in the mould

Abstract

 Compacted graphite iron castings may be consistently produced by the controlled treatment of molten iron with a magnesium-containing additive in the mould. The flow rate of the metal, composition of the nodularizing additive, and mould design are correlated with a predetermined treatment constant.

Description

[0001] This invention relates to a method of reproducibly and reliably making compacted graphite iron castings by treating molten iron with a magnesium-based nodularizing agent in the mould.

[0002] Molten iron containing dissolved carbon in amounts of about 2 to 5 percent by weight and silicon in amounts of about 0.5 to 3.0 percent by weight generally solidifies as grey iron when cast into conventional foundry sand moulds. The silicon inhibits carbide formation and promotes the precipitation of free graphite in flake form. Principally because of its carbon microstructure, grey iron is relatively hard and brittle and not as prone to shrinkage during cooling as are other cast irons. Herein, the term "grey iron" may refer to a solidified iron casting with a grey iron microstructure or the iron melt from which such a casting may be poured.

[0003] In the late 1940's, it was discovered that the addition of certain elements, principally magnesium, to grey iron melts would cause the carbon to precipitate in the form of graphite nodules rather than flakes. Iron with this microstructure is commonly known as nodular, spherulitic, or ductile iron.

[0004] Nodular iron has much higher tensile strength, ductility and better machinability than grey iron. One disadvantage is its high shrinkage during solidification. Consequently, risers are provided in nodular iron casting moulds to keep the casting cavity full as the iron cools.

[0005] Magnesium or other nodularizing elements may be introduced to the molten iron in an alloyed form as in magnesium-ferrosilicon alloys or in elemental form mixed with a suitable diluent such as iron or an iron alloy. Since sulphur in an iron melt uselessly consumes nodularizing elements without affecting carbon precipitation, iron for nodular castings is usually de-sulphurized to less than 0.02 weight percent retained sulphur before it is treated. Generally a small quantity of a rare earth metal such as cerium is added to the nodularizing agent to minimize the effect of tramp elements that inhibit nodule formation.

[0006] During the development of nodular iron, it was found that under-treatment with magnesium produced an iron wherein free carbon precipitated partially as nodules and partially as flakes. The flakes tended to be more rounded and coarser than those of untreated grey iron. This iron was found to have physical properties midway between nodular and grey irons. It has been referred to as compacted graphite or vermicular iron. The tensile strength of compacted graphite (e.g.) iron is generally higher than that of grey iron and approaches that of ductile iron. The thermal conductivity of c.g. iron is greater than that of ductile iron but less than that of grey iron. The impact and fatigue properties of c.g. iron are better than grey cast iron but slightly lower than ductile cast iron. An advantage of compacted graphite iron over nodular iron is low shrinkage as it cools. We have found that if the nodularity of a casting is kept below 70% by volume of the total carbon content, shrinkage is minimal enough to eliminate the need for risers in the mould.

[0007] While the advantages of compacted graphite iron were recognized some time ago, it was not known how to make it reliably. As noted by R. R. Oathout "Host physical and mechanical properties of compacted graphite iron are intermediate to those of grey and ductile irons. It should be noted that this iron is by no means new. It was discovered by accident during the development of ductile iron in the late 1940's. Control difficulties resulted in a lack of reproducibility that inhibited commercial usage. Magnesium-silicon additives used to produce graphite nodularity in ductile iron could not be controlled precisely enough to produce the intermediate product; usually too much nodularity or too much flake graphite was obtained", Compacted Graphite Iron for Diesel Engine Castings, Metal Progress, May 1978. This observation was also made and commented on by Lalich and LaPresta in Uses of Compacted Graphite Cast Irons, Foundry M and T, September 1978 and by Sergeant and Evans in The Production and Properties of Compacted Graphite Irons, 1978 Conference Paper.

[0008] In the 1960's, it was found that the addition of 0.15 to 0.50 weight percent retained titanium along with a nodularizing amount of magnesium and a small amount of rare earth metals could be used to predictably make compacted graphite iron. While this method is effective, it is not practical for use in foundries where nodular iron is made because of the inhibiting effect titanium has on the nodularizing process. Titanium is known as a deleterious element, that is, its presence even in trace amounts hinders the formation of good nodular graphite structures in castings. Thus its presence cannot be tolerated in iron foundries where it might contaminate melts intended for nodular iron castings. Any scrap containing titanium is unsuitable for remelt where the carbon microstructure of the castings to be poured must be controlled.

[0009] U.S. patents numbers 3,703,922 and 4,004,630 assigned to Materials and Methods Limited, England, relate to treating molten iron-in the mould to produce nodular iron castings. The practice involves retaining magnesium-ferrosilicon alloy in a treatment chamber in a mould between the downsprue and the mould cavity. Molten iron runs over the inoculant during the pour and is treated uniformly with the nodularizing.additive. Enough magnesium, generally greater than 0.4 weight percent, is retained in the metal so that the graphite assumes a totally nodular structure. U.S. patent number 4,004,630 is drawn to a method of relating the solubility of a nodularizing alloy in molten grey iron to the area of the treatment chamberin order to produce iron casting with a totally nodular graphite structure. While inoculation in the mould is an attractive process for producing nodular iron castings, before this invention it was not known how to adapt it for making compacted graphite iron. As discussed above, it was not known how to control the process to the extent that magnesium-ferrosilicon alloys, without the addition of any deleterious elements such as titanium, could be used in an inoculating process to consistently and predictably make compacted graphite iron castings.

[0010] It is therefore an object of the invention to provide a reproducible method of making low shrinkage compacted graphite iron castings by treating molten iron with a magnesium based nodularizing agent in the mould without the addition of deleterious elements. A more particular object of the invention is to provide a controlled method of making compacted graphite iron castings based on iron pour rate and mould design so that in the solidified casting 30 to 70% by volume of the free carbon is in the form of nodules and the balance is in the form of compacted graphite. A more particular object of the invention is to provide a method of making such compacted graphite iron by providing a mould having a treatment chamber for retaining a magnesium inoculant agent, the chamber size depending on the metal pour rate and a predetermined solution factor to provide proper contact area between the poured iron and the nodularizing agent.

[0011] According to the present invention, a method of consistently producing substantially shrinkage free compacted graphite iron castings by treating molten iron with a magnesium-containing inoculant in a riserless foundry mould, in which the method comprises casting molten iron containing about 2.5 to 5 weight percent free carbon and less than one weight percent sulphur into said foundry mould at a controlled pour rate, is characterised in that the method includes flowing said molten iron over and reacting it with a magnesium-containing inoculant substantially free of elements deleterious to carbon nodule formation and containing 5 to 7 weight percent magnesium such that the contact area therebetween throughout the pour is equal to said controlled pour rate divided by a treatment constant of 0.176 + 0.021 kg/cm². second (2.5 + 0.3 pounds per inch². second), and allowing the inoculant-treated iron to flow into and solidify in a casting cavity in said foundry mould to produce a solidified casting in which 30 to 70 volume percent of free carbon in the solidified casting is in the form of nodules and the balance is in the form of compacted graphite.

[0012] In a preferred embodiment of the invention, a compacted graphite iron casting may be made as follows. A mould is provided with a treatment chamber for retaining an adequate amount of nodularizing agent and a casting cavity downstream thereof. The treatment chamber is sized such that the contact area between the poured iron and the nodularizing agent in the chamber is equal to the metal pour rate in kilograms per second divided by a predetermined treatment constant of about 0.176 kg/cm^2- second (2.5 pourds per inch . second). The outlet of the treatment chamber is choked so that the flow rate through it is about 5 to 25 percent less than the pour rate of the iron.

[0013] An amount of treatment additive adequate to last throughout the pour is retained in the treatment chamber. Suitable additives should contain about 5-7 weight percent magnesium, in alloyed or unalloyed form, as the principal nodularizing agent. The additive may be in free flowing particulate form or some other form suitable for use as inoculant in-the-mould. Herein the term "inoculant" refers to an additive for molten iron that affects the precipitation of carbon as graphite as the iron solidifies. Molten iron at a suitable temperature is poured into the mould at a rate of about 6.35 to 11.34 kg (14 to 25 pounds) per second. The iron should be desulphurized and contain about 2.5 to 5 weight percent free carbon. The poured iron is inoculated in the treatment chamber and allowed to solidify in the casting cavity.

[0014] We have discovered that providing a treatment chamber having a predetermined contact area with metal poured at a controlled rate allows us to predictably and reproducibly make castings with about 30 to 70 by volume percent nodular graphite and the balance compacted graphite. The pour rate and magnesium content of the inoculant may be easily varied to control the degree of nodularity. The reliability of the method allows the elimination of risers in the mould which provides substantial savings of both the amount of poured metal and the inoculant. Moreover, the treated iron is not contaminated with any deleterious elements such as titanium.

[0015] The invention will be better understood in view of the following Figures, detailed description and examples.

Figure 1 is a sectional side view of a foundry mould suitable for practice of the invention having two treatment chambers upstream of the casting cavities for retaining a nodularizing additive.

Figure 2 is a sectional view of the treatment chambers of the mould of Figure 1 taken in the direction of the arrows along line 2:2.

Figure 3 is a photomicrograph of a casting made in accordance with the invention showing the dispersion of nodular and compacted graphite throughout an iron matrix.

[0016] In accordance with a preferred practice of the invention, a resin bonded sand mould of the type shown in Figures 1 and 2 is provided. The mould comprises pouring basin 2 in mould cope 4 and downsprue 6. Sprue runners 8 located near the bottom 10 of downs^prue 6 lead into two similar chambers 12 in drag mould 14 for retaining nodularizing treatment additives 16. Chambers 12 are substantially box-like with flat bottoms 18 and gently outwardly sloping sides 20. This provides a slightly larger contact area between poured iron and additive at the beginning of the pour and a slightly smaller one near the end. Hereafter, the contact area between poured metal and inoculant in the treatment chamber will be defined as the average contact area taken at mid depth of additive in a chamber as shown by hashed line 22 in Figure 1. Chamber outlets 24 together have areas about 5 to 25 percent smaller than the total cross sectional areas of sprue runners 8 and are therefore said to be "choked". Choked outlets 24 adjoin runners 26 which are reduced in size at 28. Downstream thereof, a second enlarged runner portion 30 is provided to entrap any slag, dross or other solid contaminants before poured metal progresses into casting cavities 32. Runners downstream of chambers 12 lie in cope mould 14 generally along the mould parting line 34. The most restricted metal flow -Ls at choked outlets 24 of chambers 12. The term mould as used herein also includes any detachable mould portion, e.g., a separate pouring basin containing a chamber for treating a pour with inoculant, which in casting position is in fluid flow relation with the mould portion containing the casting cavity.

[0017] Referring again to treatment chambers 12, we have found that they must be sized to provide a predetermined contact area between the cast metal and the nodularizing agent 16. Referring to Figure 2 taken along line 2:2 of Figure 1, a view of treatment chambers 12 is provided showing the average contact area between nodularizing additive 16 and the poured metal. In accordance with the invention, this area must be equal to the metal pour rate in kilograms per second divided by a treatment constant of from about 0.155 to 0.197, preferably 0.176, kg/cm^2- second (2.2 to 2.8, preferably 2.5, pounds per inch^2. second). Sizing the treatment chambers in accordance with this formula and choking the chamber outlets assures that the molten iron is treated with just the right amount of magnesium when the inoculating agent contains about 5 to 7 weight percent magnesium to produce a desired compacted graphite iron casting. We have found that the depth of the treatment chamber is not critical so long as it is deep enough to contain the amount of inoculant which will yield the proper residual magnesium content in the casting. The minimum depth may be determined by calculating the contact area between the iron and inoculant and then determining the number of kilograms of alloy required to treat a particular casting. This weight divided by the density of the inoculant gives the total volume of the additive. The additive volume divided by the calculated contact area is the minimum height for the treatment chamber.

[0018] The amount of magnesium retained in a cooled casting to achieve a blend of 30 to 70 percent nodular and the balance compacted graphite should be about 0.013 to 0.017 weight percent. The percent nodularity increases about proportionately with the amount of retained magnesium. It may be necessary to conduct a trial run with a particular mould in order to finely adjust either the treatment chamber dimensions or pour rate to achieve the exact desired proportions of nodular and compacted graphite. The inoculating agent retained in the treatment chamber should be at least 0.35 percent by weight of the cast metal and contain about 5 to 7 weight percent magnesium as the principal nodularizing agent. The balance of the additive weight is generally made up of higher melting diluent elements such as iron and silicon. A portion of silicon is desirable because it is a graphite promoter and carbide inhibitor. Inoculant additives of magnesium alloyed with ferrosilicon or mixtures of elemental magnesium and sized ferrosilicon particles are suitable. This invention does not require or allow-the presence of any but noneffective trace amounts of nodularity inhibiting, deleterious elements such as titanium.

[0019] The preferred pouring rate for molten iron is from about 6.35 to 11.34 kg per second (14 to 25 pounds per second) depending on the size of the casting. The pour rate is preferably slower for small castings and faster for larger castings. The pour rate chosen for a particular casting is a parameter that can be modified as desired by one skilled in the art to optimize such factors as total pour time, treatment chamber size, and pour temperature.

[0020] The preferred casting temperature is in the range of about 1316° to 1538° C (2400 to 2800° F). At these temperatures, iron retains good fluidity in a room temperature mould. Higher temperatures tend to assist in the dissolution of higher melting diluents in the treatment additives.

[0021] As in the manufacture of nodular iron, the molten iron should be desulphurized in the ladle or melting cupola before it is cast to bring the total sulphur content down to less than 0.2 weight percent. The cast metal is allowed to cool in the mould at room temperature until removal of the casting. So long as the nodularity is kept below 70 percent, there is very little shrinkage and no risers are needed in the mould to compensate for external or internal shrinkage in the casting body.

[0022] This invention will be better understood in view of the following specific example. Four runs of experimental production desulphurized iron were manually poured to form c.g. automotive exhaust manifolds with a target graphite nodularity of about 65 percent by volume. Resin bonded sand moulds for the c.g. parts were made from patterns initially used to make nodular iron moulds. The major changes in the mould patterns were the elimination of the risers (needed to compensate for shrinkage in totally nodular castings) and the sizing of the treatment chamber retaining the nodularizing additive. The total weight of iron poured in each mould was approximately 61.24 kg (135 pounds) and the minimum desired pouring time was 6 seconds. Attempts were made to control the pour rate to about 10.21 kg per second (22.5 pounds per second) subject to normal deviations of a few kilograms per second that may be encountered in manual pouring. Mechanical pourers are easier to control and would be ideal for mass production of c.g. iron castings in accordance with this invention.

[0023] The key to the subject invention is maintaining the treatment constant for an in-the mould inoculant containing 5 to 7 percent magnesium at or about 0.176 + 0.021 kg/cm^2. second (2.5 + .3 pounds per inch². second).

[0024] Each mould contained two similar treatment chambers in communication with similar casting cavities for exhaust manifolds, the poured iron being split equally between them. The desired total average reaction area of the chambers at mid depth of alloy was calculated by dividing the pour rate of 10.21 kg per second (22.5 pounds per second) by the preferred treatment constant for the inoculant of 0.176 kg/cm^2- second (2.5 pounds per inch^2- second). This number was divided by 2 since there were two treatment chambers, the area for each being about 28.90 cm² (4.48 square inches).

[0025] The additive inoculant'used was a combination of free flowing sized elemental magnesium and ferrosilicon particles of the type described in U.S. patent 4,224,069 assigned to General Motors Corporation. The additive contained about 6 weight percent magnesium and the balance 50 percent silicon-ferrosilicon. We believe that any inoculant containing 5-7 weight percent magnesium that is suitable for use as an in-the-mould nodularizing additive is adaptable to the method of making c.g. iron according to the present invention. The additive may be in the form of free flowing particles, preforms of agglomerated particles, cast solid preforms, or particles suspended in a resinous binder, for example. The density of the free flowing particulate additive was 2.104 gms/cm³ (0.076 pounds per cubic inch). The amount of additive used in each mould was 0.57 percent of the total weight of poured metal (61.24 kg (135 pounds) per casting) or 0.35 kg (0.77 pounds) total (0.021 kg (.046 pounds) magnesium). The total volume occupied by each treatment portion was therefore 0.35 divided by

or 166.35 cm (0.77 divided by 0.076 or 10.13 cubic inches). This amount was split equally between the two treatment chambers. Therefore the depth of alloy in each chamber was 83.1 cm (5.07 cubic inches) divided by 28.9 cm² (4.48 square inches) or 2.87 cm (1.13 inches). The amount of magnesium present in each mould was approximately 0.06 x 0.35 kg or 0.021 kg (0.06 x 0.77 pounds or .046 pounds).

[0026] We have found that the amount of magnesium retained in a cooled casting is about 50 percent of that taken up during the in-the-mould inoculation. For c.g. castings a retained magnesium content of 0.13 to 0.17 weight percent is needed. Therefore, at least twice that amount of magnesium should be present in the additive portion in the mould. We prefer to use an amount in excess of this minimum amount for two purposes. First, it assures that the entire pour is treated with nodularizing additive, and secondly, a small amount of retained additive in the inoculant chamber will cause a slight but visible flare at the end of the pour which is an indication that adequate additive was retained in the mould to produce a good compacted graphite iron casting. Retaining excess additive in a treatment chamber does not lead to overtreatment or other adverse effects. However, the use of excessive treatment additive is not economical.

[0027] The mould downsprue had a cross sectional area of approximately 12.9 cm² (two square inches). The total cross sectional area of the runners between the downsprue and the two additive chambers also totalled about 12.9 cm (2 square inches). The total cross sectional area of the choked exits of the reaction chambers was 0.84 times that of the downsprue, (12.9 cm²) for a total of 10.84 cm² (1.68 square inches) or 5.42 cm² (0.84 square inches) per runner. Thus, the molten iron exited from the treatment chamber at a lower rate than the pour rate of the metal. The choke served to provide adequate time for the nodularizing agent to be in contact with the poured metal so that it was uniformly and adequately treated.

[0028] The treatment chambers themselves-were shaped to provide a substantially rectangular surface for interaction between the inoculant and poured iron. Each -chamber has a box-like shape with slightly outwardly sloping walls. The measurement of the chamber at the top of the additive was 6.27 cm by 5.05 cm (2.47 by 1.99 inches), at mid depth of the additive, 6.02 cm by 4.80 cm (2.37 by 1.89 inches) and at the bottom of the chamber 5.77 cm by 4.55 cm (2.27 by 1.79 inches).

[0029] Table I shows the chemistries and pour temperatures of iron used to cast four test runs of several thousand c.g. iron exhaust manifolds. T.C. is the total carbon, Si the silicon, and S the sulphur content in weight percent.. The temperature is the pour temperature in degrees Celsius. A flat layer of the above described additive was introduced into each drag mould (approximately 0.175 kg (0.385 pounds) additive per chamber). The cope mould was then set in place and desulphurized iron poured into the mould basin by hand at a rate of about 10.21 kg per second (22.5 pounds per second). The castings were allowed tc cool at room temperature in the sand mould for about 45 minutes before removal.

[0030] Twenty castings were randomly selected from each of the first three runs and subjected to microscopic and X-ray examination. Table II sets forth the results of these tests.

[0031] Note: All castings were selected from casting batch at random.

[0032] The production is the number of castings poured. The nodularity range is the microscopically observed volume percent graphite which is in nodular form in a section of a manifold casting taken along a bolt boss. Figure 3 is a typical micrograph of a c.g. iron manifold casting made in accordance with the invention. The precipitated graphite appears as dark spots in the lighter ferritic iron matrix 40. In this casting about 65% of the graphite is in nodular form as indicated at 42 and the balance in modified flake or compacted form as indicated at 44. Runs 1 and 4 had higher nodularity than hoped for, but we believe this is due to the difficulties involved with maintaining strict control in manually poured experimental casting. Run 2 was entirely satisfactory in that the casting nodularity was consistently in the 60-70% range, 65% being the target. Some castings in Run 3 had a higher nodularity than desired.

[0033] The sample castings were X-rayed to detect any internal shrinkage which usually takes the form of bubble-like voids in the matrix. Castings with 80% or greater nodularity showed shrinkage defects. Because the moulds were riserless, excess shrinkage during cooling would be expected to produce some shrinkage defects. However, no internal or external defects due to shrinkage were found in the Run 2, 60-70% nodular c.g. iron castings.

[0034] Accordingly, we have provided the first production adaptable method of making c.g. iron castings by treating desulphurized iron in the mould with a conventional nodularizing agent. Key to our method is a controlled pour rate and a predetermined treatment chamber size based on a treatment constant of 0.176 + 0.021 kg/cm². second (2.5 + 0.3 pounds per inch². second).

[0035] While the invention has been described in terms of the specific embodiment thereof, other forms may be readily adaptable by one skilled in the art. Therefore, the invention is to be limited only as set forth in the following claims.

Claims

1. A method of consistently producing substantially shrinkage free compacted graphite iron castings by treating molten iron with a magnesium-containing inoculant in a riserless foundry mould, in which the method comprises casting molten iron containing about 2.5 to 5 weight percent free carbon and less than one weight percent sulphur into said foundry mould at a controlled pour rate, characterised in that the method includes flowing said molten iron over and reacting it with a magnesium-containing inoculant (16) substantially free of elements deleterious to carbon nodule formation and containing 5 to 7 weight percent magnesium such that the contact area therebetween throughout the pour is equal to said controlled pour rate divided by a treatment constant of 0.176 + 0.021 kg/cm². second (2.5-+ 0.3 pounds per inch^2. second), and allowing the inoculant-treated iron to flow into and solidify in a casting cavity (32) in said foundry mould to produce a solidified casting in which 30 to 70 volume percent of the free carbon in the solidified casting is in the form of nodules (42) and the balance is in the form of compacted graphite (44).

2. A method of consistently making compacted graphite iron castings according to claim 1, characterised in that the molten iron is poured into said foundry mould at a rate of from 6.35 to 11.34 kg per second (14 to 25 pounds per second),
said molten iron is caused to flow over and react with a bed (16) of said magnesium-containing inoculant such that the contact area between the iron and the inoculant throughout the pour is equal to the metal pour rate in kilograms per second divided by said treatment constant; and the flow of said metal is choked as it exits said contact area to a flow rate of 75 to 95 percent of the iron pour rate.

3. A method of consistently making compacted graphite iron castings according to claim 1, characterised in that the method includes: providing a foundry mould comprising a treatment chamber (12) for retaining the magnesium-containing inoculant (16), a choke (24) at the outlet of the treatment chamber, and a casting cavity (32) downstream thereof, the treatment chamber being sized so that the contact area between the poured iron and the inoculant in the chamber is equal to the metal pour rate in kilograms per second divided by a treatment constant of from 0.155 to 0.197 kg/cm^2- second (2.2 to 2.8 pounds per inch^2. second), and the choke (24) has an area such that the flow rate therethrough is 75 to 95 percent of the iron pour rate;
retaining the inoculant (16) in the treatment chamber (12) in an amount of at least 0.35 percent by weight of the cast metal, said additive comprising about 5-7'weight percent magnesium as the principal inoculant; and pouring molten iron into the mould at a rate of from 6.35 to 11.34 kg per second at a temperature from 1316° to 1538° C.

Drawing