US3689255A - Process for the production of cast iron with spherolites - Google Patents

Abstract

A CAST IRON WITH SPHEROIDAL GRAPHITE IS PRODUCED WITH A RELATIVELY LOW SILICON CONTENT WHICH HAS IMPROVED PLASTICDEFORMATION BEHAVIOR AND TOUGHNESS AT ROOM TEMPERATURES AND BELOW TO-40*C. COBALT AND MAGNESIUM ARE ADDED IN COMMON AND SIMULTANEOUSLY TO ACHIEVE RESIDUAL CONTENTS OF FROM 0.1 TO 1.5% AND FROM 0.001 TO 0.08% BY WEIGHT, RESPECTIVELY.

Description

United States Patent 3,689,255 PROCESS FOR THE PRODUCTION OF CAST IRON WITH SPHEROLITES Erich K. Modl, Winterthur, Switzerland, assignor to Sulzer Brothers, Ltd., Winterthur, Switzerland N0 Drawing. Filed Nov. 30, 1970, Ser. No. 93,802 Claims priority, application Switzerland, Dec. 30, 1969,

Int. Cl. C22c 37/04 U.S. Cl. 75-123 CB Claims ABSTRACT OF THE DISCLOSURE Heretofore, it has been known to treat ferrous based melts which are to be used in the production of ductile cast iron, i.e. cast iron containing spheroidal graphite, with Fe-Si (iron-silicon) master alloys containing magnesium (Mg). However, for grades having increased toughness, it has been necessary to have as low a silicon content as is possible since a high silicon content produces brittleness of a ferritic basic mass.

Further, where a treatment medium has been added to a melt to be used in the production of cast iron containing spheroidal graphite, in order to maintain the silicon content of the basic melt substantially unchanged, the treatment medium has been a silicon-free magnesium master alloy, such for example, a Ni-Mg (nickel-magnesium) or Cu-Mg (copper-magnesium). However, the nickel or copper which has thus been introduced produces no ferritization but instead favors the pearlite formation. Consequently, these known magnesium containing silicon-free master alloys should not be used for the production of ductile cast irons which are to possess a ferritic basic structure.

It has been known how to use iron-magnesium (Fe- Mg) master alloys, in the form of compressed material, as treatment additives for cast iron with spheroidal graphite. However, the use of Fe-Mg master alloys has not been reliable, and the formation of spherolites has often been imperfect. In this regard, it is noted that an unfavorable shape and a small number of spherolites lowers the mechanical characteristics of the castings made from cast iron having such spherolitic graphite. A further drawback of these specifically master alloys having light weights reside in their handling because these alloys do not, of themselves, become submerged in the basic ferrous melt, but float upon the surface, and thus impair the magnesium (Mg) yield. When used in an immersion process, the treatment costs become substantially increased.

Accordingly, it is an object of the invention to produce a cast-iron material having a relatively low silicon content and a ferritic basic structure in a cast state.

It is another object of the invention to form a casting having an improved plasic deformation behavior at room temperature and at lower temperatures.

Briefly, the invention provides a process for producing a ductile cast iron which can be used to form a casting having improved plastic-deformation behavior and improved notch-impact toughness values.

The process includes the step of adding to an initial melt at least metallic cobalt and magnesium in common and simultaneously, in quantities that lead to residual magnesium contents of 0.01 to 0.08% by weight, and to cobalt contents of 0.1 to 1.5% by weight, in the casting.

It is true that it is known how to use cobalt as an alloying element for ductile cast iron. However, the cobalt has been added before a magnesium treatment has been performed. As a result, in spite of the ferritizing effect found with a chemical composition comparable to that of a cast iron of the invention, particularly one having an equal carbon and silicon content, and in spite of an increase in the number of spherolites and an improvement of the spherolite formation, it has been found that the subsequent magnesium treatment allowed the familiar undesirable effect of the magnesium carrier to gain influence again. In addition to this, where cobalt has been used previously as an alloying element the effective cobalt content has been relatively large, i.e. in the region of approximately 3% to 6% by Weight of cobalt.

Apart from its positive characteristics, which result from the effects of the ferritic basic structure and the low silicon content, the treatment process of the invention also has the same advantages as nickel-magnesium or the copper-magnesium treatments, particularly with respect to the simple manipulation required for introduction into the melt.

By achieving substantailly lower cobalt contents, the advantages found in investigations with cobalt-alloyed cast iron with spheroidal graphite, particularly as to mechanical and technological values, are obtained to an increased degree. It has furthermore been found that with simultaneous cobalt treatment, it is possible to keep the residual magnesium content needed for the formation of the spherolites particularly low. This is of particularly great importance in order to avoid dross and its known unfavorable influence on the quality of castings.

In accordance with the invention, the magnesium may be replaced, in part, by at least one other of the elements known to promote a formation of spherolitic graphite, such for example as calcium, yttrium and/or rare earthmetals, for example, cerium, lanthanum, etc.

In order to promote still lower silicon contents (below 1.3% by weight), it is advantageous to add aluminum in quantities of up to 3% by weight to the melt, at the same time the magnesium and cobalt are added.

Since magnesium is insoluble in its solid state in cobalt, it is not possible, because of the alloying behavior of these two elements, to smelt a actual magnesium-cobalt master alloy. The addition of the treatment medium to the molten ferrous based materials can, however, be accomplished advantageously by adding compressed and/ or sintered formed items, briquettes or compressed items, that contain in finely comminuted form at least cobalt and magnesium, and are except for impurities, free of silicon. Porous cobalt-carrying substances impregnated with magnesium can also be added to the molten metal. Also, formed items, produced from comminuted or pulverized cobalt and mag nesium by the aid of binding mediums, such for example, as lime milk or cement, that do not influence the quality of the melt can be used to introduce the cobalt and magnesium.

One particular field of application for a ductile cast iron treated with a cobalt-magnesium additive is in the production of a casting that upon solidification displays an aligned dendritic crystalline growth, such for example, as a thin-walled sand casting, a continuous-length casting or a casting made in permanent molds producing a relatively great shrinkage effect, e.g. a casting made in a chill-mold.

The invention is also applicable to a cast iron with spheroidal graphite having a maximum silicon content of 2.1% by weight, a maximum phosphorous content of 0.05% by weight, and a maximum magnanese content of 0.1% by weight, for making castings having a preponderant ferritic basic structure and that at room temperature and at lower temperatures down to -40 C. display an improved plastic deformation behavior. It is hereby possible to tolerate in the ferritic basic structure, depending on the required plastic deformability, a pearlite proportion going up to about 20% as viewed at the surface of the microstructure under a 100-times optical enlargement.

The magnesium-cobalt treatment of the invention also allows a casting, apart from special cases, to be used in the cast state without further time-consuming and expensive heat-treatments.

The invention is explained in more detail in the following by the aid of the following examples.

EXAMPLE 1 In a high-frequency induction crucible furnace of suitable size, lined with masses of compressed magnesite, about 11 kilograms (kg.) of magnetic iron scrap was melted down and carburized with 0.5 kilogram (kg) of graphite.

The basic ferrous melt was of the following chemical composition: carbon (C)=3.7; silicon (Si)=1.80; manganese (Mn) =0.13; phosphorous (P) =0.01; sulphur (S) =0.008.

The basic melt was treated with a cobalt-magnesium additive medium at a temperature of 1480 C., or C. The additive medium had previously been prepared, by a powder-metallurgical technique, from a mixture of 76% powdered cobalt (99.56% Co content, grain size 40 microns tm.]), 4% cesium (Ce) powder of a similar grain size, and 20% magnesium (Mg) powder (99.8% Mg, grain size 100 MIL), the form of compressed tablets which had been cold-pressed with a pressure of 3.5 metric tons per square centimeters (cm?). The quantity of treatment medium added amounted to 1% of the weight of the melt. The additive was introduced through immersion, by means of a graphite bell, in the molten metal. The treatment proceeded smoothly, and without any special light effects. At about 1430 C., the treated melt was inoculated with 0.3% Fe-Si (75%), and was then cast into specimens for testing (Specimen Y-block size 2 in per ASTM A536 67) the casting being done in dry slightly preheated sand molds. Chemical analysis of the specimens showed: Percentages by weight: C=3.84; P=0.01; S=0.007; Si=2.25; Mn=0.15; Mg=0.027; Co=0.50.

The specimens were then investigated in the usual way. Determination of the mechanical characteristics yielded the following measured individual values: Tensile strength: 47.7 and 48.3 kg./mm. Yield strength: 31.0 and 33.2 -kg./mm. Elongation in 2 inches: :18.7 and 18.8%; BHN (Brinell hardness number). 143 and 144 kg./mm.

Investigation of the microstructure showed numerous well-formed spherolites in the preponderatingly ferritic basic structure, the pearlite prooprtion being less than 3% in the cast state.

EXAMPLE 2 A ferrous based melt, smelted as described above and whose silicon content amounted to approximately 1.7% by weight, was treated with a treatment medium made by cold-compression from cobalt, magnesium and aluminum in the following proportions: 60% cobalt; 20% magnesium; 20% aluminum. The treatment temperature of Example 1 was retained.

The specimens, cast at the same casting temperature as Y test-pieces, had the following mechanical characteristics: Tensile strength 49.4 kg./mm. Yield strength 34.3 kg./mm. Elongation in 2 in.: 16.8%; BHN: 154 kg./ mm. Charpy V-notch impact values: 1.8 ml g./cm. at 20 C.; 1.2 mkg./cm. at 40 C.

Investigation of the structure again showed well-formed spherolites in a ferritic basic structure having pearlite in a maximum proportion of 5%.

EXAMPLE 3 For the continuous casting of horizontal bars (35 to 43 mm. diameter) from ferrous material with spheroidal graphite having a preponderatingly ferritic basic structure, it is necessary periodically to re-treat the basic melt, pretreated with various kinds of Mg master alloys, in heat retaining containers with Mg-containing additives, for the purpose of compensating the fading-away effect of the initial magnesium (Mg) treatment.

When use is made of the usual Si-containing magnesium master alloys, this results in a continuous increase of Si, which impairs the plastic-deformation capacity of the ferritic continuously-cast length. If in accordance with the invention, the Mg after-treatment is done with the Si-free Mg-Co additive, then no undersirable increase of Si occurs, and the originally optimum Si content can be positively maintained.

The quantity of iron present in each case in the heatretaining container of the continuous-casting machine is, at intervals of about 1 hour given an addtion of 400 to 800 kg. of pretreated liquid metal. The treatment is carried out by the immersion process, with 3.4 kg. of Mg-Co additive at intervals of 10 to 15 minutes. The treatment temperature is 1340 C. The continuous-casting produced in this way has the following measured final values: Yield strength: 38 to 45 kg./mm. Tensile strength: 43 to 65 kg./mm. Elongation in 2 in.: 17% to 21%; BHN: kg./mm.

The microstructure showed across the entire cross-section a very uniform and preponderant ferritic solidification, with perfectly-formed small spherolites.

By using Mg-Co additives a definite high degree of uniformity of the structure occured, and. the separating-out of eutectic cementite was suppressed, while slag and dross inclusions were diminished.

The invention is obviously not limited to the above described examples. It is, of course, also easily possible to increase the already very good mechanical characteristics of the castings obtained in their cast state by heat treatment.

What is claimed is:

1. A process for the production of cast iron with spheroidal graphite comprising the step of adding at least metallic cobalt and metallic magnesium in common and simultaneously to a melt with the cobalt and magnesium being added in quantities to produce a residual cobalt content of from 0.1% to 1.5 by weight and a residual magnesium content of from 0.01% to 0.08% by weight.

2. A process as set forth in claim 1 which further comprises the step of adding at least one element from the group consisting of spherolite forming elements to the melt in common and simultaneously with the cobalt and magnesium.

3. A process as set forth in claim 2 wherein said elements include calcium, strontium, barium, scandium, thorium, yttrium and rare earth metals.

4. A process as set forth in claim 2 which further comprises the step of simultaneously adding aluminum in a quantity of up to 3 by weight of the melt with the cobalt and magnesium.

5. A process as set forth in claim 1 which further comprises the step of simultaneously adding aluminum in a quantity of up to 3 by weight of the melt with the cobalt and magnesium.

6. A process as set forth in claim 1 wherein the cobalt and magnesium are in comminuted form within a preformed silicon free item.

7. A cast iron made in accordance with claim 1 for a casting having a preponderant ferritic basic structure containing 2.1% by weight maximum of silicon, 0.05% by weight maximum phosphorous, 0.1% by weight manganese, 0.1% to 1.5% by weight of cobalt and 0.1% to 0.08% by weight of magnesium and having an improved plastic deformation behavior and improved notch-impact toughness values at room temperatures and at lower temperatures down to -40 C.

8. A process for the production of cast iron with spheroidal graphite consisting of the step of adding metallic cobalt and metallic magnesium in common and simultaneously to a ferrous melt, said cobalt being added in an amount by weight greater than the amount of added magnesium.

9. A process as set forth in claim 8 wherein the metallic cobalt is added in an amount of from 0.1% to 1.5% by weight of the total weight of the resultant melt.

10. A process as set forth in claim 8 wherein the metallic magnesium is added in an amount of from 0.01% to 0.08% by weight of the total weight of the resultant melt.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 5/1964 Great Britain.

L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R.

15 7s-130 A, 130 AB UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3: 9, 55 Dated September 5, i 1972 Erich K. Modl Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, line after "a" insert --ferrous-- Claim 1, line 7, after "weight. insert --in the resultant Y melt.-- I

Signed and sealed this 13th day of March 1973..

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-IOSO (10-69) I USCOMM-DC 6O376-P69 u.s. GOVERNMENT PRIN'HNG OFFICE: 1909 o-aes-aaa