GRADE OF DUCTILE IRON WITH REINFORCED FERRITIC MATRIX

Abstract

A grade of ductile cast iron having a tensile strength (Rm) equal to or greater than 700 MPa, an elongation at break equal to or greater than 11%, a yield strength (Rp.sub.0.2) equal to or greater than 532 MPa, and a minimum ratio of yield strength (Rp.sub.0.2) to tensile strength (Rm) of 76% in a tensile test performed on a specimen taken from a tensile bar in accordance with standard EN 1563 and having a diameter of 25 mm and a length of 200 mm.

Claims

1. A ductile cast iron with a strengthened ferritic matrix structure, wherein the strengthened ferritic matrix structure comprises more than 70% of ferrite, the ductile cast iron has a tensile strength (Rm) equal to or greater than 700 MPa, an elongation at break equal to or greater than 11%, a yield strength (Rp.sub.0.2) equal to or greater than 532 MPa and a minimum ratio of yield strength (Rp.sub.0.2) to tensile strength (Rm) of 76% in a tensile test performed on a specimen taken from a tensile bar in accordance with standard EN 1563 and having a diameter of 25 mm and a length of 200 mm, and in that the ductile cast iron is formed by the following elements, in percentage by weight: carbon, between 2.8% and 4.5%; silicon, between 3% and 5%; manganese, between 0.1% and 0.8%; sulfur, less than 0.01%; phosphorus, between 0.001% and 0.05%; copper, between 0.1% and 1%; magnesium, less than 0.1%; nickel, between 0% and 1%; chromium, between 0% and 1%; impurities, less than 0.2%; and iron, making up 100%.

2. The ductile cast iron of claim 1, wherein the percentage by weight of copper is between 0.1% and 0.8%.

3. A method for preparing a ductile cast iron according to claim 1, the method comprising: preparing a raw material containing carbon, silicon, manganese, sulfur, phosphorus, copper, magnesium and iron; melting the raw material; inoculating the raw material; applying a spheroidizing treatment; and casting the inoculated material.

4. The method of claim 3, wherein the inoculating the raw material is carried out twice, three times or four times.

5. The method of claim 3, wherein the spheroidizing treatment is carried out until a proportion of form VI graphite particles of greater than 90% is obtained.

6. The method of claim 3, wherein casting the inoculated material comprises producing a cast part, wherein the method does not include application of a heat treatment to the cast part.

7. A cast element in the form of a high-voltage power line insulator cap or a mechanical part configured for use in transport, mining or power generation, the cast element consisting essentially of a ductile cast iron according to claim 1.

8. The ductile cast iron of claim 1, wherein the percentage by weight of silicon is between 4.3% and 5%.

9. The ductile cast iron of claim 2, wherein the percentage by weight of copper is between 0.1% and 0.7%.

10. The method of claim 3, wherein the preparing the raw material further comprises forming the raw material to further contain at least one of nickel and chromium.

11. The method of claim 5, wherein the spheroidizing treatment is carried out until a proportion of form VI graphite particles of greater than 95% is obtained.

12. The method of claim 4, wherein the spheroidizing treatment is carried out until a proportion of form VI graphite particles of greater than 90% is obtained.

13. The method of claim 12, wherein casting the inoculated material comprises producing a cast part, wherein the method does not include application of a heat treatment to the cast part.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present disclosure will be better understood and further advantages will become apparent from the following description and from the accompanying drawings, in which:

[0037] FIG. 1 is a microstructure image of an SG iron with a ferritic-pearlitic matrix according the present disclosure before nital etching; and

[0038] FIG. 2 is a microstructure image of an SG iron with a ferritic-pearlitic matrix according the present disclosure after nital etching.

DETAILED DESCRIPTION

[0039] The development of ductile spheroidal graphite (SG) cast irons with a strengthened ferritic matrix stems from the objective of manufacturers to reduce variations in the mechanical properties and the machinability costs of parts.

[0040] Known traditional ductile cast irons with a predominantly pearlitic microstructure matrix have the mechanical properties shown in Table 1 based on tests carried out with grade EN-GJS-600-3.

TABLE-US-00003 TABLE 1 # Rm (MPa) Rp.sub.0.2 (MPa) A % Rp.sub.0.2/Rm (in %) 1 713 508 8.4 71.2% 2 646 452 11.8 70.0% 3 689 495 9.8 71.8% 4 708 500 8.8 70.6% 5 719 512 8.3 71.2% 6 686 485 9 70.7% 7 721 512 8.2 71.0% 8 646 452 11.8 70.0% 9 686 480 9.3 70.0% 10 725 511 8.2 70.5%

[0041] In addition to iron, the composition of these cast irons contains the percentages by weight shown in Table 2 below.

TABLE-US-00004 TABLE 2 wt % C Si Mn P S Cu Mg min 3 2.1 0.1 0.2 0.02 max 4 3 0.5 0.1 0.05 0.7 0.06

[0042] Although the mechanical properties of these cast irons are already remarkable, they do not achieve very high values of tensile strength (greater than or equal to 700 MPa), yield strength (Rp.sub.0.2/Rm greater than or equal to 76%) and elongation at break (greater than 8%, or even greater than or equal to 11%).

[0043] The mechanical characteristics of these predominantly pearlitic cast irons are also highly sensitive to the thickness of the part produced.

[0044] The ductile cast iron according to the present disclosure has a strengthened ferritic matrix. This means that its structure is predominantly ferritic (>70%) and that it can comprise pearlite in a minority proportion; the structure of the ductile cast iron according to the present disclosure can therefore also be described as a ferritic-pearlitic matrix.

[0045] The cast iron of the present disclosure achieves on average a tensile strength (Rm) greater than or equal to 700 MPa, a yield strength (Rp.sub.0.2) greater than or equal to 532 MPa, a minimum ratio of yield strength (Rp.sub.0.2) to tensile strength (Rm) of 76%, and an elongation at break that is always greater than 8% and even greater than or equal to 11% in tensile tests performed on a test specimen taken from a tensile bar in accordance with standard EN 1563 and with a diameter of 25 mm and a length of 200 mm. Four test results (A to D) are given in Table 3 below.

TABLE-US-00005 TABLE 3 # Rm (MPa) Rp.sub.0.2 (MPa) A % Rp.sub.0.2/Rm (in %) A 725 557 11.5 76.8 B 710 556 11.5 78.3 C 718 559 13 77.8 D 709 539 12 77

[0046] In addition to iron (and unavoidable impurities), the composition of this grade of cast iron contains the percentages by weight shown in Table 4. The composition of a grade according to the present disclosure is also given, by way of example, in Table 4.

TABLE-US-00006 TABLE 4 wt % C Si Mn P S Cu Mg Ni Cr min 2.8 3 0.1 0.001 0 0.1 0 0 0 max 4.5 5 0.8 0.05 0.01 1 0.1 1 1 Exam- 3.17 4.30 0.22 <0.012 0.0035 0.59 0.039 0 0 ple

[0047] In the ductile cast iron according to the present disclosure, the silicon helps to strengthen the ferritic matrix; the percentage of copper is limited to the range [0.1%-1%], or even to the range [0.1%-0.8%] in order to control the proportion of pearlite in the matrix. Preferentially, the copper content is even limited to 0.7%.

[0048] This grade of ductile cast iron reveals a ferritic-pearlitic (predominantly ferritic) microstructure, as can be seen in FIG. 1 before etching with nital (a solution of nitric acid and alcohol commonly used for the chemical etching of ferrous metals) and in FIG. 2 after etching with nital.

[0049] By virtue of its ferritic or ferritic-pearlitic structure, the mechanical properties of the ductile cast iron are much less sensitive to the thickness of the part produced, which is a major industrial advantage.

[0050] To prepare a ductile cast iron according to the present disclosure for the production of cast parts, the method first comprises a step of preparing a raw material containing carbon, silicon, manganese, sulfur, phosphorus, copper, magnesium, iron and potentially nickel and chromium, in the ranges of percentages by weight mentioned hereinabove. The method then comprises the steps of melting the raw material, inoculating the material, and applying a spheroidizing treatment, prior to the step of casting the inoculated material. These steps are carried out so that the ductile cast iron has a strengthened ferritic structure and the mechanical characteristics mentioned hereinabove, i.e., a tensile strength (Rm) greater than or equal to 700 MPa, a yield strength greater than or equal to 532 MPa, a minimum ratio of yield strength (Rp.sub.0.2) to tensile strength (Rm) of 76% and an elongation at break greater than or equal to 11%.

[0051] The products for the inoculating treatment are those listed in conventional supplier catalogs, but require special attention in terms of both composition and particle size in order to obtain the Rm, Rp.sub.0.2 and A mechanical characteristics mentioned hereinabove. In addition, the choice of inoculant must be matched to the spheroidizing treatment product (subsequent step). In particular, the active element in the inoculant (barium, strontium, zirconium) should be chosen according to the composition of the spheroidizing alloy.

[0052] The inoculation step can be carried out once, twice, three times or even four times. Typically, this is carried out each time molten metal is transferred, including in the melt stream feeding the molten metal into the mold and/or when placing an ingot in the pattern plate.

[0053] The products for the spheroidizing treatment are also those listed in conventional supplier catalogs, but require special attention in terms of both composition and particle size in order to obtain the Rm, Rp.sub.0.2 and A mechanical characteristics mentioned hereinabove. The content of active elements, such as magnesium, calcium and rare earths, should be rigorously controlled. All conventional introducing processes for spheroidization (tundish cover, sandwich, in-mold, etc.) are applicable to the present method.

[0054] As can be seen in FIG. 1, the quality of spheroidization is very good when the proportion of form VI graphite particles is greater than 90%. It is optimal when the proportion of form VI graphite particles is greater than 95%, with 5% form V particles or less, i.e., 100% nodularity. Graphite particle counting can be carried out manually or by software. For the classification of graphite particles, reference is made here to standard NF-EN-945.

[0055] The magnesium content must also be sufficient, but less than 0.1%, to ensure that the graphite particles have a spheroidal character throughout the entire range of thicknesses of the parts, without being excessive.

[0056] The mechanical characteristics of the ductile cast iron according to the present disclosure are obtained, in particular, with a high density of graphite spheroids per unit polished surface, i.e., greater than 1000/mm.sup.2 of polished surface.

[0057] It should be noted here that the composition of the ductile cast iron according to the present disclosure falls within the ranges defined by the min and max limits at the end of the production process. During the various steps (providing the raw material, inoculation, spheroidization), it will therefore be ensured that the compounds are added in such a way as to observe the ranges in the melt that is ultimately obtained.

[0058] The method also relates to the production of a molded part that does not undergo any heat treatment. Heat treatments are usually applied to cast iron parts to correct microstructural imperfections: obtaining a molded part in ductile cast iron that has the desired structure and mechanical properties directly, without the need for heat treatment, is a major economic advantage.

[0059] The ductile cast iron according to the present disclosure is used for the production of cast mechanical parts, such as a high-voltage power line insulator cap, or for the production of cast mechanical parts in the fields of transport, mining or power generation.

[0060] It goes without saying that the present disclosure is not restricted to the embodiment described above, and may be modified without departing from the scope of the invention as defined by the following claims.