EP1499750A2

EP1499750A2 - Inoculation alloy against micro-shrinkage cracking for treating cast iron castings

Info

Publication number: EP1499750A2
Application number: EP03740659A
Authority: EP
Inventors: Thomas Margaria
Original assignee: Pechiney Electrometallurgie SAS
Current assignee: Ferroglobe France SAS
Priority date: 2002-04-29
Filing date: 2003-04-24
Publication date: 2005-01-26
Also published as: BR0309658A; FR2839082B1; US20050180876A1; AU2003265514A1; JP2005528522A; WO2003093514A2; FR2839082A1; CN1665941A; KR20040097396A; WO2003093514A3; NO20045177L; ZA200408584B; MXPA04010671A; CA2484036A1

Abstract

The invention concerns inoculation alloys for treating cast iron castings containing (by wt. %) 0.005 % to 3 % of an element of the group consisting of bismuth, lead and antimony, 0.3 to 10 % of metals of rare-earth group and optionally aluminium up to 5 % and calcium up to 1.5 %, the rest being ferro-silicon, lanthanum constituting more than 90 % of the rare-earth metals contained in the composition. The inventive alloys enable efficient inoculation of cast iron and avoid occurrence of micro-shrinkage cracking in the cast parts. The alloys are conditioned in the form of slugs or powder.

Description

Inoculating alloy anti microretassure for treatment of cast iron.

Field of the invention

The invention relates to the treatment in the liquid state of cast iron intended for the manufacture of parts for which it is desired to obtain a structure free of iron carbides and an absence of micro-shrinkage.

State of the art

Cast iron is a well-known iron-carbon-silicon alloy widely used for the manufacture of mechanical parts. It is known that to obtain good mechanical properties of these parts, it is ultimately necessary to obtain an iron + graphite structure while avoiding as much as possible the formation of iron carbides of Fe ₃ C type which harden and weaken the alloy. Then it may be desired that the graphite formed is spheroidal, vermicular or lamellar, but the essential prerequisite to be fulfilled is to avoid the formation of iron carbide. To this end, the liquid iron undergoes an inoculation treatment before casting which favors the appearance of graphite on cooling rather than that of iron carbide. Inoculation treatment is therefore very important. Now, it is well known that inoculation, whatever the inoculants used, has an efficiency on liquid cast iron which decreases over time and which, in general, has already decreased by 50% after ten minutes; a person skilled in the art designates this phenomenon under the name of "fading effect". In order to obtain maximum efficiency, progressive inoculation is generally practiced, consisting of several additions of inoculants at different stages in the development of the cast iron. It is therefore common practice to inoculate liquid iron, on the one hand in a ladle with an inoculating alloy, for example in grains of size between 2 and 10 mm or between 0.4 and 2 mm, on the other hand " jet ", that is to say pouring the ladle with an inoculating alloy in grains of size between 0.2 and 0.7 mm, and finally" in the mold ", in fact in the feed channels molds, by placing inserts made of an inoculating material on the course of the liquid iron. These defined shaped inserts are called pawns. There are two types of pawns: __._ ,, __ "_

PCT / FR03 / 01295

2

- “molded” pawns obtained by molding the molten inoculant.

- the agglomerated pawns obtained from a pressed powder with generally very little binder, or even without a binder.

Molded pins are considered by those skilled in the art to be the best level of quality; however, agglomerated pawns are often preferred to them for cost reasons. The duration of the casting of a part being very short, the kinetics of dissolution of the pawns must be extremely fast.

Furthermore, those skilled in the art very often observe in the rooms the presence of voids of millimeter or micrometric dimensions designated by the name of micro-shrinkage. These defects weaken the parts; moreover, if a subsequent machining of the parts is necessary, for example to dress a surface, the fact of falling on such defects leads to the inevitable rejection of the defective parts.

One known way to avoid the appearance of micro-shrinkage in cast iron parts is the addition of lanthanum in liquid cast iron. This metal from the lanthanide group has the property of reducing the viscosity of cast iron, not only that of liquid cast iron just before its solidification begins, but also that of cast iron in the process of solidification, that is to say solid + liquid mixture. Everything happens as if, by adding lanthanum, the moving cast iron became thixotropic. Those skilled in the art can then, by correctly drawing their molds, collect the shrinkage in the feed feeder and thus obtain healthy parts.

Thus were successively placed on the market, first nodulants containing lanthanum, the use of which was reserved for nodular cast iron called GS cast iron, then inoculants of FeSi type with 45% Si and 2% La, also usable both for GS fonts and for lamellar graphite fonts called GL fonts.

The object of the invention is to provide inoculating alloys intended for the treatment of liquid iron allowing effective inoculation, in particular during treatment "in the mold", while avoiding the formation of micro-porosities in the parts obtained by molding.

Subject of the invention

The invention relates to inoculating alloys intended for the treatment of cast iron containing (by weight) from 0.005 to 3% of an element of the group bismuth, lead and antimony, from 0.3 to 10% of metals of the group rare earths and possibly aluminum up to 5% and calcium up to 1.5%, the rest being ferro-silicon, lanthanum constituting more than 90% of the rare earth metals used in its composition.

The alloy preferably contains bismuth at a content of between 0.2 and 1.5%, and preferably between 0.7 and 1.3%. The lanthanum content is advantageously between 0.3 and 8%, and preferably between 0.5 and 3%. The presence of at least 0.8% aluminum is advantageous, and its content is preferably between 1 and 3.5%.

The alloy according to the invention can be packaged in the form of a powder or a mixture of alloy powders of different compositions, or in the form of pegs molded from the molten alloy, or agglomerated from a powder. or a mixture of powders. This powder preferably has a particle size less than 1 mm, with a particle size fraction between 50 and 250 μm representing more than 35% by weight of the total, and a fraction less than 50 μm representing less than 25% of the total.

Description of the invention

Since an inoculant is intended by nature to lead to the production of cast iron in which the carbon is present in the form of graphite, it has appeared desirable to the applicant to develop an inoculant having anti-micro-shrinkage properties. Thus, inoculating alloys based on FeSi at 75% were added, first added with an anti-micro-cracking element which can be either lanthanum or germanium. With regard to germanium, the required contents range from 0.3 to 6%. As for lanthanum, they range from 0.3 to 8%, and preferably from 0.5 to 5%.

But more interesting solutions have appeared by imagining inoculating alloys in which the same element can fulfill several functions: thus it appeared to be particularly interesting, starting from an alloy such as that described in patent US 4432793 (Nobel-Bozel), based on ferro-silicon and containing up to 3% of bismuth, lead or antimony, and up to 3% of rare earths, to add to it an anti-microporosity element such as lanthanum, and to contract the formula obtained by optimizing the total of lanthanum and other rare earths in an Fe-Si-Bi-La alloy.

The Applicant has first verified that these new anti-microporous alloys, packaged in the usual particle sizes, namely between 2 and 7 mm, or between 0.4 and 2 mm for treatment in bags, and between 0.4 and 0.7 mm for jet treatment, exhibited good properties as inoculants. We then considered the preparation of pawns „, --- _. - _,. -

PCT / FR03 / 01295 4 inoculants with these same alloys. The result in terms of reduction of the micro-porosity was confirmed despite the addition of bismuth in the final melt.

Very good results have thus been obtained with molded pins made of FeSi type alloy containing:

- from 60 to 80%, and preferably from 72 to 78% of silicon,

- from 0.3 to 8%, and preferably from 0.5 to 5% of lanthanum,

- from 0.2 to 1.5%, and preferably from 0.7 to 1.3% of bismuth,

- from 0.8 to 5% o, and preferably from 1% to 3.5% of aluminum.

Examples

To carry out the examples described below, a cast iron charge was melted in an induction furnace and treated by the Tundish Cover process using a usual inoculating alloy of the FeSiMg type at 5% Mg and 1% Ca containing no rare earths, at a dose of 20 kg for 1600 kg of cast iron. The analysis of this liquid melt was as follows:

C = 3.7%, Si = 2.6%, Mn = 0.07%, P = 0.03%, S = 0.003%, Mg = 0.038%.

The performance at the macro- as well as the micro-porosity level was assessed by means of the “V” test piece casting test.

In this test, the test piece consists of a 110 mm high "N", with an angle at the top

40 °, the width of the branches of the "N" being 20 mm and the thickness of the part 20 mm.

This geometry gives a width of 80 mm at the top of the "N", a unit volume of 69 cm ³ , and a unit mass of 480 g to 500 g depending on the quality of the cast iron. On this type of part, the porosities appear selectively in the re-entrant part of the "N".

To assess the result of the test, the part is cut to mid-thickness, and the section is examined by optical microscopy to assess the surface of the porosities; the result is expressed in relative area compared to the area of the section.

Example 1

A treated cast iron ladle from the preliminary operation was inoculated in the ladle using an inoculating powder alloy with a particle size between 2 and 10 mm, of composition: "Foundry Grade", mainly Fe balance, used at the dose 200 g per ton of pig iron. This cast iron was used to cast N-shaped pieces of geometry identical to that defined in the control test, arranged in a cluster in a 36-piece sand mold fed by a supply channel where a filter made up of a refractory foam.

The parts obtained were examined by optical microscopy on a polished section to determine the structure of the metal as a function of the depth and the level of porosity.

At the heart of the branches, the density of the graphite nodules was measured at 120 / mm ² .

The average porosity of the parts was evaluated at 2.4%.

Example 2

A second pocket of treated cast iron coming from the preliminary operation was inoculated in the pocket using an inoculating alloy with a particle size between 2 and 10 mm of composition:

Si = 75.4%, Al = 0.94%, Ca ≈ 0.86%, La = 2.2%, Bi = 0.92%, balance mainly Fe, used at a dose of 200 g per tonne of melting.

This cast iron was used to cast N-shaped pieces of geometry identical to that defined in the control test, arranged in a cluster in a 36-piece sand mold fed by a supply channel where a filter made up of a refractory foam.

The parts obtained were examined by light microscopy on a polished section to determine the structure of the metal as a function of the depth and the level of porosity. At the heart of the branches, the density of graphite nodules was counted at 360 / rnm ² .

The average porosity of the parts was evaluated at 0.3%.

Example 3

A third treated cast iron ladle coming from the preliminary operation was used to cast N-shaped pieces of geometry identical to that defined in the control test, arranged in a cluster in a 36-piece sand mold fed by a channel. lead where was placed a pawn of 25 g consisting of inoculating alloy for treatment in the mold, of composition:

Si = 73.6%, Al = 3.92%, Ca = 0.78%, La = 2.1%, Bi = 0.97%, mainly Fe balance. The parts obtained were examined by optical microscopy on section polished to determine the structure of the metal based on the depth and level of porosity. At the heart of the branches, the density of the graphite nodules was counted at 320 / mm ² . The average porosity of the parts was evaluated at 0.2%.

Claims

claims

1. Inoculating alloy for casting iron, containing (by weight) from 0.005 to 3% of an element from the bismuth group, lead and antimony, from 0.3 to 10% of metals from the rare earth group and possibly from aluminum up to 5% and calcium up to 1.5%, the rest being ferro-silicon, characterized in that lanthanum constitutes more than 90% of the rare earth metals used in its composition.

2. Alloy according to claim 1, characterized in that it contains 0.3 to 8% lanthanum and 0.2 to 1.5% bismuth.

3. Alloy according to one of claims 1 or 2, characterized in that its bismuth content is between 0.7 and 1.3%.

4. Alloy according to one of claims 1 to 3, characterized in that its lanthanum content is between 0.5 and 5%.

5. Alloy according to one of claims 1 to 4, characterized in that its aluminum content is between 0.8 and 5%.

6. Alloy according to claim 5, characterized in that its aluminum content is between 1 and 3.5%.

7. Alloy according to one of claims 1 to 6, characterized in that it is packaged in powder.

8. Alloy according to one of claims 1 to 6, characterized in that it is packaged in the form of pins for treatment "in the mold".

9. Alloy according to claim 8, characterized in that the pin is obtained by molding the molten alloy.

10. Alloy according to claim 8, characterized in that the pin is obtained by agglomeration of a powder.

11. Alloy according to claim 10, characterized in that the particle size of the powder is <1 mm, the particle size fraction between 50 and 250 μm representing more than 35% by weight of the total, and the fraction less than 50 μm less than 25%.

12. Alloy according to one of claims 10 or 11, characterized in that the average composition of the alloy is obtained by mixing powders of alloys of different compositions.