Patent Application
20150284830
The present invention relates to an inoculant alloy for treating cast-iron.
Cast-iron is a well-known iron-carbon alloy widely used for manufacturing mechanical parts. Cast-iron is obtained by mixing the constituents of the alloy in the liquid state at a temperature comprised between 1320 and 1450° C. before casting in a mold and by cooling the obtained alloy.
When cooling, carbon may adopt several physicochemical structures depending on several parameters.
When carbon is associated with iron and forms iron carbide Fe3C (also called cementite), the resulting cast-iron is called white cast-iron. White cast-iron has the characteristic of being hard and brittle, which is not desirable for some applications.
If carbon appears in the form of graphite, the resulting cast-iron is called gray cast-iron. Gray cast-iron is softer and may be worked.
In order to obtain cast-iron parts having good mechanical properties, it is hence necessary to obtain a structure of the cast-iron comprising the maximum of carbon in the form of graphite and limit as much as possible the formation of these iron carbides which harden and embrittle the alloy.
In the absence of any particular inoculation treatment, carbon tends however to be associated with iron so as to form iron carbide. Hence, it is necessary to treat the cast-iron in the liquid state so as to modify the association parameters of carbon and obtain the desired structure.
To this end, the liquid cast-iron undergoes an inoculation treatment aiming to introduce in the cast-iron graphitizing components or graphitization supports commonly called germs which will promote, when the cast-iron is cooling in the mold, the apparition of graphite rather than iron carbide.
In general, the components of an inoculant hence consist of elements which promote the formation of graphite and the decomposition of iron carbide during the solidification of the cast-iron. Examples may include carbon, silicon, calcium, aluminum.
Of course, an inoculant may be designed so as to fulfill other functions and comprise to this end other components having a particular effect. The cast-iron may also undergo prior or subsequent additional treatments.
Thus, depending on the required properties, it may be desired in particular that the formed graphite is spheroidal, vermicular or lamellar.
Either graphitic form may be obtained preferably by a particular treatment of the cast-iron by means of specific components.
Thus, for example the formation of spheroidal graphite may be promoted by a treatment called nodularizer treatment mainly aiming to provide the cast-iron with an enough quantity of magnesium so that graphite can grow so as to form round particles (spheroids or nodules).
These nodularizer components are generally added in the form of a specific alloy (nodularizer alloy) prior to the inoculant treatment of the cast-iron during a particular treatment.
Thus, the nodularizer alloy allows essentially to influence the shape of the graphite nodules, whereas the inoculant product aims to increase the number of these nodules and homogenize the graphitic structures.
Mention may also be made of the addition of desulfurizing products, or products allowing specifically treating some defects of the cast-iron depending on the initial composition of the liquid cast-iron bath, such as micro-shrinkages and pinholes, likely to appear during cooling.
These treatments may be performed at once or in several times and at different moments during manufacturing the cast-iron.
Most inoculants are conventionally manufactured from a ferrosilicon alloy such as FeSi45, FeSi65 or FeSi75 while adjusting the chemistry according to the aimed composition of the inoculant. It may also consist of mixtures of several alloys.
It should be noted that the inoculation efficiency of the cast-iron part also depends on its thickness (or on the solidification rate).
In the areas with small thicknesses, which cool more quickly, a higher risk of carbides formation will be noted.
Conversely, in the areas with larger thicknesses, cooling will be slower (2 to 4 hours) and will promote the formation of graphite.
As a result, parts having areas with different thicknesses may have physicochemical structures which differ from one area to another, which is not desirable.
Furthermore, controlling germination in the areas with large thickness remains difficult and may result in obtaining a non-uniform structure.
For parts with large thicknesses, when the inoculation method is not controlled, the formation of degenerated graphite and/or chunky
graphite may reduce the mechanical properties of the cast-iron. In order to solve these defects, the smelter generally proceeds to the addition of pure Antimony in the liquid metal.
The addition of pure Antimony in the liquid metal raises accuracy problems because the rate of introduction is very low (in the order of 10 to 30 g per tonne of liquid cast-iron). The addition efficiency of pure Antimony is comprised between 50 and 80% and the effective introduced quantity is hence difficult to control.
If the quantity is not enough, degraded graphite may be formed in the structure.
Conversely, if the introduced quantity exceeds the target, antimony will tend to strongly increase the perlite proportion, which phase is not desirable in ferritic structures.
In the case of addition of pure antimony, the smelter should further associate Rare Earths (abbreviated as RE) in order to achieve a maximum improvement of the form of the graphite. Similarly, if the quantity of Rare Earths is not enough, the part will have a spiky
type graphite defect. Conversely, if the quantity of Rare Earths is highly dosed, the graphite defect will rather be of the
chunky
type, which essentially happens when the used raw materials are relatively pure.
These spiky
or
chunky
type graphite defects deteriorate the mechanical properties of cast-iron, and in particular the tensile strength and the impact resistance of the formed part.
Furthermore, the introduction of pure antimony in the liquid cast-iron causes its vaporization and thus results in a strong gassing. It has been measured that, with the addition of pure antimony, the threshold of release of antimony in the working environment is higher than 0.5 mg/m3, which is the exposition limit value (ELV set by the regulations). Hence, the operators must work with a respirator for protection from N95-type particles or higher.
The treatment of parts with small thicknesses has already been considered for developments of specific inoculants. The documents FR2511044A1, FR2855186A1 and EP0816522A1 describe such an inoculant for thin parts.
According to these documents, such an inoculant for thin parts comprises in particular a ferrosilicon-based inoculant alloy and comprising between 0.005 and 3% by weight of Rare Earths, in particular Lanthanum, as well as between 0.005 and 3% by weight of bismuth, lead or antimony with a Rare Earths/(Bismuth+Lead+Antimony) ratio comprised between 0.9 and 2.2; bismuth being particularly preferred, the descriptions of these documents covering only but bismuth.
It should be noted that these documents disclose the use of antimony only in a general manner and that they contain neither specific example nor particular value related to this element.
Among the other documents that mention the use of antimony, the following documents may be cited.
The document WO2006/068487A1 describes an inoculant comprising a phase-modifying component (inoculant function) associated with an agent for modifying the structure of the graphite which may consist of antimony. It should be noted that this structure-modifying agent is used in mixture with the inoculant compound (ferrosilicon) and not in an allied form. Furthermore, antimony is clearly referred to as being a perlite promoter, which phase is not desirable in general, as above-mentioned. The used quantity of antimony is comprised between 3 and 15%, which corresponds to a significant quantity probably at the origin of the formed perlite proportion.
The document JP2200718A describes an inoculant which consists of a mixture of ferrosilicon, antimony, calcium silicide and rare earths. Antimony is not used in an allied form.
The document JP57067146A describes a ferrosilicon-based alloy comprising between 5 and 50% by weight of antimony and up to 10% of rare earths. Besides the high proportion of antimony, this alloy is used as a perlite inhibitor, and not as an inoculant.
There are also several articles and documents dealing with a nodularizer function (the graphite form) of antimony, which is not the fundamental purpose and which does not resolve the inoculation problem (number and quality of the nodules).
Furthermore, it consists most often of a use of antimony in a mixed and non-allied form.
Hence, there is a need for an inoculant alloy which allows improving the treatment of thick parts.
To do so, the present invention aims an inoculant alloy for treating thick ferrosilicon-based cast-iron parts, containing between 0.005 and 3% by weight of rare earths, characterized in that it also contains between 0.2 and 2% by weight of antimony.
Thus, it has indeed been unexpectedly observed that antimony, when allied to rare earths in a ferrosilicon-based alloy according to the claimed proportions, would allow an effective inoculation, and with the spheroids stabilized, of thick parts without the above-mentioned drawbacks of pure antimony.
In particular, the introduction of antimony in the form of an alloy allows achieving a high efficiency of use of antimony, in the order of 97 to 99%. Hence, the effective introduced quantity is known much more precisely.
Thus, the increase of efficiency allows for an economy of the products and simplifies the management of products additions, including rare earths.
Thanks to this increase of efficiency and to the simultaneous reduction of gaseous emissions in the atmosphere, the working conditions are also improved for the operators in charge of these additions.
The use of an alloy according to the invention allows limiting the gassing of antimony between 0.1 and 0.2 mg/m3 and the use of a respirator mask is no longer necessary.
It will also be noted that the antimony/rare earths association significantly lengthens the antimony decay time. Hence, the produced effect lasts longer in the complete foundry process. It will be noted that the antimony decay time is even longer than the bismuth decay time in the inoculant alloys for thin parts.
The alloy according to the present application, upon ladle or furnace addition, may thereby allow replacing and even suppressing an additional jet or late inoculation.
The alloy according to the present application also allows particularly limiting greatly and even avoiding the formation of chunky
or
spiky
type graphite defects, but also improving the form of graphite by ensuring a nodularity greater than 95% while bringing the spheroids closer to the perfect sphere.
The alloy according to the present application allows thus ensuring a homogeneous ferrite/perlite matrix across the different thicknesses of the manufactured part, which improves in particular the conditions of a subsequent machining of the part.
Preferably, the ratio Antimony to Rare Earths will be higher than 1.4, preferably higher than 1.6, and lower than 2.5, preferably lower than 2.
According to a first variant, the inoculant alloy also comprises magnesium. It will then be a nodularizer with an additional inoculant effect.
Unexpectedly, it has in particular been observed that, unlike the already used bismuth, antimony allows achieving a better efficiency of the magnesium introduced in the cast-iron.
As regards bismuth, it is known that the latter accelerates the decantation of magnesium in cast-iron and that the latter thereby loses more active magnesium serving to the transformation of lamellar graphite into spheroidal graphite. The better assimilation of antimony in the form of a nodularizer according to the invention allows ensuring a good stability of the residual magnesium between 1350° C. and 1580° C.
According to a second variant, the inoculant alloy does not contain magnesium.
Preferably, the ratio rare earths to antimony is comprised between 0.9 and 2.2.
Preferably, the proportion by weight of Antimony is higher than 0.3%, preferably higher than 0.5%, still preferably higher than 0.8%.
Preferably, the proportion by weight of antimony is lower than 1.5%, preferably lower than 1.3%.
Advantageously, the rare earths comprise Lanthanum, preferably only but Lanthanum.
Preferably, the proportion by weight of rare earths is higher than 0.2%, preferably higher than 0.3%.
Preferably, the proportion by weight of rare earths is lower than 1.2%, preferably lower than 1%.
The present invention also relates to the use of the inoculant according to the invention.
According to a first variant of use, said inoculant is introduced in the form of a powder.
It should be noted in this regard that a drawback of the products described in the documents FR2511044A1 and EP0816522A1 has been a degradation of their grain size distribution over time during storage of the inoculant. The inoculant according to the invention has shown a high stability in the grain size distribution in some conditions.
According to a second variant, said inoculant is introduced in the form of a solid insert placed in a casting mold.
Preferably, the use of the inoculant according to the invention aims to manufacture cast-iron parts having portions with thicknesses larger than 6 mm, preferably portions with thicknesses larger than 20 mm, and still more preferably portions with thicknesses larger than 50 mm.
The present invention will be better understood in light of the description and examples which follow.
The inoculant according to the invention will be typically used in the context of an inoculation of a cast-iron bath. It may also be used for pre-conditioning said cast-iron as well as a nodularizer, if appropriate.
In the context of a typical use of an inoculant, the composition of an inoculant alloy according to the invention may for example comprise:
Of course, the inoculant may also comprise additional elements which bring particular effects depending on the required properties. More particularly, this may be the case in the context of an iron-cast pre-conditioning treatment.
As example, the inoculant alloy thus may have the following composition:
An inoculation treatment will typically consist in adding from 0.05 (preferably at least 0.1%) to 0.8% by weight of the inoculant to the cast-iron bath, in particular under the following conditions given as examples:
The grain size distribution of the inoculant according to the invention may be adapted depending on its methods of addition.
Examples may include:
The inoculant alloy may also be successfully added as an inoculant before filling the casting mold or during a ladle or late inoculation, after having adjusted the chemistry of the alloy (in particular Ba between 1.5 and 5% by weight and Ca between 0.5 and 2% by weight).
Depending on the metallurgical state of the cast-iron after treatment with the inoculant alloy according to the present application, it is possible to suppress the post-inoculation step. Indeed, maintaining the inoculation effect for prolonged time periods with the action of antimony allows significantly reducing the late inoculation treatments and even suppressing them. For example, when adding an inoculant containing the Bi/RE pair, the inoculation effect loses 30% during the first 4 minutes. Thus, late stage addition of an inoculant becomes mandatory in order to recover 100% of the inoculation effect to be achieved. This is not the case with an inoculant according to the present application.
In the context of a use as a nodularizer with an additional inoculant function, the composition of the alloy will also comprise magnesium. As example, the composition of such a nodularizer alloy with an inoculant function may be as follows:
The grain size distribution of the nodularizer (in particular with an inoculant function) according to the invention will be adapted depending on the size of the treatment ladles. For example, for ladles with 100 to 500 kg of cast-iron, preference will be given to a grain size distribution comprised between about 0.4 and about 2 mm, and even up to 7 mm. For ladles with 500 to 1000 kg of cast-iron, preference will be given to a grain size distribution comprised between about 2 and about 7 mm, or between about 10 and about 30 mm. For ladles with more than 1000 kg of cast-iron, preference will be given to a grain size distribution comprised between about 10 and about 30 mm.
Examples of use will now be described.
Foundry Reference (A1)
In accordance with the prior art, the liquid cast-iron has been treated by adding, in the induction furnace, pure antimony in a proportion of 30 g of antimony per tonne of liquid cast-iron.
Afterwards, the cast-iron has undergone a nodularizer treatment by means of a FeSiMg type nodularizer alloy comprising a third of a FeSiMg alloy comprising 2% of rare earths and two thirds of a FeSiMg alloy free of rare earths.
Finally, the cast-iron has undergone an inoculation treatment by addition, in the casting basin, of 0.1% by weight of a FeSiMnZr alloy and 0.1% of a FeSiAl alloy, the inoculant alloys being added in the form of an inoculant insert in the mold.
Use of an Inoculant Alloy According to the Invention (A2)
An inoculant alloy according to the above-mentioned composition 2 and containing (by weight proportion): Si=65% Si, Ca=1.76% Ca, Al=1.23%, Sb=0.15%; RE=0.16%, Ba=7.9%; has been used in a proportion of 0.15% by weight of cast-iron.
The step of addition of pure antimony has been suppressed and the nodularizer treatment has been simplified by using only but a FeSiMg nodularizer alloy which does not contain rare earths.
Comparative Results
The foundry A, treated with an inoculant according to the present application, has shown an increase of the tensile elongation on test samples for a EN-GJS-400-15 grade.
Foundry Reference (B1)
In accordance with the prior art, the liquid cast-iron has been treated by adding, in the induction furnace, pure antimony in a proportion of 20 g of antimony per tonne of liquid cast-iron.
Afterwards, the cast-iron has undergone a nodularizer treatment by means of a FeSiMg type nodularizer alloy comprising 1% by weight of rare earths and introduced in the cast iron in the form of a cored wire.
Finally, the cast-iron has undergone an inoculation treatment by addition, in the casting basin, of 0.15% by weight of a FeSiBiRE alloy.
Use of an Inoculant Alloy According to the Application (B2)
An inoculant alloy according to the above-mentioned composition 2 and containing, as previously: Si=65% Si, Ca=1.76% Ca, Al=1.23%, Sb=0.15%, RE=0.16%, Ba=7.9%; has been used in a proportion of 0.15% by weight of cast-iron.
The step of addition of pure antimony has been suppressed and the nodularizer treatment has been simplified by using only but a nodularizer alloy FeSiMg which does not contain rare earths (also introduced in the form of a cored wire).
Comparative Results
Chunky
graphite defect
As regards the impact resistance results, the cast-iron B2 has achieved results in compliance with the requirements.
Foundry Reference (C1)
In accordance with the prior art, the liquid cast-iron has been treated by adding, in the induction furnace, pure antimony in a proportion of 25 g of antimony per tonne of liquid cast-iron.
Afterwards, the cast-iron has undergone a nodularizer treatment by means of a FeSiMg type nodularizer alloy comprising 6.7% by weight of magnesium as well as 1.2% of calcium and 0.98% of rare earths.
Finally, the cast-iron has undergone a late inoculation treatment by addition of 0.12% by weight of a FeSiMnZrBa alloy having a grain size distribution comprised between 0.2 and 5 mm.
Use of an Inoculant Alloy According to the Application with a Nodularizer Function (C2)
A nodularizer alloy with an inoculant function according to the above-mentioned composition 3 has been used.
As with the previous examples, the step of addition of pure antimony has been suppressed.
The nodularizer treatment has been performed by means of a FeSiMg type alloy according to the composition 3 of the present application and comprising 6.4% by weight of magnesium as well as 1.3% of calcium, 0.6% of antimony and 1.2% of rare earths.
A complementary inoculation has been performed according to a late inoculation method with 0.09% of a FeSiAlCa alloy and 0.009% of a FeSiMnZrBa alloy.
Comparative Results
Chunky
When using a nodularizer according to the present application, it has been noted that Chunky
graphite defects have disappeared in all controlled parts.
Thus, it has been possible to carry out the additional inoculation (late inoculation) by use of a more economical FeSiAlCa type inoculant.
Foundry Reference (D1)
In accordance with the prior art, the liquid cast-iron has been treated by adding, in the induction furnace, pure antimony in a proportion of 30 g of antimony per tonne of liquid cast-iron.
Afterwards, the cast-iron has undergone a nodularizer treatment by means of a FeSiMg type nodularizer alloy comprising 9.1% by weight of magnesium as well as 1.4% of calcium and 1.1% of rare earths.
Finally, the cast-iron has undergone an inoculation treatment by addition of a 10 kg insert per tonne of cast-iron of a FeSiMnZr inoculant alloy.
Use of an Inoculant Alloy According to the Application (D2)
An inoculant alloy according to the above-mentioned composition 2 and containing: Si=65% Si, Ca=1.76% Ca, Al=1.23%, Sb=0.15%, RE=0.16%, Ba=7.9%; has been used in the form of a 10 kg insert, in the same way as the reference.
As with the previous examples, the step of addition of pure antimony has been suppressed.
The nodularizer treatment has been performed by means of a same alloy as the reference, namely by using a FeSiMg type nodularizer alloy comprising 9.1% by weight of magnesium as well as 1.4% of calcium and 1.1% of rare earths.
Comparative Results
Chunky
graphite defect
The cast-iron D allows preparing a cast-iron EN-GJS-400-18-LT grade, used in particular in the wind power field. The use of the inoculant according to the application has allowed significantly increasing the impact resistance.
Foundry Reference (E1)
The liquid cast-iron has undergone a nodularizer treatment by means of a FeSiMg type nodularizer alloy comprising 9.1% by weight of Magnesium as well as 0.8% of Bismuth and 0.7% of rare earths.
Afterwards, the cast-iron has undergone an inoculation treatment according to a late inoculation method by addition of 0.18% of a FeSiMnZr alloy having a grain size distribution comprised between 0.2 and 5 mm.
Use of an Inoculant Alloy According to the Application with a Nodularizer Function (E2)
A nodularizer alloy according to the above-mentioned composition 3 has been used. The used alloy is a FeSiMg type alloy comprising 9.1% of magnesium as well as 0.75% of antimony and 0.5% of rare earths.
Afterwards, the cast-iron has undergone an additional inoculation treatment according to a late inoculation method by addition of 0.17% of a FeSiMnZr alloy having a grain size distribution comprised between 0.2 and 5 mm.
Comparative Results
Chunky
As mentioned above, it has been observed that, by replacing bismuth with antimony, the efficiency of magnesium in the cast-iron E has increased.
The foundry reference (F1) and the test (F2) using an inoculant alloy according to the application have been realized in accordance with the example 4 and the foundry D by inoculating massive parts.
Comparative Results
It has been observed that, thanks to the achieved high efficiency, it is possible to better control the added antimony quantity. The foundry F2 has allowed for a significant economy by reducing by 31.5% the doses of antimony to be added.
The foundry reference (G1) and the test (G2) using an inoculant alloy according to the application have been realized in accordance with the example 4 and the foundry D by inoculating massive parts.
Comparative Results
It has been observed that, thanks to the inoculant according to the present application, the antimony release is significantly limited and much lower than the regulatory threshold 0.5 mg/m3. The work conditions are thereby improved.
Foundry Reference (H1)
In accordance with the prior art, the liquid cast-iron has been treated by adding, in the induction furnace, pure antimony in a proportion of 15 g of antimony per tonne of liquid cast-iron.
Afterwards, the cast-iron has undergone a nodularizer treatment by means of nodularizer cored wire (13 mm diameter, 32% of Mg, 1.2% of RE, 230 g/m of powder).
Finally, the cast-iron has undergone a late inoculation treatment by addition, to the casting jet, of 0.15% by weight of a FeSiMnZr alloy.
Use of an Inoculant Alloy According to the Application (H2)
An inoculant alloy according to the above-mentioned composition 1 [containing Si=64% Si, Ca=1.64% Ca, Al=1.15%, Sb=0.5%, RE=0.3%] has been used in a proportion of 0.2% by weight of cast-iron.
The step of addition of pure antimony has been suppressed and the nodularizer treatment has been simplified by using only but a FeSiMg nodularizer alloy which does not contain rare earths (also introduced in the form of a cored wire).
Comparative Results
Chunky
Upon the impact resistance results, the cast-iron H2 has achieved results in compliance with the requirements.
Although the invention has been described with particular embodiments, it goes without saying that it is not limited thereto and that it comprises all technical equivalents of the described means as well as their combinations if these are within the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12/60817 | Nov 2012 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2013/052710 | 11/12/2013 | WO | 00 |
