FR2458597A1 - Cast iron contg. aluminium and opt. nodular graphite - for mfg. electrical resistors withstanding high temps. and with low temp. coefft. of resistance - Google Patents

Abstract

The alloy has a resistivity of 2.5-3.5 ohm.mm/m, and a temp. coefft. of resistance (TCR) below 0.03% and usually 0.015-0.03%. The compsn. is 3-27% Al, 1.0-5.0% C, 1-9% Si, and 0.5-6.0% Mn; and the alloy may also contain 0.4-3% Cu, 1-8% Ni and 0.4-6% Cr, where the Ni is used together with Cu or Cr. The alloy pref. contains 0.3-2.0% Mo added to increase its high temp. strength; and/or 0.1-3% each of Ti, Zr and/or V; and/or 0.05-1% N2, Mg, or lanthanides. One pref. alloy contains 3-8% Al, 2-5% C, 2-9% Si, 0.5-3% Mn. The alloying elements are pref. added to the molten iron in an electric furnace, and inoculation may be used to obtain nodular graphite in the castings. The resistivity given above is combined with high resistance to oxidn. and corrosion and good mechanical properties.

Description

 ALLOY BASED ON IRON AND ALUMINUM.

 The present invention relates to iron-aluminum alloys; it also relates to the application of these alloys, taking into account their electrical characteristics, to the manufacture of elements of electrical resistances and, more particularly, of resistors for strong currents; it also relates to the process for the preparation of these alloys.

 The resistivity and the magnetic permeability of a ferrous material depend on the characteristics of the various phases which constitute them and therefore, on the chemical composition, on the homogeneity, on the distribution of the shape, the dimension and the state of equilibrium of said phases.

In the case of iron-aluminum alloys, taking into account that aluminum is notably soluble in iron, at ambient temperature, up to a percentage of the order of about 32% (by weight) or, expressed as a ratio atomic, about 60%, aluminum exerts a certain influence on the properties of pure iron, in particular on the properties of resistivity and permeability. Thus, for example, in the case of an alloy of 20% by weight of aluminum, the resistivity of iron goes from 0.11 to 1.8 ohm.m2 / m approximately, while the permeability, initially of order of about 100 for pure iron, falls to a value slightly greater than 1, so that this alloy is called "non-magnetic iron".

 Other results to which the presence of aluminum in the iron leads, and which prove to be of great interest, are the reduction in the specific weight (of the order of approximately 5 kg / dm 3), as well as characteristics of corrodibility (in particular when hot), on the one hand, and the increase in mechanical strength and expandability, on the other hand.

When the aluminum content increases, the toughness at low temperature comes to be reduced due to the gradual displacement towards the higher temperature compared to the ambient conditions of the transition point of resilience.

 In the manufacture of electrical resistances, it is therefore necessary to measure the aluminum content and the addition of specific additives, to control the rate of harmful impurities, to regulate the conditions of melting, of plastic working and heat treatment, where appropriate, as well as the geometrical shape and dimension of the elements intended to be used for the manufacture of the resistance elements, this in order to exhale the favorable effects of the presence of aluminum and to reduce, on the other hand, its adverse effects.

The qualities sought in a material intended to be used for the manufacture of electrical resistances are in particular: - the high resistivity, - the low temperature coefficient, - the optimal resistance to hot corrosion, and - the good resistance to atmospheric corrosion , - sufficiently high mechanical characteristics, in particular
with regard to thermal fatigue, a low specific weight and a
high specific heat.

 It is also preferable that the alloy does not exhibit ferromagnetism, although this is not decisive for the use for which it is intended according to the present invention.

 The subject of the present invention is an improvement in iron and aluminum alloys making it possible to optimize the abovementioned qualities by acting on the chemical composition and / or the state of structure, as well as by implementing a process for defined manufacturing.

 The iron-aluminum alloys according to the present invention, preferably intended, although not exclusively, for the manufacture of elements of electrical resistance and, more particularly, of elements of electrical resistance for high current, are characterized by the fact that '' they contain a percentage of aluminum between 3 and 27% by weight, and that they also contain additives among which mention may be made of carbon, silicon and manganese, optionally combined with copper, nickel and chromium , depending on the properties sought for the intended application, these alloys having a high electrical resistance, of the order of 3.5 ohms. mm2 / m, a low thermal coefficient, at most equal to 0.03%, and in general between 0.02 and 0.03% per centigrade degree, a low specific weight, of the order of 5'kq / dn3, sufficiently high mechanical and corrosion resistance characteristics up to 1 000C.

 The process for the production of ferromagnetic alloys in accordance with the present invention is characterized in that the basic composition is produced in a cupola, or in general in any suitable combustion furnace, then the other additives are added thereto, the treatment involving possible desulphurization in an electric furnace and, where appropriate, other technological steps in order to optimize the qualities of the alloy.

 As regards the chemical composition of the alloys, the addition of carbon is obviously one of the main factors. Carbon, as results from experience, must be present in the form of graphite and not in the form of carbide so that maximum resistivity is obtained and at the same time avoids the brittleness and high hardness inherent in ferrous alloys containing carbon, whether fully combined or in the form of cementite.

 From these experimental considerations, the holder was led to note that it is expedient to obtain a high resistance in masses of iron-aluminum-carbon alloys to have a carbon content close to that of eutectic and that the carbon either in the elementary state.

 To prevent foam graphite from forming and an unacceptable porosity to be obtained in the molten mass, it is necessary to control the casting temperature, the addition of "inoculants" and to accelerate the cooling of the mass itself. .

 Under these conditions, carbide formation in the alloy can take place, so it is more than ever necessary to subject the mass to an annealing in order to cause the decomposition of the combined carbon into elementary carbon and ferrite; for example, in a cast iron containing 25% of alloy, an annealing at 920 "C causes this decomposition: the hardness is reduced from 400 to 260 HB, with an increase in resistivity of approximately 10%.

In another sample containing 24% aluminum, in addition to the abovementioned reduction in hardness, a notable increase in resistivity was observed, which goes from 1.95 to 2.60 ohms / mm2 / m, and similar behavior. appeared on a 20% aluminum cast iron, which was annealed at 650oC. there is no doubt that it is useful in specific cases to subject the resistance elements to an annealing treatment, under suitable conditions of temperature and time, to increase the resistivity of the material because, in the case where the alloy contains a high percentage of aluminum (more than 12%), there is a reduction in conductivity due to a modification of the structure.

Given that aluminum leads to an increase in the coefficient of thermal expansion of iron, its addition can lead to risks of rupture by fatigue when the resistance elements, having complex sections and / or high thicknesses, are subjected to frequent temperature differences resulting from their discontinuous operation.

it is therefore necessary that the aluminum content is not more than 15%, a value which is given here for information, so that the coefficient of expansion is not more than 16w10-6 cm / cm / C (against 11, 5.10-6 in the case of pure iron and 12.5-10-6 in the case of common cast iron, values given for information only;
In the case of iron-aluminum-carbon alloys, having an aluminum content close to 15%, we obtain what are called white cast irons which are consequently fragile and have a lower resistivity.

Correction factors have been sought and it has been found that the introduction of each of the additives tends to eliminate such a defect, this action being particularly favorable in the case of the addition of silicon which has been observed to increase the resistivity iron. For example, in a basic composition containing 14% of aluminum, if one proceeds to the addition of 3.7% of silicon, one obtains a structure containing carbon present predominantly in the form of graphite and one obtains the coefficient about 15.10-6. and a resistivity of the order of 2.80 ohms / mm2 / m, while the hardness, which is the index of the rate of graphitation of the cast iron, amounts to 210 HB, whereas, in the absence of silicon, the hardness of the alloy is 435 HB.

 Iron-aluminum alloys have a lower temperature coefficient of resistivity than that of pure iron; the addition of correction agent can accentuate the phenomenon up to a threshold for which the resistivity decreases with the increase in the temperature of the mass.

For example, in a composition based on 14.6% aluminum and 4.6% silicon, upe chemical resistance is obtained which decreases with increasing temperature; the temperature coefficient is modified by -0.02% per degree centigrade.

 The shape of graphite generally exerts a notable effect on the properties of ferrous alloys and, with the aim of improving the mechanical characteristics of the elements intended to be used in resistors, it has been investigated what is the influence of the refining of the graphite or its formation in the form of a spheroid, by varying the operating conditions during the tests.

Thus, in the refining, tests were carried out which consisted in adding calcium silicide to the inoculants in percentages suitable for the other alkali metals. These tests made it possible to note that the addition of 0.6X of Ca2Si in the presence of 2, 'of barium leads to a remarkable reduction in the presence of the graphite lamellae in the melt, lamellae which have been found to be more thinner and more dispersed.

The refining treatment obtained using other additives, for example by adding nitrogen in the form of ferrous alloys or decomposing compost, appeared to be particularly suitable for the manufacture of resistance elements with maximum cross-section.

 In the case of spheroidization, the presence of aluminum in the alloy made the treatment more complex, and therefore it was necessary to develop a specific spheroidization treatment.

The addition, for example, of elements of the group of rare earths to magnesium, commonly used in alloys and, less frequently, in the metallic state, for the spheroidization of graphite, appeared entirely suitable, in particular for alloys with high aluminum content.

For the implementation of the spheroidization, it is very important to adjust the thermal conditions of the bath and to control the sulfur content of the alloy. For example, in the case where 0.3% of spheroidization agent is added, consisting of an alloy of 15% of magnesium and Fe-Ce-La, at 35% TR, in equal parts, in the bath at 1420 ° C., and proceeds to a later addition of
Ca2Si, we obtain3 in the case of a 6% aluminum alloy, a melt containing graphite in the finely spheroidal state, which has improved mechanical characteristics compared to those obtained for an analogous alloy untreated and therefore, a resistance to bending increased by 25% and an energy absorbed during rupture by dynamic bending increased by 55%.

 Another improvement to which this spheroidization treatment leads is the hot behavior of the alloy, since this makes it possible to increase the working temperature up to a value greater than 1000 C.

 an attempt has also been made to use the treatment known as "duplex", suitably adapted, applied to the base composition (Fe, C, Si, Mn) obtained with cupola, which is then subjected to the final treatment involving the addition of additives to a fourelectric, which was implemented on the aforementioned treatment involving refining and or spheroidization. In the case where the sulfur content in the alloy obtained in the cupola is relatively high, the bath is sometimes subjected to a desulfurization treatment, in particular if the alloy is intended to be subjected to the spheroidization step.

Thanks to this process, economic advantages and improvements in the electrical and mechanical qualities of the material obtained are obtained.

 In addition to carbon and silicon, the favorable effects of which have already been described above, it is also possible to add other additives in order to improve the desired qualities of the materials intended to be used for the manufacture of resistors; without unacceptably reducing the resistivity of the alloy.

 In these tests, for example, nickel, copper, manganese, chromium, molybdenum, titanium, zinc, niobium, vanadium, nitrogen, magnesium, and other earth group elements few have been used.

Thus: -Nickel, in addition to refining the graphite and increasing the resistance
tivity of the alloy, leads to a significant improvement in toughness.

We have for example found that in an alloy containing 6% aluminum, 2.5%
of silicon and 6% of nickel, this mixture gives rise to test tubes of
round section with a diameter of 20 mm resistance to bending impact
dynamic of around 100 kg / cm, value comparable to that of mechanical cast iron
fuck. The nickel content can be increased to a higher value for
obtain tougher alloys with higher strength,
around 2 ohms / mm2 / m and free of ferromagnetism.

- Copper exerts from the point of killing the graphitization of analogous effects
to those of nickel, but its addition should be limited to a few percent
due to its tendency to separate in elementary form. Given that
this element gives the surface of the iron, in a moderately atmosphere
corrosive, a dense rust patina with ennobled interface
oxide-metal, it is necessary to obtain corrosion resistance
more suitable in an industrial atmosphere, to introduce for example
in the iron-aluminum alloy from 0.5 to 3% 'of copper.

- Manganese exerts a favorable effect on the increase in resistivity.

- Chromium constitutes a coadjuvant of aluminum to confer resistance
to hot oxidation and mechanical stresses; it has anti effects
graphitization, so it is necessary to limit the quantities that
we add to alloys with high carbon content, at a few percentages
and on the other hand to introduce elements of graphitization consisting prin
mainly nickel. In low-iron-aluminum alloys
carbon, we can increase, on the other hand, the chromium content up to 6%,
however possibly subjecting the melt to a refining of its
grain by addition of nitrogen, zirconium or other rare earth metals,
elements that we cite without limitation.

- Tests have also been made on the addition of molybdenum to increase
the mechanical strength of the iron-aluminum alloy by adding 0.5: i,
molybdenum. We obtain resistance elements which do not show any
significant deformation, even after long periods of hot use.

They give rise to a modest anti-graphitization action which one can, from
timely, correct by adding additive, such as for example
nickel.

- Titanium, zirconium, niobium, vanadium are general elements
carbide contents which, however, in limited content, have a hardening effect
ment on the matrix and, in the case of alloys containing carbon,
a graphite refining effect, provided that the content is very
low.

In the case of iron-aluminum alloys, their action is exerted for example
on the raw structure of the casting, leading to a fracture
with fine grains, as we have seen from mass
molten 7% aluminum alloy to which 0.5% was added to the bath
zirconium or 1% titanium.

- Nitrogen has a favorable action at low contents on the refining of graphite
or grain in the iron-aluminum or iron-aluminum-chrome alloy.

- Rare earth metals have been tried, either to modify the shape of
graphite, in particular in spheroidization, to control the ac
harmful to the presence of impurities, such as for example sulfur,
since sulphides actually have a less harmful effect on toughness
of the alloy.

- In order to increase the mechanical resistance of the alloy according to the pre
sente invention, we studied the action of impurities consisting of phosphorus
and sulfur on ferrous materials, which revealed the need for
site to limit their percentages respectively <0.10 and <0.02%.

 In order to obtain resistance elements having constant properties which prove to be completely independent of the melting and casting treatment, it has been investigated whether it is possible to sinter iron-aluminum alloy powder containing less binder.

 For example, an alloy was prepared containing 14% aluminum, 2% carbon, 3% silicon, 0.5% manganese, which was subjected to mechanical spraying. The powder thus obtained, passed through a sieve to remove larger grains as well as grains that are too fine, was subjected to a lubrication treatment by adding, for example, sodium stearate, then to a compression force of 3.5. tonnes / cm2 in a mold.

 The form removed from the mold was subjected to sintering at 11000C for 1 hour in a neutral atmosphere. A material is obtained having a density of approximately 6 kg / dm3, a resistivity of 2.8 ohms / mm2 / m and sufficient mechanical strength. In order to increase the resistivity, 5% silicon, therefore 99%, in the form of 1 mm granules, are mixed with the powder of the above-mentioned alloy.

After sintering, it is found that the grains adhere to the porous matrix according to a phenomenon of division while retaining their individuality; the specific gravity was brought down to approximately 5.5 kg / dm 3 and the resistivity was increased to 3.2 ohms. mm2 / m approximately.

 Other advantages and characteristics of the present invention will appear on reading the following description and examples given by way of illustration but not limitation.

EXAMPLE 1
The following composition is prepared (in weight proportions):
. Al .................. 6.5 5
c ...................... 3.0%
if ........................ 3.5%
. Mn ...................... 0.7%
Mo ........................ o, 4%
. Ti, Zr, Va, of the order of ........... 0.2%.

An alloy is thus obtained having the following physical properties:
- Resistivity ............... 2,51chms.mm / m
- Temperature index .............. 0.0003
- Density .................... 6.8 kg / dm3
- Dimensional stability and
resistance to oxidation
hot: .................... ~ 700 C.

EXAMPLE 2
An alloy is prepared in accordance with the present invention having the following composition (in weight proportions):
Al ...................... 7.5%
c ......................... 3.0%
if 4.5%
Mn
Ni ................. 3.0%
Cr - 1.1%
Mo .................... 0.145%
N and rare earths, of the order of, 0.08%.

An alloy is thus obtained having the following physical properties:
- Resistivity .................. 2,95olms.mm / m
- Temperature index ........ 0.002
- Density ............................ 6.6 kg / dm3
- Dimensional stability and
resistance to oxidation
hot: ...................... ~ 800 C.

EXAMPLE 3
An alloy is prepared in accordance with the present invention having the following composition (in weight proportions):
. Al ........................ 7.5%
c 3.0%
. If ......................... 4.5%
. Mn ......................... 2.0%
Ca 2.5%
Mo ............................... 0.6%
. Ti, Zr, Va, of the order of ............ 0.6%.

An alloy is thus obtained having the following physical properties:
- Resistivity ................... 3.1 ohms.mm2 / m
- Temperature index ........... 0.002
- Density ................... 6.6 kg / dm3
- Dimensional stability and
resistance to oxidation
hot: ..................... 800 C.

EXAMPLE 4
An alloy is prepared in accordance with the present invention having the following composition (in weight proportions):
. Al ............................... 7.4%
. C ............................... 3.1%
. If ............................... 4.5%
. Mn ............................... 2.5%
. Ni ............................... 4.0%
Cr. ,, 1.2%
. Cu .............................. 2.8%
Mo
Ti, Zr, Va, of the order of ............. 0.5%
N and rare earths, of the order of ,,, 0.2.

An alloy is thus obtained having the following physical properties:
- Resiativity .................... 3.50 ahms.mm/m
- Temperature index .......... 0.002
- Density .......................... 6.65 kg / dm3 - dimensional t3tabi1îé ct;
resistance to oxidation
hot: .................... 900 C.

EXAMPLE 5
An alloy is prepared in accordance with the present invention having the following composition (in weight proportions):
. Al ........................... 23%
. C ........................... 2.0%
. If ........................... 5.0%
. Mn ........................... 2.0%
Mo ........................................... 0.6%
. Ti, Zr, Va, of the order of ............... 0.5%.

An alloy is thus obtained having the following physical properties:
- Resistivity ................................ 2.85 ohms, mm2 / m
- Temperature index ..................... 0.0001
- Density ...................... 5.7 kg / dm3
- Dimensional stability and
resistance to oxidation
hot:; ~ 950 C.

EXAMPLE 6
An alloy is prepared in accordance with the present invention having the following composition (in weight proportions):
. Al ....................................... 26%
. C ....................................... 1.5%
. If ....................................... 5.0%
. Mn ........................................ 2.2%
. Ni ........................................ 4.0%
Cr ......................................... 5.0%
. Mo .............................................. 0.5 %
. N and rare earths, of the order of ................ 0.2%.

An alloy is thus obtained having the following physical properties:
- Resistivity .................................... 2.95 olums.mm/m
- Temperature index .......................... 0.0001
- Density ........................................ 5.65 kg / mm
- Dimensional stability and
resistance to oxidation
hot: ......................................... ~ 050 C.

EXAMPLE 7
An alloy is prepared in accordance with the present invention having the following composition (in weight proportions):
- Al ......................................... 25%
. c .......................................... 1.8%
If ........................... 5.5%
Mn, 2.0%
Cu ............................... 2.5%.

Wn alloy is thus obtained having the following physical properties:
- Resistivity 3.1 ohms. Mm2 / m
- Temperature index ........................ 0.001
- Density ............................ 5.6 kg / dm3
- Dimensional stability and
resistance to oxidation
hot: ........................... 950 C.

EXAMPLE 8
An alloy is prepared in accordance with the present invention having the following composition (in weight proportions):
. Al ............................... 27%
c ................................... 2.0%
If ............................. 5.3%
Mn .................................... 2.5%
. Ni ................................... 4.2%
Cr .................................... 4.6%
. Cu .................................... 2.6%
. Mo .................................... 0.55%
Ti, Zr, Va, in the order of ............ 0.3%
N and rare earths, of the order of ...... 0.2%.

An alloy is thus obtained having the following physical properties:
- Resistivity ........................... 3.52 ohms.mm2 / m
- Temperature index ............... 0.0001
- Density ......................... 5.65 kg / dm
- Dimensional stability and
resistance to oxidation
hot: .......................... ~ 1000 C.

 Of course, the present invention is in no way limited to the embodiments described and shown; it is susceptible of numerous variants accessible to those skilled in the art, depending on the applications envisaged and without departing from the spirit of the invention.

Claims (17)

 1.- As a new industrial product, an alloy based on iron and aluminum, having the following physical properties:  - resistivity between 2.5 and 3.5 ohm.mm2 / m,  - temperature index less than 0.03% and, in general,  between 0.015 and 0.03%, this alloy being characterized in that its aluminum content is between 3 and 27% and in that it contains the following elements:  - C from 1.0 to 5.0%  If from 1.0 to 9.0%  Mn from 0.5 to 6.0%, and in that it can also contain one or more of the elements belonging to the list comprising: - copper (0.4 to 3%),  -nickel (1.0 to 8%), and  - chromium (0.4 to 6%), nickel and copper being present in a combined form, while copper, on the one hand, and nickel-chromium on the other hand, are present alone or in combination .  2. An iron-aluminum alloy according to claim 1, characterized in that it also contains molybdenum in a weight percentage of between 0.3 and 2.0%, this element being added with the aim of increasing the resistance. hot mechanical of the alloy.  3. An iron-aluminum alloy according to claim 1 or 2, characterized in that it also contains titanium, zirconium and vanadium, alone or in combination, in weight proportions of between 0.1 and 3.0% .  4.- Iron-aluminum alloy according to any one of claims 1 to 3, characterized in that it can also contain nitrogen, magnesium or rare earth metals, alone or in combination, in weighty proportions between 0.05 and 1.0%.  5. An iron-aluminum alloy according to any one of claims 1 to 4, characterized in that aluminum is present in weight proportions comprised between 3 and 8%, and in that the elements carbon, silicon, manganese are respectively present in the following proportions:  C from 2.0 to 5.0%  If from 2.0 to 9.0%  Mn from 0.5 to 3.0%, the resistivity being able to reach a value of 2.5 ohms. Mm2 / m.  6. An iron-aluminum alloy according to claim 5, characterized in that it also contains:  . 2.0 to 4.0 nickel,  . 0.8 to 1.5% chromium, and the resistivity reaches a value of 2.95 ohms. Mm2 / m.  7. An iron-aluminum alloy according to claim 5, characterized in that it also contains, in weight proportions:  . 1.5 to 3.0% copper, and in that the resistivity of the alloy reaches 3.0 ohms. Mm2 / m.  8. An iron-aluminum alloy according to claim 5, characterized in that it also contains, in weight proportions:  . 1.5 to 6.0% nickel,  0.6 to 2.5% chromium,  1.5 to 3.0% copper, the resistivity of the alloy reaching a value of 3.5 ohms. Mm2 / m.  9. An iron-aluminum alloy according to any one of claims 1 to 4, characterized in that the aluminum is present in a weight proportion of the order of 13 to 27%, and in that it contains the elements following expressed in weight proportions:  . 1.0 to 3.0% carbon,  1.0 to 7.0% silicon,  0.5 to 3.0% manganese, the resistivity of the alloy reaching a value of the order of 2.5 ohms. Mm2 / m.  10.- Iron-aluminum alloy according to claim 9, characterized in that it also contains:  1.0 to 8.0% nickel,  0.5 to 6.0% chromium, and in that the resistivity of the alloy reaches a value of the order of 2.9 ohms. Mm2 / m.  11. An iron-aluminum alloy according to claim 9, characterized in that it also contains:  1.0 to 3.0% copper, and in that the resistivity of the alloy reaches a value of the order of 3.1 ohms m / m.  12. An iron-aluminum alloy according to claim 9, characterized in that it also contains:  . 1.5 to 6.0% nickel,  0.6 to 2.5% chromium,  1.0 to 3.0% copper, the resistivity of the alloy reaching a value of the order of 3.5 ohms. Mm / m.  13.- Electric resistors, in particular resistors for strong currents, characterized in that they contain the alloy according to any one of claims 1 to 12.  14.- A method of producing iron-aluminum alloy according to any one of claims 1 to 12, characterized in that the basic cast iron is treated exclusively in the electric furnace and that the additives are gradually added in order of decreasing affinities with respect to carbon, that is to say the elements generating carbide before those which generate graphite, then in that the bath, before casting, is subjected to a degassing treatment in the aim of reducing as much as possible its gas content, in particular hydrogen, present in the melt and, optionally, an inoculation treatment to modify the shape and the distribution of the graphite in the melt, then in that the temperature of the bath in the oven is brought to a temperature of the order of 1400 to 15000C, then in that one proceeds to the casting in a period of less than 6 minutes, preferably less than 4 minutes.  15.- Method according to claim 14, characterized in that the basic cast iron is treated with cupola and the addition of the additives is carried out in an electric oven.  16.- Method according to claim 14 or 15, characterized in that said mass, before casting, is subjected to an annealing treatment at a temperature between 600 and 10000C to eliminate internal tensions and obtain the maximum formation of graphite.  17.- Method according to any one of claims 14 to 16, characterized in that the alloy, prior to casting, is subjected to a refining or spherodization treatment of graphite to improve the mechanical characteristics of the alloy .