U.S. patent application number 10/359590 was filed with the patent office on 2003-12-04 for cobalt based alloy, article made from said alloy and method for making same.
This patent application is currently assigned to ISOVER SAINT GOBAIN. Invention is credited to Bernard, Jean-Luc, Berthod, Patrice, Liebaut, Christophe.
Application Number | 20030221756 10/359590 |
Document ID | / |
Family ID | 29585796 |
Filed Date | 2003-12-04 |
United States Patent
Application |
20030221756 |
Kind Code |
A1 |
Berthod, Patrice ; et
al. |
December 4, 2003 |
Cobalt based alloy, article made from said alloy and method for
making same
Abstract
The invention relates to a cobalt-based alloy having mechanical
strength at high temperature, in particular in an oxidizing or
corrosive medium, essentially comprising the following elements
(the proportions being shown as percentage by weight of the alloy):
26 to 34% Cr, 6 to 12% Ni, 4 to 8% W, 2 to 4% Ta, 0.2 to 0.5% C,
less than 3% Fe, less than 1% Si, less than 0.5% Mn and less than
0.1% Zr, the remainder being composed of cobalt and inevitable
impurities, the molar ratio of tantalum with respect to carbon
being of the order of 0.4 to 1. Application to articles which are
mechanically stressed at high temperature, in particular articles
which. can be used for the preparation or the conversion or glass
under hot conditions.
Inventors: |
Berthod, Patrice;
(Pont-a-Mousson, FR) ; Liebaut, Christophe;
(Mercurey, FR) ; Bernard, Jean-Luc; (Giencourt,
FR) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
ISOVER SAINT GOBAIN
Courbevoie
FR
|
Family ID: |
29585796 |
Appl. No.: |
10/359590 |
Filed: |
February 7, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10359590 |
Feb 7, 2003 |
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09308650 |
Aug 6, 1999 |
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09308650 |
Aug 6, 1999 |
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PCT/FR98/02056 |
Sep 24, 1998 |
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Current U.S.
Class: |
148/674 ;
420/436 |
Current CPC
Class: |
C22C 19/07 20130101;
C03B 37/047 20130101; C22F 1/10 20130101 |
Class at
Publication: |
148/674 ;
420/436 |
International
Class: |
C22C 019/07 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 29, 1997 |
FR |
97/12088 |
Claims
1. Cobalt-based alloy having mechanical strength at high
temperature, in particular in an oxidizing or corrosive medium,
essentially comprising the following elements (the proportions
being shown as percentage by weight of the alloy):
10 Cr 26 to 34% Ni 6 to 12% W 4 to 8% Ta 2 to 4% C 0.2 to 0.5% Fe
less than 3% Si less than 1% Mn less than 0.5% Zr less than
0.1%
the remainder being composed of cobalt and inevitable impurities,
the molar ratio of tantalum with respect to carbon being of the
order of 0.4 to 1.
2. Alloy according to claim 1, in which the proportions of the
elements are within the following ranges:
11 Cr 28 to 32% Ni 8 to 10% W 5 to 7% Ta 2.5 to 3.5% C 0.3 to
0.45%
3. Alloy according to claim 1 or 2, in which the molar ratio of
tantalum with respect to carbon is of the order of 0.45 to 0.9.
4. Alloy according to claim 3, in which the elements are in
proportions of the order of:
12 Cr 29% Ni 8.5% C 0.38% W 5.7% Ta 2.9%
5. Alloy according to claim 1, in which the elements are in
proportions of the order of:
13 Cr 28% Ni 8.5% C 0.22% W 5.7% Ta 3%
6. Alloy according to any one of claims 1 to 5, characterized in
that it exhibits a non-continuous intergranular phase of
carbides.
7. Article, in particular an article which can be used in
particular for the preparation or the conversion, under hot
conditions, of glass, made of an alloy according to any one of the
preceding claims, in particular by founding.
8. Article according to claim 7, obtained by founding and having
been subjected to a heat treatment after casting the alloy.
9. Article according to any one of claims 6 to 8, consisting of a
fibre-drawing spinner for the manufacture of mineral wool.
10. Process or the manufacture of an article according to claim 8,
comprising the casting of the molten alloy in an appropriate mould
and a heat treatment of the moulded article comprising a first
annealing at a temperature of 1100 to 1250.degree. C. and a second
annealing at a temperature of 850 to 1050.degree. C.
Description
[0001] The present invention relates to a cobalt-based alloy having
mechanical strength at high temperature, in particular in an
oxidizing or corrosive medium, such as molten glass, which can be
used in particular for the production of articles for the
preparation and/or the conversion of glass under hot conditions,
such as components of machines for the manufacture of glass wool by
fibre-drawing molten glass.
[0002] The fibre-drawing technique consists in allowing liquid
glass to fall continuously within an assembly of revolving parts
rotating at very high rotational speed around their vertical axis.
Halted in its initial fall by the bottom of an internal part known
as a distributing "cup", the glass spreads out under the effect of
the centrifugal force against the cylindrical wall, pierced with
holes, of this same part. These holes allow the glass to pass
through which, still under the effect of the centrifugal force,
will be distributed against the wall known as a "band" of an
external part known as a "spinner", which is also pierced with
holes, these being smaller than the preceding holes. The glass,
still under the effect of the centrifugal force, passes through the
band of the spinner everywhere in the form of filaments of molten
glass. An annular burner situated above the outside of the spinner,
producing a descending stream of gas running along the external
wall of the band, diverts these filaments downwards while drawing
them. The latter subsequently "solidify" in the form of glass wool.
The parts known as "cup" and "spinner" are fibre-drawing tools
which are very much stressed thermally (thermal shocks during
startups and shutdowns), mechanically (centrifugal force, erosion
due to passage of the glass) and chemically (oxidation and
corrosion by, molten glass and by hot gases exiting from the burner
for the disc).
[0003] By way of indication, the operational temperature is of the
order of at least 1000.degree. C. for the glass to exhibit a
suitable viscosity.
[0004] Under these conditions, the main forms of damage to these
components are: deformation by hot creep of the vertical walls, the
appearance of horizontal or vertical cracks, or the wear by erosion
of the fibre-drawing orifices, which require the pure and simple
replacement of the components. Their constituent materials
therefore has to be resistant for a sufficiently long production
time to remain compatible with the technical and economic
constraints of the process.
[0005] A suitable material is disclosed in the document
FR-A-2,536,385. It is a superalloy based on nickel reinforced by
chromium and tungsten carbides of the (W,Cr).sub.23C.sub.6 type
present in two forms: eutectic carbides distributed at the grain
boundaries in a continuous intergranular network ensuring the
overall stiffness; and fine carbides (secondary precipitates)
distributed in a dense and homogeneous way in the grains of the
nickel matrix, contributing resistance to intragranular creep.
[0006] Resistance to oxidation and to corrosion at the temperature
of use is provided by the high chromium content of the alloy, which
forms a protective chromic oxide Cr.sub.2O.sub.3 layer at the
surface of the part in contact with the oxidizing medium. Continual
diffusion of the chromium towards the corrosion front makes
possible the renewal of the layer of Cr.sub.2O.sub.3 oxides in the
event of cracking or other damage.
[0007] The operating temperatures at which this alloy can be used
with success are, however, limited to a maximum value of the order
of 1000 to 1050.degree. C. Beyond this maximum temperature, the
material displays a lack of both mechanical strength, with the
appearance of cracks, and of resistance to corrosion, the cracks
allowing the corrosive medium to penetrate into the material.
[0008] This problem of rapid deterioration at relatively high
temperature makes it impossible to use this type of alloy for the
manufacture of mineral wool from very viscous glasses (such as
basalt) which cannot be fibre-drawn at temperatures below
1100.degree. C.
[0009] To meet this need for a material exhibiting good mechanical
strength and good resistance to oxidation and to corrosion by glass
at very high temperatures, provision has been made for the use of
superalloys based on cobalt, an element with an intrinsic strength
superior to that of nickel.
[0010] These alloys always comprise chromium or resistance to
oxidation, as well as generally carbon and tungsten, in order to
obtain a reinforcing effect by precipitation of carbides. They also
include nickel in solid solution, which stabilizes the crystal
lattice of the cobalt as face-centred cubic at all
temperatures.
[0011] The presence alone of these elements is not sufficient,
however, to achieve the expected properties and numerous attempts
have been made to further improve the properties of cobalt-based
alloys.
[0012] These attempts are generally based on the addition of
reactive elements to the composition of the alloy.
[0013] Thus, FR-A-2,699,932 discloses a cobalt-based alloy
including rhenium which can additionally comprise, in particular,
niobium, yttrium or other rare-earth metals, boron and/or hafnium.
U.S. Pat. No. 4,765,817 discloses an alloy based on cobalt,
chromium, nickel and tungsten which also comprises boron and
hafnium. FR-A-2,576,914 also uses hafnium. EP-A-0,317,579 discloses
an alloy which includes boron and is devoid of hafnium but which
comprises yttrium. U.S. Pat. No. 3,933,484 also relates to an alloy
including baron. U.S. Pat. No. 3,984,240 and U.S. Pat. No.
3,980,473 disclose the use of yttrium and dysprosium.
[0014] These elements are very expensive and their poor efficiency
of incorporation generally makes it necessary to overdose them in
the working of the alloy, which correspondingly increases the share
of the starting materials in the cost of the material. In this
respect, it should be noted that a number of these documents teach
the use of high chromium contents (of the order of 35 to 36%),
which is also expensive.
[0015] The presence of these very highly reactive elements requires
that the alloy be prepared by the difficult technology of melting
and casting under vacuum, with equipment requiring a significant
investment.
[0016] Furthermore, these alloys still exhibit a marked risk of
brittleness at high temperature in corrosive medium, such as molten
glass.
[0017] The need thus remains for a novel alloy having good
mechanical properties at high temperature, in particular in
oxidizing and/or corrosive medium, such as molten glass, which is,
in addition, easy and relatively inexpensive to prepare.
[0018] This aim and others which will become apparent subsequently
was achieved by the invention by virtue of an alloy essentially
comprising the following elements, the proportions being shown as
percentage by weight of the a alloy:
1 Cr 26 to 34% Ni 6 to 12% W 4 to 8% Ta 2 to 4% C 0.2 to 0.5% Fe
less than 3% Si less than 1% Mn less than 0.5% Zr less than
0.1%
[0019] the remainder being composed of cobalt and inevitable
impurities, the molar ratio of tantalum with respect to carbon
being of the order of 0.4 to 1.
[0020] The invention makes it possible, by virtue of a very precise
selection of the proportions of the constituent elements of the
alloy, more particularly carbon and tantalum, to optimize the form
of reinforcement of the alloy. Thus, it may be generally said that,
although the alloy according to the invention exhibits a relatively
low carbon content with respect to the prior art, the reinforcement
by precipitation of carbides was able to be improved by optimizing
the distribution of the carbides within the material.
[0021] The description which will follow gives further details on
the importance of the constituents of the alloy and of their
respective proportions.
[0022] Cobalt, which constitutes the base or the alloy according to
the invention, contributes, by its refractory nature (melting point
equal to 1495.degree. C.), an intrinsic mechanical strength at high
temperature of the matrix.
[0023] Nickel, present in the alloy in the form of a solid solution
as element which stabilizes the crystalline structure of the
cobalt, is used in the usual range of proportions of the order of 6
to 12%, advantageously of 8 to 10%, by weight of the alloy.
[0024] Chromium contributes to the intrinsic mechanical strength of
the matrix in which it is present partly in solid solution. It also
contributes to the reinforcement of the alloy in the form of
carbides of M.sub.23C.sub.6 type with M=(Cr,W) which are present at
the grain boundaries, where they prevent grain-over-grain slip, and
inside the grains in the form of a fine dispersion, where they
contribute resistance to intragranular creep. In all its forms,
chromium contributes to the resistance to corrosion as precursor
chromium oxide forming a protective layer at the surface exposed to
the oxidizing medium. A minimum amount of chromium is necessary for
the formation and the maintenance of this protective layer. An
excessively high chromium content is, however, harmful to the
mechanical strength and to the toughness at high temperatures,
because it results in an excessively high stiffness and an
excessively low ability to elongate under stress which are
incompatible with the stresses at high temperature.
[0025] Generally, the chromium content of an alloy according to the
invention will be from 26 to 34% by weigh., preferably of the order
of 28 to 32% by weight, advantageously of approximately 29 to 30%
by weight.
[0026] Tungsten participates with chromium in the formation of
intergranular and intragranular (Cr,W).sub.23C.sub.6 carbides but
is also found in solid solution in the matrix where this heavy atom
locally distorts the crystal lattice and impedes, indeed blocks,
the progression of the dislocations when the material is subjected
to a mechanical stress. A minimum amount is desirable, in
combination with the chromium content, in order to promote carbides
of M.sub.23C.sub.6 type to the detriment of chromium carbides
Cr.sub.7C.sub.3, which are less stable at high temperature. While
this element has beneficial effects on the mechanical strength, it
nevertheless exhibits the disadvantage of being oxidized at high
temperature in the form of very volatile compounds, such as
WO.sub.3. An excessively high amount of tungsten in the alloy is
reflected by a generally unsatisfactory behaviour with respect to
corrosion.
[0027] A good compromise is achieved according to the invention
with a tungsten content of the order of 4 to 8% by weight,
preferably of the order of 5 to 7% by weigh, advantageously of the
order of 5.5 to 6.5% by weight.
[0028] Tantalum, also present in solid solution in the cobalt
matrix, makes an additional contribution to the intrinsic strength
of the matrix, in a way similar to tungsten. In addition, it is
capable of forming, with carbon, TaC carbides present at the grain
boundaries which contribute an intergranular reinforcement,
complementing the (Cr,W).sub.23C.sub.6 carbides, in particular at
very high temperature (for example, of the order of 1100.degree.
C.) , due to their greater stability at high temperature. The
presence of tantalum in the alloy according to the invention also
has a beneficial effect on the resistance to corrosion.
[0029] The minimum tantalum content which makes it possible to
obtain the desired strength is of the order of 2%, it being
possible for the upper limit to be chosen to approximately 4%. The
amount of tantalum is advantageously of the order of 2.5 to 3.5% by
weight, in particular of 2.8 to 3.3%.
[0030] Another essential constituent of the alloy is carbon,
necessary for the formation of the metal carbide precipitates. The
present inventors have demonstrated the influence of the carbon
content on the properties of the alloy.
[0031] Surprisingly, whereas the prior art teaches the use of
carbon in relatively high contents, greater than 0.5% by weight, a
lower carbon content gives excellent mechanical properties at high
temperature with very good resistance to oxidation and to
corrosion, despite the low proportion of carbides which results
therefrom.
[0032] According to the invention, a carbon content in the range
from 0.2 to 0.5% by weight is sufficient to produce a sufficiently
dense precipitation of carbides for effective intergranular and
intragranular mechanical reinforcement. It would seem, in
particular, that intergranular carbides, which are distributed
non-continuously at the grain boundaries of the alloy, contribute
advantageously to the mechanical properties by opposing
grain-over-grain slip or creep, without, for all that, promoting
the propagation of cracks, as can be the case with carbides in
general.
[0033] The carbon content is advantageously of the order of 0.3 to
0.45% by weight, preferably of the order of 0.35 to 0.42% by
weight.
[0034] According to the invention, the relatively low content of
carbides is compensated for, on the one and, by a suitable
(non-continuous) distribution of the intergranular carbides and, on
the other hand, by a suitable "quality" of carbides, namely the
presence of a certain proportion of tantalum carbides at the grain
boundaries.
[0035] The inventors have discovered that the nature of the metal
carbides constituting the intergranular phases depends on the Ta/C
atomic ratio and that a molar ratio of tantalum with respect to
carbon of at least approximately 0.4 makes it possible to
precipitate, at the grain boundaries, a sufficient proportion of
TaC with respect to the M.sub.23C.sub.6 carbides.
[0036] The presence of intergranular carbides of M.sub.23C.sub.6
type which are rich in chromium remains desirable in order to allow
a degree of diffusion of chromium along the grain boundaries and
the invention consequently provides for a Ta/C molar ratio of the
order of 0.4 to 1 (corresponding to a ratio by weight of the order
of 6.0 to 15.1). Preferably, the Ta/C molar ratio is from 0.45 to
0.9, very advantageously from 0.48 to 0.8, in particular of the
order of 0.5 to 0.7 (ratio by weight preferably from 6.8 to 13.6,
very advantageously from 7.2 to 12.1, in particular of the order of
7.5 to 10.6).
[0037] Thus, the strength of the alloy according to the invention
is optimized by the presence of two types of carbides with
complementary properties, both from the viewpoint of mechanical
properties and of resistance to corrosion: (Cr,W).sub.23C.sub.6,
which acts as chromium source and as mechanical reinforcement up to
high temperatures; and TaC, which takes over the mechanical
reinforcement at very high temperature and which opposes, under
oxidizing and/or corrosive conditions, the penetration of the
oxidizing or corrosive medium respectively.
[0038] The constituents shown above are sufficient to ensure the
excellent properties of the alloy according to the invention,
without resorting to additional elements which are expensive or at
least very reactive, requiring great precautions during
preparation, such as boron, yttrium or other rare-earth metals,
hafnium, rhenium, and the like. Such elements could optionally be
incorporated in the alloy according to the invention but it would
not be a preferred embodiment since the advantages related to the
cost and to the ease of manufacture would be lost.
[0039] Nevertheless, the alloy can comprise other conventional
constituent elements or inevitable impurities. It generally
comprises:
[0040] silicon as deoxidizer of the molten metal during the
preparation and the moulding of the alloy, in a proportion of less
than 1% by weight;
[0041] manganese, also a deoxidizer, in a proportion of less than
0.5% by weight;
[0042] zirconium as scavenger of undesirable elements, such as
sulphur or lead, in a proportion of less than 0.1% by weight;
[0043] iron, in a proportion which can range up to by weight
without detrimentally affecting the properties of the material;
[0044] the cumulative amount of the other elements introduced as
impurities with the essential constituents of the alloy
("inevitable impurities") advantageously represents less than 1% by
weight of the Composition of the alloy.
[0045] A particularly preferred example of alloy according to the
invention has a composition in which the elements are in
proportions of the order of:
2 Cr 29% Ni 8.5% C 0.38% W 5.7% Ta 2.9% Fe <3% Si <1% Mn
<0.5% Zr <0.1% Impurities <1% Co remainder
[0046] preferably devoid of B, Hf, Y, Dy, Re and other rare-earth
metals.
[0047] Another preferred alloy according to the invention has a
composition in which the elements are in proportions of the order
of:
3 Cr 28% Ni 8.5% C 0.22% W 5.7% Ta 3% Fe <3% Si <1% Mn
<0.5% Zr <0.1% Impurities <1% Co remainder
[0048] preferably devoid of B, Hf, Y, Dy, Re and other rare-earth
metals.
[0049] The alloy according to the invention, when it is devoid of
highly reactive elements, such as B, Hf or rare-earth metals,
including Y, Dy and Re, can be shaped very easily by standard
melting and casting with conventional means, in particular by
induction melting under an at least partially inert atmosphere and
casting in a sand mould.
[0050] After casting, the desired microstructure can advantageously
be achieved by a two-stage heat treatment:
[0051] a stage of solution forming heat treatment comprising an
annealing at a temperature of 1100 to 1250.degree. C., in
particular of the order of 1200.degree. C., for a time which can
range in particular from 1 to 4 hours, advantageously of the order
of 2 hours; and
[0052] a stage of precipitation of carbides comprising an annealing
at a temperature of 850 to 1050.degree. C., in particular of the
order of 1000.degree. C., for a time which can range in particular
from 5 to 20 hours, advantageously of the order of 10 hours.
[0053] Another subject-matter of the invention is a process for the
manufacture of an article by founding from an alloy as described
above, with the above heat treatment stages.
[0054] The process can comprise at least one cooling stage, after
the casting and/or after the first stage of heat treatment, as well
as on conclusion of the heat treatment.
[0055] The intermediate and/or final coolings can be carried out,
for example, by cooling with air, in particular with a return to
ambient temperature.
[0056] The alloy according to the invention can be used to
manufacture all kinds of parts which are stressed mechanically at
high temperature and/or operated in an oxidizing or corrosive
medium. Further subject-matters of the invention are such articles
manufactured from an alloy as described above, in particular by
founding.
[0057] Mention may in particular be made, among such applications,
of the manufacture of articles which can be used for the
preparation or the transformation of glass under hot conditions,
for example fibre-drawing spinners for the manufacture of mineral
wool.
[0058] The notable mechanical strength at high temperature in
corrosive medium of the alloy according to the invention makes it
possible to very substantially increase the lifetime of equipment
for shaping molten glass.
[0059] The invention is illustrated by the following examples and
the single figure, which represents a microphotograph of the
structure of an alloy according to the invention.
EXAMPLE 1
[0060] A molten charge with the following composition is prepared
via the induction melting technique under an inert atmosphere (in
particular argon) and is subsequently shaped by simple casting in a
sand mould:
4 Cr 29.0% Ni 8.53% C 0.38% W 5.77% Ta 2.95% Remainder: Fe <3%
Si <1% Mn <0.5% Zr <0.1% others summed <1%
[0061] the rest being composed of cobalt.
[0062] The casting is followed by a heat treatment comprising a
stage of solution forming heat treatment for 2 hours at
1200.degree. C. and a stage of precipitation of the secondary
carbides for 10 hours at 1000.degree. C., each of these stationary
phases finishing with cooling with air to ambient temperature.
[0063] The microstructure of the alloy obtained, revealed by
optical or electron microscopy according to conventional
metallographic techniques and optionally x-ray microanalysis, is
composed of a cobalt matrix, stabilized as a face-centred cubic
structure by the presence of nickel, comprising various elements in
solid solution: Cr, Ta, W, as well as various carbides present
within the grains and at the grain boundaries. This structure is
visible in the single Figure: the grain boundaries, which do not
appear in the microphotograph with the magnification used, have
been represented by the fine lines 1. Within the grains delimited
by the boundaries 1, the intragranular phase is composed of fine
secondary carbides 2 of (Cr,W).sub.23C.sub.6 type precipitated
evenly in the matrix, which appear in the form of small points. At
the grain boundaries, there is found a dense but non-continuous
intergranular phase composed of eutectic (Cr,W).sub.23C.sub.6
carbides 3, which appear as dark, and of TaC tantalum carbides 4,
which appear in the form of small clear islets well separate from
one another.
[0064] With a molar ratio of tantalum with respect to carbon in the
composition of the alloy equal to 0.51, the intergranular phase is
approximately 50% by volume composed of chromium and tungsten
carbides 3 and approximately 50% composed of tantalum carbides
4.
[0065] The properties of mechanical strength at high temperature of
the alloy were evaluated in the following three tests:
[0066] measurement of the tensile stress at fracture (in MPa) at
900.degree. C. on a cylindrical test specimen with a total length
of 40 mm comprising two ends for attachment to the tensioning
device each 9 mm long and an intermediate working part with a
diameter of 4 mm and a length of 22 mm, with a tensioning rate of 2
mm/min;
[0067] measurement of the tensile elongation at fracture (in %) at
900.degree. C. under the above conditions;
[0068] measurement of the creep strength (in hours) at 1050.degree.
C. under 35 MPa on a cylindrical test specimen with a total length
of 80 mm comprising two attachment ends, each 17.5 mm long, and an
intermediate working part with a diameter of 6.4 mm and a length of
45 mm.
[0069] The properties of resistance to oxidation with air and to
corrosion by glass were evaluated in a test consisting in rotating
a cylindrical test specimen, with a diameter of 10 mm and a length
of 100 mm, half immersed in a bath of molten glass of following
type at 1080.degree. C. for 125 hours. The result is given by the
depth (in mm) of the eroded region at the level of the test
specimen-molten glass-hot air triple point. The composition of the
glass is approximately as follows (in parts by weight):
5 SiO.sub.2 Al.sub.2O.sub.3 Fe.sub.2O.sub.3 CaO MgO Na.sub.2O
K.sub.2O B.sub.2O.sub.3 SO.sub.3 64.7 3.4 0.17 7.2 3 15.8 1 4.5
0.25
[0070] The results are collated in Table 1 below.
[0071] The ability of this alloy to be used to constitute a device
for the shaping of molten glass was evaluated in the application to
the manufacture of glass wool. A fibre-drawing spinner with a
diameter of 400 mm and of conventional shape was manufactured by
casting and heat treatment as above and then used under industrial
conditions for fibre-drawing a first glass at a temperature of
1080.degree. C.
[0072] The spinner is used until its shutdown is decided upon
following the ruin of the spinner indicated by visible
deterioration or by the quality of fibre produced becoming
unsatisfactory. The lifetime (in hours) of the spinner thus
measured is 540 hours.
[0073] Under the same conditions, the lifetime of a fibre-drawing
spinner made of a nickel-based superalloy is 150 h, for a
nickel-based alloy according to Patent FR-A-2,536,385 of the
following composition which has been subjected to the same heat
treatment for the precipitation of carbides as that of Example
1:
6 Ni 54.5 to 58% by weight Cr 27.5 to 28.5% W 7.2 to 7.6% C 0.69 to
0.73% Si 0.6 to 0.9% Mn 0.6 to 0.9% Fe 7 to 10% Co <0.2%
[0074] The microstructure of this alloy is composed of a nickel
matrix comprising carbides of M.sub.23C.sub.6=(W,Cr).sub.23C.sub.6
type distributed homogeneously in the matrix, forming a continuous
intergranular phase.
[0075] The alloy of Example 1 in particular makes possible, by
virtue of its excellent creep strength and of its very good
resistance to corrosion, a consequent increase in the lifetime of
the spinner, multiplied by a factor of 3.6 with respect to the
conventional alloy.
EXAMPLE 2
[0076] Another alloy according to the invention with the following
composition is prepared as in Example 1 and its properties are
evaluated in the same way:
7 Cr 28.2% Ni 8.60% C 0.22% W 5.71% Ta 3.04% remainder: Fe <3%
Si <1% Mn <0.5% Zr <0.1% others summed <1%
[0077] the rest being composed of cobalt.
[0078] Its microstructure is distinguished from that of Example 1
by intergranular phases which are still non-continuous but less
dense, due to the lower carbon content, and which are composed
mainly of TaC tantalum carbides (Ta/C molar ratio=0.91).
[0079] The results of the tests of mechanical behaviour and of
behaviour with respect to corrosion appear in Table 1.
[0080] This alloy is notable in particular for its mechanical
properties, especially a vary significant hot ductility, reflected
by the elongation at fracture at 900.degree. C., and a very
creditable creep strength, increased tenfold with respect to a
conventional nickel-based alloy.
[0081] Its ability to withstand thermal shock makes it an
advantageous material for constituting fibre-drawing spinners for
the manufacture of glass wool, as is shown by a fibre-drawing test
under industrial conditions: despite the tendency towards corrosion
of the alloy of Example 2, the lifetime of the disc is
approximately 720 hours. The brittleness resulting from the attack
by the glass was compensated for by the good mechanical properties
of the alloy. Under the same conditions (different from those of
Example 1), the lifetime of a spinner made of conventional
nickel-based superalloy shown in Example 1 is only 250 h.
8 TABLE 1 EX. 1 EX. 2 Tensile stress at fracture at 287 247
900.degree. C. (MPa) Tensile elongation at 34 38 fracture at
900.degree. C. (%) Creep strength at 1050.degree. C. 954 335 under
35 MPa (h) depth of the eroded region in 0.0 0.6 a bath of molten
glass (mm)
COMPARATIVE EXAMPLES 1 TO 9
[0082] Other alloys were prepared by way of comparison by choosing
contents of the constituent elements outside the ranges
characteristic of the invention. Their compositions are listed in
Table 2: for each alloy, the content or contents not in accordance
with the invention has/have been underlined.
9 TABLE 2 Co Ni C Cr W Ta COMP. EX. 1 0 base 0.44 30.1 4.65 3.37
COMP. EX. 2 base 8.23 0.19 30.0 5.78 1.85 COMP. EX. 3 base 8.86
0.98 29.0 0.0 2.87 COMP. EX. 4 base 8.45 0.39 29.7 2.94 0.02 COMP.
EX. 5 base 8.74 0.37 28.2 5.59 5.84 COMP. EX. 6 base 8.14 0.33 25.7
5.97 4.17 COMP. EX. 7 base 9.16 0.38 39.9 6.34 2.62 COMP. EX. 8
base 7.58 0.35 29.1 3.06 3.80 COMP. EX. 9 base 7.96 0.34 29.2 8.87
2.88
[0083] The alloy of Comparative Example 1 only differs from an
alloy according to the invention in its matrix, which is of nickel
instead of being composed of cobalt. Although the form of
reinforcement is the same as for an alloy according to the
invention (carbon content and Ta/C ratio in accordance with the
invention), this alloy has a creep strength 30 times lower and a
weaker ductility (with an elongation at fracture 3 times lower)
than the alloy according to the invention.
[0084] The alloy of Comparative Example 2 has a creep strength of
only 74 hours under the conditions specified above and exhibits a
very strong tendency towards corrosion with an eroded region with a
depth of 0.83 mm in the test with the rotating test specimen. This
poor behaviour is explained by the somewhat low carbon and
excessively low tantalum content, which results in a low density of
carbides M.sub.23C.sub.6 and TaC, providing an insufficient
intergranular and intragranular reinforcement, and in an
excessively low availability of chromium at the grain boundaries,
limiting the rate of diffusion of the chromium atoms towards the
corrosion front.
[0085] The alloy of Comparative Example 3 also exhibits a very
strong tendency towards corrosion with an eroded region with a
depth of 0.80 mm, despite its high carbon content. The
characterization of the microstructure of the alloy has shown the
existence of a very dense and continuous intergranular network of
carbides, composed of 80% chromium carbides and 20% tantalum
carbides. Like the nickel-based superalloy discussed in Example 1,
this alloy is disadvantaged by its excessively high carbon content
and has a poorer performance than the alloy according to the
invention reinforced by a non-continuous intergranular phase of
carbides. In addition, in the complete absence or tungsten, the
chromium carbides are less resistant at high temperature than the
eutectic carbides (Cr,W).sub.23C.sub.6, resulting in a greater
mechanical weakness at high temperature.
[0086] The alloy of Comparative Example 4 has a mediocre creep
strength of the order of 200 hours with a substantial tendency
towards corrosion (erosion depth of 0.33 mm). This example
illustrates the importance of the tantalum carbides in the
mechanical strength and the resistance to corrosion. This is
because this alloy is characterized by a virtual absence of
tantalum, which results in the exclusive precipitation of chromium
carbides. The deterioration in the mechanical performance at high
temperature, due to the lack of more refractory tantalum carbides
and also to the relatively low tungsten content, does not make
possible to compensate for the weakness with respect to corrosion
and makes the material incompatible with uses at high temperature
in corrosive medium (in contrast to the alloy of Example 2, which
compensates for the tendency towards corrosion by excellent
mechanical properties at high temperature).
[0087] The alloy of Comparative Example 5 has a microstructure
exhibiting a dense and homogeneous intergranular precipitation
composed exclusively of tantalum carbides, due to the very high
tantalum content and to the Ta/C molar ratio greater than 1. As all
the chromium is, for this reason, in solid solution in the matrix,
the protective chromium oxide layer is not formed under good
conditions, apparently as a result of an excessively slow diffusion
of the matrix chromium, resulting in a substantial erosion in the
corrosion test.
[0088] The alloy of Comparative Example 6 is itself also very
sensitive to corrosion with an eroded region with a depth of 2.50
mm in the test with the revolving test specimen. This time it is
the excessively low chromium content which is responsible for this
behaviour, in the sense that it is insufficient to provide for the
formation and the maintenance of the surface Cr.sub.2O.sub.3 layer.
In addition, the relatively high tantalum content does not promote
the formation of a sufficient amount of intergranular chromium
carbides.
[0089] The alloy of Comparative Example 7 has itself an excessively
high chromium content which causes its solidification
microstructure to change to a different metallurgical system from
the other alloys, with a secondary precipitation in the form of
acicular precipitates and a dense intergranular network composed of
chromium carbides and of chromium compounds. For this reason, it
exhibits an excessively great stiffness, reflected by an elongation
at fracture of only 1.5%.
[0090] The alloy of Comparative Example 8 has a tensile stress at
fracture at 900.degree. C. of 257 MPa and a creep strength of
approximately 300 hours with a certain tendency towards corrosion
(erosion depth 0.40 mm) . As the density of the carbides is fixed
by the carbon content, the low tungsten content of this alloy is
reflected by a lower degree of hardening in solid solution,
resulting in the low tensile mechanical strength under hot
conditions and the low creep strength.
[0091] The alloy of Comparative Example 9 has a very strong
tendency towards corrosion with an erosion depth of 1.50 mm in the
corrosion test. The excessively great presence of tungsten in the
composition results in a significant modification of the material
at high temperature by oxidation of the tungsten in the form of
volatile compounds of WO.sub.3 type, responsible for the
deterioration in the behaviour with respect to corrosion.
[0092] As shown by the preceding examples, the good mechanical
strength at high temperature in the presence of a corrosive medium
of the alloys according to the invention, obtained by careful
selection of the contents, in particular, of chromium, tungsten and
especially of carbon and tantalum, is the result of the following
combination: reinforcement of the grain boundaries due to the
intergranular tantalum carbides and optionally to the intergranular
chromium and tungsten carbides; blockage of cracking by the
non-continuous dispersion of a limited amount of intergranular
chromium and tungsten carbides; blockage of the penetration of the
corrosive medium by the presence of tantalum carbides; availability
of chromium in the precipitated form.
[0093] The invention which has been described in the more
particular case of the shaping of molten glass is in no way limited
to this specific application and generally relates to all fields
where materials with good resistance to high temperature are
required.
* * * * *