SK284724B6 - Cobalt based alloy, article made from said alloy and method for making same - Google Patents

Abstract

The invention concerns an alloy based on cobalt provided with mechanical resistance to high temperature, in particular in an oxidising or corrosive environment, comprising essentially the following elements (the proportions mentioned being expressed in weight percentage of the alloy): 26 to 34 % of Cr; 6 to 12 % of Ni; 4 to 8 % of W; 2 to 4 % of Ta; 0.2 to 0.5 % of C; less than 3 % of Fe; less than 1 % of Si; less than 0.5 % of Mn; less than 0.1 % of Zr; the remainder consisting of cobalt and unavoidable impurities, the mol ratio of tantalum relative to carbon being of the order of 0.4 to 1. The invention is applicable to articles mechanically stressed at high temperature; in particular articles used for the preparation and hot transformation of glass, which is produced, form this alloy and method for making of this product.

Description

Field of the invention

The present invention relates to a cobalt-based alloy characterized by mechanical strength at high temperature, in particular in an oxidizing or corrosive environment, for example in a molten glass environment, useful in particular for the production of articles used for heat treatment and / or glass transformation at high temperatures, for example, of glass wool spinning machines. The invention also relates to an article, particularly applicable to the heat treatment or transformation of glass made from this alloy, and to a method of making the article.

BACKGROUND OF THE INVENTION

The spinning method consists in letting the liquid glass continuously fall inside a set of rotating elements rotating at a very high rotational speed about its vertical axis. The glass, which in its initial fall is stopped by the bottom of the inner element called the distribution bowl, is spilled by the centrifugal force against the perforated cylindrical wall of the element. The glass passes through these openings and, still under the influence of centrifugal force, extends against a cylindrical wall called a "belt" of the outer element, referred to as a "spinner", which is also perforated by openings that are smaller than the previous openings. Still under the influence of centrifugal force, the glass passes through the web of the spinning element in all directions in the form of fused glass fibers. A circular burner situated above the outer casing of the spinner produces a gas stream descending along the outer strip wall. It deflects these fibers downwards while extending them. The fibers then "solidify" in the form of glass wool. Component called "basket" and "spinning" element are very stressed elements in glass spinning, both thermally (thermal shocks during start and stop) as well as mechanically (centrifugal force, erosion caused by glass passage) and chemically (oxidation and corrosion due to melted glass and hot gases flowing from the burner to the spinner).

The operating temperature is approximately at least 1000 ° C to give the glass the desired viscosity.

Under these conditions, the main types of damage to these elements, which require simple replacement of the components, are the following: deformation of the vertical walls by the flow of hot molten glass, the formation of horizontal or vertical cracks, wear of holes used for erosion spinning. Thus, the structural material of the component must resist for a sufficiently long manufacturing time to be compatible with the technical stresses and economic requirements of the process.

A suitable material is described in document FR-A-2 536 385. It is a high-alloy nickel-based alloy reinforced with chromium and tungsten carbides of the (W, Cr) type ₂₃ C ₆ , which are present in two forms: eutectic carbides distributed on contact surfaces grains in an intergranular continuous lattice for overall strength, and fine carbides (secondary precipitated) distributed densely and homogeneously in the grains of the nickel matrix imparting resistance to the intragranular flow.

Resistance to oxidation and corrosion at the temperature of use is ensured by the high chromium content of the alloy, which forms a protective layer of Cr ₂ O ₃ on the surface of the components in contact with the oxidizing environment. Permanent diffusion of chromium to the corrosion front allows the Cr ₂ O ₃ oxide layer to recover in the event of tearing or other damage.

The operating temperatures at which this alloy can be used successfully are more or less limited by a maximum value in the range of about 1000 to 1050 ° C. Above this maximum temperature, the material exhibits a lack of resistance, both mechanically due to the formation of cracks and corrosively, because the cracks allow the corrosive environment to penetrate into the interior of the material.

The problem of this rapid degradation at a relatively high temperature prevents the use of this type of alloy in the production of mineral wool from very viscous glasses (e.g., molten basalt) which cannot be fiberized at temperatures below 1100 ° C.

In order to respond to this need to develop a material having good mechanical resistance and resistance to oxidation and corrosion by glass at very high temperatures, it has been proposed to use highly alloyed cobalt-based alloys, an element whose intrinsic strength is higher than that of nickel.

These alloys also contain chromium to provide resistance to oxidation and generally contain carbon and tungsten to provide a reinforcing effect by carbide precipitation. These alloys also contain nickel in solid solution, which stabilizes the crystalline cobalt lattice to the center-center cube at all temperatures.

The presence of these elements alone is more or less insufficient to achieve the desired properties, so many other attempts have been made to improve the properties of the cobalt-based alloys.

These attempts generally involve adding reaction elements to the alloy composition.

These efforts result in a solution according to French patent FR-A-2 699 932, which discloses a cobalt-based alloy containing rhenium, which may additionally contain, in particular, niobium, yttrium or other rare elements, boron and / or hafnium. U.S. Pat. US-A-4 765 817 discloses an alloy based on cobalt, chromium, nickel, tungsten, also containing boron and hafnium. In French patent no. FR-A-2 576 914 also uses hafnium. In European patent no. EP-A-0 317 579 discloses a boron-containing alloy not containing hafnium but containing yttrium. U.S. Pat. US-A-3 933 484 also relates to a boron-containing alloy. U.S. Pat. US-A-3 984 240 and US-A-3 980 473 disclose the use of yttria and dysprosia.

These elements are very expensive and their poor incorporation efficiency generally forces them to overdose in the production of alloys, which further increases the share of raw materials in the cost of the material. In this respect, it has been noted that many of these documents recommend the use of high chromium contents (approximately 35 to 36%) equally expensive.

The presence of these elements with very high reactivity leads to the production of an alloy by complicated technology of melting and casting under vacuum, by means of a device that requires considerable investment.

In addition, these alloys pose a considerable risk of high temperature brittleness in a corrosive environment such as a molten glass environment.

Thus, there remains a need in the art to develop a novel alloy with good mechanical properties at high temperature, particularly in an oxidizing and / or corrosive environment, such as a molten glass environment, which, moreover, would be simple and relatively inexpensive to manufacture.

SUMMARY OF THE INVENTION

The present invention provides a cobalt-based alloy that exhibits high temperature mechanical strength, particularly in an oxidizing or corrosive environment, which essentially comprises the following elements (amounts are given by weight based on the weight of the alloy):

Cr 26 to 34% Ni 6 to 12% W 4 to 8% the 2 to 4% C 0.2 to 0.5% fe less than 3% Are you less than 1% Mn less than 0.5% Zr less than 0.1%

wherein the remainder consists of cobalt and the necessary impurities, wherein the molar ratio of tantalum to carbon is of the order of 0.4 to 1.

In a preferred embodiment, in this alloy, the ratio of the individual elements to each other is as follows (in weight percent based on the weight of the alloy):

Cr 28 to 32% Ni 8 to 10% W 5 to 7% the 2.5 to 3.5% C 0.3 to 0.45%

According to another preferred embodiment, the molar ratio of tantalum to carbon in the alloy is in the range of from 0.45 to 0.9.

According to a further preferred embodiment, the elements are contained in the alloy in the following amounts (in weight percent based on the weight of the alloy):

Cr 29% Ni 8.5% C 0.38% W 5,7% the 2.9%

According to an even more preferred embodiment, in this alloy the elements are contained in the following amount (in weight percent based on the weight of the alloy): _____________________

Cr 28% Ni 8.5% C 0.22% W 5,7% the 3%

The alloy of the present invention preferably comprises a discontinuous intergranular phase of carbides.

It is also within the scope of the present invention to provide an article, in particular an article useful for the heat treatment or transformation of glass, which is characterized in that it is made from some type of specified alloy, in particular by casting.

Preferably, the product is obtained by casting and subjected to a heat treatment after casting the alloy.

According to a further preferred embodiment, the product according to the invention is formed by a rotary spinner for producing mineral wool.

The present invention also relates to a process for the production of a specified product, which comprises casting a molten alloy into a suitable mold and heat treating a shaped product comprising first annealing at 1100-1250 ° C and second annealing at 850-1050 C.

The present invention makes it possible to optimize the method of reinforcing the alloy due to the very precise ratio of the elements constituting the individual components of the alloy, in particular carbon and tantalum. In this way, it can generally be stated that, although the alloy according to the invention has a relatively low carbon content compared to the prior art alloys, it has been possible to achieve solidification of the materials by carbide precipitation while optimizing the distribution of carbides within the material.

In the following description, further details are given regarding the importance of the presence of the individual alloy constituents and their relative proportions.

Cobalt, which forms the basis of the alloy according to the invention, due to its refractory nature (melting point equals 1495 ° C), causes the internal mechanical resistance of the matrix at high temperature.

The nickel present in the alloy in the form of a solid solution as a stabilizing element of the cobalt crystalline structure is usually used in the range of about 6 to 12% by weight, preferably 8 to 10% by weight of the alloy.

Chromium contributes to the intrinsic mechanical strength of the matrix in which it is partially present in the solid solution. It also contributes to the reinforcement of the alloy in the form of carbides of the type M ₂₃ C ₅ where M = (Cr, W) present on the grain contact surfaces where they prevent the grain from slipping together, and inside the grains in the form of fine dispersion where they cause intragranular flow resistance. In all these forms, chromium contributes to corrosion resistance as a precursor to chromium oxide, which forms a protective layer on the surface exposed to the oxidizing environment. A minimum amount of chromium is required to form and maintain this protective layer. Too high a chromium content is more or less detrimental to the mechanical strength and toughness to high temperatures because it leads to too high stiffness and too low an elongation capability that is incompatible with high temperature stress.

The chromium content of the alloy according to the invention is generally 26 to 34% by weight, preferably about 28 to 32% by weight, in particular about 29 to 30% by weight.

Tungsten precipitates with chromium and is involved in the formation of intergranular and intragranular carbides (Cr, W) _{23C 6} , but is also found in a solid solution in a matrix in which this heavy atom locally deforms the crystalline lattice and prevents, almost blocks, spreading dislocations when the material is subjected to mechanical stress. A minimum amount of this tungsten is desired, in combination with the chromium content, to favor the formation of M ₂₃ C ₆ carbides over the disadvantageous Cr ₇ C ₃ chromium carbides, which are less stable at high temperature. Although this element has beneficial effects on mechanical strength, it is more or less a disadvantage since it oxidizes at high temperature to form highly volatile compounds, for example WO ₃ . Too high an amount of tungsten in the alloy results in a generally unsatisfactory corrosion behavior.

A good compromise is achieved according to the invention at a tungsten content of about 4 to 8% by weight, preferably at a content of between about 5 and 7% by weight, especially between about 5.5 and 6.5% by weight.

Tantalum, also present in the cobalt matrix in the solid solution additionally contributes to the internal strength of the matrix, similar to tungsten. In addition, it is capable of forming with the carbon the TaC carbides present on the grain contact surfaces which, due to their higher stability at high temperature, contribute to intergranular reinforcement complementary to carbides (Cr, W) ₂₃ C ₆ , especially at very high temperatures (e.g. C). The presence of tantalum in the alloy according to the invention also has a good effect on the corrosion resistance.

The minimum tantalum content to achieve the desired strength is about 2% by weight, the upper limit may be about 4% by weight. The amount of tantalum is preferably in the range of 2.5 to 3.5% by weight, in particular 2.8 to 3.3% by weight.

Another major constitutive element of the alloy is carbon, which is essential for the formation of metal carbide precipitates. The effect of the carbon content on the alloy properties has been demonstrated.

Surprisingly, it has been found, although earlier work recommended that carbon be used in relatively high contents higher than 0.5% by weight, that at a lower carbon content excellent mechanical properties at high temperature and very good oxidation and corrosion, despite the lower carbide ratio resulting therefrom.

According to the present invention, a carbon content in the range of 0.2 to 0.5% by weight is sufficient to achieve a sufficiently dense carbide precipitation for effective mechanical intergranular and intragranular strengthening. In particular, it appears that intergranular carbides that are discontinuously distributed on the grain contact surfaces of the alloy advantageously contribute to improved mechanical properties by preventing grain displacement or creep without simultaneously promoting crack propagation as can in the case of carbides occur.

The carbon content is preferably about 0.3 to 0.45 wt%, preferably about 0.35 to 0.42 wt%.

The relatively low carbide content of the present invention is compensated by, on the one hand, the appropriate distribution of (discontinuous) intergranular carbides and, on the other hand, by adapting the "quality" of carbides, i.e. the presence of a certain amount of tantalum carbides on the grain interface.

It has been found that the nature of the metal carbides constituting the intergranular phase depends on the Ta / C atomic ratio and that the molar ratio of tantalum to carbon of at least about 0.4 allows the precipitation of a sufficient proportion of TaC on the grain contact surfaces relative to the M ₂ 3C carbides. ₆ .

The presence of chromium-rich intergranular carbides of the type M ₂₃ C ₆ remains desirable as it allows some diffusion of chromium along the grain contact surfaces, and therefore a molar Ta / C molar ratio of about 0.4 to 1 (corresponding to a weight ratio of about 6) , 0-15.1). The Ta / C molar ratio is preferably in the range of 0.45 to 0.9, in particular in the range of 0.48 to 0.8, most preferably about 0.5 to 0.7 (weight ratio preferably in the range of 6.8 to 13 6, especially 7.2 to 12.1, most preferably about 7.5 to 10.6).

In this way, the strength of the alloy according to the invention is optimized by the presence of two types of carbides with complementary properties, both with respect to mechanical properties and with respect to corrosion resistance: (Cr, W) _{23C 6} , which acts as a chromium source and causes mechanical strengthening up to high temperature, and TaC, which causes mechanical strengthening only at very high temperatures and resists the penetration of oxidizing or corrosive environments under oxidative and / or corrosive conditions.

Said elements are sufficient to ensure the excellent properties of the alloy according to the invention, and it would be necessary to use additional expensive or very reactive elements requiring high safety in production, such as boron, yttrium or other rare earth metals, hafhium, rhenium and the like . These elements may optionally be added to the alloy according to the invention, but in this case it is not a preferred embodiment since the advantages in terms of cost and ease of manufacture would be lost.

The alloy may more or less contain the usual constitutional elements or necessary impurities. It generally includes:

- silicon as a molten metal reducing agent in the production and casting of an alloy, in an amount of less than 1% by weight,

- manganese as well as reducing agent, in an amount of less than 0,5% by weight,

- zirconium as a scavenger of undesirable elements, such as sulfur or lead, in an amount of less than 0,1% by weight,

- iron up to 3% by weight without affecting the material properties,

- the cumulative amounts of other elements introduced as impurities with the necessary constitutive elements of the alloy ("unavoidable impurities") are preferably less than 1% by weight in the composition of the alloy

A particularly preferred example of an alloy according to the invention has a composition with the following representation of elements:

Cr 29% Ni 8.5% C 0.38% W 5,7% the 2.9% fe <3% Are you <1% Mn <0,5% Zr <0,1% impurities <1% What Rest

preferably free of B, Hf, Y, Dy, Re and other rare earth elements.

Another preferred alloy according to the invention has a composition with the following representation of elements:

Cr 28% Ni 8.5% C 0.22% W 5,7% the 3% fe <3% Are you <1% Mn <0,5% Zr <0,1% impurities 1% What Rest

preferably free of B, Hf, Y, Dy, Re and other rare earth elements.

If the alloy according to the invention does not contain highly reactive elements such as B, Hf and rare earth elements such as Y, Dy, Re, it can be shaped very simply by conventional melting and casting using conventional means, in particular by induction melting in at least partially inert atmosphere. and casting into a sand mold.

After casting, the desired microstructure can advantageously be achieved by two-stage heat treatment:

a solution preparation phase comprising annealing at a temperature of 1100 to 1250 ° C, in particular at a temperature of about 1200 ° C, for 1 to 4 hours, preferably about 2 hours, and

a carbide precipitation phase comprising annealing at a temperature of 850 to 1050 ° C, in particular about 1000 ° C, for 5 to 20 hours, preferably about 10 hours.

It is also an object of the present invention to provide a process for producing an article from this alloy by melting as described hereinbefore with said heat treatment stages.

The process may include at least one cooling phase, after the casting and / or after the first heat treatment phase, as well as after the heat treatment has been completed.

Cooling during or at the end of the process may be, for example, air cooling, especially to ambient temperature.

The alloy of the invention can be used to manufacture any components mechanically stressed at high temperature and / or intended to work in an oxidizing or corrosive environment. Accordingly, it is within the scope of the present invention to make articles made of the described alloy, in particular by melting.

Among these applications, mention may be made in particular of the manufacture of articles useful for the heat treatment or transformation of glass, for example rotary spinning heads for the production of mineral wool.

The remarkable mechanical strength of the inventive alloy at high temperature in a corrosive environment makes it possible to significantly increase the service life of the molten glass forming apparatus.

DETAILED DESCRIPTION OF THE INVENTION

The invention is illustrated by the following examples and one figure which represents a photomicrograph of a structure of an alloy according to the invention.

Example 1

By the method of induction melting in an inert atmosphere (especially argon atmosphere), a molten batch was prepared with the following composition, which was then shaped by simple casting into a sand mold:

Cr 29.0% Ni 8.53% C 0.38% W 5.77% the 2,95% | residual: fe <3% Are you <1% Mn <0,5% Zr <0,1% Other <1%

the remainder being cobalt.

The casting was followed by a heat treatment comprising a solution phase of 1200 ° C for 2 hours and a secondary carbide precipitation phase of 1000 ° C for 10 hours, each of which was terminated by air cooling down to ambient temperature.

The microstructure of the alloy obtained by optical or electronic microscopy, according to classical metallographic methods and optionally by X-ray microanalysis, consisted of a cobalt matrix stabilized in a flat centered cube structure by the presence of nickel containing various elements in solid solution: Cr, Ta, W as well as various carbides present inside. and at the grain contact point. This structure is shown in the figure: point of contact between the grains, which are seen in the photomicrograph magnification used are shown by a thin line in the first place within the grains bounded by the contact area 1, the intragranular phase formed by fine secondary carbides of type 2 (Cr, W) ₂₃ C _{6 are} precipitated uniformly in the matrix, which appear in the form of small dots. At the grain contact points there is a dense but discontinuous (discontinuous) intergranular phase consisting of eutectic carbides 3 (Cr, W) ₂₃ C ₆ , which are visible in darker places, and tantalum carbides TaC 4, which are visible in the form of small bright islands well separated.

At a tantalum to carbon molar ratio of 0.51 in the alloy composition, the intergranular phase is comprised of about 50 volume% chromium and tungsten carbides 3 and about 50% tantalum carbides 4.

The mechanical strength of the alloy at high temperature was evaluated by the following three tests:

- by measuring the tensile stress at break (in MPa) at 900 ° C using a cylindrical test specimen with a total length of 40 millimeters having both ends of 9 millimeters attached in a tensiometric instrument and a center effective portion of 22 millimeters in length and diameter 4 millimeters at a pull speed of 2 millimeters / minute,

- measuring the relative elongation at break (in%) at 900 ° C under the above conditions,

- measuring the creep rupture strength (in hours) at 1050 ° C and MPa using a cylindrical test specimen with a total length of 80 millimeters having both ends fixed at 17.5 millimeters and a center effective section of 45 millimeters in length and diameter 6.4 millimeters.

The resistance to oxidation in air and to corrosion by glass was evaluated by testing a cylindrical test specimen of 100 millimeters in length and a diameter of 10 millimeters rotated for 125 hours half-immersed in a molten glass bath of the following type at a temperature of 1080 ° C. The result is given by the depth of the eroded zone (in millimeters) determined by the point of the triple point of the test sample - molten glass - hot air. The composition of the glass was approximately as follows (in parts by weight):

SiO ₂ Al2O3 Fe _; O ₃ CaO MgO Najo K ₅ O B2O3 SO ₃ 64.7 3.4 017 7.2 3 15.8 1 4.5 0.25

The results are shown in Table 1 below.

The fitness of this alloy for use as a fused glass forming apparatus has been evaluated through a glass wool application. A glass spinner having a diameter of 400 millimeters of classical shape was made by casting and heat treatment as described above and then used under industrial conditions for glass spinning at 1080 ° C.

This spinning element was used until it was decided to discontinue production because of obvious damage to the spinning element in a visible manner or because the quality of the fiber produced has become inadequate.

The lifetime of this spinning element (in hours) thus measured was 540 hours.

Under the same conditions, the lifetime of the spinning element of a high-alloy nickel-based glass was 150 hours, in the case of a nickel-based alloy according to French patent FR-A-2,536,385 with the following composition (in percent by weight) which were subjected to the same heat treatment to precipitate carbides as the alloy of Example 1:

Ni 54.5 to 58% Cr 27.5 to 28.5% W 7.2 to 7.6% C 0.69 to 0.73% Are you 0.6 to 0.9% Mn 0.6 to 0.9% fe 7 to 10% What <0,2%

The microstructure of this alloy consisted of a nickel matrix containing M ₂₃ C ₆ = = (W, Cr) ₂₃ C ₆ carbides distributed homogeneously in the matrix and forming an intergranular continuous phase.

The alloy of Example 1, in particular due to its creep rupture strength and its very good corrosion resistance, allows a subsequent increase in the service life of the spinning element, multiplied by a factor of 3.6 compared to a conventional alloy.

Example 2

According to this example, another alloy of the present invention was prepared with the following composition, using the same procedure as in Example 1 and evaluating its properties in the same way:

Cr 28.2% Ni 8,60% C 0.22% W 5.71% the 3.04% | residual: fe <3% Are you <1% Mn <0,5% Zr <0,1% Other <1%

the remainder being cobalt.

The microstructure of this alloy differed from the microstructure of the alloy in Example 1 by the intergranular phases as well as discontinuous but less dense due to the lower carbon content and mainly consisting of tantalum carbides TaC (molar ratio Ta / C = 0.91).

The results of the mechanical behavior and corrosion experiments are shown in Table 1.

This alloy is particularly distinguished by its mechanical properties, in particular by its high hot ductility, which is reflected in an improvement in the relative elongation at break at 900 [deg.] C. and a very good creep strength of ten times that of a conventional nickel-based alloy.

The ability of this alloy to withstand thermal shocks makes it a preferred material for making glass spinning elements for glass wool production, as evidenced by the glass spinning experiment under industrial conditions: despite the tendency against corrosion of the alloy of Example 2, the spinning disk life was approximately 720 hours. Glass embrittlement was compensated by the good mechanical properties of the alloy. Under the same conditions (different from those in Example 1), the service life of the spinning element of the classic high-alloy nickel-based alloy in Example 1 was only 250 hours.

Table 1

Example 1 Example 2 Tensile stress at break at 900 ° C (MPa) 287 247 Relative elongation at break at 900 ° C (%) 34 38 Creep strength at 1050 ° C and pressure of 35 MPa (hours) 954 335 Depth of eroded zone in molten glass bath (mm) 0.0 0.6

Comparative Examples 1 to 9

For comparison, other alloys were made using constitutive element contents outside the characteristic ranges used according to the invention. The compositions of these alloys are shown in Table 2: for each alloy, contents that are not within the scope of the present invention are underlined.

Table 2

What Ni C Cr W the Porov.pr. 1 0 basis 0.44 30.1 4.65 3.37 Porov.pr. 2 basis 8.23 0.19 30.0 5.78 1.85 Porov.pr. 3 basis 8.86 0.98 29.0 12 2.87 Porov.pr. 4 basis 8.45 0.39 29.7 2.94 0.02 Cf. 5 basis 8.74 0.37 28.2 5.59 5.84 Porov.pr. 6 basis 8.14 0.33 25.7 5.97 4.17 Porov.pr. 7 basis 9.16 0.30 39.9 6.34 2.62 Porov.pr. 8 basis 7.58 0.35 29.1 3.6 3.80 Porov.pr. 9 basis 7.96 0.34 29.2 8.87 2.88

The alloy of Comparative Example 1 differed from the alloy of the invention only in its matrix which was nickel instead of cobalt. Although the reinforcement method was the same as the inventive alloy (carbon content and Ta / C ratio conforms to the invention), the alloy had a creep rupture strength 30 times lower and a lower ductility (with a relative elongation at break 3 times lower) than an alloy according to the invention.

The alloy of Comparative Example 2 had a creep strength of only 74 hours under the conditions specified above and a very strong corrosion tendency, the eroded zone had a depth of 0.83 millimeter in the rotating test sample. This undesirable behavior is explained by a slightly lower carbon content and a very low tantalum content, which results in low density of M ₂₃ C ₆ and TaC carbides causing insufficient intergranular and intragranular hardening and very low chromium availability at the grain contact points, thereby limiting atomic diffusion rates. chromium to the forehead of corrosion.

The alloy of Comparative Example 3 also has a very strong tendency to corrosion, the depth of the eroded zone in the experiment was 0.80 millimeters despite the high carbon content. The description of the alloy microstructure showed the existence of an intergranular very dense and continuous network of carbides consisting of 80% chromium carbides and 20% tantalum carbides. Like the high-alloy nickel-based alloy shown in Example 1, this alloy is disadvantageous because of its very high carbon content, and is less preferred than the alloy of the invention reinforced by the intergranular discontinuous (discontinuous) carbide phase. Moreover, in the complete absence of tungsten, chromium carbides are less resistant to high temperature than eutectic carbides (Cr, W) _{23C 6} , resulting in higher mechanical brittleness at high temperature.

The alloy of Comparative Example 4 had a creep strength of approximately 200 hours and a strong tendency to corrosion (the eroded zone had a depth of 0.33 millimeters). This example demonstrates the importance of tantalum carbides for mechanical strength and corrosion resistance. This is because this alloy is characterized by the almost complete absence of tantalum, which leads to the exclusive precipitation of chromium carbides. The deterioration of the mechanical properties at high temperature due to the lack of tantalum carbides with higher heat resistance and relatively low tungsten content does not compensate for the high tendency of the alloy to corrosion and makes this material incompatible with high temperature use in a corrosive environment (unlike the alloy of Example 2 tendency to corrosion by excellent mechanical properties at high temperature).

The alloy of Comparative Example 5 had a microstructure with dense and homogeneous intergranular precipitation exclusively of tantalum carbides, given by a very high tantalum content and a Ta / C molar ratio higher than 1. The total amount of chromium was in the solid solution matrix, These conditions did not evolve, evidently due to the very slow diffusion of the chromium matrix, resulting in significant erosion when conducting the corrosion test.

The alloy of Comparative Example 6 also has a very high corrosion tendency, with the depth of the eroded zone being 2.50 millimeters when conducting the rotating test sample. In this case, a very low chromium content is responsible, which is not sufficient to ensure the formation and maintenance of the Cr ₂ O ₃ protective layer. Moreover, the relatively high tantalum content does not promote the formation of a sufficient amount of intergranular chromium carbides.

The alloy of Comparative Example 7 has a very high chromium content which causes its solidification microstructure to be transformed into another metallurgical system of other alloys with secondary precipitation in the form of needle precipitates and a dense intergranular network formed by chromium carbides and chromium compounds. For this reason, this alloy has a very high stiffness, which has an elongation at break of only 1.5%.

The alloy of Comparative Example 8 has a tensile at break of 257 MPa at a temperature of 900 ° C and a creep strength of approximately 300 hours and a certain tendency to corrosion (erosion depth 0.40 millimeters). The carbide density is given by the carbon content, the low content of tungsten in this alloy results in a lower degree of hardening in the solid solution, which results in low mechanical tensile strength and low creep strength.

The alloy of Comparative Example 9 has a very strong corrosion tendency with an erosion depth of 1.50 millimeters when conducting a corrosion test. Too high a tungsten content in the composition leads to a significant modification of the material at high temperature by oxidizing tungsten in the form of volatile compounds of the WO ₃ type, which are responsible for the deterioration of the corrosion behavior.

From the results given in the previous examples, it is clear that the good mechanical strength of the alloys of the invention at high temperature in the presence of a corrosive environment, achieved by a precise selection of the individual element contents, especially chromium, tungsten and especially carbon and tantalum. the contact points of the grains caused by tantalum carbides and optionally intergranular chromium and tungsten carbides, preventing crack propagation by discontinuous dispersion of some limited amount of intergranular chromium and tungsten carbides, preventing the penetration of a corrosive environment due to the presence of tantalum carbides,

The invention, which has been described for the particular case of molding molten glass, is not limited to this specific application and relates generally to areas where materials with good high temperature resistance are required.

Claims (9)

1. A cobalt-based alloy which exhibits high-temperature mechanical strength, in particular in an oxidizing or corrosive environment, characterized in that it contains the following elements (quantities are given in percent by weight of the alloy): Cr 26 to 34% Ni 6 to 12% W 4 to 8% the 2 to 4% C 0.2 to 0.5% fe less than 3% Are you less than 1% Mn less than 0.5% Zr less than 0.1% the remainder being cobalt and unavoidable impurities, and the molar ratio of tantalum to carbon is of the order of 0.4 to 1. An alloy according to claim 1, characterized in that in this alloy the ratio of the individual elements to each other is in the following range (in weight percent based on the weight of the alloy): Cr 28 to 32% Ni 8 to 10% W 5 to 7% the 2.5 to 3.5% C 0.3 to 0.45% An alloy as claimed in claim 1 or 2, characterized in that the tantalum to carbon molar ratio is in the range of 0.45 to 0.9. An alloy according to claim 3, characterized in that the elements are present in the alloy in the following amounts (in weight percent based on the weight of the alloy): Cr 29% Ni 8.5% C 0.38% W 5,7% the 2.9% An alloy according to claim 1, characterized in that the elements are contained in the following amount (in weight percent based on the weight of the alloy): a continuous intergranular phase of carbides. An article, in particular an article usable for the heat treatment or transformation of glass, characterized in that it is made from an alloy according to any one of the preceding claims, in particular by casting. Product according to claim 7, characterized in that it is obtained by casting and subjected to a heat treatment after casting the alloy. Product according to one of Claims 6 to 8, characterized in that it consists of a rotary spinning head for the production of mineral wool. A method of manufacturing an article according to claim 8, which comprises casting the molten alloy into a suitable mold and heat treating the shaped article, comprising first annealing at 1100-1250 ° C and second annealing at 850-1050 ° C.