Patent Publication Number: US-2003221756-A1

Title: Cobalt based alloy, article made from said alloy and method for making same

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° 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) 23 C 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 2 O 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 2 O 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° 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° 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:  
                                                      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° 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 23 C 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) 23 C 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 23 C 6  type to the detriment of chromium carbides Cr 7 C 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 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) 23 C 6  carbides, in particular at very high temperature (for example, of the order of 1100° 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 23 C 6  carbides.  
       [0036] The presence of intergranular carbides of M 23 C 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) 23 C 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:  
                                                      Cr   29%           Ni    8.5%           C    0.38%           W    5.7%           Ta   2.9%           Fe   &lt;3%           Si   &lt;1%           Mn   &lt;0.5%           Zr   &lt;0.1%           Impurities   &lt;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:  
                                                      Cr   28%           Ni   8.5%           C   0.22%           W   5.7%           Ta   3%           Fe   &lt;3%           Si   &lt;1%           Mn   &lt;0.5%           Zr   &lt;0.1%           Impurities   &lt;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° C., in particular of the order of 1200° 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° C., in particular of the order of 1000° 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:  
                                                      Cr   29.0%           Ni   8.53%           C   0.38%           W   5.77%           Ta   2.95%           Remainder: Fe   &lt;3%           Si   &lt;1%           Mn   &lt;0.5%           Zr   &lt;0.1%           others summed   &lt;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° C. and a stage of precipitation of the secondary carbides for 10 hours at 1000° 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) 23 C 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) 23 C 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° 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° C. under the above conditions;  
     [0068] measurement of the creep strength (in hours) at 1050° 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° 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):  
                                                               SiO 2     Al 2 O 3     Fe 2 O 3     CaO   MgO   Na 2 O   K 2 O   B 2 O 3     SO 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° 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:  
                                                      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   &lt;0.2%                      
 
     [0074] The microstructure of this alloy is composed of a nickel matrix comprising carbides of M 23 C 6 =(W,Cr) 23 C 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:  
                                                      Cr   28.2%           Ni   8.60%           C   0.22%           W   5.71%           Ta   3.04%           remainder: Fe   &lt;3%           Si   &lt;1%           Mn   &lt;0.5%           Zr   &lt;0.1%           others summed   &lt;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° 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.  
                           TABLE 1                                   EX. 1   EX. 2                                                        Tensile stress at fracture at   287   247           900° C. (MPa)           Tensile elongation at   34   38           fracture at 900° C. (%)           Creep strength at 1050° 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.  
                                           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 23 C 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) 23 C 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 2 O 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° 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 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.