EP0171336B1 - Cobalt-containing austenitic stainless steel, highly resistant against impringement attack - Google Patents

Cobalt-containing austenitic stainless steel, highly resistant against impringement attack Download PDF

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EP0171336B1
EP0171336B1 EP85420115A EP85420115A EP0171336B1 EP 0171336 B1 EP0171336 B1 EP 0171336B1 EP 85420115 A EP85420115 A EP 85420115A EP 85420115 A EP85420115 A EP 85420115A EP 0171336 B1 EP0171336 B1 EP 0171336B1
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stainless steel
containing stainless
steel alloy
cavitation
weight
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EP0171336A1 (en
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Raynald Simoneau
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Hydro Quebec
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt

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  • the present invention relates to an austenitic cobalt stainless steel having a very high resistance to high intensity erosive cavitation making it very particularly useful for the manufacture or repair of parts of hydraulic machines.
  • the invention also relates to the parts of hydraulic machines thus made or covered with said cobalt stainless steel.
  • cavitation phenomenon in particular experienced by hydraulic machines such as turbines, pumps, propellers, valves, or exchangers, is a drawback well known to specialists.
  • cavitation phenomenon the phenomenon by which a cavity or a bubble of vapor is formed in a liquid when the local pressure drops below the vapor pressure. When the pressure rises above that of the vapor, the gas or vapor bubble suddenly implodes. This implosion is accompanied by powerful physical phenomena, in particular a microjet which follows the bubble and whose speed can reach the values of several hundreds of meters per second.
  • the best solution consists in using parts entirely made of stainless steel. Another solution is to weld one or more layers of stainless steel on all the surfaces of the carbon steel parts subject to cavitation phenomena of low intensity to thereby avoid the synergistic effect of cavitation erosion and galvanic corrosion.
  • austenitic stainless steels essentially consisting of approximately 15.0% to 18.5% by weight of chromium, from approximately 10% to approximately 22.5% by weight of cobalt, up to approximately 0, 2% by weight of carbon up to approximately 1.5% by weight of manganese up to approximately 0.75% by weight of silicon, up to approximately 0.15% by weight of nitrogen, the balance being iron .
  • DE-C-607 384 describes iron-chromium-cobalt alloys which contain less than 0.3% of carbon, from 15 to 22% of chromium, from 6 to 16% of manganese, from 1 to 10% of cobalt, the the remainder consisting of iron accompanied by the usual impurities.
  • the present invention is directly related to the discovery that low hardness cobalt stainless steels containing as little as 8% by weight of cobalt have an erosive cavitation resistance as good as that of alloys containing up to '' at 65% cobalt, provided that at least 60% by weight of said stainless steels with low cobalt content is, at room temperature, in a cubic phase with a metastable centered face having a sufficiently low stacking fault energy therein so that it can transform under the effect of cavitation into a compact hexagonal phase s and / or into martensite a showing a fine jaw 'deformation.
  • the present invention has for its first object an austenitic cobalt stainless steel having a high resistance to erosive cavitation, of the type consisting of: the remaining percentage consisting of Fe and the usual impurities, said steel being characterized in that its content of elements known as ferritizing agents (Cr, Mo, Si), in elements known as austenitizing agents (C, N, Co, Ni, Mn) and, among these ferritating and austenitic elements, in elements known to increase or decrease the energy of stacking fault, is suitably chosen and adjusted so that at least 60% by weight of the steel is, at the ambient temperature, in a cubic phase with a metastable centered face y having a sufficiently low stacking energy that it can transform under the effect of cavitation into a compact hexagonal phase E or into martensite a showing a end deformation chewing.
  • ferritizing agents Cr, Mo, Si
  • austenitizing agents C, N, Co, Ni, Mn
  • Co stainless steel according to the invention has a low carbon content (less than 0.3%).
  • the fact that this steel also has a excellent resistance to cavitation despite this low carbon content is compatible with the above-mentioned result of observations made by KC Anthony and AI, namely the observation that the high resistance to cavitation of STELLITE-6 type alloys is retained even if the carbon content of these alloys is reduced from 1.3 to 0.25%.
  • At least 60% by weight of the cobalt stainless steel according to the invention must be, at ambient temperature, in a cubic phase with a centered face which is both metastable and has the lowest possible energy. lack of stacking.
  • the metastability of the face-centered cubic austenitic phase therein is an essential element of the invention, since it is absolutely necessary that the steel is capable, under the effect of cavitation, of being transformed into a compact hexagonal phase e and / or martensite a.
  • phase y the content of the steel in known ferritating (Cr, Mo, Si) and austenitic (C, N, Co, Ni, Mn) elements respectively must be properly selected and adjusted so as to just stabilize the austenite (that is to say the y phase) in particular in the case of rapid cooling of the steel, to promote a transformation induced by cavitation of this y phase into the ⁇ and / or martensite phase .
  • the stainless steel according to the invention must show a fine cavitation-induced chewing, which chewing is specific to crystals with a low energy of stacking fault.
  • the elements known to increase the energy of stacking fault one can quote Ni and C.
  • those known to lower the EFE one can - quote Co, Si, Mn and N. Of course, these last elements will have to be chosen in priority to obtain the desired result, namely a low EFE.
  • Cobalt is undoubtedly one of the most interesting insofar as it has the advantage, in addition to lowering EFE, to maintain the metastability of the austenitic phase of steel over a large concentration range.
  • the stainless steel according to the invention which contains less than 30% by weight of cobalt and up to 70% by weight of iron can thus have a stacking fault energy as low as that of alloys with a high cobalt content, and a substantially identical end-of-deformation coupling (see in particular the article by DA Woodford et al, “A deformation Induced Phase Transformation Involving a Four-Layer Stacking Sequence in Co-Fe Alloy ", Met. Trans., Vol. 2, page 3223, 1971 where it is stated that in Fe-Co alloys, only 15% by weight of iron is sufficient to make completely disappear the transformation induced by cavitation from phase y to phase e).
  • chromium has a very strong interaction with cobalt and iron to promote the formation of low energy crystals due to stacking failure.
  • the surface layer of the Fe-Cr-Co-C alloys according to the invention shows, after exposure to cavitation, a very fine latticework network in the compact hexagonal phase (phase e) or martensite a.
  • phase e the compact hexagonal phase
  • martensite a The presence of this fine and continuous chewing obtained under exposure to cavitation explains the high resistance to cavitation of the alloy, which, by its chewing, has an effective means of absorbing the energy of cavitation shocks by deformation of its crystal structure.
  • This fine chewing is also an excellent means of accommodating high stresses and thus delaying the creation and propagation of fatigue cracks.
  • the localized hardening associated with this fine chewing ensures an extension of the chewing to the whole exposed surface at the beginning of the exposure to cavitation (incubation period).
  • the austenitic cobalt stainless steel according to the invention advantageously consists of: the remaining percentage consisting of Fe and the usual impurities.
  • a particularly interesting stainless steel covered by this preferred embodiment is that consisting of 10% by weight of Co, 18% by weight of Cr, and 0.3% by weight of C, the remaining percentage consisting of Fe and usual impurities. It turns out that this particular steel is not only very effective, but one of the cheapest. It can in particular be noted that the composition of this steel is substantially equivalent to the composition of stainless steels of the standard 300 series, the only difference residing in the absence of nickel (known to increase the energy of EFE stacking fault) replaced by an increased amount of Co (known to lower EFE).
  • the austenitic cobalt stainless steel according to the invention advantageously consists of: the remaining percentage consisting of Fe and the usual impurities.
  • Co stainless steel according to the invention is soft. This steel is less expensive than conventional alloys with a high Co content such as STELLITE 6 or STELLITE 21, while having substantially the same resistance to cavitation. As a result, the stainless steel according to the invention offers an economical alternative to alloys of the STELLITE 21 type currently used to protect hydraulic machines against the effects of erosive cavitation. Welding wires or electrodes made from the steel according to the invention can be used to repair damage due to cavitation. Hydraulic machine parts or whole groups can also be cast or completely covered with this steel which is cheaper than the Stellite is capable of being hot and cold rolled for the development of the manufacture of machine elements hydraulic with high resistance to cavitation.
  • another subject of the invention is the use of the steel according to the invention for the manufacture or recovery of parts intended for the manufacture or repair of hydraulic machines as well as the manufacture of wires welding for the manufacture or repair of hydraulic machines.
  • the stainless steel parts according to the invention have a cavitation resistance at least equal to the parts made of harder alloys of the STELLITE-1 or -6 type. Since the stainless steels according to the invention are soft, they are much easier to grind. In fact, the parts according to the invention have all the advantages of parts made from soft alloys with a high Co content, of the STELLITE-21 type, but at a lower cost.
  • the resistance of the steels and alloys tested to erosive cavitation was measured by ultrasonic cavitation test according to standard ASTM-G32.
  • the losses in weight of 16 mm cylindrical samples vibrating at 20 kHz at a double amplitude of 50 ⁇ m in distilled water at 22 ° C were measured every half hour for six hours using a electric scale accurate to tenth of a milligram.
  • the materials tested are listed in Table 1 below, where their composition is also found. nominal, their manufacturing process, their hardness and their original crystallographic structure.
  • the experimental Co # 1 Co # 25 alloys listed in the previous table were prepared by melting on a water-cooled copper plate in a small laboratory arc furnace an appropriate mixture of several of the following constituents: steel carbon, 304 stainless steel, STELLITE-21, ferrochrome, electrolytic cobalt, ferromanganese and ferrosilicon. It should be noted that the compositions of these experimental alloys, with the exception of Co # 7,12 and 15 which were tested for reference, all fall within the composition range of cobalt stainless steel according to the invention.
  • Metallographic observations were made by taking optical and electron micrographs on the eroded surfaces of the samples after various periods of exposure to cavitation.
  • the surfaces of the samples in question were originally electrochemically polished and cleaned with acid.
  • microhardness measurements were carried out by applying a pyramidal diamond to the eroded surface of the samples after various periods of exposure to cavitation, until this surface was too bumpy to allow measurements.
  • the longest wavelength CuK " has been chosen so that the diffraction occurs only on a thin surface layer (of the order of 10 to period d incubation so that surface erosion has just started.
  • Table 1 as well as Figures 1 and 2 provide the results of the erosive cavitation tests carried out by the Inventor. These results clearly demonstrate that stainless steel 308 has a resistance to cavitation twice that of carbon steel 1020 and that all of the experimental Co-Cr-Fe alloys with the exception of Co # 5, 7 and 11 to 15 have a much better resistance to cavitation (of the order of 10 to 50 times greater) than stainless steel 308 although they have only a slightly higher hardness.
  • the table above shows that the 1020 carbon steel sample is the only material which did not show any phase transformation induced by deformation after exposure to cavitation. As expected, only a small portion of the eroded surface of the austenitic 308 stainless steel sample was transformed into martensite. It is interesting to note that on this steel, the exposure to cavitation modified the texture of the surface by eroding the oriented surface grains (200), the oriented grains (111) showing superior resistance.
  • Stainless steel 301 which was partially martensitic when welded, had its surface completely transformed into martensite under the effect of cavitation.
  • the alloy Co # 5 (10% cobalt) which was essentially ferritic when melted with a small percentage of austenite, was almost completely transformed into martensite under exposure to cavitation.
  • the alloy Co # 3 (20% cobalt) which was austenitic when melted, was transformed superficially into the compact hexagonal phase ⁇ , with a small percentage of martensite, while the surface of the sample in STELLITE 21 was transformed from less important in ⁇ phase only.
  • the Co # 6 alloy (10% cobalt, 18% chromium) has shown excellent resistance to cavitation with an induced transformation into martensite a rather than in phase E.
  • the alloys Co # 11 to 15 which were martensitic in the state as cast (see Table 1), did not show the best resistance to cavitation.
  • the degree of transformation induced by cavitation follows the following increasing order: 1020 (approximately 0%), Co # 5 (approximately 10%), 308 (approximately 15%), 301 (approximately 75%) STELLITE 21 (approximately 75%), Co # 3 (approximately 90%), Co # 6 (approximately 90%).
  • the hardening induced by cavitation follows substantially the same order.
  • FIG. 16a shows that there is a significant increase in the surface hardness of the most resistant alloys during the incubation period. No strain hardening was measured on the soft ferrite of the carbon steel sample.
  • the experimental alloy Co # 3 which, when melted, is softer than STELLITE 21, showed the strongest hardening, with a final hardness higher than that of STELLITE 21. This hardness increased very quickly at the beginning of the period of 'incubation.
  • microhardness at depth shows that the hardening by deformation due to cavitation is limited to a very thin surface layer (less than 50 wm), which makes this kind of measurement very difficult.
  • the Co # 3 alloy (20% cobalt) exhibits a phase transformation induced by cavitation as well as a more pronounced work hardening than STELLITE 21 (65% cobalt) which is known to be very stable.
  • This Co # 3 alloy also appears to have a resistance on the upper cavitation, even if this alloy has a lower initial hardness (23 RC compared to 30 RC for STELLITE 21).
  • the composition that stainless steels must have to offer the best possible resistance to cavitation can include various hardeners such as molybdenum, to maintain the same degree of phase transformation.
  • the content of the cobalt stainless steel according to the invention in elements known as ferritisants (Cr, Mo, Si) and austenitisants (C, N, Co, Ni, Mn) must be appropriately chosen and adjusted so as to barely stabilize the austenite, particularly in the case of rapid cooling, to thus promote a transformation induced by cavitation from phase a to phase ⁇ or to martensite , the high resistance to cavitation of the steels according to the invention resulting mainly from their composition where the elements known to increase the stacking fault energy, namely carbon and nickel, are replaced as much as possible by known elements to lower this stacking fault energy such as Co, Si, Mn and N and thus lead to a finer deformation coupling.
  • ferritisants Cr, Mo, Si
  • austenitisants C, N, Co, Ni, Mn
  • the cobalt stainless steels according to the invention can advantageously be used for the manufacture and repair of parts or groups of hydraulic machines, such as turbines, pumps, valves, etc. They can be used either as covers welded to carbon steel, or as basic materials, cast or in the form of sheet metal, for the manufacture of machines made of stainless steel. These steels can furthermore be hot or cold rolled and be developed in welding wires or electrodes to replace the much more expensive STELLITE 21 used to repair cavitation damage in hydraulic turbines.

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Abstract

A soft, austenitic stainless steel alloy showing a high cavitation erosion resistance making it particularly useful for the manufacture and/or repair of hydraulic machine components. The alloy comprises from 8 to 30% by weight of Co; from 13 to 30% by weight of Cr; from 0.03 to 0.3% by weight of C; up to 0.3% by weight of N; up to 3% by weight of Si; up to 1% by weight of Ni; up to 2% by weight of Mo; and up to 9% by weight of Mn; the balance being substantially Fe. The amount of the above mentioned elements that are respectively known as ferrite formers (Cr, Mo, Si) and as austenite formers (C, N, Co, Ni, Mn) and, among said austenite and ferrite formers, the amount of each of the elements that are respectively known to increase and lower the stack fault energy, are respectively selected and balanced so that at least 60% by weight of the alloy is, at ambient temperature, in a metastable, face centered cubic phase having a stack fault energy low enough to make it capable of being transformed under cavitation exposure to a fine deformation twinning, hexagonal close pack epsilon -phase and/or alpha -martensitic phase.

Description

La présente invention a pour objet un acier inoxydable austénitique au cobalt ayant une très grande résistance à la cavitation érosive de forte intensité le rendant tout particulièrement utile pour la fabrication ou la réparation de pièces de machines hydrauliques. L'invention a également pour objet les pièces de machines hydrauliques ainsi faites ou recouvertes dudit acier inoxydable au cobalt.The present invention relates to an austenitic cobalt stainless steel having a very high resistance to high intensity erosive cavitation making it very particularly useful for the manufacture or repair of parts of hydraulic machines. The invention also relates to the parts of hydraulic machines thus made or covered with said cobalt stainless steel.

Le phénomène de cavitation que subissent notamment les machines hydrauliques telles que les turbines, pompes, hélices, vannes, ou échangeurs, est un inconvénient bien connu des spécialistes. Par phénomène de cavitation. on entend le phénomène par lequel une cavité ou une bulle de vapeur se forme dans un liquide lorsque la pression locale descend au-dessous de la pression de vapeur. Lorsque la pression remonte au-dessus de celle de la vapeur, la bulle gaz ou de vapeur implose soudainement. Cette implosion s'accompagne de phénomènes physiques puissants notamment d'un microjet qui suit la bulle et dont la vitesse peut atteindre les valeurs de plusieurs centaines de mètres par seconde.The cavitation phenomenon in particular experienced by hydraulic machines such as turbines, pumps, propellers, valves, or exchangers, is a drawback well known to specialists. By cavitation phenomenon. the phenomenon by which a cavity or a bubble of vapor is formed in a liquid when the local pressure drops below the vapor pressure. When the pressure rises above that of the vapor, the gas or vapor bubble suddenly implodes. This implosion is accompanied by powerful physical phenomena, in particular a microjet which follows the bubble and whose speed can reach the values of several hundreds of meters per second.

Lorsqu'un tel microjet rencontre une paroi, son énergie cinétique est transformée en une onde de choc localisée capable de déformer la surface métallique la plus dure et ainsi produire une érosion mécanique importante. L'intensité des contraintes locales produite par ces impulsions peut s'étendre sur une très grande gamme dépendant des conditions de nature du liquide, de la température et de la présence de gaz étranger, du taux de variation de pression et de la vitesse du liquide. Ces chocs répétés érodent la surface métallique par propagation de fissures par fatigue (déformation élastique) ou par déformation plastique conduisant à un arrachement de particules de faibles dimensions.When such a microjet encounters a wall, its kinetic energy is transformed into a localized shock wave capable of deforming the hardest metallic surface and thus producing significant mechanical erosion. The intensity of the local stresses produced by these pulses can extend over a very wide range depending on the conditions of the nature of the liquid, the temperature and the presence of foreign gas, the rate of change of pressure and the speed of the liquid. . These repeated shocks erode the metal surface by propagation of cracks by fatigue (elastic deformation) or by plastic deformation leading to tearing of small particles.

L'observation des dommages sur plusieurs groupes et pièces de machines hydrauliques de même que le résultat d'essais accélérés de cavitation érosive ultrasonique effectués par divers spécialistes, inclus le présent inventeur, ont montré que les pièces des machines hydrauliques, et plus particulièrement les pièces des.turbines hydrauliques sont généralement sujettes à une gamme assez large d'intensités de cavitation, laquelle gamme peut être divisée en deux catégories impliquant des solutions différentes. L'une de ces catégories couvre les phénomènes de cavitation érosive de faible intensité. L'autre de ces catégories couvre les phénomènes de cavitation de forte intensité.The observation of damage on several groups and parts of hydraulic machines as well as the result of accelerated tests of ultrasonic erosive cavitation carried out by various specialists, including the present inventor, have shown that the parts of hydraulic machines, and more particularly the parts Hydraulic turbines are generally subject to a fairly wide range of cavitation intensities, which range can be divided into two categories involving different solutions. One of these categories covers the phenomena of erosive cavitation of low intensity. The other of these categories covers the phenomena of high intensity cavitation.

Les phénomènes de cavitation de faible intensité que subissent les machines hydrauliques et plus particulièrement les turbines' hydrauliques, se produisent généralement sur de larges surfaces et affectent principalement les aciers au carbone, laissant les aciers inoxydables pratiquement inattaqués. Ce type de cavitation produit une érosion lente des aciers au carbone, laquelle érosion est accélérée par des phénomènes de corrosion et/ou de couplage galvanique qui se produisent avec les alliages nobles, tels que des aciers inoxydables.The low intensity cavitation phenomena experienced by hydraulic machines, and more particularly hydraulic turbines, generally occur over large areas and mainly affect carbon steels, leaving stainless steels practically unaffected. This type of cavitation produces a slow erosion of carbon steels, which erosion is accelerated by corrosion and / or galvanic coupling phenomena which occur with noble alloys, such as stainless steels.

Pour remédier à cette partie du problème, la meilleure solution consiste à utiliser des pièces entièrement faites en acier inoxydable. Une autre solution consiste à souder une ou plusieurs couches d'acier inoxydable sur toutes les surfaces des pièces en acier au carbone sujettes à des phénomènes de cavitation de faible intensité pour ainsi éviter l'effet synergétique d'érosion de cavitation et de corrosion galvanique.To remedy this part of the problem, the best solution consists in using parts entirely made of stainless steel. Another solution is to weld one or more layers of stainless steel on all the surfaces of the carbon steel parts subject to cavitation phenomena of low intensity to thereby avoid the synergistic effect of cavitation erosion and galvanic corrosion.

De leur côté, les phénomènes de cavitation de forte intensité se produisent plutôt sur les pièces de machines hydrauliques ou les groupes opérant sous des pressions ou vitesses d'eau supérieures, uniquement sur des petites surfaces localisées, telles que, par exemple, la partie arrière des aubes des turbines. Ce type de cavitation produit une érosion rapide même sur des matériaux de forte résistance tels que des aciers inoxydables austénitiques, avec des vitesses de perforation de l'ordre de 0,1 à 10 mm par an.On the other hand, the phenomena of high intensity cavitation rather occur on parts of hydraulic machines or groups operating under higher water pressures or speeds, only on small localized surfaces, such as, for example, the rear part. turbine blades. This type of cavitation produces rapid erosion even on materials of high resistance such as austenitic stainless steels, with perforation speeds of the order of 0.1 to 10 mm per year.

Pour contourner cette partie du problème, des matériaux ayant une forte résistance à la cavitation sont nécessaires. Des aciers durs tels que des alliages à base de cobalt de type STELLITE-1 et STELLITE-6 (marques de commerce), des bronzes d'aluminium ou des matériaux polymères à haute résistance tel que le NYLON 66 (marque de commerce), ont été essayés avec succès et sont utilisés pour certaines applications particulières. Il s'avère toutefois que ces applications particulières sont relativement limitées en pratique, essentiellement parce que la plupart des matériaux connus de forte résistance sont difficiles à usiner et à utiliser, en plus d'être relativement chers.To get around this part of the problem, materials with high resistance to cavitation are needed. Hard steels such as STELLITE-1 and STELLITE-6 cobalt-based alloys (trademarks), aluminum bronzes or high-strength polymer materials such as NYLON 66 (trademark) have been successfully tried and are used for certain specific applications. However, it turns out that these particular applications are relatively limited in practice, essentially because most of the known materials of high resistance are difficult to machine and use, in addition to being relatively expensive.

Il a été reconnu récemment que certains alliages peuvent montrer une très grande résistance à la cavitation sans pour autant être très durs. K. C. Anthony et al dans leur article « The effect of composition and microstructure on cavitation erosion resistance •, 5th Int. Conf. of Erosion by Solid and Liquid Impact, article 67, Cambridge Angleterre, septembre 1979, ont, en particulier, démontré que, dans le cas des alliages à base de cobalt connus sous la marque STELLITE, la résistance à la cavitation érosive n'est pas modifiée si l'on abaisse la concentration de carbone de l'alliage de 1,3 à 0,3 % avec, comme résultat, une diminution de la dureté de 40 à 25 RC. Ce résultat surprenant a conduit à essayer des alliages mous à base de cobalt à faible teneur en carbone, tel que le Stellite 21, pour réparer des dommages causés par la cavitation dans des turbines hydrauliques. Ces essais ont montré que les allliages mous à base de cobalt sont beaucoup plus efficaces que les aciers inoxydables austénitiques 308 à 301 lorsqu'ils sont soudés à la surface des pièces pour réparer les dommages causés par cavitation érosive de forte intensité. Plus particulièrement encore, les alliages testés se sont avérés être beaucoup plus faciles à meuler ce qui est très important pour de telles réparations et, bien que de prix plus élevé, plus économiques à l'usage du fait qu'ils résistent plus de dix fois plus longtemps que les aciers, inoxydables et ainsi réduisent substantiellement le nombre de réparations.It has been recognized recently that some alloys can show very high resistance to cavitation without being very hard. KC Anthony et al in their article “The effect of composition and microstructure on cavitation erosion resistance •, 5th Int. Conf. of Erosion by Solid and Liquid Impact, article 67, Cambridge England, September 1979, have in particular demonstrated that, in the case of cobalt-based alloys known under the brand name STELLITE, the resistance to erosive cavitation is not modified if the carbon concentration of the alloy is lowered from 1.3 to 0.3% with, as a result, a reduction in hardness from 40 to 25 RC. This surprising result led to testing soft, low-carbon cobalt-based alloys, such as Stellite 21, to repair cavitation damage in hydraulic turbines. These tests have shown that soft cobalt-based alloys are much more effective than austenitic stainless steels 308 to 301 when welded to the surface of parts to repair damage caused by high intensity erosive cavitation. More particularly still, the alloys tested have proved to be much easier to grind, which is very important for such repairs and, although of higher price, more economical to use. the fact that they last more than ten times longer than stainless steels and thus significantly reduce the number of repairs.

Le fait que les alliages mous notamment à base de cobalt puissent avoir une forte résistance de cavitation n'a pas encore été expliqué de façon satisfaisante. A l'origine, la tenue supérieure à l'érosion des alliages au cobalt du type STELLITE-6 a été attribuée à l'existence d'une transformation martensitique induite par déformation, laquelle transformation absorberait un pourcentage important de l'énergie incidente de cavitation. Toutefois, des essais ultérieurs ont démontré que la contribution de cette transformation martensitique à la résistance à l'érosion des alliages est, si elle existe, mineure (voir, par exemple, D.A. Woodford, « Cavitation-Erosion-induced Phase Transformation in Alloys », Met. Trans. Vol. 3, page 1137, mai 1972 ; et S. Vaidya et al, « The role of Twinning in the Cavitation Erosion of Cobalt Single Crystals •, Met. Trans. A., Vol. 11 A, page 1139, juillet 1980). En fait, ces essais ont plutôt démontré que toute amélioration dans la tenue d'un alliage à l'érosion est associée à une diminution de l'énergie de faute d'empilement (E.F.E.) des cristaux de l'alliage. Il a donc été suggéré que le mode de glissement plan que l'on trouve dans les matériaux à faible E.F.E. retarde le développement des contraintes localisées nécessaires à l'amorce de fissures, avec, de là, une amélioration de la résistance de l'alliage à la fatigue.The fact that soft alloys especially based on cobalt can have a high cavitation resistance has not yet been satisfactorily explained. Originally, the superior resistance to erosion of cobalt alloys of the STELLITE-6 type was attributed to the existence of a martensitic transformation induced by deformation, which transformation would absorb a large percentage of the incident cavitation energy. . However, subsequent tests have demonstrated that the contribution of this martensitic transformation to the erosion resistance of alloys is, if it exists, minor (see, for example, DA Woodford, "Cavitation-Erosion-induced Phase Transformation in Alloys" , Met. Trans. Vol. 3, page 1137, May 1972; and S. Vaidya et al, "The role of Twinning in the Cavitation Erosion of Cobalt Single Crystals •, Met. Trans. A., Vol. 11 A, page 1139, July 1980). In fact, these tests have rather shown that any improvement in the resistance of an alloy to erosion is associated with a reduction in the stacking fault energy (E.F.E.) of the crystals of the alloy. It has therefore been suggested that the plane sliding mode found in materials with low E.F.E. delays the development of the localized stresses necessary for the initiation of cracks, with, from there, an improvement in the resistance of the alloy to fatigue.

S. Vaidya et al ont également suggéré dans leur article ci-dessus mentionné que la présence d'un mâclage fin est responsable de l'excellente résistance à l'érosion du cobalt sous forme hexagonale compacte (H.C. en abrégé, ou encore phase s), ladite forme H.C. étant une forme stable à basse température du cobalt provenant d'une transformation allotropique se produisant à 420 °C pour le cobalt pur à l'origine sous une forme cubique à face centrée (forme C.F.C. en abrégé, ou encore phase y).S. Vaidya et al also suggested in their article above mentioned that the presence of a fine chewing is responsible for the excellent resistance to erosion of cobalt in compact hexagonal form (HC for short, or phase s) , said form HC being a stable form at low temperature of cobalt originating from an allotropic transformation occurring at 420 ° C. for pure cobalt originally in a cubic form with centered face (CFC form for short, or phase y ).

U.S.A. 2 990 275 décrit des aciers inoxydables austénitiques essentiellement constitués d'environ 15,0 % à 18,5 % en poids de chrome, d'environ 10 % à environ 22,5 % en poids de cobalt, jusqu'à environ 0,2 % en poids de carbone jusqu'à environ 1,5 % en poids de manganèse jusqu'à environ 0,75 % en poids de silicium, jusqu'à environ 0,15 % en poids d'azote, le solde étant du fer.USA 2,990,275 describes austenitic stainless steels essentially consisting of approximately 15.0% to 18.5% by weight of chromium, from approximately 10% to approximately 22.5% by weight of cobalt, up to approximately 0, 2% by weight of carbon up to approximately 1.5% by weight of manganese up to approximately 0.75% by weight of silicon, up to approximately 0.15% by weight of nitrogen, the balance being iron .

DE-C-607 384 décrit des alliages fer-chrome-cobalt qui renferment moins de 0,3 % de carbone, de 15 à 22 % de chrome, de 6 à 16 % de manganèse, de 1 à 10 % de cobalt, le reste étant constitué par du fer accompagné des impuretés habituelles.DE-C-607 384 describes iron-chromium-cobalt alloys which contain less than 0.3% of carbon, from 15 to 22% of chromium, from 6 to 16% of manganese, from 1 to 10% of cobalt, the the remainder consisting of iron accompanied by the usual impurities.

La présente invention est directement liée à la découverte du fait que des aciers inoxydables au cobalt à faible dureté contenant aussi peu que 8 % en poids de cobalt possèdent une résistance à la cavitation érosive aussi bonne que celle, excellente, que possèdent les alliages contenant jusqu'à 65 % de cobalt, à condition qu'au moins 60 % en poids desdits aciers inoxydables à faible teneur en cobalt soit, à température ambiante, dans une phase cubique à face centrée métastable y ayant une énergie de faute d'empilement suffisamment faible pour qu'elle puisse se transformer sous l'effet de la cavitation en une phase hexagonale compacte s et/ou en de la martensite a montrant un mâclage fin' de déformation.The present invention is directly related to the discovery that low hardness cobalt stainless steels containing as little as 8% by weight of cobalt have an erosive cavitation resistance as good as that of alloys containing up to '' at 65% cobalt, provided that at least 60% by weight of said stainless steels with low cobalt content is, at room temperature, in a cubic phase with a metastable centered face having a sufficiently low stacking fault energy therein so that it can transform under the effect of cavitation into a compact hexagonal phase s and / or into martensite a showing a fine jaw 'deformation.

Plus particulièrement, l'invention est liée à la découverte du fait que les alliages mous Fe-Cr-Co-C qui possèdent un mâclage fin induit par la cavitation, lequel mâclage est spécifique aux cristaux à faible énergie de faute d'empilement (E.F.E.), possèdent également une résistance efficace à la cavitation au moyen des divers mécanismes suivants :

  • - écrouissage et accomodation de contraintes élevées, retardant l'initiation de fissures de fatigues ;
  • - extension du mâclage plan à toute la surface de l'alliage conservant ce dernier lisse durant toute une période d'incubation, et ;
  • - absorption continue de l'énergie de cavitation incidente par la production d'une grande densité de dislocation et de particules érodées fines conduisant ainsi à de faibles taux d'érosion.
More particularly, the invention is linked to the discovery of the fact that the soft Fe-Cr-Co-C alloys which have a fine cavitation-induced weaving, which weaving is specific to crystals with low energy of stacking fault (EFE ), also have an effective resistance to cavitation by means of the following various mechanisms:
  • - work hardening and accommodation of high stresses, delaying the initiation of fatigue cracks;
  • extension of the planar covering over the entire surface of the alloy retaining the latter smooth during an entire incubation period, and;
  • - continuous absorption of the incident cavitation energy by the production of a high density of dislocation and fine eroded particles thus leading to low rates of erosion.

Sur la base de cette découverte, la présente invention a pour premier objet un acier inoxydable austénitique au cobalt possédant une forte résistance à la cavitation érosive, du type consistant en :

Figure imgb0001
le pourcentage restant étant constitué de Fe et des impuretés habituelles, ledit acier étant caractérisé en ce que sa teneur en éléments connus comme ferritisants (Cr, Mo, Si), en éléments connus comme austénitisants (C, N, Co, Ni, Mn) et, parmi ces éléments ferritisants et austénitisants, en éléments connus pour augmenter ou abaisser l'énergie de faute d'empilement, est adéquatement choisie et ajustée de façon à ce qu'au moins 60 % en poids de l'acier soit, à la température ambiante, dans une phase cubique à face centrée métastable y ayant une énergie de faute d'empilement suffisamment faible pour qu'elle puisse se transformer sous l'effet de la cavitation en une phase hexagonale compacte E ou en de la martensite a montrant un mâclage fin de déformation.On the basis of this discovery, the present invention has for its first object an austenitic cobalt stainless steel having a high resistance to erosive cavitation, of the type consisting of:
Figure imgb0001
 the remaining percentage consisting of Fe and the usual impurities, said steel being characterized in that its content of elements known as ferritizing agents (Cr, Mo, Si), in elements known as austenitizing agents (C, N, Co, Ni, Mn) and, among these ferritating and austenitic elements, in elements known to increase or decrease the energy of stacking fault, is suitably chosen and adjusted so that at least 60% by weight of the steel is, at the ambient temperature, in a cubic phase with a metastable centered face y having a sufficiently low stacking energy that it can transform under the effect of cavitation into a compact hexagonal phase E or into martensite a showing a end deformation chewing.

Comme on peut le constater à la lecture de la composition ci-dessus, l'acier inoxydable au Co selon l'invention a une faible teneur en carbone (inférieure à 0,3 %). Le fait que cet acier ait également une excellente résistance à la cavitation malgré cette faible teneur en carbone est compatible avec le résultat ci-dessus mentionné des observations faites par K. C. Anthony et AI, à savoir l'observation du fait que la forte résistance à la cavitation des alliages de type STELLITE-6 est conservée même si la teneur en carbone de ces alliages est diminuée de 1,3 à 0,25 %.As can be seen from reading the above composition, Co stainless steel according to the invention has a low carbon content (less than 0.3%). The fact that this steel also has a excellent resistance to cavitation despite this low carbon content is compatible with the above-mentioned result of observations made by KC Anthony and AI, namely the observation that the high resistance to cavitation of STELLITE-6 type alloys is retained even if the carbon content of these alloys is reduced from 1.3 to 0.25%.

Tel que précédemment indiqué, au moins 60 % en poids de l'acier inoxydable au cobalt selon t'invention doit être, à température ambiante, dans une phase cubique à face centrée qui soit à la fois métastable et ait la plus faible énergie possible de faute d'empilement. La métastabilité de la phase austénitique cubique à face centrée y est un élément essentiel de l'invention, puisqu'il est absolument nécessaire que l'acier soit capable, sous l'effet de la cavitation, d'être transformé en phase hexagonale compacte e et/ou en martensite a. Pour obtenir la métastabilité nécessaire de la phase y, la teneur de l'acier en éléments respectivement connus ferritisants (Cr, Mo, Si) et austénitisants (C, N, Co, Ni, Mn) doit être adéquatement choisie et ajustée de façon à tout juste stabiliser l'austénite (c'est-à-dire la phase y) notamment dans le cas d'un refroidissement rapide de l'acier, pour promouvoir une transformation induite par cavitation de cette phase y en phase ε et/ou martensite.As previously indicated, at least 60% by weight of the cobalt stainless steel according to the invention must be, at ambient temperature, in a cubic phase with a centered face which is both metastable and has the lowest possible energy. lack of stacking. The metastability of the face-centered cubic austenitic phase therein is an essential element of the invention, since it is absolutely necessary that the steel is capable, under the effect of cavitation, of being transformed into a compact hexagonal phase e and / or martensite a. To obtain the necessary metastability of phase y, the content of the steel in known ferritating (Cr, Mo, Si) and austenitic (C, N, Co, Ni, Mn) elements respectively must be properly selected and adjusted so as to just stabilize the austenite (that is to say the y phase) in particular in the case of rapid cooling of the steel, to promote a transformation induced by cavitation of this y phase into the ε and / or martensite phase .

Tel qu'indiqué également ci-dessus, l'acier inoxydable selon l'invention doit montrer un mâclage fin induit par la cavitation, lequel mâclage est spécifique aux cristaux à faible énergie de faute d'empilement. Pour obtenir cette faible énergie de faute d'empilement, il est nécessaire de tenir compte de la capacité de chaque élément à abaisser ou augmenter l'énergie de faute d'empilement, et d'ajuster la teneur respective des divers éléments choisis pour constituer l'acier de façon à ce que l'énergie de faute d'empilement de l'ensemble des éléments combinés soit suffisamment faible pour qu'on ait un mâclage fin de déformation lorsque l'acier est sujet à la cavitation. Parmi les éléments connus pour augmenter l'énergie de faute d'empilement, on peut citer Ni et C. Parmi ceux connus pour abaisser l'E.F.E., on peut - citer Co, Si, Mn et N. Bien sûr, ces derniers éléments devront être choisis en priorité pour obtenir le résultat voulu, à savoir une faible E.F.E.. Parmi des éléments connus pour abaisser l'E.F.E., le Cobalt est sans doute un des plus intéressants dans la mesure où il a l'avantage, en plus d'abaisser l'E.F.E., de conserver la métastabilité de la phase austénitique de l'acier sur une grande gamme de concentration.As also indicated above, the stainless steel according to the invention must show a fine cavitation-induced chewing, which chewing is specific to crystals with a low energy of stacking fault. To obtain this low stack fault energy, it is necessary to take into account the capacity of each element to lower or increase the stack fault energy, and to adjust the respective content of the various elements chosen to constitute the steel so that the stacking fault energy of all of the combined elements is low enough to have a fine warping when the steel is subject to cavitation. Among the elements known to increase the energy of stacking fault, one can quote Ni and C. Among those known to lower the EFE, one can - quote Co, Si, Mn and N. Of course, these last elements will have to be chosen in priority to obtain the desired result, namely a low EFE. Among elements known to lower the EFE, Cobalt is undoubtedly one of the most interesting insofar as it has the advantage, in addition to lowering EFE, to maintain the metastability of the austenitic phase of steel over a large concentration range.

L'exigence pour l'acier selon l'invention de montrer un mâclage fin induit par la cavitation est compatible avec le résultat des observations faites par S. Vaidya état (voir ci-dessus) qui ont attribué la forte résistance de la cavitation des alliages à forte teneur en cobalt à la faible énergie de faute d'empilement de ces alliages et à leur mâclage plan de déformation. Il est à noter cependant qu'il est tout à fait surprenant au vu de l'état de la technique que l'acier inoxydable selon l'invention qui contient moins de 30 % en poids de cobalt et jusqu'à 70 % en poids de fer puisse ainsi posséder une énergie de faute d'empilement aussi faible que celle des alliages à forte teneur en cobalt, et un mâclage à fin de déformation sensiblement identique (voir notamment l'article de D.A. Woodford et al, « A deformation Induced Phase Transformation Involving a Four-Layer Stacking Sequence in Co-Fe Alloy », Met. Trans., vol. 2, page 3223, 1971 où il est indiqué que dans les alliages Fe-Co, seulement 15 % en poids de fer est suffisant pour faire complètement disparaître la transformation induite par cavitation de la phase y en phase e). Une explication possible à ce phénomène particulier est que, dans l'acier inoxydable selon l'invention, le chrome a une très forte interaction avec le cobalt et le fer pour promouvoir la formation de cristaux à faible énergie de faute d'empilement.The requirement for the steel according to the invention to show a fine cavitation induced chewing is compatible with the result of the observations made by S. Vaidya state (see above) which attributed the high resistance of the cavitation of the alloys with a high cobalt content at the low stacking fault energy of these alloys and with their warping plane of deformation. It should be noted, however, that it is quite surprising in view of the state of the art that the stainless steel according to the invention which contains less than 30% by weight of cobalt and up to 70% by weight of iron can thus have a stacking fault energy as low as that of alloys with a high cobalt content, and a substantially identical end-of-deformation coupling (see in particular the article by DA Woodford et al, “A deformation Induced Phase Transformation Involving a Four-Layer Stacking Sequence in Co-Fe Alloy ", Met. Trans., Vol. 2, page 3223, 1971 where it is stated that in Fe-Co alloys, only 15% by weight of iron is sufficient to make completely disappear the transformation induced by cavitation from phase y to phase e). One possible explanation for this particular phenomenon is that, in the stainless steel according to the invention, chromium has a very strong interaction with cobalt and iron to promote the formation of low energy crystals due to stacking failure.

La couche de surface des alliages Fe-Cr-Co-C selon l'invention montre, après exposition à la cavitation, un réseau très fin de mâclage dans la phase hexagonale compacte (phase e) ou la martensite a. La présence de ce mâclage fin et continu obtenu sous exposition à la cavitation explique la forte résistance à la cavitation de l'alliage, qui, de par son mâclage, possède un moyen efficace d'absorber l'énergie des chocs de cavitation par déformation de sa structure cristalline. Ce mâclage fin est également un excellent moyen d'accomoder les contraintes élevées et ainsi retarder la création et la propagation de fissures de fatigue. L'écrouissage localisé associé à ce mâclage fin assure une extension du mâclage à toute la surface exposée au début de l'exposition à la cavitation (période d'incubation). Ceci explique pourquoi la surface exposée demeure aussi plate et lisse durant la période d'incubation, si on la compare à la surface de fort relief que l'on obtient avec des matériaux plus déformables. Des surfaces plus lisses sont, en effet, moins sujettes à attaque par les microjets tangentiels localisés qui se produisent lors de chaque implosion due à la cavitation. Ainsi, pendant la période d'incubation, le seul relief de surface que subissent les aciers inoxydables au cobalt selon l'invention est le mâclage fin de déformation ci-dessus mentionné. Ce mâclage fin conduit à de très faibles taux d'érosion compte tenu du fait que les particules érodées à la jonction des mailles sont très fines. La quantité importante de surfaces nouvellement créées pour une quantité donnée de métal perdue par érosion est un autre moyen efficace d'absorber l'énergie de cavitation incidente.The surface layer of the Fe-Cr-Co-C alloys according to the invention shows, after exposure to cavitation, a very fine latticework network in the compact hexagonal phase (phase e) or martensite a. The presence of this fine and continuous chewing obtained under exposure to cavitation explains the high resistance to cavitation of the alloy, which, by its chewing, has an effective means of absorbing the energy of cavitation shocks by deformation of its crystal structure. This fine chewing is also an excellent means of accommodating high stresses and thus delaying the creation and propagation of fatigue cracks. The localized hardening associated with this fine chewing ensures an extension of the chewing to the whole exposed surface at the beginning of the exposure to cavitation (incubation period). This explains why the exposed surface remains so flat and smooth during the incubation period, if we compare it to the surface of strong relief which we obtain with more deformable materials. Smoother surfaces are, in fact, less prone to attack by localized tangential microjets which occur during each implosion due to cavitation. Thus, during the incubation period, the only surface relief undergone by the cobalt stainless steels according to the invention is the above-mentioned fine deformation chewing. This fine chewing leads to very low rates of erosion taking into account the fact that the particles eroded at the junction of the meshes are very fine. Another effective means of absorbing incident cavitation energy is the large amount of newly created surfaces for a given amount of metal lost through erosion.

Selon un mode préféré de réalisation, l'acier inoxydable austénitique au cobalt selon l'invention consiste avantageusement en :

Figure imgb0002
le pourcentage restant étant constitué de Fe et des impuretés habituelles.According to a preferred embodiment, the austenitic cobalt stainless steel according to the invention advantageously consists of:
Figure imgb0002
 the remaining percentage consisting of Fe and the usual impurities.

Bien entendu, la teneur en chacun des éléments ci-dessus mentionnés est adéquatement choisie et ajustée tel qu'expliqué ci-dessus.Of course, the content of each of the above-mentioned elements is suitably chosen and adjusted as explained above.

Un acier inoxydable tout particulièrement intéressant couvert par ce mode préféré de réalisation est celui consistant en 10 % en poids de Co, 18 % en poids de Cr, et 0,3 % en poids de C, le pourcentage restant étant constitué de Fe et des impuretés habituelles. Il s'avère en effet que cet acier particulier est non seulement très efficace, mais un des moins chers. On peut en particulier noter que la composition de cet acier est sensiblement équivalente à la composition des aciers inoxydables de la série standard 300, la seule différence résidant dans l'absence de nickel (connu pour augmenter l'énergie de faute d'empilement E.F.E.) remplacé par une quantité accrue de Co (connu pour abaisser l'E.F.E.).A particularly interesting stainless steel covered by this preferred embodiment is that consisting of 10% by weight of Co, 18% by weight of Cr, and 0.3% by weight of C, the remaining percentage consisting of Fe and usual impurities. It turns out that this particular steel is not only very effective, but one of the cheapest. It can in particular be noted that the composition of this steel is substantially equivalent to the composition of stainless steels of the standard 300 series, the only difference residing in the absence of nickel (known to increase the energy of EFE stacking fault) replaced by an increased amount of Co (known to lower EFE).

Selon un autre mode préféré de réalisation, l'acier inoxydable austénitique au cobalt selon l'invention consiste avantageusement en :

Figure imgb0003
le pourcentage restant étant constitué de Fe et des impuretés habituelles.According to another preferred embodiment, the austenitic cobalt stainless steel according to the invention advantageously consists of:
Figure imgb0003
 the remaining percentage consisting of Fe and the usual impurities.

Tel que précédemment indiqué, l'acier inoxydable au Co selon l'invention est mou. Cet acier est moins cher que les alliages conventionnels à forte teneur en Co tels que le STELLITE 6 ou le STELLITE 21, tout en ayant sensiblement la même résistance à la cavitation. Il en résulte que l'acier inoxydable selon l'invention offre une alternative économique aux alliages de type STELLITE 21 utilisés actuellement pour protéger les machines hydrauliques contre les effets de cavitation érosive. Des fils ou électrodes de soudure faits à partir de l'acier selon l'invention peuvent être utilisés pour réparer des dommages dus à la cavitation. Des pièces de machines hydrauliques ou des groupes entiers peuvent également être coulés .ou complètement recouverts de cet acier qui est moins cher que le Stellite est est capable d'être laminé à chaud et à froid pour le développement de la fabrication d'éléments de machines hydrauliques à forte résistance à la cavitation.As previously indicated, Co stainless steel according to the invention is soft. This steel is less expensive than conventional alloys with a high Co content such as STELLITE 6 or STELLITE 21, while having substantially the same resistance to cavitation. As a result, the stainless steel according to the invention offers an economical alternative to alloys of the STELLITE 21 type currently used to protect hydraulic machines against the effects of erosive cavitation. Welding wires or electrodes made from the steel according to the invention can be used to repair damage due to cavitation. Hydraulic machine parts or whole groups can also be cast or completely covered with this steel which is cheaper than the Stellite is capable of being hot and cold rolled for the development of the manufacture of machine elements hydraulic with high resistance to cavitation.

A la lumière de ce qui précède, l'invention a pour autre objet l'utilisation de l'acier selon l'invention pour la fabrication ou le recouvrement de pièces destinées à la fabrication ou la réparation de machines hydrauliques ainsi que la fabrication de fils de soudage pour la fabrication ou la réparation de machines hydrauliques.In the light of the above, another subject of the invention is the use of the steel according to the invention for the manufacture or recovery of parts intended for the manufacture or repair of hydraulic machines as well as the manufacture of wires welding for the manufacture or repair of hydraulic machines.

Les pièces en acier inoxydable selon l'invention ont une résistance à la cavitation au moins égale aux pièces faites d'alliages plus durs du type STELLITE-1 ou -6. Les aciers inoxydables selon l'invention étant mous, ils sont beaucoup plus faciles à meuler. En fait, les pièces selon l'invention ont tous les avantages des pièces faites à partir d'alliages mous à forte teneur en Co, du type STELLITE-21, mais à moindre coût.The stainless steel parts according to the invention have a cavitation resistance at least equal to the parts made of harder alloys of the STELLITE-1 or -6 type. Since the stainless steels according to the invention are soft, they are much easier to grind. In fact, the parts according to the invention have all the advantages of parts made from soft alloys with a high Co content, of the STELLITE-21 type, but at a lower cost.

D'autres avantages et caractéristiques de la présente invention ressortiront mieux à la lecture de la description qui va suivre d'essais effectués par l'inventeur, en référence aux dessins annexés, dans lesquels :

  • - la figure 1 est une courbe comparative des pertes en poids dues à la cavitation en fonction du temps pour divers types d'aciers et d'alliages au Co ;
  • - la figure 2 est un diagramme donnant le taux d'érosion moyen de divers alliages au Co, inclus ceux selon l'invention, à partir d'essais de cavitation ultrasonique ;
  • - les figures 3 à 14 montrent des spectres de diffraction aux rayons X montrant la transformation de phase induite par cavitation érosive que subissent divers alliages au Co ;
  • - la figure 15 est un diagramme de comparaison montrant le taux d'érosion par cavitation, la transformation de phase induite et l'écrouissage de divers alliages au Co, et
  • - la figure 16 donne deux courbes comparatives de. la microdureté de surface en fonction du temps de cavitation et de la microdureté en fonction de la profondeur d'échantillons d'aciers et d'alliages au Co sujets à l'érosion.
Other advantages and characteristics of the present invention will emerge more clearly on reading the description which follows of tests carried out by the inventor, with reference to the appended drawings, in which:
  • - Figure 1 is a comparative curve of weight losses due to cavitation as a function of time for various types of steels and alloys of Co;
  • - Figure 2 is a diagram giving the average erosion rate of various Co alloys, including those according to the invention, from ultrasonic cavitation tests;
  • FIGS. 3 to 14 show X-ray diffraction spectra showing the phase transformation induced by erosive cavitation which various alloys of Co undergo;
  • FIG. 15 is a comparison diagram showing the rate of erosion by cavitation, the induced phase transformation and the work hardening of various alloys of Co, and
  • - Figure 16 gives two comparative curves of. surface microhardness as a function of cavitation time and microhardness as a function of the depth of samples of steels and Co alloys subject to erosion.

Procédures expérimentalesExperimental procedures

Les résultats expérimentaux ainsi que les données ci-après rapportées ont été obtenus comme suit :The experimental results and the data reported below were obtained as follows:

a) résistance à la cavitation érosive de forte intensité :a) resistance to high intensity erosive cavitation:

La résistance des aciers et alliages testés à la cavitation érosive a été mesurée par essai de cavitation ultrasonique selon la norme ASTM-G32. Les pertes en poids d'échantillons cylindriques de 16 mm vibrant à 20 kHz sous une double amplitude de 50 ¡.¡.m dans de l'eau distillée à 22 °C furent mesurées chaque demi-heure pendant six heures au moyen d'une balance électrique précise au dixième de milligramme. Les matériaux testés sont listés dans le tableau 1 suivant, où l'on trouve également leur composition nominale, leur procédé de fabrication, leur dureté et leur structure cristallographique originale.

Figure imgb0004
The resistance of the steels and alloys tested to erosive cavitation was measured by ultrasonic cavitation test according to standard ASTM-G32. The losses in weight of 16 mm cylindrical samples vibrating at 20 kHz at a double amplitude of 50 µm in distilled water at 22 ° C were measured every half hour for six hours using a electric scale accurate to tenth of a milligram. The materials tested are listed in Table 1 below, where their composition is also found. nominal, their manufacturing process, their hardness and their original crystallographic structure.
Figure imgb0004

Les alliages au Co expérimentaux Co # 1 Co # 25 énumérés dans le tableau précédent furent préparés en faisant fondre sur une plaque de cuivre refroidie à l'eau dans un petit four à arc de laboratoire, un mélange approprié de plusieurs des constituants suivants : acier au carbone, acier inoxydable 304, STELLITE-21, ferrochrome, cobalt électrolytique, ferromanganèse et ferrosilicium. Il est à noter que les compositions de ces alliages expérimentaux, à l'exception des alliages Co # 7,12 et 15 qui furent testés pour fin de référence, tombent tous dans la fourchette de composition de l'acier inoxydable au cobalt selon l'invention.The experimental Co # 1 Co # 25 alloys listed in the previous table were prepared by melting on a water-cooled copper plate in a small laboratory arc furnace an appropriate mixture of several of the following constituents: steel carbon, 304 stainless steel, STELLITE-21, ferrochrome, electrolytic cobalt, ferromanganese and ferrosilicon. It should be noted that the compositions of these experimental alloys, with the exception of Co # 7,12 and 15 which were tested for reference, all fall within the composition range of cobalt stainless steel according to the invention.

b) Autres mesures :b) Other measures:

Des observations métallographiques, des mesures de microdureté et des spectres de diffraction aux rayons X ont été effectués après diverses périodes d'exposition à la cavitation.Metallographic observations, microhardness measurements and X-ray diffraction spectra were made after various periods of exposure to cavitation.

Les observations métallographiques ont été effectuées par la prise de micrographies optiques et à balayage électronique sur les surfaces érodées des échantillons après diverses périodes d'exposition à la cavitation. Les surfaces des échantillons en question étaient à l'origine polies électrochimiquement et nettoyées à l'acide.Metallographic observations were made by taking optical and electron micrographs on the eroded surfaces of the samples after various periods of exposure to cavitation. The surfaces of the samples in question were originally electrochemically polished and cleaned with acid.

Les mesures de microdureté ont été effectuées par application d'un diamant pyramidal sur la surface érodée des échantillons après diverses périodes d'exposition à la cavitation, jusqu'à ce que cette surface soit trop bosselée pour permettre des mesures.The microhardness measurements were carried out by applying a pyramidal diamond to the eroded surface of the samples after various periods of exposure to cavitation, until this surface was too bumpy to allow measurements.

Pour obtenir les spectres de diffraction aux rayons X, la longueur d'onde la plus longue CuK" a été choisie de façon à ce que la diffraction ne se fasse que sur une mince couche de surface (de l'ordre de 10 a période d'incubation de façon à ce que l'érosion de surface ait tout juste commencé.To obtain the X-ray diffraction spectra, the longest wavelength CuK " has been chosen so that the diffraction occurs only on a thin surface layer (of the order of 10 to period d incubation so that surface erosion has just started.

Essais de cavitation érosiveErosive cavitation tests

Le tableau 1 ainsi que les figures 1 et 2 fournissent les résultats des essais de cavitation érosive effectués par l'Inventeur. Ces résultats démontrent clairement que l'acier inoxydable 308 a une résistance à la cavitation deux fois supérieure à celle de l'acier au carbone 1020 et que tous les alliages expérimentaux Co-Cr-Fe à l'exception des alliages Co # 5, 7 et 11 à 15 ont une bien meilleure résistance à la cavitation (de l'ordre de 10 à 50 fois supérieure) que l'acier inoxydable 308 bien qu'ils n'aient une dureté que très légèrement supérieure.Table 1 as well as Figures 1 and 2 provide the results of the erosive cavitation tests carried out by the Inventor. These results clearly demonstrate that stainless steel 308 has a resistance to cavitation twice that of carbon steel 1020 and that all of the experimental Co-Cr-Fe alloys with the exception of Co # 5, 7 and 11 to 15 have a much better resistance to cavitation (of the order of 10 to 50 times greater) than stainless steel 308 although they have only a slightly higher hardness.

Diffraction aux rayons XX-ray diffraction

Le résultat des essais de diffraction aux rayons X effectués par l'Inventeur est illustré sur les figures 3 à 15 et résumé sur le tableau 2 ci-après.

Figure imgb0005
Figure imgb0006
 The result of the X-ray diffraction tests carried out by the inventor is illustrated in Figures 3 to 15 and summarized in Table 2 below.
Figure imgb0005
Figure imgb0006

Le tableau ci-avant montre que l'échantillon d'acier au carbone 1020 est le seul matériau qui n'a montré aucune transformation de phase induite par déformation après exposition à la cavitation. Tel que prévu, une faible portion seulement de la surface érodée de l'échantillon en acier inoxydable austénitique 308 a été transformée en martensite. Il est intéressant de noter que sur cet acier, l'exposition à la cavitation a modifié la texture de la surface en érodant les grains de surface orientés (200), les grains orientés (111) montrant une résistance supérieure.The table above shows that the 1020 carbon steel sample is the only material which did not show any phase transformation induced by deformation after exposure to cavitation. As expected, only a small portion of the eroded surface of the austenitic 308 stainless steel sample was transformed into martensite. It is interesting to note that on this steel, the exposure to cavitation modified the texture of the surface by eroding the oriented surface grains (200), the oriented grains (111) showing superior resistance.

L'acier inoxydable 301 qui était partiellement martensitique lorsque soudé, a eu sa surface complètement transformée en martensite sous l'effet de la cavitation. L'alliage Co # 5 (10 % de cobalt) qui était essentiellement ferritique lorsque fondu avec un petit pourcentage d'austénite, a été presque complètement transformé en martensite sous l'exposition à la cavitation. L'alliage Co # 3 (20 % de cobalt) qui était austénitique lorsque fondu, a été transformé superficiellement en phase hexagonale compacte ε, avec un petit pourcentage de martensite, alors que la surface de l'échantillon en STELLITE 21 a été transformée de façon moins importante en phase ε seulement. De façon tout à fait surprenante, l'alliage Co # 6 (10 % cobalt, 18 % chrome) a montré une excellente résistance à la cavitation avec une transformation induite en martensite a plutôt qu'en phase E. A l'opposé, les alliages Co # 11 à 15 qui étaient martensitiques à l'état tel que coulé (voir tableau 1), n'ont pas montré la meilleure résistance à la cavitation.Stainless steel 301 which was partially martensitic when welded, had its surface completely transformed into martensite under the effect of cavitation. The alloy Co # 5 (10% cobalt) which was essentially ferritic when melted with a small percentage of austenite, was almost completely transformed into martensite under exposure to cavitation. The alloy Co # 3 (20% cobalt) which was austenitic when melted, was transformed superficially into the compact hexagonal phase ε, with a small percentage of martensite, while the surface of the sample in STELLITE 21 was transformed from less important in ε phase only. Quite surprisingly, the Co # 6 alloy (10% cobalt, 18% chromium) has shown excellent resistance to cavitation with an induced transformation into martensite a rather than in phase E. Conversely, the alloys Co # 11 to 15 which were martensitic in the state as cast (see Table 1), did not show the best resistance to cavitation.

Au vu des résultats ci-dessus, on peut constater que le degré de transformation induite par la cavitation suit l'ordre croissant suivant : 1020 (environ 0 %), Co # 5 (environ 10 %), 308 (environ 15 %), 301 (environ 75 %) STELLITE 21 (environ 75 %), Co # 3 (environ 90 %), Co # 6 (environ 90 %).In view of the results above, it can be seen that the degree of transformation induced by cavitation follows the following increasing order: 1020 (approximately 0%), Co # 5 (approximately 10%), 308 (approximately 15%), 301 (approximately 75%) STELLITE 21 (approximately 75%), Co # 3 (approximately 90%), Co # 6 (approximately 90%).

Tel qu'illustré sur la figure 15, le durcissement induit par cavitation suit sensiblement le même ordre.As illustrated in FIG. 15, the hardening induced by cavitation follows substantially the same order.

MicroduretéMicrohardness

Les résultats des mesures de microdureté effectuées par l'inventeur sont présentés sur la figure 16. La figure 16a montre qu'il y a une importante augmentation de la dureté de la surface des alliages les plus résistants au cours de la période d'incubation. Aucun durcissement de déformation n'a été mesuré sur la ferrite molle de l'échantillon en acier au carbone. L'alliage expérimental Co # 3 qui, lorsque fondu, est plus mou que le STELLITE 21, a montré le plus fort durcissement, avec une dureté finale supérieure à celle du STELLITE 21. Cette dureté a augmenté très rapidement au début de la période d'incubation.The results of the microhardness measurements carried out by the inventor are presented in FIG. 16. FIG. 16a shows that there is a significant increase in the surface hardness of the most resistant alloys during the incubation period. No strain hardening was measured on the soft ferrite of the carbon steel sample. The experimental alloy Co # 3 which, when melted, is softer than STELLITE 21, showed the strongest hardening, with a final hardness higher than that of STELLITE 21. This hardness increased very quickly at the beginning of the period of 'incubation.

La mesure de microdureté en profondeur telle que rapportée sur la figure 16 démontre que le durcissement par déformation dû à la cavitation est limité à une couche de surface très mince (inférieure à 50 wm), ce qui rend ce genre de mesure très difficile.The measurement of microhardness at depth as reported in FIG. 16 shows that the hardening by deformation due to cavitation is limited to a very thin surface layer (less than 50 wm), which makes this kind of measurement very difficult.

MicrographiesMicrographs

Diverses micrographies ont été prises de la surface de certains échantillons après exposition à la cavitation. Sur la surface de l'échantillon en acier carbone 1020, des traces de chocs d'implosion de cavitation conduisant à une surface bosseuse plus rugueuse, ont été observées par éclairage de Nomarski. La densité des trous et des bosses a augmenté rapidement avec la durée d'exposition à la cavitation pour atteindre plus que 2000/mm2 en seulement 30 secondes. La ferrite s'est avérée beaucoup plus déformée que la perlite, les particules de ferrite étant cassées et arrachées des noyaux de perlite plus durs en seulement 30 minutes.Various micrographs were taken from the surface of some samples after exposure to cavitation. On the surface of the 1020 carbon steel sample, traces of cavitation implosion shocks leading to a rougher bumpy surface were observed by Nomarski lighting. The density of holes and bumps increased rapidly with the duration of exposure to cavitation, reaching more than 2000 / mm 2 in just 30 seconds. Ferrite was found to be much more deformed than perlite, with ferrite particles being broken and torn from harder perlite cores in just 30 minutes.

A peu près la même rugosité a été observée sur l'échantillon en acier inoxydable austénitique 308, si ce n'est que la cassure et l'arrachement des particules se produisent plus tard et de façon plus homogène sur cet échantillon de phase unique. Très peu de lignes martensitites elles-mêmes très pâles ont pu être observées sur la surface déformée. Sur l'échantillon en acier inoxydable 301, les aiguilles de martensite brute présentes dans l'échantillon tel que soudé se trouvèrent amincies et multipliées par les chocs de cavitation. La rugosité fut moins prononcée et de beaucoup plus petites fissures furent observées après 30 minutes. Sur deux des alliages au cobalt essayés, à savoir le STELLITE 21 (65 % de cobalt) et l'alliage Co # 3 (environ 25 % de cobalt) des résultats tout à fait différents furent observés. Les trous causés par les chocs se sont avérés plus petits, et n'ont pas conduit à une rugosité prononcée de la surface. De nombreuses lignes de mâclage, qui ont déjà été identifiées comme telles dans la littérature dans le cas du cobalt pur, sont apparues très rapidement, après seulement quelques secondes d'exposition à la cavitation. Au fur et à mesure que le temps de cavitation augmentait, la densité des lignes de mâclage et le mâclage de déformation ont augmenté, pour conduire à un réseau très dense de mâcles jointives très fines après une ou deux heures. A ce moment, l'érosion a commencé, avec arrachement de petites particules carrées produites par fissuration de l'interface à la jointure des grains ou des mâcles. Dans l'alliage expérimental Co # 3, la surface de mâclage était séparée par de petites régions interdentitriques faites en martensite, qui semblaient être très fines et posséder une résistance à la cavitation aussi bonne que les zones de mâclage parallèle les plus fines. Comme dans le cas du cobalt pur, l'orientation des grains qui favorient le mâclage parallèle le plus fin dans le plan hexagonal compact dense (0001) a montré la plus forte résistance à la cavitation. Ce même mâclage fin a également été observé dans la martensite pure de l'alliage transformé Co # 6.Roughly the same roughness was observed on the austenitic stainless steel sample 308, except that the breakage and tearing of the particles occur later and more homogeneously on this single phase sample. Very few martensitite lines, themselves very pale, could be observed on the deformed surface. On the 301 stainless steel sample, the martensite needles crude present in the sample as welded were thinned and multiplied by cavitation shocks. The roughness was less pronounced and much smaller cracks were observed after 30 minutes. On two of the cobalt alloys tested, namely STELLITE 21 (65% cobalt) and Co # 3 alloy (approximately 25% cobalt) quite different results were observed. The holes caused by the shocks were found to be smaller, and did not lead to a pronounced roughness of the surface. Numerous chewing lines, which have already been identified as such in the literature in the case of pure cobalt, appeared very quickly, after only a few seconds of exposure to cavitation. As the cavitation time increased, the density of the chewing lines and the deformation chewing increased, leading to a very dense network of very fine joined males after one or two hours. At this time, erosion started, with the removal of small square particles produced by cracking of the interface at the grain or male joint. In the experimental alloy Co # 3, the chewing surface was separated by small interdentric regions made of martensite, which appeared to be very fine and to have a resistance to cavitation as good as the thinnest parallel chewing zones. As in the case of pure cobalt, the orientation of the grains which favor the finest parallel chewing in the dense compact hexagonal plane (0001) showed the highest resistance to cavitation. This same fine chewing has also been observed in the pure martensite of the transformed alloy Co # 6.

L'observation de micrographies prises sur des coupes faites dans les échantillons a également montré une zone de fissuration et de déformations plus grande (environ 30 wm) pour l'acier au carbone 1020. Cette zone était beaucoup plus petite dans le cas des aciers inoxydables (quelques microns). Pour les échantillons en alliages au cobalt, la couche de surface sujette à cavitation a semblé très mince (inférieure à 1 µm). Aucun mâclage induit par cavitation n'a.pu être observé sur ces coupes. Il est intéressant de noter comment les zones observées juste en dessous de la surface ont semblé non affectées par les chocs de cavitation, comme si le mâclage de surface était un moyen extrêmement efficace d'absorber les chocs de cavitation pour protéger l'échantillon. De façon générale, plus la résistance à la cavitation d'un échantillon s'est avérée grande, plus sa surface s'est avérée demeurer lisse sous exposition à la cavitation.The observation of micrographs taken on sections made in the samples also showed a larger cracking and deformation zone (around 30 wm) for 1020 carbon steel. This zone was much smaller in the case of stainless steels (a few microns). For the cobalt alloy samples, the surface layer subject to cavitation appeared very thin (less than 1 µm). No cavitation-induced chewing could be observed on these sections. It is interesting to note how the areas observed just below the surface appeared unaffected by cavitation shocks, as if surface chewing were an extremely effective means of absorbing cavitation shocks to protect the sample. In general, the greater the resistance to cavitation of a sample, the more its surface has been found to remain smooth under exposure to cavitation.

Les essais, mesures et observations ci-dessus rapportés montrent clairement que tous les alliages expérimentaux selon l'invention, à l'exception de l'alliage Co # 5, 7 et 11 à 15, ont une résistance à la cavitation sensiblement identique à celle excellente des alliages à forte teneur en cobalt, tel que le STELLITE 21.The tests, measurements and observations above reported clearly show that all of the experimental alloys according to the invention, with the exception of alloy Co # 5, 7 and 11 to 15, have a resistance to cavitation substantially identical to that of excellent alloys with a high cobalt content, such as STELLITE 21.

Les données ci-dessus, notamment le résultat des essais de diffraction aux rayons X, montrent également que l'excellente résistance à la cavitation des alliages au cobalt selon l'invention peut être attribuée au réseau fin de mâclage par déformation accompagnant la transformation en phase hexagonale compacte ε ou en martensite a, ce mâclage induit par cavitation étant spécifique aux cristaux à faible énergie de faute d'empilement.The above data, in particular the result of the X-ray diffraction tests, also show that the excellent resistance to cavitation of the cobalt alloys according to the invention can be attributed to the fine network of masking by deformation accompanying the phase transformation. compact hexagonal ε or martensite a, this cavitation-induced masking being specific to crystals with low energy of stacking fault.

Le fait qu'aucun mâclage fin et qu'une faible résistance à la cavitation ont été observés sur les alliages expérimentaux Co # 5 et 11 à 15 qui étaient principalement soit ferritiques, soit martensitiques avant d'être sujets à exposition à la cavitation, semble indiquer que la transformation induite par cavitation de la phase C.F.C. y en une phase H.C. e et/ou en de la martensite a montrant un mâclage fin de déformation, est essentiel pour obtenir une forte résistance à la cavitation. Cette exigence à son tour implique que l'acier inoxydable selon l'invention soit dans une phase austénitique métastable à température ambiante.The fact that no fine coiling and low resistance to cavitation were observed on experimental alloys Co # 5 and 11 to 15 which were mainly either ferritic or martensitic before being exposed to cavitation, seems indicate that the cavitation-induced transformation of the CFC phase y in a phase H.C. e and / or in martensite a showing a fine deformation chewing, is essential to obtain a strong resistance to cavitation. This requirement in turn implies that the stainless steel according to the invention is in an austenitic phase which is metastable at ambient temperature.

Si la stabilité de la phase austénitique de l'acier est trop bonne, la transformation de phase sous exposition à la cavitation sera faible. Ainsi, par exemple, l'alliage Co # 3 (20 % de cobalt) présente une transformation de phase induite par cavitation ainsi qu'un écrouissage plus prononcé que le STELLITE 21 (65 % de cobalt) qui est connu comme étant très stable. Cet alliage Co # 3 s'avère également posséder une résistance sur la cavitation supérieure, même si cet alliage a une dureté initiale inférieure (23 RC par rapport à 30 RC pour le STELLITE 21). A ce sujet, on peut noter que la composition que doivent avoir les aciers inoxydables pour offrir la meilleure résistance possible à la cavitation peut inclure divers durcisseurs tels que du molybdène, pour maintenir le même degré de transformation de phase.If the austenitic phase stability of the steel is too good, the phase transformation under exposure to cavitation will be weak. Thus, for example, the Co # 3 alloy (20% cobalt) exhibits a phase transformation induced by cavitation as well as a more pronounced work hardening than STELLITE 21 (65% cobalt) which is known to be very stable. This Co # 3 alloy also appears to have a resistance on the upper cavitation, even if this alloy has a lower initial hardness (23 RC compared to 30 RC for STELLITE 21). In this regard, it can be noted that the composition that stainless steels must have to offer the best possible resistance to cavitation can include various hardeners such as molybdenum, to maintain the same degree of phase transformation.

Il est donc clair que, tout comme dans le cas de l'acier inoxydable 301, la teneur de l'acier inoxydable au cobalt selon l'invention en éléments connus comme ferritisants (Cr, Mo, Si) et austénitisants (C, N, Co, Ni, Mn) doit être adéquatement choisie et ajustée de façon à à peine stabiliser l'austénite particulièrement dans le cas d'un refroidissement rapide, pour ainsi promouvoir une transformation induite par la cavitation de la phase a en phase ε ou en martensite, la forte résistance à la cavitation des aciers selon l'invention résultant principalement de leur composition où les éléments connus pour augmenter l'énergie de faute d'empilement, à savoir le carbone et le nickel, sont remplacés autant que possible par des éléments connus pour abaisser cette énergie de faute d'empilement tels que Co, Si, Mn et N et ainsi conduire à un mâclage de déformation plus fin.It is therefore clear that, as in the case of 301 stainless steel, the content of the cobalt stainless steel according to the invention in elements known as ferritisants (Cr, Mo, Si) and austenitisants (C, N, Co, Ni, Mn) must be appropriately chosen and adjusted so as to barely stabilize the austenite, particularly in the case of rapid cooling, to thus promote a transformation induced by cavitation from phase a to phase ε or to martensite , the high resistance to cavitation of the steels according to the invention resulting mainly from their composition where the elements known to increase the stacking fault energy, namely carbon and nickel, are replaced as much as possible by known elements to lower this stacking fault energy such as Co, Si, Mn and N and thus lead to a finer deformation coupling.

Les aciers inoxydables au cobalt selon l'invention peuvent avantageusement être utilisés pour la fabrication et la réparation de pièces ou de groupes de machines hydrauliques, tels que des turbines, des pompes, des robinets, etc. Ils peuvent être utilisés soit comme recouvrements soudés sur de l'acier au carbone, soit comme matériaux de base, coulés ou sous forme de tôle, pour la fabrication de machines toutes faites en acier inoxydable. Ces aciers peuvent en outre être laminés à chaud ou à froid et être développés en fils ou électrodes de soudage pour remplacer le STELLITE 21 beaucoup plus cher utilisé pour réparer les dommages de cavitation des turbines hydrauliques.The cobalt stainless steels according to the invention can advantageously be used for the manufacture and repair of parts or groups of hydraulic machines, such as turbines, pumps, valves, etc. They can be used either as covers welded to carbon steel, or as basic materials, cast or in the form of sheet metal, for the manufacture of machines made of stainless steel. These steels can furthermore be hot or cold rolled and be developed in welding wires or electrodes to replace the much more expensive STELLITE 21 used to repair cavitation damage in hydraulic turbines.

On doit noter qu'aucun traitement thermique ou mécanique spécial n'est requis, dans l'état tel que coulé ou soudé, pour obtenir la meilleure résistance à la cavitation de ces aciers inoxydables austénitiques au cobalt. S'ils doivent être déformés à froid, pour des besoins de mise en forme de fil ou de tôle par exemple, on doit alors leur faire subir un traitement thermique de recuit à haute température, comme pour les aciers inoxydables austénitiques standards. Leur meilleure formabilité que les alliages à base de cobalt est un autre avantage économique surtout pour la fabrication en fil de soudage.It should be noted that no special thermal or mechanical treatment is required, in the state as cast or welded, to obtain the best resistance to cavitation of these austenitic cobalt stainless steels. If they are to be deformed cold, for the purpose of shaping wire or sheet for example, they must then be subjected to a heat treatment of annealing at high temperature, as for standard austenitic stainless steels. Their better formability than cobalt-based alloys is another economic advantage especially for the manufacture of welding wire.

Claims (23)

1. An austenitic Co-containing stainless steel alloy showing a high cavitation erosion resistance, of the type consisting of :
Figure imgb0024
the balance consisting of Fe and the usual impurities, characterized in that the amount of the above mentioned elements that are respectively known as ferrite formers (Cr, Mo, Si) and as austenite formers (C, N, Co, Ni, Mn) and, amongst said austenite and ferrite formers, the amount of each of the elements that are respectively known to increase and lower the stacking fault energy, are respectively selected and balanced so that at least 60 % by weight of the alloy is, at ambient temperature, in a metastable, face centered cubic phase having a stacking fault energy low enough to make it capable of being transformed under cavitation exposure to a fine deformation twinning, hexagonal close pack e-phase and/or a-martensitic phase.
2. An austenitic Co-containing stainless steel alloy showing a high cavitation erosion resistance, of the type consisting of :
Figure imgb0025
the balance consisting of Fe and the usual impurities, characterized in that the amount of the above mentioned elements that are respectively known as ferrite formers and as austenite formers and, amongst said austenite and ferrite formers, the amount of each of the elements that are respectively known to increase and lower the stacking fault energy, are respectively selected and balanced so that at least 60 % by weight of the alloy is, at ambient temperature, in a metastable, face centered cubic phase having a stacking fault energy low enough to make it capable of being transformed under cavitation exposure to a fine deformation twinning, hexagonal close pack e-phase and/or a-martensitic phase.
3. A Co-containing stainless steel alloy as claimed in claim 2, consisting of :
Figure imgb0026
the balance consisting of Fe and the usual impurities.
4. A Co-containing stainless steel alloy as claimed in claim 2, consisting of :
Figure imgb0027
the balance consisting of Fe and the usual impurities.
5. A Co-containing stainless steel alloy as claimed in claim 2, consisting of :
Figure imgb0028
the balance consisting of Fe and the usual impurities.
6. A Co-containing stainless steel alloy as claimed in claim 2, containing of :
Figure imgb0029
the balance consisting of Fe and the usual impurities.
7. A Co-containing stainless steel alloy as claimed in claim 2, consisting of :
Figure imgb0030
the balance consisting of Fe and the usual impurities.
8. A co-containing stainless steel alloy as claimed in claim 2, consisting of :
Figure imgb0031
the balance consisting of Fe and the usual impurities.
9. A Co-containing stainless steel alloy as claimed in claim 2, consisting of :
Figure imgb0032
the balance consisting of Fe and the usual impurities.
10. A Co-containing stainless steel alloy as claimed in claim 2, consisting of :
Figure imgb0033
the balance consisting of Fe and the usual impurities.
11. An austenitic Co-containing stainless steel alloy showing a high cavitation erosion resistance, of the type consisting of :
Figure imgb0034
the balance consisting of Fe and the usual impurities, characterized in that the amount of the above mentioned elements that are respectively known as ferrite formers (Cr, Mo, Si) and as austenite formers (C, N, Co, Ni, Mn) and, amongst said austenite and ferrite formers, the amount of each of the elements that are respectively known to increase and lower the stacking fault energy, are respectively selected and balanced so that at least 60 % by weight of the alloy is, at ambient temperature, in a metastable, face centered cubic phase having a stacking fault energy low enough to make it capable of being transformed under cavitation exposure to a fine deformation twinning, hexagonal close pack e-phase and/or a-martensitic phase.
12. A Co-containing stainless steel alloy as claimed in claim 11, consisting of :
Figure imgb0035
the balance consisting of Fe and the usual impurities.
13. A Co-containing stainless steel alloy as claimed in claim 11, consisting of :
Figure imgb0036
Figure imgb0037
the balance consisting of Fe and the usual impurities..
14. A Co-containing stainless steel alloy as claimed in claim 11, consisting of :
Figure imgb0038
the balance consisting of Fe and the usual impurities.
15. A Co-containing stainless steel alloy as claimed in claim 11, consisting of :
Figure imgb0039
the balance consisting of Fe and the usual impurities.
16. A Co-containing stainless steel alloy as claimed in claim 1, consisting of
Figure imgb0040
the balance consisting of Fe and the usual impurities.
17. Use of an austenitic Co-containing stainless steel alloy showing a high cavitation erosion resistance as defined in claim 1, for making or covering a Co-containing stainless steel component for use in the manufacture or repair of a hydraulic machine.
18. Use of an austenitic Co-containing stainless steel alloy showing a high cavitation erosion resistance as defined in claim 2, for making or covering a Co-containing stainless steel component for use in the manufacture or repair of a hydraulic machine.
19. Use of an austenitic Co-containing stainless steel alloy showing a high cavitation erosion resistance as defined in claim 11, for making or covering a Co-containing stainless steel component for use in the manufacture or repair of a hydraulic machine.
20. Use of an austenitic Co-containing stainless steel alloy showing a high cavitation erosion resistance as defined in claim 1, for the manufacture of welding wires for use in the manufacture or repair of hydraulic machines.
21. Use of an austenitic Co-containing stainless steel alloy showing a high cavitation erosion resistance as defined in claim 2, for the manufacture of welding wires for use in the manufacture or repair of hydraulic machines.
22. Use of an austenitic Co-containing stainless steel alloy showing a high cavitation erosion resistance as defined in claim 11, for the manufacture of welding wires for use in the manufacture or repair of hydraulic machines.
EP85420115A 1984-06-28 1985-06-24 Cobalt-containing austenitic stainless steel, highly resistant against impringement attack Expired EP0171336B1 (en)

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CA457755 1984-06-28
CA000457755A CA1223140A (en) 1984-06-28 1984-06-28 Austenitic cobalt stainless steel exhibiting ultra high resistance to erosive cavitation

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EP0171336B1 true EP0171336B1 (en) 1988-08-17

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JPH0542495B2 (en) 1993-06-28
CN85104938A (en) 1987-01-07
NO852315L (en) 1985-12-30
CA1223140A (en) 1987-06-23
ES544717A0 (en) 1986-07-16
JPS6115949A (en) 1986-01-24
ATE36561T1 (en) 1988-09-15
EP0171336A1 (en) 1986-02-12
BR8503121A (en) 1986-03-18
US4588440A (en) 1986-05-13

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