Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
  • C22C19/051—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W
  • C22C19/056—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 10% but less than 20%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/02—Making non-ferrous alloys by melting
  • C22C1/023—Alloys based on nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/10—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of nickel or cobalt or alloys based thereon
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
  • F05C—INDEXING SCHEME RELATING TO MATERIALS, MATERIAL PROPERTIES OR MATERIAL CHARACTERISTICS FOR MACHINES, ENGINES OR PUMPS OTHER THAN NON-POSITIVE-DISPLACEMENT MACHINES OR ENGINES
  • F05C2201/00—Metals
  • F05C2201/04—Heavy metals
  • F05C2201/0433—Iron group; Ferrous alloys, e.g. steel
  • F05C2201/0466—Nickel

Definitions

• the invention relates to the field of nickel-based superalloys, intended in particular for the manufacture of parts for terrestrial or aeronautical turbines, for example turbine disks.
• Improved turbine performance calls for increasingly high performance alloys at high temperatures. In particular, they must be able to withstand operating temperatures of the order of 700 ° C.
• alloys allowing implementation by conventional means.
• These include the nickel base superalloy known under the name UDIMET 720, as described in particular in US-A-3,667,938 and US-A-4,083,734. This superalloy typically has the composition, described in percentages by weight. :
• carbon content may rise to 0.15% for high carbon versions
• the 718 PLUS has a lower Co content.
• the forgeability of this alloy has been improved by considerably reducing the volume fraction of the gamma phase.
• the lowering of the volume fraction of the gamma phase has, however, been to the detriment of the hot mechanical properties and the performance of the parts in general, which, in fact, are significantly lower than those of the previously mentioned alloys.
• the use of 718 PLUS alloy is therefore limited to certain applications whose requirements in terms of thermomechanical stresses are less critical.
• the 718 PLUS alloy has a high content of Nb (between 4 and 8%), which is harmful for its chemical homogeneity during processing.
• Nb is an element that leads to significant segregations at the end of solidification. These segregations can lead to the formation of manufacturing defects (white spots). Only narrow and precise reflow rate windows during the ingot development can reduce these defects.
• the development of the 718 PLUS therefore involves a process that is complex and difficult to control. High levels of Nb in superalloys are also known to be quite detrimental to crack propagation at high temperatures.
• the object of the invention is to propose an alloy having a low cost of production, that is to say with a lower cost of alloying elements than that of alloys of the UDIMET 720 type, and whose forging ability would be increased compared to alloys of the UDIMET 720 type, and while having at high temperatures (700 ° C) high mechanical properties, ie higher than those of the 718 PLUS .
• it is intended to propose an alloy whose composition would make it possible to obtain a compromise between high mechanical properties at high temperature and an acceptable cost of production for the abovementioned applications.
• This alloy should also be obtainable under conditions of preparation and forging not too restrictive to make this obtaining reliable.
• the subject of the invention is a nickel-base superalloy of the following composition, the contents of the various elements being expressed in percentages by weight:
• the remainder being composed of nickel and impurities resulting from the preparation, and such that the composition satisfies the following equations in which the contents are expressed as atomic percentages:
• composition satisfies the following equation in which the contents are expressed as atomic percentages:
• it contains, in weight percentages, between 3 and 12% of Fe,
• composition is expressed in percentages by weight:
• composition of the alloy is expressed in percentages by weight:
• these superalloys comprise a gamma phase fraction of between 30 and 44%, preferably between 32 and 42%, and the solvus of the gamma phase of the superalloy is less than 1145.degree.
• the composition of the alloy satisfies the following equation, in which the contents of the elements are calculated in the gamma matrix at 700 ° C. and are expressed as an atomic percentage:
• the content of Cr (expressed in atomic percentage) is, in the gamma matrix at 700 ° C., greater than 24 at%.
• the content of Mo + W (expressed as an atomic percentage) is> 2.8 at% in the gamma matrix.
• the invention also relates to a superalloy nickel piece, characterized in that its composition is of the previous type.
• It may be an aeronautical or land gas turbine component.
• the invention is based on a precise balancing of the composition of the alloy to obtain both mechanical properties, ease of forging and preferably a material cost of the alloy as moderate as possible, making the alloy suitable for economical production by the conventional ingot rail of parts that can operate under high mechanical and thermal stresses, particularly in land and aeronautical turbines.
• FIG. 1 shows the respective forges (represented by the necking) measured on reworked and homogenized ingots, at temperatures of 1000 to 1180 ° C., of alloys according to FIG. the invention and a UDIMET 720 type reference alloy to which the invention aims to substitute.
• the alloy according to the invention While offering good mechanical properties, the alloy according to the invention has good forging abilities with limited contents of elements generating the gamma phase, and especially Nb, to also avoid problems of segregation in the process. development.
• An alloy according to the invention is for example forgeable in the field of the supersolvus of the alloy which ensures a better homogeneity of the metal and significantly reduce the costs associated with the forging process.
• a superalloy according to the invention makes it possible, in addition to reducing the costs associated with the raw materials, to reduce the costs relating to the production processes and to the thermomechanical treatment processes (forging and stamping) of a piece made in this superalloy.
• the alloys obtained according to this invention are generally relatively low cost, in any case at a lower cost than UDIMET type alloys 720, while exhibiting high mechanical properties at high temperatures, that is to say higher than alloys of the 718 PLUS type.
• the lowering of the Co content below 1 1% makes it possible to considerably reduce the cost of the alloy, Co being among the alloying elements present massively in the invention, the most expensive one.
• the lowering of the Co content is, on the one hand, offset by an adjustment of the
• the inventors have found that an addition of Fe as a partial substitution of the Co content (relative to alloys of the UDIMET 720 or TMW-4 type) also made it possible to significantly reduce the cost of the alloy.
• an optimum content of Co was between 7 and 1 1%, better 7 to 10%, to achieve a significant increase in mechanical properties such as creep resistance while maintaining a low cost of raw materials. , preferably by adding 3 to
• the target ratio of the sum of the Ti, Nb and Ta contents and of the Al content makes it possible to ensure solid solution hardening of the gamma phase while avoiding the risk of occurrence of a switched phase in the alloy that could alter its ductility.
• a minimum fraction of gamma phase (preferably 30%, better 32%) is desired to obtain a very good creep and tensile strength at 700 ° C.
• the fraction and the solvus of the gamma phase must, however, preferably be respectively less than 44% (better 42%) and 1 145 ° C so that the alloy retains good forgeability, and also that the alloy can be partly forged in the supersolvus domain, that is to say at a temperature between the gamma solvus and the melting start temperature.
• the proportions of the phases present in the alloy were determined, by the inventors and as a function of the composition, using phase diagrams obtained by thermodynamic calculations (using the THERMOCALC software commonly used by metallurgists).
• the parameter Md which is usually used as an indicator of the stability of the superalloys, must be less than 0.901 in order to give the alloy according to the invention optimum stability.
• the composition can therefore be adjusted to reach an Md ⁇ 0.901 without harming the other mechanical properties of the alloy.
• the alloy may be unstable, to give rise, during prolonged use, to the precipitation of harmful phases, such as the sigma and mu phases which weaken the alloy.
• composition according to the invention makes it possible to maintain a value of TCP
• Topical close-packed topologically compact phases such as mu + sigma phases whose content is expressed as molar percentage of phase) less than 6% at 700 ° C in the alloy. This value makes it possible to confirm that the superalloy according to the invention has a very good microstructural stability at high temperatures.
• the cobalt content has been limited to levels below 11%, better below 10%, for economic reasons, insofar as this element is one of the most expensive of those involved in the composition of the alloy (see FIG. equation (1) where this element has the second highest weighting after Ta).
• a minimum content of 7% is desired in order to maintain a very good creep resistance.
• the substitution of nickel or cobalt with iron has the advantage of significantly reducing the cost of the alloy.
• the addition of iron promotes the precipitation of the harmful sigma phase for ductility and notch sensitivity.
• the iron content of the alloy must therefore be adjusted so as to obtain a significant cost reduction while guaranteeing a very stable alloy at high temperature (equations (2), (7)).
• the Fe content is in the general case between traces and 12%, but is preferably between 3 and 12%, better between 3 and 9%, better still between 3.6 and 7%.
• weight contents of these elements are from 1.3 to 2.8%, better still 1.8 to 2.8% for Al, 2.5 to 4.5%, better 2.8 to 4.2% for Ti, 0.5 to 2.5%, better 0.5 to 2% for the sum Ta + Nb.
• the precipitation of the gamma phase in the nickel-based alloys is essentially the presence of aluminum in sufficient concentration
• the elements Ti, Nb and Ta can promote the appearance of this phase if they are present in the nickel. alloy with a sufficient concentration: the elements aluminum, titanium, niobium and tantalum are so-called "gamma-genes" elements.
• the stability domain of the gamma phase (whose gamma solvend of the alloy is representative) and the gamma phase fraction are therefore a function of the sum of the atomic concentrations at% of aluminum, titanium, niobium and tantalum.
• the fraction and the solvus of the gamma phase must, however, preferably be lower than 44% and 1145 ° C., respectively, so that the alloy retains good forgeability, and can also be partly forged in the supersolvus domain. that is to say at a temperature between the gamma solvus and the melting start temperature.
• a fraction of phase ⁇ 1 and a solvus temperature exceeding the upper limits mentioned above would make it more difficult to implement the alloy by the conventional ingot channel, which could mitigate one of the advantages of the invention.
• the contents of aluminum, titanium, niobium and tantalum are such that the ratio between the sum of the contents of titanium, niobium and tantalum, and the aluminum content, is greater than or equal to 0, 7 and less than or equal to 1, 3.
• solid solution hardening in the gamma phase provided by Ti, Nb and Ta is even higher than the ratio (Ti at% + Nb at% + Ta at%) / Al at% is high.
• a ratio greater than or equal to 1 will be preferred to ensure better cure.
• Niobium is preferably present in a higher proportion than tantalum since tantalum has a higher cost and atomic mass than niobium. Equations (1), (4) and (5) account for these conditions.
• the Mo content must be between 2 and 5% and the W content between 1 and 4%. Optimally, the Mo content is between 2 and 4% and the W content between 1, 5 and 3.5%.
• Molybdenum and tungsten provide a strong hardening of the gamma matrix by the effect of solid solution.
• the Mo and W contents must be carefully adjusted to obtain optimal hardening without causing the precipitation of fragile sigma or mu type intermetallic compounds.
• These phases when they develop in excessive quantities, lead to significant reduction in the ductility and mechanical strength of the alloys.
• excessive levels of Mo and W strongly alter the forgeability of the alloy and considerably reduce the forgeability range, that is to say the temperature range where the alloy tolerates significant deformations for the alloy. hot shaping.
• These elements have, in addition, high atomic masses, and their presence results in a significant increase in the density of the alloy, which is not desirable for aeronautical applications in particular. Equations (2), (7) and (8) account for these conditions.
• Chromium is essential for the oxidation and corrosion resistance of the alloy and thus plays a key role in the resistance of the alloy to the effects of the environment at high temperatures.
• the chromium content (14 to 17% by weight) of the alloys of the invention was determined so as to introduce a minimum concentration of 24 at% Cr in the gamma phase at 700 ° C., taking into account the fact that an excessively high content of chromium favors the precipitation of harmful phases such as the sigma phase and therefore deteriorates the hot stability. Equations (2), (3) and (7) account for these conditions.
• the B content is between 0.0030 and 0.030%.
• the Zr content is between 0.01 and 0.06%.
• the content of C is between traces and 0.1%, optimally between traces and 0.07%.
• minor elements of carbon, boron and zirconium form segregations at the grain boundaries, for example in the form of borides or carbides. They help increase the strength and ductility of alloys by trapping harmful elements such as sulfur and modifying the chemical composition at grain boundaries. Their absence would be detrimental. However, excessive levels lead to a reduction in the melting temperature and greatly alter the forgeability. It must therefore be kept within the limits that have been said.
• Table 1 compositions of the samples tested in the laboratory Examples 1 to 4 were prepared by VIM melting to produce 10 kg ingots.
• Examples 5 to 10 were prepared by VIM melting and then VAR reflow to produce ingots of 200 kg.
• Reference Example 1 corresponds to a conventional 718 PLUS alloy.
• Reference Example 4 is out of the invention because of a too high Nb content which theoretically corresponds to the Nb content beyond which the delta phase is likely to appear.
• Example 5 contains more Fe, Co and C and less than Mo and W;
• Example 7 contains less Fe and Co and more Mo and W
• Example 8 is less loaded with alloying elements such as Al, Co, Mo, Ti and more loaded with Fe;
• Example 9 is more loaded with alloying elements such as Al, Ti, Nb and less loaded with Fe and W;
• Example 10 has a ratio (Ti at% + Nb at%) / Al at% lower and has more W, less Co and less Fe;
• Reference Example 2 contains more Ti and Nb and less Al, for an equal gamma phase fraction; the ratio (Ti at% + Nb at%) / Al at% is higher.
• Example 3 contains more AI and Nb and Ti, so a higher gamma phase fraction
• Example 4 for an equal gamma phase fraction, contains more Nb and less Ti.
• Table 2 shows additional characteristics of the alloys tested, with their main mechanical properties: tensile strength Rm, yield strength Rpo, 2, elongation at break A, creep life at 700 ° C under a stress of 600 MPa.
• the mechanical properties are given in relative values with respect to those of reference example 1 which is of the usual 718 PLUS type.
• Table 2 Complementary Characteristics and Mechanical Properties of the Samples
• the tensile strength and creep life of the alloys of the invention are all substantially greater than those of the 718 PLUS alloy (Example 1), while the cost of alloy is comparable or inferior.
• the gain in tensile strength, yield strength and creep is lower for Example 8, but the cost of this alloy is much lower than that of 718 PLUS.
• Examples 2 and 4 which are not part of the invention, show a decrease in the hot ductility compared to that obtained with 718 PLUS, which is manifested by a lower elongation at break.
• the alloys of the invention have a raw material cost which is less than or equal to that of 718 PLUS, and they are therefore much less expensive than the UDIMET 720, whose raw material cost, calculated according to the same criteria, would rise to 26.6 € / kg.

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