Classifications

• • 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
  • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
  • C22F1/18—High-melting or refractory metals or alloys based thereon
  • C22F1/183—High-melting or refractory metals or alloys based thereon of titanium or alloys based thereon

Definitions

• thermomechanical part made of a titanium alloy, and thermomechanical part resulting from these processes
• the invention relates to a thermal treatment method of a thermomechanical part made of a TA6Zr4DE titanium alloy, a manufacturing method comprising such a heat treatment method and a thermomechanical part resulting from these processes.
• the invention is particularly, but not exclusively, applicable to rotating parts of turbomachines, such as discs, journals and wheels, and in particular to high-pressure compressor discs.
• the high pressure compressor discs are obtained by stamping in the beta domain of the titanium alloy.
• an alloy called “6242” which comprises about 6% aluminum, 2% tin, 4% zirconium and 2% molybdenum. It is more specifically the TA6Zr4DE alloy according to the metallurgical nomenclature.
• This stamping is carried out at approximately 1030 ° C.
• This stamping step is followed by a heat treatment process comprising a solution step in the alpha / beta domain of the alloy at a temperature of 970 ° C., corresponding to the beta transus temperature -30 ° C., for one hour.
• This dissolution step is followed by an oil quenching step or in a water-polymer mixture. Then we realize an income treatment to
• this heat treatment process leads to an alloy having a coarse microstructure which is not conducive to good strength of the titanium alloy, in particular according to a creep under stress test imposed for a certain holding time, in particular for a range of operating temperature between -50 ° C. and +250 ° C.
• the application in the aeronautical field, and in particular for a high pressure compressor disk is very conducive to this phenomenon of "dwell effect" because during the take-off and landing phases, the engines are subject to operating conditions in the temperature and stress range corresponding to this phenomenon. This phenomenon can lead to ignition premature fatigue cracks, or even the rupture of the room.
• the object of the present invention is to provide a heat treatment process for a thermomechanical part made of a titanium alloy that can be used industrially and that makes it possible to overcome the drawbacks of the prior art and in particular to provide the possibility of to limit the extent of the "dwell effect" phenomenon.
• the heat treatment process is characterized in that a dissolution step is carried out at a temperature of between ⁇ transus - 20 ° C. and ⁇ transus - 15 ° C. for a duration of 4 to 8 hours.
• This temperature condition corresponds to a maximum temperature of about 985 ° C.
• This difference with respect to the ⁇ -transus temperature is a safety margin, which is linked to the possible difference between the measured temperature and the actual temperature of the alloy, making it possible to ensure that the temperature remains below the temperature beta transition.
• This dissolution step is performed for 4 to 8 hours depending on the size of the room.
• the idea underlying the present invention corresponds to the fact that it has been found that there exist within the material zones or colonies, conducive to the phenomenon of "dwell effect". It is found that such colonies are formed of elongated grains of alpha phase, needle-like, relatively big and joined together. Generally, such grains have a length of several millimeters over a width of the order of 200 to 300 microns. Such colonies constitute locations at which, when stresses are accumulated, a large concentration of dislocations occurs which, when activated, without any particular thermal effect, can cause slips between the grains, which can lead to to breaks.
• the present invention proposes to implement a heat treatment making it possible to refine the microstructure, in particular the size of the aforementioned needles, in order to minimize the effects of the "dwell effect", and this by reducing the extent of free circulation of the dislocations , to minimize their accumulation and, in this way, the risk of breakage of the room.
• the solution-making step is carried out for a much longer period than that usually performed.
• the piece is allowed to come closer, even to reach, its microstructural equilibrium, which makes it possible to reduce the size, in length and thickness, of the needles of the colonies likely to cause the "dwell effect” .
• This treatment makes it possible to obtain a finer microstructure than that of the prior art, and thus to minimize the consequences of the "dwell effect".
• thermomechanical properties of the material does not have the consequence, contrary to the prevailing prejudices in this field of metallurgy, to affect the thermomechanical properties of the material.
• the inventors have, in the context of the invention presented here, implemented a heat treatment process whose solution solution stage was carried out for a much longer duration than that practiced usually, without the material resulting from the entire heat treatment process having thermomechanical characteristics, and in particular imposed fatigue fatigue properties, lower than those of the materials resulting from the treatment process thermal of the prior art.
• the present invention proposes to carry out this dissolution step at a temperature relatively close to the beta transition temperature, while remaining strictly lower than the latter, and this in order to obtain a microstructure of the final piece in the classes of alpha / beta, almost alpha and alpha.
• thermomechanical parts in particular discs for high pressure compressor, having on the one hand durability greater than that of the parts obtained according to the techniques previously used, but also having thermomechanical characteristics (traction, creep, stress fatigue imposed during a holding time, etc.) at least as good, while minimizing the risks of fatigue rupture.
• the thermal treatment method according to the invention allows a gain of a factor of about two on the resistance to "dwell" (cyclic loading with hold-load time - creep - at each cycle) compared to a method of treatment, as shown in the tests described below.
• this process according to the invention also comprises a step according to which, after the dissolution step, a quenching step of the workpiece is carried out at a cooling rate greater than 200 ° C./min, and preferably, between 300 and 450 ° C.
• this cooling rate is the largest possible and preferably greater than or of the order of 400 ° C./min.
• the method further comprises the following steps:
• a tempering step is carried out at a temperature of the order of 595 ° C. for a duration of the order of 8 hours, and then
• a cooling step is carried out in air.
• the present invention also relates to a method of manufacturing a thermomechanical part made of a titanium alloy, by stamping in the ⁇ domain, comprising such a heat treatment process.
• thermomechanical part made of a titanium alloy whose manufacturing process comprises the aforementioned heat treatment process or resulting from the manufacturing process which has just been presented.
• this titanium thermomechanical part forms a rotating part of a turbomachine, and in particular a compressor disk, especially a high-pressure compressor.
• the present invention also relates to a turbomachine equipped with a thermomechanical part according to one of the definitions given above.
• FIG. 1 shows the microstructure obtained according to the conventional thermal treatment method of the invention
• FIG. 2 shows the microstructure obtained according to the conventional thermal treatment process of the prior art modified by a faster quenching speed
• FIG. 3 shows the microstructure obtained according to the heat treatment method according to the present invention
• FIG. 4 shows the microstructure obtained according to the heat treatment method according to the present invention with a faster quenching speed
• FIG. 5 shows the results of a cyclic load creep test with a hold time for a part resulting from the process of the prior art and for a part obtained by the process according to the invention.
• the present invention relates to all types of titanium alloy stabilized in temperature: titanium alloys of the beta, alpha / beta, almost alpha and alpha (this is called the structure of the finished part).
• This stamping step is followed by a heat treatment process comprising a solution step in the alpha / beta domain of the alloy at a temperature of 970 ° C., corresponding to the temperature of beta-trans-30 ° C., during one hour.
• This dissolution step is followed by an oil quenching step or in a water-polymer mixture (cooling rate in the order of
• a material having the microstructure visible in FIG. 1 is obtained, having colonies consisting of beta phase needles parallel to one another. These needles have an elongate section visible in the figure often extending over several hundred micrometers.
• the visible microstructure corresponds to that of a titanium alloy identical to that of FIG. 1, having undergone the aforementioned heat treatment with the following two differences: - the dissolution temperature is beta transus -20 0 C
• the quenching rate used during the heat treatment process is significantly faster: 400 0 C / min instead of 200 0 C / min, using for example a water quenching instead of quenching the oil, and taking care to avoid the extra thicknesses of materials by a possible prior machining of the thickest areas.
• parallel needle colonies have needles more dissimilar in size and in particular there are fewer large needles. Nevertheless, even in fewer numbers, it is expected that these large needles are sufficient in number for the phenomenon of "dwell effect" causes accumulations of dislocations likely to cause risks of rupture.
• FIG. 3 or FIG. 4 these are the microstructures obtained according to the process according to the present invention. More precisely, with respect to the conventional thermal treatment method previously discussed in connection with FIG. 1, the processing implemented to achieve the microstructure of FIG. 3, provides:
• the needles are all smaller in section, their remaining length less than 100 microns, and generally of the order of 50 micrometers.
• the decrease in the size of the needles is accompanied by a decrease in their volume and the contiguous surfaces between needles, which hampers the ability to move defects such as dislocations or gaps, which run thus smaller distances and less opportunities to accumulate.
• quenching at a higher speed of 400 ° C./min instead of 200 ° C./min was also carried out. .
• the microstructures are more freeze to a smaller size than those which generate the damages of the material. This avoids the accumulation of needles or grains, in the form of large parallel needle packets which, like a single grain, concentrate defects at the edge of their interface.
• FIG. 5 is a graph indicating the ratio of elongation deformation under cyclic loading with hold time as a function of the number of cycles, until breaking.
• Curve A represents the result of this test for materials obtained according to the heat treatment process of the prior art and in accordance with the microstructure of FIG.
• Curve B represents the result of this test for materials obtained according to the heat treatment method of the present invention and in accordance with the microstructure of FIG. 4. This standardized test thus shows that the heat treatment process of the present invention makes it possible to practically double the number of cycles before rupture since one goes from 5500 cycles to 10000 cycles.
• the present invention makes it possible, surprisingly, in particular by extending the duration of the solution-making step, to significantly improve the fatigue test life with hold time. This is mainly due to the fact that this elongation makes it possible to refine the microstructure and in particular to reduce the size of the alpha phase needles forming the colonies that are sensitive to the "dwell effect" phenomenon.
• longer dissolution times for example 8 hours
• thinner pieces for which the quenching speed of 400 ° C. is chosen. / min can be reached, shorter dissolution times (eg 4 hours) can be applied.
• the increase of the dissolution temperature favors the solution of the coarse primary alpha phase in order to transform it into a beta phase.
• the transus beta temperature of the alloy since it is fundamental not to exceed the transus beta temperature of the alloy, we will choose a temperature that does not exceed the temperature transus beta -15 0 C.
• This upper limit of the dissolution temperature is chosen according to the precision of the knowledge the transus beta temperature and the class of the process furnaces.
• forging sub-transus that is to say above the beta transition temperature, one will of course choose a solution temperature higher than the forging temperature.

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• Chemical & Material Sciences (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Turbine Rotor Nozzle Sealing (AREA)
• Structures Of Non-Positive Displacement Pumps (AREA)
• Heat Treatment Of Articles (AREA)

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• 2006
  • 2006-03-30 FR FR0651111A patent/FR2899241B1/fr active Active
• 2007
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  • 2007-03-30 WO PCT/FR2007/051046 patent/WO2007113445A2/fr active Application Filing
  • 2007-03-30 JP JP2009502173A patent/JP5525257B2/ja active Active
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