EP0318887A1 - Procédé pour augmenter la résistance à la fatigue et diminuer la tendance à la fissuration à haute température d'une pièce en superalliage à base de nickel durci par dispersion d'oxydes - Google Patents

Procédé pour augmenter la résistance à la fatigue et diminuer la tendance à la fissuration à haute température d'une pièce en superalliage à base de nickel durci par dispersion d'oxydes Download PDF

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Publication number
EP0318887A1
EP0318887A1 EP88119778A EP88119778A EP0318887A1 EP 0318887 A1 EP0318887 A1 EP 0318887A1 EP 88119778 A EP88119778 A EP 88119778A EP 88119778 A EP88119778 A EP 88119778A EP 0318887 A1 EP0318887 A1 EP 0318887A1
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EP
European Patent Office
Prior art keywords
surface zone
cold
oxide dispersion
fatigue strength
weight
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EP88119778A
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German (de)
English (en)
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EP0318887B1 (fr
Inventor
Wilhelm Ebeling
Mohamed Yousef Dr. Nazmy
Markus Staubli
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General Electric Switzerland GmbH
ABB Asea Brown Boveri Ltd
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BBC Brown Boveri AG Switzerland
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    • 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

Definitions

  • the invention relates to the improvement of the mechanical properties of oxide dispersion-hardened nickel-based superalloys with overall optimal properties with regard to high-temperature strength, long-term stability, ductility and service life.
  • the fatigue, creep behavior and tendency to embrittlement of governing mechanisms play an important role in the course of temperature and load cycles of a component.
  • it relates to a method for increasing the fatigue strength and reducing the susceptibility to cracking at high temperatures and when going through a temperature cycle of a workpiece made of an oxide dispersion-hardened, in the form of coarse longitudinally oriented stem crystals or in the form of a single crystal nickel-based superalloy.
  • Gas turbine blades are exposed to complex thermal and mechanical stresses during operation.
  • the rapidly changing load and the switching off and restarting of the gas turbine place extremely high demands on the blades.
  • the blade material is subjected to creep, static and dynamic stresses, various types of fatigue and thermal shock in a wide temperature range. This shows that the fatigue that occurs with small numbers of load changes but large amplitudes in the mechanically and thermally cycled cycles is particularly dangerous and that the service life of the workpiece is reduced in an apparently disproportionate manner.
  • the heat-resistant oxide dispersion-hardened nickel-based superalloys are characterized by low ductility, especially in the transverse direction of the crystals, especially in the state of coarse, longitudinally oriented stem crystals. It could be shown that the cracking under fatigue stress at high temperatures and when passing a kriti temperature range always begins at the surface of the workpiece (see "Thermal fatigue of materials and components", pp. 123-140). The properties and the behavior of the surface zone of the highly stressed gas turbine blade are therefore of crucial importance for its service life.
  • the invention is based on the object of specifying a method for increasing the fatigue strength and reducing the susceptibility to cracking at high temperatures and when passing through critical temperature and load cycles of a workpiece made of an oxide dispersion-hardened nickel-based superalloy.
  • the process should be simple and inexpensive and should be applicable to workpieces of any shape.
  • This object is achieved in that the ductility of the surface zone of the workpiece is increased in the method mentioned at the outset by cold working to a depth of at least 100 ⁇ m by at least one value which corresponds to a plastic elongation of 2% at room temperature.
  • FIG. 1 shows a diagram in which the course of the elongation at break obtained in the creep test is shown as a function of the degree of cold deformation.
  • the alloy was in the form of coarse, longitudinally oriented stem crystals with an average length of 15 mm, 3.5 mm width and 1.5 mm thickness.
  • the degree of cold deformation is plotted on the abscissa in the form of the plastic pre-expansion ⁇ under tensile stress.
  • the creep tests were carried out at the constant temperature of 950 ° C under the constant tensile stress of 230 MPa.
  • the respective elongation at break ⁇ R when the samples are torn is plotted on the ordinate.
  • the increase in ⁇ R as a function of the pre-stretching ⁇ is striking. It is a measure of the ductility of the material in this state, which is completely surprising, since comparatively non-dispersion hardened super alloys show the opposite behavior.
  • FIG. 2 shows a diagram in which the course of the time t R until breakage in the creep test is plotted as a function of the degree of cold deformation (pre-expansion ⁇ ). Above a pre-stretch of 2.5%, the specimen breaks under the load of 230 MPa at 950 ° C in a very short time: there is practically no creep resistance and the conditions of the short-term tensile test at elevated temperature are actually in front of you. The material behaves completely ductile, in contrast to its properties as a heat-resistant alloy in the initial state. The extremely plastic Behavior of the material in this changed state allows it to reduce stress peaks and absorb otherwise dangerous deformations that lead to crack formation.
  • the pre-stretch ⁇ is given as a parameter in%. There is an almost linear relationship between the time reached at break and the corresponding elongation at break, taking into account different levels of pre-stretch, up to a critical pre-stretch of approx. 2.5%. If the pre-stretching ⁇ is further increased (not shown in this diagram), then only the elongation at break ⁇ R increases (see FIG. 1), the time t R until the fracture is practically irrelevant (short-term test). With a pre-stretching ⁇ of approx. 3%, the plastic cold deformability of the material in the state of coarse stem crystals is exhausted. Continued cold forming would lead to breakage.
  • FIG. 4 relates to a schematic perspective illustration of a gas turbine blade, the blade of which is cut transversely to show the cross section.
  • 1 is the blade root, 2 the blade, whose longitudinal stem crystals 3 are made visible (for example by macro-etching). 4 shows the undeformed core in cross section (unetched). 5 is the cold worked surface zone (e.g. shot peening or rolling).
  • thermo fatigue for example: isothermal, shifted phases, counter phase, asynchronous cycles, etc.
  • Platelet-shaped test specimens with the dimensions 15 mm x 40 mm x 2.5 mm were machined from an oxide dispersion-hardened nickel-base superalloy with the trade name MA 6000 from INCO.
  • the platelets were now cold-deformed to varying degrees by subjecting them to tensile stress in the longitudinal direction.
  • the platelets were then subjected to a creep test: under a constant tensile load of 230 MPa and a temperature of 950 ° C., both the book elongation ⁇ R and the time t R until the sample broke were determined.
  • the results are shown in Figures 1, 2 and 3. It can be seen from this that a pre-stretch of approx. 2% increases the elongation at break ⁇ R to almost 3 times the value of the untreated sample.
  • the time t R until the break occurs occurs to less than half. Since the elongation at break ⁇ R is a measure of the ductility, ie the deformability, it could be concluded that the susceptibility of a component to cracking during operation can be significantly reduced under changing conditions. With degrees of cold deformation corresponding to a pre-stretching ⁇ of more than 2.5%, the material practically took on a "pseudo-superplastic" character: the samples broke in a very short time and showed elongation at break ⁇ R of over 10%.
  • Cylindrical test specimens were machined from the alloy MA 6000 (composition see above!) In the state of longitudinally oriented stem crystals.
  • the cylinder axis was parallel to the longitudinal direction of the crystals.
  • the test specimens had a diameter of 5 mm and a length of 25 mm. They were rolled on a lathe in such a way that the surface area was cold formed to an average depth of approx. 100 ⁇ m by 2%. Accordingly, the total cross section of 19.0 mm2 was 92% (18.03 mm2) from the undeformed core and 8% (1.57 mm2) from the deformed surface zone.
  • test specimens were then subjected to a fatigue test, the temperature and the load being changed periodically and synchronously according to the diagram in FIG. 5. Accordingly, the tensile stress was in phase with the upper limit temperature, while the compressive stress was in phase with the lower limit temperature.
  • the load was set so that a maximum tensile stress of 100 MPa at 900 ° C alternated with a maximum compressive stress of 100 MPa to 300 ° C in the respective test specimen.
  • the untreated comparative samples broke under the conditions mentioned after an average of 800 to 1200 load changes, while the test specimens with a cold-formed surface zone withstood 5000 and more load changes until they broke.
  • Cylindrical test specimens were machined from an oxide dispersion hardened nickel-based superalloy.
  • the test specimens had a diameter of 6 mm and a length of 35 mm. They were shot peened all around for 6 minutes under a jet pressure of 8 bar.
  • the steel balls used had a diameter of 0.3 to 0.5 mm.
  • the surface zone was cold-worked to a depth of approximately 150 ⁇ m by an average of 2.5%.
  • the total cross section of 28.4 mm2 consisted of 90% (25.57 mm2) of the undeformed core and 10% (2.83 mm2) of the cold-formed surface zone.
  • test specimens were subjected to a similar fatigue test as described in Example 2.
  • the entire cycle lasted 20 minutes instead of 10 minutes (see FIG. 5).
  • the heating and cooling times each took 4 minutes, the holding times 6 minutes.
  • the upper temperature limit was 980 ° C, the lower 350 ° C.
  • the non-cold-formed comparative samples broke under the conditions mentioned after an average of 300 to 500 load changes, while the test specimens with the cold-formed surface zone lasted at least 2000 load changes until they broke.
  • a gas turbine blade made of the alloy MA 6000 in the state of longitudinally oriented stem crystals was subjected to cold working subject in the surface zone.
  • the surface of the airfoil was shot peened on all sides for 10 minutes under a blasting pressure of 10 bar.
  • the steel balls used had an average diameter of 0.4 mm.
  • the surface zone was cold worked down to a depth of 200 ⁇ m by an average of 3%.
  • the total cross-section of approx. 1150 mm2 therefore consisted of 96.5% (1110 mm2) from the undeformed core and 3.5% (40 mm2) from the cold-formed surface zone.
  • the fatigue test consisted of a kind of thermal shock test with simultaneous periodic application of a tensile load at the upper and a pressure load at the lower temperature. The entire cycle lasted 30 minutes. The heating-up time was 6 minutes, the cooling-down time was 4 minutes and the holding times were 10 minutes each. The highest temperature reached at the top of the airfoil was 1000 ° C, the lowest 400 ° C. At the foot of the airfoil, the temperatures were 850 ° C respectively. 320 ° C. The maximum tensile stresses at the upper temperatures were 120 MPa, the maximum compressive stresses 80 MPa, always acting in the longitudinal axis of the blade.
  • a non-cold-formed gas turbine blade was tested as a reference body.
  • the first clearly visible cracks appeared after 400 to 600 cycles, while the cold-formed blades still showed no visible cracks after 2000 cycles.
  • the airfoil was separated from a gas turbine blade made of the oxide-dispersion-hardened nickel-base superalloy specified in Example 3 and cold-hardened on the surface by repeatedly rolling a hardened steel roller of 30 mm in diameter along the surface lines.
  • the material was in the form of longitudinal, coarse stem crystals.
  • a cold-formed surface zone with an average depth of 150 ⁇ m was created by the rolling.
  • the cold deformation was approximately 2.5%.
  • the total cross-section of around 1100 mm2 thus consisted of 97.3% (1070 mm2) of the undeformed core and 2.7% (30 mm2) of the cold-formed surface zone.
  • the airfoil cutout was subjected to a fatigue test.
  • the direction of force application was parallel to the longitudinal axis of the blade, which was also the longitudinal axis of the stem crystals.
  • the synchronous temperature and load change cycle lasted a total of 60 min.
  • the tension was changed between 100 MPa tension and 100 MPa pressure in such a way that the maximum tension with the maximum temperature of 1050 ° C and the maximum compression stress with the minimum temperature of 450 ° C coincided.
  • the cycle was thus carried out according to the following scheme: - Heating from 450 ° C to 1050 ° C with simultaneous voltage changes from -100 MPa to +100 MPa: 10 min - Hold at 1050 ° C and +100 MPa: 20 min - Cooling from 1050 ° C to 450 ° C with simultaneous voltage change from +100 MPa to -100MPa: 10 min - Hold at 450 ° C and -100 MPa: 20 min Total cycle 60 min
  • the untreated comparative samples broke under the conditions mentioned after an average of 250 to 300 load changes, while the test specimens with the cold-formed surface withstood more than 1000 load changes until they broke. At least three to four times the lifespan of the untreated blades with regard to thermal fatigue under a low number of load cycles is to be expected.
  • the invention is not restricted to the examples of performance. Increasing the fatigue strength and reducing the susceptibility to cracking at high temperatures and when going through a temperature cycle of a workpiece consisting of an oxide dispersion-hardened nickel-base superalloy is accomplished by increasing the ductility (deformability) of its surface zone.
  • the latter is cold worked to a depth of at least 100 ⁇ m at room temperature by at least one value which corresponds to a plastic elongation of 2%.
  • the cold forming is carried out by mechanical processing in the form of targeted milling, turning or grinding or by targeted blasting with solid (shot peening) or liquid particles.

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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)
EP88119778A 1987-12-01 1988-11-28 Procédé pour augmenter la résistance à la fatigue et diminuer la tendance à la fissuration à haute température d'une pièce en superalliage à base de nickel durci par dispersion d'oxydes Expired - Lifetime EP0318887B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
CH4674/87A CH676126A5 (fr) 1987-12-01 1987-12-01
CH4674/87 1987-12-01

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EP0318887A1 true EP0318887A1 (fr) 1989-06-07
EP0318887B1 EP0318887B1 (fr) 1993-08-11

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EP88119778A Expired - Lifetime EP0318887B1 (fr) 1987-12-01 1988-11-28 Procédé pour augmenter la résistance à la fatigue et diminuer la tendance à la fissuration à haute température d'une pièce en superalliage à base de nickel durci par dispersion d'oxydes

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EP (1) EP0318887B1 (fr)
CH (1) CH676126A5 (fr)
DE (1) DE3883173D1 (fr)

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3505130A (en) * 1966-06-13 1970-04-07 Orenda Ltd Method for improving fatigue strength in turbine blades
GB2074194A (en) * 1980-04-21 1981-10-28 Gen Electric Composite grained cast article and method
EP0115092A2 (fr) * 1983-02-01 1984-08-08 BBC Brown Boveri AG Elément de construction à résistance élevée contre la corrosion et l'oxydation, réalisé en un superalliage à durcissement par dispersion et procédé pour sa fabrication
EP0074918B1 (fr) * 1981-09-10 1987-07-01 United Technologies Corporation Procédé de grenaillage et polissage simultanés

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3505130A (en) * 1966-06-13 1970-04-07 Orenda Ltd Method for improving fatigue strength in turbine blades
GB2074194A (en) * 1980-04-21 1981-10-28 Gen Electric Composite grained cast article and method
EP0074918B1 (fr) * 1981-09-10 1987-07-01 United Technologies Corporation Procédé de grenaillage et polissage simultanés
EP0115092A2 (fr) * 1983-02-01 1984-08-08 BBC Brown Boveri AG Elément de construction à résistance élevée contre la corrosion et l'oxydation, réalisé en un superalliage à durcissement par dispersion et procédé pour sa fabrication

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
J.O. ALMEN et al.: "Residual stresses and fatigue in metals",1963, McGRAW-Hill Book Co., Inc., Seiten 46-58, New York, US; Kapitel 5 "Methods of producing residual stresses", Seiten 59-80: Kapitel 6 "Mechanical proceeding" *
Z. MATALLKDE., Band 77, Nr. 5, Mai 1986, Seiten 322-337; B. SCHOLTES et al.: "Auswirkungen mechanischer Randschichtverformungen auf das Festigkeitsverhalten metallischer Werkstoffe" *

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DE3883173D1 (de) 1993-09-16
EP0318887B1 (fr) 1993-08-11
CH676126A5 (fr) 1990-12-14

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