Method of determining if an alloy article has any remaining working life

A method of determining if an alloy article (28) has any remaining working life comprises taking a sample from an alloy article (28) and removing all metal matrix material from the sample to leave the carbide particles. The carbide particles are analysed using x-ray diffraction to determine the ratio of the amount of the M.sub.23 C.sub.6 carbide phase to the amount of the MC carbide phase. This ratio is compared with a database, containing the ratio of the amount of the M.sub.23 C.sub.6 carbide phase to the amount of the MC carbide phase as a function of temperature of heat treatment and time of heat treatment, to determine the temperature of heat treatment and the time of heat treatment of the sample of the alloy article (28). This is compared with a database of heat treatment temperatures and associated heat treatment times corresponding to the full working life of the alloy article (28) to determine if the alloy article (28) has any remaining working life.

The present invention relates generally to a method of determining if an
 alloy article has any remaining working life, particularly to a method of
 determining if a nickel superalloy article for gas turbine engines has any
 remaining working life.
 One of the main difficulties in determining if gas turbine engine
 components, particularly turbine blades and turbine vanes, have any
 remaining working life is that the gas turbine engines may be operated
 with different operating cycles. Hence a prediction of the service life of
 a gas turbine engine component is often made with little, or no, knowledge
 of the history of the use of the gas turbine engine component.
 One known method of determining if gas turbine engine components have any
 remaining working life performs creep tests on a representative number of
 components. However, the resulting scatter in the test data makes the
 remaining working life estimate to be conservative.
 An additional problem is that it is difficult to make allowances for the
 starting conditions of the gas turbine engine components. There may be a
 considerable variation in the initial microstructure of the gas turbine
 engine components because of the variability of the casting process.
 Accordingly the present invention seeks to provide a method of determining
 if an alloy article has any remaining working life which reduces or
 overcomes the above mentioned problems.
 Accordingly the present invention provides a method of determining if an
 alloy article has any remaining working life comprising the steps of:
 (a) taking at least one sample from an alloy article,
 (b) removing substantially all metal matrix material from the at least one
 sample to leave the carbide particles,
 (c) analysing the carbide particles using x-ray diffraction, identifying
 the x-ray peaks of the main carbide phases from the x-ray diffraction
 spectra,
 (d) determining the ratio, or difference, of the amount of a first carbide
 phase to the amount of a second carbide phase,
 (e) providing a database containing the ratio, or difference, of the first
 carbide phase to the second carbide phase as a function of temperature of
 heat treatment and time of heat treatment,
 (f) comparing the ratio, or difference, of the amount of the first carbide
 phase to the amount of the second carbide phase determined in step (d)
 with the database in step (e) to determine the temperature of heat
 treatment and the time of heat treatment of the sample of the alloy
 article,
 (g) comparing the temperature of heat treatment and the time of heat
 treatment of the sample of the alloy article determined in step (f) with a
 plurality of different heat treatment temperatures and associated heat
 treatment times corresponding to the full working life of the alloy
 article to determine if the alloy article has any remaining working life.
 Preferably step (d) comprises determining the ratio, or difference, of the
 amount of the M.sub.23 C.sub.6 carbide phase to the amount of the MC
 carbide phase.
 Step (d) may comprise determining the ratio, or difference, of the amount
 of the M.sub.23 C.sub.6 carbide phase to the amount of the M.sub.6 C
 carbide phase.
 Step (d) may comprise determining the ratio, or difference, of the amount
 of the M.sub.6 C carbide phase to the amount of the MC carbide phase.
 Preferably step (d) comprises determining the ratio of the integrated
 intensity of the x-ray peak of the first carbide phase to integrated
 intensity of the x-ray peak of the second carbide phase.
 Step (b) comprises dissolving substantially all the metal matrix material
 from the at least one sample in an electrochemical cell to leave the
 carbide particles.
 Step (b) comprises dissolving substantially all the metal matrix material
 in an electrochemical cell having a solution comprising hydrochloric acid,
 tartaric acid and methanol.
 Preferably the alloy article is a nickel base superalloy, a cobalt base
 superalloy or an iron base superalloy.
 Preferably the alloy article is a turbine blade or a turbine vane.
 Preferably step (a) comprises removing the sample from the leading edge of
 the turbine blade or turbine vane. Preferably step (a) comprises removing
 the sample from a predetermined position on the leading edge of each
 turbine blade or each turbine vane.
 The alloy article may comprise an alloy comprising 10 wt % Co, 9 wt % Cr,
 5.5 wt % Al, 10 wt % W, 2.5 wt % Ta, 1.5 wt % Ti, 1.5 wt % Hf, 0.15 wt % C
 and the balance Ni plus incidental impurities.
 The alloy article may comprise an alloy comprising 16 wt % Cr, 8.5 wt % Co,
 3.4 wt % Al, 2.6 wt % W, 1.7 wt % Ta, 3.4 wt % Ti, 1.7 wt % Mo, 0.8 wt %
 Nb, 0.11 wt % C and the balance Ni plus incidental impurities.
 The alloy article may comprise 8.3 wt % Cr. 10 wt % Co, 0.7 wt % Mo, 10 wt
 % W, 3 wt % Ta, 5.5 wt % Al, 1 wt % Ti, 0.14 wt % C, 1.5 wt % Hf and the
 balance Ni plus incidental impurities.

An industrial gas turbine engine 10, shown in FIG. 1, comprises in axial
 flow series an inlet 12, a compressor section 14, a combustion chamber
 assembly 16, a turbine section 18, a power turbine section 20 and an
 exhaust 22. The turbine section 18 is arranged to drive the compressor
 section 14 via one or more shafts (not shown). The power turbine section
 20 is arranged to drive an electrical generator 24 via a shaft 26,
 alternatively the power turbine section may drive a pump or a propeller.
 The operation of the gas turbine engine is quite conventional and will not
 be discussed further.
 The turbine section 18 and the power turbine section 20 comprises turbine
 blades 28 and turbine vanes 30, a turbine blade 28 is shown more clearly
 in FIG. 2. The turbine blade 28 comprises a root portion 32, a platform
 portion 34 and an aerofoil portion 36. The aerofoil portion 36 has a
 leading edge 38, a trailing edge 40, a concave pressure surface 42 and a
 convex suction surface 44.
 The turbine blades 28 and turbine vanes 30 are generally cast from nickel
 based superalloys. The turbine blades 28 or turbine vanes 30 may be
 conventionally cast or directionally solidified or cast as single
 crystals.
 The turbine blades, or turbine vanes, may be provided with environmental
 protective coatings and/or thermal barrier coatings.
 Conventionally cast and directionally solidified nickel based superalloy
 components, turbine blades 28 and turbine vanes 30 contain carbon, which
 leads to the precipitation of a number of carbide phases within the
 microstructure.
 It is desirable, periodically during servicing of gas turbine engines, to
 know if the turbine blades 28 or turbine vanes 30 in the gas turbine
 engine 10 have any remaining working life and how much remaining working
 life the turbine blades 28 or turbine vanes 30 have. If the turbine blades
 28 or turbine vanes 30 do not have any remaining working life, they must
 be replaced with new turbine blades 28 or turbine vanes 30. If the turbine
 blades 28 or 30 have some remaining working life they may still be used
 until their full working life has expired.
 In order to determine if the turbine blades 28, or turbine vanes 30, have
 any remaining working life a number of samples of the alloy from which the
 turbine blades 28 and turbine vanes 30 are cast are given the same heat
 treatment as the turbine blades 28, or turbine vanes 30. The samples were
 subsequently heat treated at a range of temperatures that the turbine
 blades 28 and turbine vanes 30 are likely to experience in operation. Some
 of the samples at each heat treatment temperature were examined at 50 day
 intervals up to a period of 250 days. However samples may be examined at
 any suitable time interval and for any time duration.
 The samples taken at each heat treatment temperature for each 50 day time
 period were examined by firstly removing all the metal matrix material
 from the samples to leave only the carbide phases in the samples. The
 metal matrix material is removed by placing the samples one at a time in
 an electrochemical cell and dissolving all the metal matrix material to
 leave the carbide phases. The electrochemical cell is set up to dissolve
 the gamma and gamma prime phases in the nickel superalloy, that is metal
 and intermetallic phases. The electrochemical cell contained a solution of
 10% hydrochloric acid, 1% tartaric acid and methanol and was operated with
 a current of 0.02 A for up to 16 hours. The electrochemical cell may be
 operated using other suitable solutions and currents. The sample may be
 agitated by placing the electrochemical cell in an ultrasonic bath to
 ultrasonically agitate the sample. The surface of the sample is
 periodically washed with methanol. The carbide phases, or carbide phase
 particles, are removed from the solution in the electrochemical cell by
 filtering the solution and collecting the carbide phase particles on a
 filter paper, for example an amorphous glass microfiber filter paper.
 The carbide phase particles from each sample are examined in an x-ray
 diffractometer, for example a Philips Xpert diffractometer with a copper
 x-ray tube operated at 40 kV and 40 mA. Each sample was scanned over
 2.theta. values ranging from 10-140.degree.. The widths of the divergence
 and receiving slits were 1.degree. and 0.1.degree. respectively.
 The x-ray diffraction spectra of each sample was analysed using
 conventional techniques, for example a Philips Profit software, to
 determine the ratios, or differences, of the amounts of different carbide
 phases present by integrating the intensity of the x-ray peaks
 corresponding to the different carbide phases. Thus a database containing
 the ratios, or differences, of the amounts of different carbide phases at
 different heat treatment temperatures for different heat treatment times
 is produced.
 In order to determine if the turbine blades 28, or turbine vanes 30, have
 any remaining working life one or more samples are taken from the turbine
 blades 28, or turbine vanes 30, which have operated in the turbine 18, or
 turbine 20, of the gas turbine engine 10 for an unknown period of time and
 at an unknown operating temperatures. Preferably the samples are removed
 from a predetermined position at the leading edge of each turbine blade
 28, or turbine vane 30.
 All the metal matrix material from the samples of the turbine blades 28, or
 turbine vanes 30, is removed to leave only the carbide phases in the
 samples. The metal matrix material is removed by placing the samples one
 at a time in an electrochemical cell and dissolving all the metal matrix
 material to leave the carbide phases. The electrochemical cell contained a
 solution of 10% hydrochloric acid, 1% tartaric acid and methanol and was
 operated with a current of 0.02 A for up to 16 hours. The electrochemical
 cell may be operated using other suitable solutions and currents. The
 sample may be agitated by placing the electrochemical cell in an
 ultrasonic bath to ultrasonically agitate the sample. The surface of the
 sample is periodically washed with methanol. The carbide phases, or
 carbide phase particles, are removed from the solution in the
 electrochemical cell by filtering the solution and collecting the carbide
 phase particles on a filter paper, for example an amorphous glass
 microfiber filter paper.
 The carbide phase particles from each sample are examined in an x-ray
 diffractometer, for example a Philips Xpert diffractometer with a copper
 x-ray tube operated at 40 kV and 40 mA. Each sample was scanned over
 2.theta. values ranging from 10-140.degree.. The widths of the divergence
 and receiving slits were 1.degree. and 0.1.degree. respectively.
 The x-ray diffraction spectra of each sample was analysed using
 conventional techniques, for example Philips Profit software to determine
 the ratios, or differences, of the amounts of different carbide phases
 present by integrating the intensity of the x-ray peaks corresponding to
 the different carbide phases.
 The ratio, or difference, of the amount of a first carbide phase to the
 amount of a second carbide phase in the samples is compared with the
 database of ratios, or K differences, of the amount of the first carbide
 phase to the second carbide phase to determine the temperature of heat
 treatment and the time of heat treatment of the sample of the turbine
 blade 28, or turbine vane 30.
 A model is provided to assist with the determination of unknown metal
 temperatures by comparison with the experimental database. The model
 contains a description of the reaction kinetics of the carbide phases
 during heat treatment at various heat treatment temperatures for various
 heat treatment times. The model embodies in it a description of the
 thermodynamic equilibrium state of the carbide phases in the alloy as a
 function of temperature. The model, which is based on fundamental
 scientific principles, enables the fitting of curves to the experimental
 points in the database using empirical constants to enable interpolation
 to temperatures for which no experimental data exists, for example in
 between the temperatures at which the samples in the database were
 exposed. Alternatively, the use of standard curve fitting techniques to
 the experimental data would also be possible.
 The temperature of heat treatment and the time of heat treatment of the
 sample of the turbine blade 28, or turbine vane 30, is compared with a
 plurality of different heat treatment temperatures and associated heat
 treatment times corresponding to the full working life of the turbine
 blade 28, or turbine vane 30, to determine if the turbine blade 28, or
 turbine vane 30, has any remaining working life. If it is determined that
 the turbine blades 28, or turbine vanes 30, have some remaining life, they
 may be left in the turbine 18, 20 or the turbine blades 28 and/or turbine
 vanes 30 may removed from the turbine 18, 20 and the existing coating is
 stripped and a new coating is applied an the turbine blades 28, or turbine
 vanes 30, are put back into the turbine 18, 20.
 It is to be noted that the present invention is applicable to the metal
 matrix material of the article and not any environmental protective
 coating or thermal barrier coating applied to the article.
 EXAMPLE 1
 Samples of MARM-002 were heat treated by heat treating at a temperature of
 1190.degree. C. for 15 minutes, followed by a heat treating at a
 temperature of 870.degree. C. for 18 hours. This is the normal heat
 treatment applied to MARM-002 components before service use.
 Some of the samples were heat treated at a temperature of 700.degree. C.
 for 50 days, 100 days, 150 days, 200 days and 250 days respectively. Some
 of the samples were heat treated at a temperature of 800.degree. C. for 50
 days, 100 days, 150 days, 200 days and 250 days respectively. Some of the
 samples were heat treated at a temperature of 900.degree. C. for 50 days,
 100 days, 150 days, 200 days and 250 days respectively. Some of the
 samples were heat treated at a temperature of 1000.degree. C. for 50 days,
 100 days, 150 days, 200 days and 250 days respectively.
 Each of the samples was placed in an electrochemical cell to remove all the
 metal matrix material from the carbide phase particles. The carbide phase
 particles from each sample were analysed by x-ray diffraction to determine
 the ratio of the amount of a first carbide phase to the amount of a second
 carbide phase to produce the database of ratios of amounts of different
 carbide phases at different heat treatment temperatures for different heat
 treatment times.
 In particular it has been found that in MARM-002 that there are three main
 types of carbide phases present, these are M.sub.6 C, M.sub.23 C.sub.6 and
 MC. The M.sub.6 C carbide phase has a composition of approximately
 (W.sub.0.45, Cr.sub.0.25, Ni.sub.0.2, Co.sub.0.1).sub.6 C, the M.sub.23
 C.sub.6 carbide phase has a composition of approximately (Cr.sub.20,
 W.sub.2, Co)C.sub.6 and the MC carbide phase a composition of
 approximately (Ti,Ta,Hf,W,Zr)C, however, there are several compositions
 for MC.
 It has been found that on heat treatment the amount of M.sub.23 C.sub.6
 progressively increases on exposure to heat treatment at temperatures of
 800.degree. C. and above. As the temperature approaches the maximum
 stability limit of approximately 1020.degree. C. the amount of M.sub.23
 C.sub.6 progressively decreases. On heat treatment at temperatures of
 950.degree. C. and above the amount of M.sub.6 C increases. It is seen by
 comparing FIGS. 3, 4 and 5 that the peaks of the x-ray spectra
 corresponding to the M.sub.23 C.sub.6 carbide phase 50A, 50B and 50C
 respectively and the peaks of the x-ray spectra corresponding to the
 M.sub.6 C carbide phase 52C generally increase with heat treatment
 temperature. Conversely, the amount of MC carbide phase progressively
 decreases on heat treatment at temperatures of 800.degree. C. and above.
 It can also be seen from FIGS. 3, 4 and 5 that there is a switch over in
 the intensities of the different types of MC carbide phase peaks 46A, 46B
 and 46C corresponding to hafnium rich MC carbide phase and 48A, 48B and
 48C corresponding to a titanium/tantalum rich MC carbide phase with
 increasing temperature.
 In the case of MARM-002 the ratio of the amount of the M.sub.23 C.sub.6
 carbide phase to the amount of the MC carbide phase is determined for
 temperatures above 750.degree. C. to 900.degree. C. and it may be possible
 to determine the ratio of the amount of the M.sub.6 C carbide phase to the
 amount of the MC carbide phase above 900.degree. C. It may also be
 possible to determine the ratio of the amount of the M.sub.6 C carbide
 phase to the amount of the M.sub.23 C.sub.6 carbide phase above
 900.degree. C. The ratio is determined using carefully selected peaks in
 the x-ray spectra to ensure that the peak has minimum error introduced due
 to contributions from overlapping peaks, but also that the peak has
 maximum intensity.
 Samples were taken from the leading edges of MARM-002 turbine blades and
 were processed in an electrochemical cell to leave the carbide phase
 particles. The carbide phase particles were analysed by x-ray
 diffractometer and the ratio of the amount of the M.sub.23 C.sub.6 carbide
 phase to the amount of the MC carbide was determined from the x-ray
 spectra. These ratios were compared to those in the database to determine
 the heat treatment temperature and heat treatment time of the turbine
 blade 28, or turbine vane 30, samples which gives an indication of the
 amount of the working life used at that temperature of heat treatment.
 It has been found that this provides an accuracy of prediction of the heat
 treatment temperature of within 25.degree. C. for the heat treatment
 temperature range 800.degree. C. to 900.degree. C.
 The amount of working life used is then compared to the actual working life
 of a turbine blade, or turbine vane, for that particular temperature to
 determine if the turbine blade 28, or turbine vane 30, has any remaining
 working life or whether new turbine blades 28, or turbine vanes 30, need
 to be installed in the turbine of the gas turbine engine 10.
 MARM-002 is a nickel based alloy produced by the Martin-Marietta
 Corporation of Bethesda, Md., USA. MARM-002 has a nominal composition of
 10 wt % Co, 9 wt % Cr, 5.5 wt % Al, 10 wt % W, 2.5 wt % Ta, 1.5 wt % Ti,
 1.5 wt % Hf, 0.15 wt % C and the balance Ni plus incidental impurities.
 The invention is also applicable to the nickel based superalloys MARM-247
 and IN738, and may be applicable to other nickel based alloys, cobalt
 based alloys or iron based alloys which comprise carbon and carbide
 forming elements, such as Cr, W, Hf etc in the form of carbide phases.
 MARM-247 is a nickel based alloy produced by the Martin-Marietta
 Corporation of Bethesda, Md., USA. MARM-247 has a nominal composition
 comprising 8.3 wt % Cr, 10 wt % Co, 0.7 wt % Mo, 10 wt % W, 3 wt % Ta, 5.5
 wt % Al, 1 wt % Ti, 0.14 wt % C, 1.5 wt % Hf and the balance Ni plus
 incidental impurities.
 IN738 is a nickel based alloy and has a nominal composition comprising 16
 wt % Cr, 8.5 wt % Co, 3.4 wt % Al, 2.6 wt % W, 1.7 wt % Ta, 3.4 wt % Ti,
 1.7 wt % Mo, 0.8 wt % Nb, 0.1 wt % C and the balance Ni plus incidental
 impurities.
 It is preferred that the samples are taken from the leading edge of the
 turbine blades 28, or turbine vanes 30, however, it may be possible to
 take samples from other suitable regions of the turbine blades 28, or
 turbine vanes 30.
 Although specific ratios of the amount of carbide phases have been
 described it is possible to use the inverse of any of these ratios. It is
 also possible to use the difference of the amounts of carbide phases.