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NDT methods for revealing anomalies and defects in gas turbine blades Author: J. Pitkänen VTT Manufacturing Technology Espoo,Finland Co-authors : T. Hakkarainen, H. Jeskanen, P. Kuusinen,K. Lahdenperä & P. Särkiniemi VTT Manufacturing Technology M. Kemppainen Technical University of Helsinki M. Pihkakoski Helsinki Energy Espoo,Finland Contact

ABSTRACT

Energy produced by gas turbines is increasing all over the world. One important part in these gas turbines is the turbine blading. Improved blades and vanes are being introduced while the industry also uses the old generation of blade types. Applicable NDT methods are needed for all blade types. Many NDT-methods can be applied in laboratory environment. This overview describes a variety of methods for blade inspection such as dye penetrant, eddy current, radiography and ultrasonic methods. Some of these methods are considered on the basis of the experience gained in measurements. One new UT-method is described, easily transferable for practical inspections on site. A wide aperture VTT probe has been developed for the purpose. With this special probe different blade properties can be measured, such as coating thickness, detection and size of cracks, delaminations as well as mechanical properties.

INTRODUCTION

The lifetime of GT-blades is very important to utility. The lifetime of parts, where the temperature is high is most critical, because of thermal stresses, corrosion, oxydation and erosion. These factors reduce the lifetime of a blade. The lifetime can vary suddenly in use, because of many unforeseen incidents occuring in environment of the blades are. Also some variation in production or in repair can deteriorate the materials and in same time shorten the lifetime of the component.

Dye penetrant testing is the NDT method most frequently used for inspecting gas turbine blades and vanes. It is often recommended to use fluorescent dye penetrants for inspection. After some time in service the blade surface is often corroded. In these cases fluorescent dye penetrant is not recommended and normal dye penetrant is more usable. The drawback of dye penetrant testing is its suitability only for surface opening cracks. Subsurface defects are not detected. When a crack is found with dye penetrant testing, it is not possible to estimate its depth, and here other methods are needed.

There are many methods which can be used for detection of cracks, ageing and degradation of a coating, and for thickness measurement. Thermal methods can be used for measurement of wallthickness and detection of near surface anomalies like delaminations (Aladin 1996, Carl et al 1998a), problems in cooling channels (Carl 1998b).

It has been observed that for instance in MCrAlY-coatings the permeability in base material as well as the in coating are changing during the lifetime of a component. In MCrAlY-coating the decrease of Cr-content and /or b -phase change the magnetic properties to more ferromagnetic. Even though the effect is not large, it can be detected in some cases (Czech et al 1998). Czech et al measured the magnetic permeability with a coil in conjuction with permanent magnets. So they could estimate the lifetime of coating. This is also possible with the eddy current method developed by ENEL (Antonelli 1998 a, b). The method developed by ENEL can also tbe used for the determination of the coating thickness of MCrAlY-coatings.

EDDY CURRENT TECHNIQUES

Eddy current technique has been used for detecting and sizing of a crack. It is best to calibrate with a calibration block made from the same material as the real component. This means more costs but gives certainly most reliable results. Ludwig et al (1998) have reported that open cracks and tight cracks can be separated from each other with eddy current technique.
However from a crack which goes through grain boundaries is clearly more difficult to estimate the size of a crack. This was according Ludwig et al (1998) problem the estimate the life time of a cracked blade. Figure 1 shows eddy current signals from three cracks that were found with dye penetrant testing.

a) 3 surface breaking cracks on a turbine blade	b) Eddy current signals from cracks.
Fig 1: Eddy current signals from the three cracks shown in the blade .

Measuring the thickness of a coating is simple with eddy current technique, if there are no strong changes in permeability. In addition the geometry of a blade can complicate the measurement. Figure 2 shows the thickness measurement from a first stage X45 vane. The coating was nonconducting ceramic material. In this case the measurement can be easily to carried out with conventional eddy current technique. More demanding is to measure metallic coatings, with a good conductive like MCrAlY-coatings. The point for thickness measurement calibration has to be chosen very carefully, see figure 2, because a strong variation is observed in base material depending on base material location in blade. This of course has an effect on the measurement. The method developed by ENEL (Antonelli et al, 1998) doesn't need calibration blocks to carry out thickness measurement.

Fig 2: EC thickness measurement of the coating from a 1. stage vane, material X45 with ceramic coating.

During the lifetime of a material properties can vary drastically as shown in figure 3. In these cases the strong effect from the permeabitility cannot be corrected easily. In this case the coating from IN738 blade is damaged. The damaged area is gives a similar signal to ferritic steel. While in the undamaged area the measured signal was more like one from stainless steel (AISI 316). The changes of the material properties on the surface of a blade make the inspection more difficult both in the case of cracks and in measuring the thickness of the coating.

Fig 4: Thickness measurement principle The time of flight of leaky Rayleigh wave varies according to the thickness of the coating

ULTRASONIC TECHNIQUES

Ultrasonic tehcniques used for turbine blade measurement are normally applied to detect cracks in the shoe of the blade. In this case we consider only leaky the Rayleigh wave technique, which can be utilized for turbine blade measurement with contact ultrasonic probe developed by VTT Manufacturing Technology. This method has been studied by Kauppinen (1997). With this probe it is possible to measure thickness of the coating, figs 4 and 5.

Fig 5: Thickness measurement with the time of flight of leaky Rayleigh wave from a MCrAlY- coated turbine blade

The developed probe is also suitable for detection of cracks , if the leaky Rayleigh wave is penetrating into the base material. In case where leaky Rayleigh wave doesn't penetrate into the base material, it is still possible to use normal Rayleigh wave. The leaky Rayleigh wave decays rapidly when crack like defect are present. In the figure 6. we see that with leaky wave mode it is easy to detect even small cracks down to 50m m. In depth the resolution depends on the frequency (wavelength) of the probe.

Fig 6: (a) MCrAl-coated GT-blade, in which the EDM- notch depths vary between 50mm - 500mm (b) ultrasonic B-scan image in pitch-catch mode. (c) Ultrasonic B-scan image in pulse-echo mode from the blade with EDM-notches

Fig 7: Material X45 is measured with contact wide aperture probe is shown in fig. 7. The center frequency of the probe was 12 MHz. The plate is shown in upper picture and C-scan result in lower picture

Same technique is used for thermal fatique crack detection in fig 7. As it is clearly seen two cracks in this plate can be detected with this contact probe. The plate is about 3 mm thick and one of the cracks is extending throughout the wall (in the middle of the plate).

Other application possibilities for developed probe type are detection of delaminations, measurement of material wave velocity and elasticity.

RADIOGRAPHIC TECHNIQUES

Radiographic methods can be divide to normal through wall measurement with film or with real time radioscopy, X-ray diffraction method, compton effect measurement, X-ray tomography, neutron tomography, neutron radiography, positron annhilation. VTT has used real time radioscopy equipment for measuring turbine blades and Helsinki University of Technology has carried out some X-ray diffraction measurements.

In X-ray measurement the radiation decays differently in various materials and discontiniuties. The changes are affected by density variation, thickness variation, variation in composition of the material and from lack of material (corrosion, cracks). In figure 8 the realtime radioscopy equipment is shown. The inspected object is located between the source and the image intensifier. The object is radiated and in the image intensifier measured radiation is digitized through video card. This information can be saved on the hard disc or CD-rom. These measurement has been carried out with X-ray tube of 160 kV.

Fig 8: The PC- real time radioscopy system in evaluating radiographic results

Fig 9: X-ray picture from the inside structures of the gas turbine blades

Fig 10: Cooling channel crossing, which could cause from the edm-manufacturing of r the channels.

In pictures cracks, pores, geometrical thicknesses, material density variations can be detected, figure 9. Especially cooling holes are clearly seen in the pictures, figure 10. Pores have been detected mostly in radiography measuments. The sizes of those pores are about same as the dimensions of the cooling channels. The blocks in the cooling channels can cause extra stress to turbine blade. The cooling of the blade is changed drastically and the temperature distribution on the blades changed can rapidly damage the blade.

Residual stresses were measured from a turbine blade material In 939. The measuring instrumet was XSTRESS 3000 (Stresstech Oy) X-ray diffraction equipment. The radiation was focussed to the object with help of a collimator, the diameter of which was 3 mm. Exposure time was either 20 s or 40 s and y -oscillation ±5°. In the measurement point was the 2 j -rotation ±20° with 10 angles. With this arrangement exposure time for one point was 400s or 2000s. With this measurement the variation in half width of X-ray diffraction spectrum according to the stress state were measured. In figure 11 the measured X-ray diffraction spectrum is shown from the area 2 in table 1, which was thermally loaded. In table 1 are shown the measured residual stresses from 3 areas in the same turbine blade. Additional X-ray diffraction spectrum can give some information about the degradation of the coating and also from the lifetime of the blade.

Area n:o	sx (MPa)	sy (MPa)
1	99	-215
2	64	-339
3	-946	-776
Table 1: Residual stress measurements from 3 areas of a turbine blade

Fig 11: The half width value of the X-ray diffraction spectrum from residual stress measurement

CONCLUSION

There are a lot of methods available for determining the properties of turbine blades. Some of these methods can be used to complete the information received from the dye penetrant testing.

Most of the methods are available in laboratory stage only. But there are some techniques, like eddy current and ultrasonic techniques, which can be utilized in practical inspections on site.

The driving force of NDT-evaluation is of course the utilities' need to optimize the plant operation and to minimise the risks of service damage.

REFERENCES

1. Aladin ,1996 Aladin Wärmewellenmikroskop, Charakterisierung von Materialeigenschaften und Mikrostrukturen mittels Wärmewellen, Datenblatt 010296, IzfP-FHG, Saarbrücken 4p.
2. Antonelli, G., Ruzzier, M. & Cernushi, F., 1998a A Calibration Free Electromagnetic Technique for NDT of Metallic Coatings, 7th European Conference on non-destructive testing, Copenhagen, 26. - 29, 7p.
3. Antonelli, G., Ruzzier, M. & Necci, F., 1998b Thickness Measurement of MCrALY High-Temperature Coatings by Frequency Scanning Eddy Current Technique, Journal of Engineering for Gas Turbines and Power, Transactions of ASME 120 (1998) July, p. 537 - 542.
4. BEMI, 1997 BEMI Barkhausenrausch- und Wirbelstrommikroskop, Neue Möglichkeiten in der Werkstoffprüfung, Datenblatt 010497, IzfP-FHG, Saarbrücken 4p.
5. Carl., V., Becker, E. & Sperling, A., 1998a Thermography inspection system for gas turbine blades, 7th European Conference on non-destructive testing, Copenhagen, 26. - 29. May p. 2658 - 2665.
6. Carl., V., Becker, E. & Sperling, A., 1998b Quantitative Wallthickness Measurement with Impuls-Video-Thermography. 7th European Conference on non-destructive testing, Copenhagen, 26. - 29. May p. 1918 - 1924.
7. Coste, J.-F., Lakestani, F. & Vortrefflich, W., 1996 Thickness Determination of Gas Turbine Metallic Coatings Using Rayleigh Waves The Materials Challenge, News Bulletin of the institute for Advanced Materials, number 9, February 1996, p. 23 - 25.
8. Czech, N., Kirchner, F. and Stamm, W., 1996 Life assesment of service stressed high temperature coatings by permeability measurements Elevated temperature coatings: Science and Technology II, The minerals & Materials society, ed. Dahotre, N. B. and Hampikian, J. M., s. 361 - 371.
9. Impuls-Video-Thermographie, 1996 Impuls-Video-Thermographie, Charakterisierung von Materialeigenschaften und Mikrostrukturen, Datenblatt 010496, IzfP-FHG, Saarbrücken, 2p.
10. Kauppinen, P., 1997 The evaluation of integrity and elasticity of thermallys prayed ceramic coatings by ultrasonics,Dissertation, Helsinki University of Technology (Espoo, Finland), 14th of November, 1997, 130p.
11. Lakestani, F., Coste, J.-F. & Denis, R., 1995 Application of ultrasonic Thickness Measurement of Metallic coatings, NDT International 28(1995)3, p. 171 - 178.
12. Ludwig. K., Daalmans, G. M., Bär, L. , Lohmann H. P. and Becker, E., 1998 Crack depth determination in gas turbine blades by eddy current technique. 7th European conference on non-destructive testing, Copenhagen, 26. - 29. May 1998, p. 1888 - 1898.

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