Abstract:
A method and system for ultrasonically inspecting turbine wheels and other disk-shaped articles having a central opening and multiple secondary openings radially spaced outward from the central opening. At least one ultrasonic transducer is placed in at least one of the secondary openings of the wheel. The transducer is configured and oriented to perform a pulse-echo diagnostic technique on the wheel by emitting ultrasonic signals that intersect radials of the wheel at angles of approximately ninety degrees to the radials. The ultrasonic signals intersect the radials at points so that a plurality of points are located on a plurality of radials. The points define a locus of points through the volume of the wheel between the central opening and the secondary opening, such that the volume is inspected for defects. The transducer receives ultrasonic signals that are reflected from one or more points when a defect is encountered.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to ultrasonic inspection methods and systems. More particularly, this invention relates to a method and system for ultrasonically inspecting a disk-shaped article having a central opening and a plurality of circumferentially-spaced secondary openings that are radially spaced outward from the central opening, such as a turbine wheel or disk, wherein ultrasonic signals are generated and received by transducers placed in the secondary openings.  
         [0003]     2. Description of the Related Art  
         [0004]     Ultrasonic inspection techniques have been widely used to perform nondestructive testing on articles formed of materials having an intrinsically coarse grain structure, which results in anisotropic and nonuniform acoustic properties. Particular examples of such articles include forged super-alloy turbine wheels (rotors) used in gas and steam turbines. In the hostile operating environments of gas and steam turbines, the structural integrity of the turbine wheel is of great importance in view of the high mechanical stresses that wheels must be able to continuously withstand at extremely high temperatures.  
         [0005]     Ultrasonic inspection techniques employed with turbine wheels have typically involved inspecting each forging from a plane perpendicular to the highest operating stresses, i.e., ultrasonic transducers are placed on the fore and/or aft surfaces transverse to the rotational axis of a turbine wheel. With this approach, the ultrasonic energy is generated in a direction substantially perpendicular to the orientation of the most common defects, which tend to lie in axial-radial planes of a turbine wheel. Two ultrasonic testing techniques have been conventionally used, a first of which is a pitch-catch technique using two transducers placed on the fore and aft surfaces of the wheel. One of the transducers serves to generate an ultrasonic signal, while ultrasonic signals reflected from acoustical discontinuities being received by the second transducer. The second common technique is referred to as pulse-echo and makes use of a single transducer to both generate the ultrasonic signal and receive reflected signals.  
         [0006]     Inspections targeting axial-radial defects are performed routinely on steam turbine discs with shrunk-on wheels. Stress corrosion cracking typically develops at the surface of the central bore, and such cracks can be located using the pitch-catch technique in which the ultrasonic signals are targeted at the bore and the keyway surfaces at the perimeter of the wheel. This technique is not intended for full volumetric inspection of the wheel. A nondestructive testing method using the pitch-catch technique is also disclosed in U.S. Pat. No. 5,189,915 to Reinhart et al., whereby wheel blade attachment locations can be inspected ultrasonically from the bore of the wheel. Another pitch-catch technique has been developed for gas turbine wheels in the as-forged condition, at which time the wheels have parallel sides so that the technique is relatively simple to execute.  
         [0007]     A pitch-catch inspection technique using transducers placed on the fore and aft faces of a gas turbine wheel following final machining would also seem the most likely technique to detect flaws having an axial-radial orientation throughout the wheel volume. However, axial-radial defects are difficult to detect in gas turbine wheels due to their complex geometry resulting from the final machining operation, which includes the machining of circumferentially-spaced bolt holes spaced radially from the central bore. A pitch-catch inspection of such a wheel would require a complex movement of the transducers and control of transducer angles to ensure an effective examination. Such an approach would be further exacerbated by the high sonic noise produced by the large material grain size as well as a tendency for the acoustic pulse to be steered by flow lines produced during the forging process.  
         [0008]     In view of the above, it would be desirable if an improved method were available for ultrasonically inspecting articles having complex geometries, such as gas turbine wheels following final machining.  
       SUMMARY OF INVENTION  
       [0009]     The present invention provides a method and system for ultrasonically inspecting turbine wheels and other disk-shaped articles having a central opening and a plurality of secondary openings radially spaced outward from the central opening. The method and system make use of at least one ultrasonic transducer placed in at least one of the secondary openings of the article. The ultrasonic transducer is configured and oriented to perform a pulse-echo diagnostic technique on the article by emitting ultrasonic signals that intersect radials of the article at angles of approximately ninety degrees to the radials, with the result that the ultrasonic signals intersect the radials at points so that a plurality of the points are located on a plurality of the radials. The points define a locus of points through the volume of the article between the central opening and the secondary opening, such that the volume is inspected for defects.  
         [0010]     In view of the configuration and operation of the ultrasonic transducer, the method of this invention includes placing at least one ultrasonic transducer in at least a first of the secondary openings of the article, after which the transducer is caused to emit ultrasonic signals that intersect radials of the article at angles of approximately ninety degrees to the radials, such that a plurality of intersection points are located on a plurality of radials. The transducer is then used to receive reflected ultrasonic signals that return (are reflected) from one or more of the points, e.g., as a result of the presence of a defect at the point.  
         [0011]     In view of the above, it can be seen that the present invention provides an inspection process that, when applied to a turbine wheel after final machining, makes use of holes spaced radially outward from the central hub bore. Such holes are commonly present as bolt holes machined near the rim area of a turbine wheel. When the pulse-echo ultrasonic technique is conducted from each bolt hole location, the existence of any axial-radial flaws within the wheel volume between the bolt hole and the central hub bore can be determined along the locus of points inspected with the ultrasonic signals, which includes the critical volume of the wheel immediately adjacent the central hub bore. As such, the present invention provides a method for ultrasonically inspecting turbine wheels and other articles having complex machined geometries.  
         [0012]     The ultrasonic transducer is preferably one of multiple ultrasonic transducers assembled together as a unit that includes a specially shaped base whose shape conforms to the diameter of the hole in which the unit is placed. The method of this invention also preferably encompasses a technique by which the proper orientation of the transducers can be determined within the hole, and a method of calibrating the amplitude of reflected sound from a defect to an equivalent reference reflector.  
         [0013]     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]      FIG. 1  shows a face of a gas turbine wheel in which an ultrasonic transducer unit is installed for performing a nondestructive ultrasonic inspection of the wheel in accordance with this invention.  
         [0015]      FIG. 2  is a partial cross-sectional view of the wheel of  FIG. 1  along lines  22 .  
         [0016]      FIG. 3  is a view in the direction of line  33  of  FIG. 2 , showing in greater detail a transducer unit placed in a bolt hole of the wheel in accordance with a first embodiment of the invention.  
         [0017]      FIG. 4  schematically represents a transducer unit in accordance with a second embodiment of the invention.  
         [0018]      FIGS. 5, 6  and  7  graphically illustrates the ultrasonic inspection technique of the invention.  
         [0019]      FIG. 8  is a distance-gain-size diagram of a type used in a calibration technique of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]     A gas turbine wheel  10  of a type known in the art is illustrated in  FIGS. 1 and 2 . The wheel  10  is represented as being in the final-machined condition, and generally includes a rim  12 , a hub  14 , and a web  16  between the rim and hub  12  and  14 . The rim  12  is configured for the attachment of turbine blades (not shown) in accordance with known practice. A hub bore  18  in the form of a through-hole is centrally located in the hub  14  for mounting the wheel  10  on a turbine axis, and therefore the axis of the hub bore  18  coincides with the axis of rotation of the wheel  10 . A plurality of bolt holes  20  are machined through the web  16  at locations that are equal radial distances from the hub bore  18  (center-to-center), as well as circumferentially equally spaced from each other (center-to-center). The axes of the bolt holes  20  are shown as parallel to the axis of the hub bore  18 .  
         [0021]     From  FIGS. 1 and 2 , the complexity of the geometry of the wheel  10  is evident, making any ultrasonic inspection of the wheel  10  using conventional methods very difficult. According to the present invention, ultrasonic inspection of the wheel  10  is performed from the bolt holes  20  near the perimeter of the wheel  10 , instead of the fore and aft faces of the wheel  10  or from the hub bore  18 , as has been suggested in the past. For this purpose, a transducer unit  22  is shown in  FIGS. 2 and 3  as being configured to fit within the bolt holes  20  and closely contact the curved surfaces of the bolt holes  20 . As depicted in  FIGS. 2 and 3 , the unit  22  comprises several rectangular ultrasonic transducers  24  aligned in a row and mounted on a semi-cylindrical wedge  26 . The wedge  26  provides a flat surface  28  for mounting the transducers  24 , and a curved surface  30  whose radius of curvature is closely matched to that of the bolt holes  20 . By forming the wedge  26  of the same (or similar) material as the wheel  10 , the sound velocity in the wedge  10  is the same or nearly the same as in the wheel  10 , so that the effect of the curvature of the bolt hole  20  is minimal. A suitable couplant, such as light weight turbine oil, can be applied between the curved surface  30  of the wedge  26  and the mating surface of the bolt hole  20  to further reduce the effect of the interface between the transducers  24  and the bolt hole  20 . The wedge  26  is shown in  FIG. 2  as being equipped with a handle  32  to facilitate placement and removal of the unit  22  from the bolt holes  20 .  
         [0022]     The transducers  24  can be of any type suitable for use in ultrasonic inspection procedures, such as a style RHP gamma series (or equivalent) available from Krautkramer, Inc. The width of the transducers  24  are preferably maximized to fit into the bolt holes  20 , with minimal separation between elements. The height of each transducer  24  is preferably equal to or greater than its width in order to promote the signal-to-noise response from a defect reflector. In the configuration shown in  FIGS. 2 and 3 , the transducers  24  are arranged on the transducer unit  22  as a linear array, and as such can share a single pulser. The transducers  24  produce a longitudinal wave at a fixed angle of zero degrees normal to the surface of the wedge  26  on which the transducers  24  are mounted. Groups of the transducers  24  can be pulsed simultaneously or multiplexed to simulate movement in the axial direction. The entire group of transducers  24  can be rotated to provide radial coverage along a loci  40  of points  36  ( FIG. 5 ).  
         [0023]     According to one aspect of the invention, the position of the transducer unit  22  for proper orientation of the transducers  24  is determined so that the ultrasonic signals generated by the transducers  24  intersect axial-radial type defects throughout the volume of the wheel  10  between the bolt holes  20  and the hub bore  16 , as shown in  FIGS. 5 and 6 . In  FIG. 5 , the center-to-center distance between each bolt hole  20  and the hub bore  18  is identified as a constant distance R along a radial of the wheel  10 .  FIG. 5  further represents an ultrasound signal transmitted from one of the bolt holes  20  along a path  38  that is perpendicular to a second radial  34  of the wheel  10 , intersecting the radial  34  at a point  36  located a distance d from the center of the bolt hole  20  and located a distance r from the center of the hub bore  18 . The distances R, d and r define sides of a right triangle with an angle of ninety degrees between the sides corresponding to the radial  34  and signal path  38  (distances r and d, respectively), an angle α between the sides corresponding to the signal path  38  and the radial between the bolt hole  20  and the hub bore  18  (the distances d and R, respectively), and an angle θ between the sides corresponding to the radial  34  intersected by the signal and the radial between the bolt hole  20  and the hub bore  18  (the distances r and R, respectively). With this relationship, r=R sin α and d=R sin θ. The triangle depicted in  FIG. 5  is just one of essentially an infinite number of triangles defined by ultrasonic signals intersecting an infinite number of radials  34  oriented at different angles θ from the radial through the hub bore  18  and the bolt hole  20  from which the signals are broadcast. The locus  40  of points  36  where the sound beam from the bolt hole  20  intersects these radials  34  of the wheel  10  at ninety degrees (perpendicular) is defined by the following equation:
 
cos 2  θ+cos 2  α=1
 
         [0024]     The above equation defines the inspection coverage because at these positions the sound beam is perpendicular to a defect with orientation in the axial-radial plane. The transducer position within the bolt hole  20  is determined by physically rotating the transducer unit  22  within the hole  20 . For example, the unit  22  is initially positioned in the hole  20  to maximize the reflection from the hub bore  18 , and then rotated to follow the locus  40  of points  36  identified by the above equation. The maximized reflection from the bore  18  can be used to calibrate the inspection so indications can be sized relative to an equivalent flat bottom hole (FBH) reflector, as discussed below.  
         [0025]      FIG. 6  plots multiple loci  40  of points  36  resulting from ultrasonic signals transmitted from over half of the bolt holes  20  of the wheel  10 , and evidences that these loci  40  of points  36  provide coverage of a substantial annular-shaped volume  42  of the wheel  10  lying between the hub bore  18  and the circle along which the bolt holes  20  lie. The volume  42  is defined by only those loci  40  of points  36  established by perpendiculars between the bolt holes  20  and the wheel radials  34 . With sound beam spread that inherently occurs, the volume  42  of the wheel  10  covered by the loci  40  can be increased slightly depending on the particular transducer parameters used. In any event, the volume  42  of material that can be inspected positively coincides with the region within the wheel  10  in which critical defects typically occur, as represented in  FIGS. 6 and 7 .  
         [0026]     According to another aspect of this invention, a method is provided for calibrating the amplitude of reflected sound to standard calibration reflectors, known as flat bottom holes (FBH). The method involves establishing a predetermined amplitude response from an ultrasonic signal reflected from the wall of the hub bore  18 , which permits a relationship between the indication amplitude and the FBH size to be calculated using the following equation:
 
 A   f   =λd   2   P   f ( b/R ) 1/2 /(2( R−b ) P   R )
 
 where A f  is the area of the FBH, λ is the wavelength of sound in the turbine wheel  10 , d is the sound path distance between the transducer  24  and an indication (defect), P f  is the amplitude of the reflected ultrasonic signal from the indication, b is the radius of the hub bore  18 , R is the distance between the centers of the hub bore  18  and the bolt holes  20 , and P R  is the amplitude of the reflected calibration ultrasonic signal from the hub bore  18 . This equation is derived as follows. The sound pressure along the axis of one of the transducers  24  is:
 
 P=p   0 2 sin(Π(( D/ 2) 2   +d   2 ) 1/2   −d )/λ)
 
 where P is the on-axis pressure in the direction of sound travel, p 0  is the original pressure at the material surface, and D is the diameter of the transducer&#39;s disc-shaped oscillator (J. Krautkramer et al., Ultrasonic Testing of Materials, 4 th  edition, Springer-Verlag (1990), p. 70). For large values of d and D/λ, and for non-circular transducers, this may be simplified as
 
 P=p   0   A/λd 
 
 where A is the area of the oscillator (J. Krautkramer et al. at p. 71). Using this relationship, the sound pressure measured from the large bore reflection can be expressed as:
 
 P   R   =p   0   A   T ( b/R ) 1/2 /2λ( R−b )
 
 and the sound pressure measured from a flaw is given by:
 
 P   f   =p   0 ( A   T   /λd )×( A   f   /λd )
 
 where A T  is the area of the oscillator and A f  is the area of the flaw. Solving the last two equations for A f  yields the above-identified expression for determining the area of an equivalent flat bottom hole (FBH), namely,
 
 A   f   =λd   2   P   f ( b/R ) 1/2 /(2( R−b ) P   R )
 
         [0032]     In an alternative embodiment of the invention, the linear array transducer unit  22  represented in  FIGS. 2 and 3  is replaced with a phased array transducer unit  122  represented in  FIG. 4 . As the term phased array is conventionally understood in the art, the transducer unit  122  comprises a series of individual ultrasonic transducers  124  arranged in a row and acoustically isolated from each other. Contrary to the linear array unit  22  of  FIGS. 2 and 3 , each transducer  124  has its own electrical connection and pulser (not shown), and each produces its own radio-frequency time/amplitude response (A-scan), which can then be summed and graphically displayed. The angle of the ultrasonic beam  138  generated by each transducer  124  is varied (steered) as well as the mode and focus of the beam  138  by controlling the timing of the pulse and reception for each individual transducer  124 . The advantage of using the phased array transducer  122  in the inspection method of this invention is the ability to focus the ultrasonic signal at a specific target  136  at a specific depth. This focusing of the sound beam  138  counteracts the negative effect that the curvature of the bolt holes  20  has on the sound field, which would disperse the sound beam  138  and reduce the sound amplitude reflected from a defect.  
         [0033]     The phased array transducer unit  122  represented in  FIG. 4  comprises eight transducers  124 . The actual number of elements (N) used with the method of this invention will vary depending on the spacing between elements (k), the wavelength (λ) of sound in the metal, the sound beam divergence angle (α) to the target  136 , and the active length (L) of the transducer unit  122 . This relationship is given by the following equation:
 
 N= ( L+k ) sin α/(0.44λ +k  sin α)
 
         [0034]     The optimum length (L) of the unit  122  can be determined by solving the following equation for L using the maximum distance (d) from the center of the bolt hole  20  to the target  136  and the radius (r) of the bolt hole  20 :
 
 d   2   =r   2 −2 d r  cos( L/ 2 r )− d   2  sin 2 ( L/ 2 r )/ sin 2  α=0
 
         [0035]     The calibration of the phased array transducer unit  122  of  FIG. 4  can be accomplished using a Distance-Gain-Size (DGS) technique that relates the amplitude of reflected sound from the hub bore  18  of the turbine wheel  10  to the amplitude response from known size flat bottom holes (FBH) at varying distances from the unit  122 . A sample DGS diagram for flat and parallel sided geometry is provided in  FIG. 8  as an example of the method. The backwall response in this case would be replaced by the reflection from the turbine wheel  10 . The DGS diagram can be obtained through computer modeling of the sound field responses or can be determined empirically using geometrically equivalent calibration blocks containing machined FBH reflectors.  
         [0036]     While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configurations of the units  22  and  122  and of the article (e.g., wheel  10 ) being inspected with the units  22  and  122  could differ from that shown. Therefore, the scope of the invention is to be limited only by the following claims.