Patent Publication Number: US-2012043962-A1

Title: Method and apparatus for eddy current inspection of case-hardended metal components

Description:
BACKGROUND OF THE INVENTION 
     Embodiments of the present invention relate to the inspection of components using eddy current technology and, more particularly, to apparatus and methods for inspecting components having a complex geometric shape. 
     Metal components, and particularly steel components such as gears, shafts, mechanical joints, and the like, are often heat treated to produce a hardened layer penetrating some small depth into the surface of the component to improve strength and resistance to wear. This is commonly referred to as “case-hardening.” Measuring the case-hardened depth profile is important for quality control to prevent part wear and breakage. 
     One known quality control method involves sectioning sample components and performing micro-hardness mapping in order to validate the case-hardening process. This process is time consuming and costly, and components can still have undetected defects. 
     Another quality control method involves using eddy current inspection. It is usually used to detect discontinuities or flaws on the surface of a component. Eddy currents are induced within the component under inspection by alternating magnetic fields created in a drive coil of a probe placed in close proximity to the component. Changes in the flow of eddy currents are caused by the presence of a discontinuity or a crack in the test specimen. The altered eddy currents produce a secondary magnetic field which is received by a sense coil in the eddy current probe which in turn converts the altered secondary magnetic field to an electrical signal which may be recorded for analysis. 
     There are also commercial instruments available to measure the case depth of cylinders or similar shapes using eddy current coils which encircle the test specimen. However, such instruments can only determine the average case depth of the whole part or only an average along a circumferential direction. Such equipment cannot determine local case depth values. Further, if the test objects are of complex shape and cannot be encircled by the probe, they cannot be measured using these commercial probes. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other drawbacks of the prior art are addressed by the present invention, embodiments of which provide a non-destructive method to measure the local case depth of surface-hardened steel components using eddy current techniques. “Case depth” refers to the depth of a hardened surface layer of a component comprising the hardened surface layer on top of a less-hard (e.g., unhardened) layer. 
     An embodiment of the invention relates to a method for determining a case depth of a hardened layer on a surface of a metal object. The method includes placing an eddy current probe in a selected location on the surface. The eddy current probe is used to generate a time-varying eddy current in the object. Using the eddy current probe, the time-varying eddy current is measured, and a signal representative of the measured time-varying eddy current is provided to a computer. Using the computer, the measured time-varying eddy current is compared to a correlation of measured eddy currents to known case depths. The method further includes determining the case depth at the location of the probe based on the correlation. 
     According to another aspect of the invention, an apparatus for determining a case depth at a location on a surface of a metal object includes an eddy current probe, a computer, and signal processing equipment. The eddy current probe includes at least one drive coil and at least one sense coil. The signal processing equipment is operably connected to the computer and the eddy current probe. The signal processing equipment is operable to drive the at least one drive coil in response to the computer and to generate output signals representative of measured eddy currents produced by the at least one sense coil. The computer is programmed to: (i) command the signal processing equipment to generate a time-varying eddy current in the metal object using the at least one drive coil; (ii) receive signals representative of a measured time-varying eddy current from the signal processing equipment; (iii) compare the measured time-varying eddy current to a correlation of measured eddy currents to known case depths; and (iv) determine the case depth at the location of the probe based on the correlation. 
     According to yet another aspect of the invention, an apparatus is provided for determining a case depth at a location in a surface of a metal object having a shape which comprises a plurality of teeth, each tooth having a land adjoined by spaced-apart flanks, wherein the flanks define recessed roots between adjacent lands. The apparatus includes a first housing, an eddy current probe, and a spring element. The first housing includes a body with at least one foot protruding there from. The at least one foot is configured to engage the flanks so as to retain the first housing in a stable orientation relative to the metal object. The eddy current probe is carried by the first housing, and comprises: a probe housing enclosing at least one drive coil and at least one sense coil, and electrical cabling connected to the drive and sense coils. The spring element is disposed between the first housing and the eddy current probe and arranged to urge the eddy current probe away from the first housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a schematic view of an eddy current inspection system constructed in accordance with an aspect of the present invention; 
         FIG. 2  is a cross-sectional view of an “air-referenced” eddy current probe; 
         FIG. 3  is a cross-sectional view of a “standard-referenced” eddy current probe; 
         FIG. 4  is a cross-sectional view of an “side-by-side-referenced” eddy current probe; 
         FIG. 5  is a schematic cross-sectional view of an eddy current probe fixture constructed in accordance with an aspect of the present invention; 
         FIG. 6  is a schematic cross-sectional view of alternative eddy current probe fixture constructed in accordance with an aspect of the present invention; 
         FIG. 7  is a schematic cross-sectional view of another alternative eddy current probe fixture constructed in accordance with an aspect of the present invention; 
         FIG. 8  is a schematic cross-sectional view of yet another alternative eddy current probe fixture constructed in accordance with an aspect of the present invention; 
         FIG. 9  is a block diagram showing an eddy current case depth measurement process; and 
         FIG. 10  is a schematic view showing a variety of probe shapes positioned adjacent to an exemplary gear. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates schematically the components of an eddy current inspection system or apparatus  10  suitable for carrying out a method of the present invention for determining a case depth of a hardened layer in a surface of a metal object, according to an embodiment of the invention. The system includes a computer  12 , an eddy current probe  14 , and signal processing equipment  16  operably interconnected between the computer  12  and the probe  14 . 
     As used herein the term “computer” includes any device capable of executing a programmed instruction set. For example, a conventional microcomputer (sometimes referred to as a personal computer or “PC”) may be used. To provide portability, a “laptop”-type computer may be used. Alternatively, the computer may be a microprocessor or microcontroller-based device that is built in or otherwise integrated with other components. As explained in more detail below, the computer  12  is used for functions such as transfer function computation, case depth calculation, and signal display. 
     The signal processing equipment  16  may include, for example, a digital-to-analog (D/A) converter  18 , a power amplifier  20 , signal preconditioner circuit(s)  22 , and an analog-to-digital (A/D) converter  24 , all of which are depicted functionally, with the understanding that known types of hardware are commercially available to perform each of these discrete functions. The signal processing equipment  16  serves as a substitute for a conventional stand-alone eddy current (“EC”) instrument. 
       FIG. 2  illustrates the general configuration of an eddy current probe  14 , which is suitable for measuring case depth, in particular for curved surfaces, such as gear teeth. The probe is shown disposed on to the surface “S” of a test specimen “T.” As used herein, the term “test specimen” refers generally to any metallic component having a case-hardened surface, which is to be tested for determining case depth, hardness, or otherwise. Examples of particularly suitable test specimens include turbine rotor dovetail slots, gear teeth, and the like. The probe  14  has a probe housing  26  that encloses two sets of reflection coils  28  and  30 , connected differentially in a known manner. The size and shape of the probe housing  26  may be varied to suit a particular application. In the illustrated example, each set of reflection coils  28  and  30  includes a generally cylindrical driver coil  32  surrounded by a generally cylindrical pickup coil  34  (also called a “sense” coil). However, the coils could be rectangular or “U”-shaped as well. The sense coil  34  and the driver coil  32  are aligned coaxially. Optionally, a ferrite magnetic core  36  is disposed inside the driver coil  32  to improve the coil sensitivity at low frequencies. A multi-conductor cable  38  provides a connection to the signal processing equipment  16 . In use, the coil sets  28  and  30  may be placed in tangential or normal orientations to the surface S. 
     It is also possible to vary the shape of the eddy current probe depending on the geometry of the components under test.  FIG. 10  illustrates several different probe shapes, including representative cylindrical probes  31  positioned both normally (a primary axis of the probe is perpendicular to the component surface) and tangentially (a primary axis of the probe is parallel to the component surface) to a gear “G” having a number of spaced-apart teeth, adjacent to the tooth pitchlines “P” and roots “R,” and two different types of conformally-shaped probes  33  and  35  positioned adjacent to the roots R. 
     As shown in  FIG. 2 , the probe  14  is “air-referenced,” meaning one set of the reflection coils  30  is placed at the tip  40  of the probe housing  26  which would be placed in contact with or adjacent to the surface S during testing, while the other reflection coil set  28  is positioned a substantial distance away from the tip  40  (and thus a substantial distance from the surface S). The total output from the probe  14  is the difference between the two sense coils  34 . Thus, the induction voltage directly coupled from the driver coils  32  is cancelled when the probe  14  is in ambient air, distant from the test specimen T. With the tip  40  touching the surface S, one set of coils  30  will be on the test specimen T and the other set  28  will be located away from the test specimen T. In this situation the differential voltage output of the two sense coils  34  reflects the test specimen&#39;s presence and variation. 
     The probe  14  may be referenced in different configurations to suit a particular application. For example,  FIG. 3  shows a “standard-referenced” probe  114  similar in construction to the probe  14  with two coil sets  28  and  30 , but incorporating a standard  42  of an alloy which is similar to or the same as the test specimen T, but which lacks a hardened surface. The standard  42  is located in the probe housing  126  adjacent to the coil set  28 . This type of referencing allows the probe  114  to “see” the difference between a test specimen T and the standard  42 . This configuration reduces the common mode signal and allows one to increase the preamplifier gain on the instrument, thus increasing the signal to noise ratio, reducing varying signal components due to temperature change, etc., and increasing sensitivity to the case depth variation than the air-referenced probe  14  described above. 
     Another embodiment is shown in  FIG. 4 . A “standard-referenced” probe  214  is similar in construction to the probe  114  with two coil sets  28  and  30 , and includes a standard  42 . The coil set  30  and the standard  42  are mounted side-by-side near the tip  240  of the probe housing  226 . The other coil set  28  is mounted above the standard  42  and next to the coil set  30 . The side-by-side coil configuration keeps the coil sets  28  and  30  at similar proximity to the test specimen T. Any heat transfer from the test specimen T to the probe  214  will heat the coil sets  28  and  30  almost identically, which will minimize probe signal drift due to temperature change. 
     It is particularly desirable to measure the case depth of components such as dovetail slots in turbine rotors, gear teeth in turbine gear sets, and the like. Such components commonly include a surface having alternating peaks (or lands) and valleys (or recesses). For example,  FIG. 5  illustrates a portion of a gear G having a number of spaced-apart teeth, each having a top land “L” and flanks “F.”  FIGS. 5-9  illustrate some specific fixture configurations that are particularly useful for gear tooth case depth evaluation. 
       FIG. 5  shows a probe fixture or apparatus  300  having a first housing  302  that is generally in the shape of an inverted “U” and that includes a body  304  and a pair of spaced-apart protruding feet  306  and  308 . One of the feet  306  is generally wedge-shaped with opposed front and back faces  310  and  312 . (“Wedge-shaped” means trapezoidal, and/or presenting two opposed, non-parallel surfaces/faces.) A recess  314  is formed in the front face  310 . Biasing means or device such as the illustrated elastic block  316  are positioned in the recess  314 . An eddy current probe  318  is disposed in the recess  314 , encapsulated by the elastic block  316 . It will be understood that the probe  318  may be constructed like any of the example probes  14 ,  114 , or  214  described above. A spring element  320  such as the illustrated coil spring or an elastic member biases the probe  318  outwards. Optionally, small resilient pads or bumpers  322  may be provided on the bottom edges or surfaces of the feet  306  and  308 . A multi-conductor cable or other electrical cabling  324  provides a connection to external electrical equipment, such as the signal processing equipment described above and/or to a switch  326  which may be used to trigger various data operations. The probe fixture  300  is used by placing the feet  306  and  308  between teeth of the gear G. The action of the elastic block  316  (biasing device) forces the foot  306  into a position of firm contact with the flank F of the gear teeth. Simultaneously, the spring element  320  urges the probe  318  into firm contact with the flank F. This places the probe  318  in a known position relative to the gear G and in solid contact therewith. 
       FIG. 6  shows another probe fixture or apparatus  400  including a first housing  402  having a body  404  and a wedge-shaped foot  406  protruding from a lower surface  408  of the body  404 . The foot  406  has opposed front and back faces  410  and  412 . A recess  414  is formed in the body  404  and an eddy current probe  418  is disposed in the recess  414 . The probe  418  may be constructed like any of the example probes  14 ,  114 , or  214  described above. The probe  418  is able to translate along its longitudinal axis. A spring element  420  such as the illustrated coil spring or an elastic member biases the probe  418  outwards. A biasing means or device such as the illustrated elastic block  422  is positioned on the back face  412 . A multi-conductor cable or other electrical cabling  424  provides a connection to external electrical equipment, such as the signal processing equipment described above and/or a switch  426  which may be used to trigger various data operations. The probe fixture  400  is used by placing the foot  406  between two teeth of the gear G. The action of the biasing means or device  422  forces the foot  406  into a position of firm contact with the flank F of the gear tooth. Simultaneously, the spring element  420  urges the probe  418  into firm contact with another portion of the gear G, such as the flank F of the adjacent gear tooth. 
       FIG. 7  shows another probe fixture or apparatus  500  including a first housing  502  that defines a reference surface  508 . A wedge-shaped foot  506  with opposed front and back faces  510  and  512  protrudes from the reference surface  508 . A recess  514  is formed in the foot  506  communicating with the front face  510 , and a probe  518  is disposed in the recess  514 . The probe  518  may be constructed like any of the example probes  14 ,  114 , or  214  described above. The probe  518  is able to translate towards or away from the front face  510 . A spring element  520  such as the illustrated coil spring or an elastic member biases the probe  518  towards the front face  510 . Biasing means or device such as the illustrated leaf spring  522 , a coil spring, or an elastic block is positioned on the back face  512 . A multi-conductor cable or other electrical cabling  524  provides a connection to external electrical equipment, such as the signal processing equipment described above and/or a switch  526  which may be used to trigger various data operations. The probe fixture  500  is used by placing the reference surface  508  against a portion of the gear G, such as the top lands L. The foot  506  protrudes between two adjacent gear teeth. The action of the biasing means or device  522  forces the foot  506  into a position of firm contact with the flanks F of the gear teeth. Simultaneously, the spring element  520  urges the probe  518  into firm contact with the flank F. 
       FIG. 8  shows yet another probe fixture or apparatus  600  including a first housing  602  that defines a reference surface  608 . A probe  618  similar in construction to the probes  14 ,  114 , or  214  described above protrudes from the reference surface  608 . The probe  618  is mounted on an axis  616  so that it can pivot towards or away from a forward end  610  of the housing  602 . A spring element  620  such as the illustrated coil spring or an elastic member biases the probe  618  towards the forward end  610 . A stop  628  protrudes from the reference surface  608  between the forward end  610  and the probe  618 . A means or device such as the illustrated threaded rod  630  is provided for adjusting the longitudinal position of the stop  628 . A multi-conductor cable or other electrical cabling  624  provides a connection to external electrical equipment, such as the signal processing equipment described above. The probe fixture  600  is used by placing the reference surface  608  against a portion of the gear G, such as the top lands L of the gear teeth. The fixture  600  is then translated until the stop  628  makes firm contact with another portion of the gear G, such as the illustrated flank F of the gear tooth. 
       FIG. 9  is a block diagram showing an eddy current case depth measurement method or process, which may be carried out using the apparatus described above, according to an embodiment of the invention. As a first step, a set of calibration samples having a known case depth is provided. The calibration samples are made of the same alloy as the test specimens to be inspected, have the same local geometry as the test specimens, and are heat treated to the same condition. For example, if a steel gear or similar component is to be tested, several steel calibration samples each representative of the profile of a single tooth or several teeth of the gear may be used. An example of a portion of a gear is shown in  FIG. 5 . The case depth of the calibration samples may be determined by destructive methods such as indenter tests or by sectioning and micro-hardness mapping. At block  1000 , the probe  14  is placed on each of the calibration samples, an eddy current is generated, and a measured eddy current is recorded. 
     The basic process of obtaining a “measured eddy current” using the inspection system  10  is the same both for calibration samples and for actual test specimens, and will described in general. Initially, a digital signal generated by the computer  12  and processed through the D/A converter  18  and power amplifier  20  (see  FIG. 1 ) is used to excite drive coils  32  of the eddy current probe (see  FIG. 2 ), while the probe  14  is contacting or adjacent to a location on a surface of a metal object (e.g., the calibration sample or the test specimen). As a result, an eddy current is generated in the metal object. Depending on the particular application, the induced current may be continuous or it may be an essentially short-duration pulse of electrical current. (In an embodiment, “short-duration” means an on portion of the pulse (current&gt;0) is up to and including 300 ms in duration; in another embodiment, “short-duration” means the total period of each pulse (current=0 and current&gt;0) is no more than 300 ms in duration.) The signal generation may be triggered by operation of the switches  326 ,  426 ,  526 , or  626  described above. 
     In an embodiment, the inspection system  10  is operated in a “burst” mode. In this mode, the drive coils  32  of the probe  14  are driven only for a short time window when taking a measurement. The output of the probe  14  is sensitive to temperature, and drive current passing through the drive coil  32  heats the probe  14 . Limiting the time of operation reduces the heat generated in the probe  14 , thus limiting the probe&#39;s temperature rise. For example, the “on” time may be limited to a significant temperature rise of about 0.5° C. (0.9° F.) or less. This burst mode operation avoids the typical long time required for a probe  14  to warm up and achieve a stable temperature when operated continuously. The driven or “on” time is selected based on the signal frequency to give a few cycles of excitation. The latency time until the next burst may be 10 times that of the “on” time as an example. 
     The sense coils  34  sense the eddy current as a voltage. For example, an eddy current might produce a signal ranging from +500 mV to −500 mV in the sense coils  34  for a particular test specimen. It is noted that a sense coil  34  that measures eddy current may produce either a voltage or a current indicative of the eddy current. Therefore, “a measured eddy current,” as used herein, includes any measured representation of the eddy current, whether the representation is in the form of a voltage, a current, or a digitized value. The measured eddy current signal is processed through the signal preconditioner  22  and A/D converter  24  (see  FIG. 1 ) and subsequently passed to the computer  12  as a digitized signal representative of the measured eddy current. The digitized value may be stored in volatile or nonvolatile memory of the computer  12  for further processing. 
     Referring back to  FIG. 9 , at block  1002 , once the calibration samples have been measured, the measured eddy currents are correlated to the known case depth values. The correlation is embodied in a calibration curve that is generated using a stored program in the computer  12 , showing the relationship between output and sample case depth. The curve can take the form of a stored table of eddy current measurements and corresponding case depth values (i.e., a “lookup table”) or graphical representation in memory of the computer  12 . The substance of the calibration curve can use a piecewise linear algorithm, linear fitting, or any other known technique suitable for characterizing the correspondence between the measurements and the known case depth values. The result is a transfer function of the type y=ƒ(x) (see block  1004 ). 
     Once the transfer function is created and stored, the probe  14  is placed on a test specimen (for example a gear G) and measured eddy current values are generated as described above and provided to the computer  12  (block  1006 ). The measured eddy current values are then fed to the transfer function (block  1008 ) and a case depth of the unknown test specimen T is calculated or otherwise determined using the transfer function. Case depth output (block  1010 ) can be displayed in real time and/or saved in a data file and/or printed. In practice, a fixture such as the fixtures  300 ,  400 ,  500 , or  600  described above may be used to take multiple local case depth measurements at selected spaced-apart points on a component and to compare the measurements against the component&#39;s manufacturing specifications, thereby verifying the quality of the case-hardening process. 
     In addition to determining the case depth as described above, embodiments of the invention described above may be used to obtain a hardness profile (that is, a graph or other representation of the hardness measurement versus the depth from the surface S of a test specimen T). Portions of a test specimen T having different hardness will react differently depending upon the frequency of the current driving the drive coils  32 . By testing a set of calibration samples each having a known hardness, using a range of drive current frequencies, a calibration curve and transfer function for hardness can be developed as described above for the case depth calibration curve. The calibration curve may be embodied in a stored lookup table. Test specimens T may then be tested using a range of drive current frequencies to generate eddy currents in the object having a plurality of selected frequencies and to generated a measured eddy current. The transfer functions are then applied to the measured eddy currents to determine both case depth and hardness at each depth. This multi-frequency testing may be done by performing sequential measurements on the same test specimen T using different frequencies, or by simultaneously applying multiple drive frequencies and sensing the response for each frequency. 
     An embodiment relates to an apparatus for determining a case depth at a location in a surface of a metal object. The apparatus includes a first housing having a body with at least one foot protruding therefrom, the at least one foot configured to engage the flanks so as to retain the first housing in a stable orientation relative to the metal object. “Stable” orientation is defined as the at least one foot engaging the flanks at three or more non-colinear points (i.e., not all the points are in the same line) and/or at two or more non-colinear lines. 
     The foregoing has described embodiments of apparatus and methods for eddy current inspection of components having a complex geometric shape. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the embodiments of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims. 
     In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” “third,” “upper,” “lower,” “bottom,” “top,” “up,” “down,” etc. are used merely as labels, and are not intended to impose numerical or positional requirements on their objects. As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property.