It is anticipated, that through exposing some of the problems encountered in industrial applications of eddy current technology, technical assistance may be generated through the theoretical community. The specific problem examined involves the development, storage\u27and utilization of calibration masters. Although the use of probes versus coils greatly reduces the number of calibration masters in that probes are not sensitive to overall geometry and mass variations, the eddy current response is influenced by near probe geometry variations. At present, an empirically derived algorithm is relied upon to characterize this phenomenon although it is hoped that an appropriate theoretical model will be developed. In addition to geometry, the non-linear permeability present in ferromagnetics, and the microstructural and compositional variations greatly increase the complexity of constructing such a model

Jeskey, G. V.

Vaccaro, F. P.

English

It is anticipated, that through exposing some of the problems encountered in industrial applications of eddy current technology, technical assistance may be generated through the theoretical community. The specific problem examined involves the development, storage'and utilization of calibration masters. Although the use of probes versus coils greatly reduces the number of calibration masters in that probes are not sensitive to overall geometry and mass variations, the eddy current response is influenced by near probe geometry variations. At present, an empirically derived algorithm is relied upon to characterize this phenomenon although it is hoped that an appropriate theoretical model will be developed. In addition to geometry, the non-linear permeability present in ferromagnetics, and the microstructural and compositional variations greatly increase the complexity of constructing such a model.</p

Jeskey, G.

Vaccaro, F.

Digital Repository @ Iowa State University (ISU)

Industrial Problems Encountered in the Eddy Current Inspection of Steel

Jeskey, G

Vaccaro, F

INDUSTRIAL PROBLEMS ENCOUNTERED IN THE EDDY CURRENT INSPECTION OF STEEL G. V. Jeskey, F. P. Vaccaro The Timken Company Canton, OH 44706 ABSTRACT It is anticipated, that through exposing some of the problems encountered in industrial appli-cations of eddy current technology, technical assistance may be generated through the theoretical community. The specific problem examined involves the development, storage'and utilization of cali-bration masters. Although the use of probes versus coils greatly reduces the number of calibration masters in that probes are not sensitive to overall geometry and mass variations, the eddy current response is influenced by near probe geometry variations. At present, an empirically derived algo-rithm is relied upon to characterize this phenomenon although it is hoped that an appropriate theoretical model will be developed. In addition to geometry, the non-linear permeability present in ferromagnetics, and the microstructural and compositional variations greatly increase the com-plexity of constructing such a model. There are two kinds of talks one encounters at technical conferences such as this one, those which expose and those which solve problems. This presentation will be of the former type and will attempt to elucidate in a more de-tailed fashion some of the difficulties encountered in the eddy current materials sorting of steel. It is hoped that con-structive feedback will be generated from the theoretical community, to the benefit of those of us in industry. The specific problem ex-amined today will involve the influence of steel component curvature on eddy current response. It might be helpful to review exactly how the influence of component curvature on probe response became a concern and how the problem is being dealt with presently. The important theoretical factors in developing a model descriptive of the phenomenon will be examined. Most of those present here should be familiar with the basic differences between through coil and probe eddy current inspection techniques. Yet the relative advantages for specific applications are not always appreci-ated. In Table 1 a detailed comparison is presented for the specific application of heat treatment materials sorting. Both techniques provide excellent sorting capabilities, are available in relatively inexpensive commercial systems and provide inspection speed capa-cities comparable to many production operations. The coil technique, however, does not focus on the component region of interest whereas the probe can be often located to examine the area of concern. In view of this insensitivity to overall geometry variations, the use of probes, in many applications, substantially reduces mastering requirements. 548 The most serious limitation in the use of probes involves the strict demands for probe-specimen orientation control, and the dependence of probe response on local component geometry variations. It is with this last point that we are primarily concerned in this presentation, namely the dependence of eddy current response on component curvature. The populations of eddy current response for four different carburized steel cylinders are shown in Fig. 1. In these data a comparison is being made between properly and improperly heat treated material. In directing our atten-tion to the influence of curvature, it is ob-served that the eddy current response increases statistically with diameter. This may be inter-preted as an increase in the penetration of flux and a corresponding increased interaction with the detection coil in the probe. A similar influence of curvature is observed for a hollow cone (see Fig. 2). The variation in probe response due to curvature introduces a mastering difficulty tending to decrease the advantage that probes enjoy over through coil techniques. This effect is presently accommodated through a normali-zation procedure in which empirically derived algorithms adjust the mastering response (see Figs. 3 and 4). The curves generated from these algorithms are shown in Fig. 5 in relation to the pertinent statistical data. From an engineering stand-point, this approach is satisfactory in estab-lishing sorting limits. However, through the development of a viable theoretical model, in-creased confidence, predictability, and the suppression of the amounts of statistical data required may be obtained. There are four major areas requ1r1ng consideration in constructing an appropriate theoretical model. They are: 1. The nature and distribution of the probe field. 2. The nature and distribution of the field generated eddy currents. 3. The construction of a mathematical model accommodating the specific boundary.conditions as well as the nature and distributions of field and eddy currents. 4. The influences of simplifying approxima-tions on the representative character of the proposed models. The nature and distribution of the field are dependent upon the specific probe charac-teristics. These characteristics include coil current, frequency, number of turns, coil area, orientation, and the influence of core material. In addition, the probe may be composed of mul-tiple coils for separating the field generating primary from the detecting secondary, or in a differential arrangement to suppress extraneous noise. The influence of probe characteristics could prove to be one of the most difficult considerations in any theoretical treatment. The origin, nature and distribution of the eddy currents will be dependent upon the field, the probe-specimen geometry and the electromagnetic properties of the steel speci-men. There are few theoretical attempts to explain eddy current phenomena in ferromag-netic, non-linear permeability (~) materials in contrast to linear, non-ferromagnetic con-ductors. Analytical solutions have been primarily restricted to assumed linear con-ditions. Utilizing the concepts of a linear re-lationship between the phasor quantities B and H and a representation of hysterisis through complex permeability (in which the B/H charac-teristic is elliptical through the introduction of a phase lag 8h between Band H), D. O'Kelly (1, 2, 3, 4) has calculated the flux distri-butions and the hysterisis and eddy current losses in steel plates (Fig. 6). The prin-ciples applied to the analysis of the funda-mental component of B and H were also applied to harmonics providing additional accuracy (2). One other popular approach involves the numeri-cal analyses of network models in which non-linear effects are accounted for in a descre-tized fashion (5, 6). For non-linear conditions, many numeri~al techniques have been utilized with digital computers to obtain solutions. However, most finite element computations have ignored hys-teresis effects or employed representation techniques. As one can see, there are several available theoretical approaches to choose from in modeling eddy currents while taking into account non-linear effects. 549 In the treatments above, it is commonly assumed that the material is homogeneous and isotropic. In actual eddy current inspection of steel, microstructural and compositional variations will greatly influence electromag-netic properties. These effects are especially pronounced when the process involves special compositional treatments such as carburizing, in which a hardened surface layer is created through the impregnation of carbon (Fig. 7). The resulting carbon gradient produces both a compositional effect and will significantly alter the microstructure. Carbon, both in solution and in precipi-tated forms·, reduces permeabi 1 ity ( ~) (Fig. 8) and the saturation induction B~ (Fig. 9) while the coercive force He (Fig. 10} and hysteresis loss (Fig. 19) are increased (7). Heat treat-ment and thermal history have a synergistic influence with composition on the electromag-netic properties. This synergism influences the electromagnetic properties in steels in the same manner as it controls the mechanical properties, namely through regulating micro-structure. Many of the variations in magnetic properties are the result of morphological differences and not simply base composition changes. As one might expect, these combined effects are more pronounced at higher carbon 1 evel s. The precise manner in which the electro-magnetic properties of steel depend upon compo-sition and microstructure may be classified under four headings: 1. The carbon and alloy content of the primary magnetic phases (i.e. martensite or ferrite). 2. The structure of the primary ferromagnetic phases. 3. The volume percent and distribution of non-ferromagnetic carbides. 4. The volume percent and distribution of re-tained non-ferromagnetic (paramagnetic) austen-ite. The influence of non-magnetic phases has been aptly described in "Metallurgy and Magne-tism" by James Stanley ( 7): "Non-ferromagnetic phases or inclusions such as graphite in cast iron or slag in wrought iron have an effect on the mag-netic properties similar to that produced by cutting an air gap in a solid magnetic ring." "A similar effect takes place when the gaps are internal which is in reality what happens when a non-magnetic phase or inclusion is present. This is shown in Fig. 43" (Fig. 12) "for the case of steel with different amounts of carbon (the cementite of the pearlite is rela-tively non-magnetic)." Although carbon is one of the major modi-fiers of electromagnetic properties it is by no means the only one. Silicon, aluminum and other alloying elements are known to reduce saturation values and permeability (Fig. 14). The influence of these alloy additions is also very dependent upon the heat treatment. Composition and microstructure will cor-respondingly affect electrical conductivity (Fig. 15). Increasing alloy additions, when in solution, will generally decrease conductivity. When microstructural factors become important, the effects become more difficult to predict. Nevertheless, the eddy currents generated will be affected through variations in conductivity. Extensive efforts have been made at Timken Research to determine the electromagnetic pro-perties of the steels of interest. This effort has included an examination of the influence of composition and heat treatment. Preliminary results may be qualitatively summarized as follows. In Fig. 16 the B - H characteristic curves for core and case structures are compared. The case displays substantially reduced magnetic properties. This decrease is either the result of high carbon content, high carbide content or the presence of retained austenite. Further exploration of this reduction in properties will be performed in order to understand the individual contributions of the carbon and re-tained austenite. The effects of over-tempering are presented in Fig. 17 for core and case respectively. The drastic increase in magnetic properties in the case is attributed to the decomposition of re-tained austenite into bainite, although the precipitation of carbides and the associated migration of the BCT martensite structure to the equilibrium BCC ferrite structure may also be playing a significant role. In the core, similar increases occur although in view of the originally high properties the improvement with over-tempering is not as dramatic. The increase observed in the core may be attributed to the precipitation of carbides and the corresponding formation of ferrite having characteristically high magnetic properties. These hysteresis curve effects need to be more fully understood as they depend upon alloy content and heat treatment. The frequency of inspection is also a parameter of great impor-tance in case carburized component inspection in view of the influence that frequency has on depth of penetration. Once the specific conditions of probe field, material properties and geometry are well de-fined, the development of a comprehensive mathe-matical model and the selection of appropriate approximations are all that remain. These two factors were indirectly examined in the previous discussions regarding the available analytical models. Any attempt at combining all of the 550 above considerations in constructing a theore-tical model for the effect of curvature on the eddy current response in case carburized steel may prove intractable. Nevertheless, continued research efforts are scheduled at The Timken Company and substantial benefits are antici-pated. For those engaged in similar efforts, or who may be able to provide insight into this complex problem, the development of future cor-respondence would be greatly appreciated by the authors. REFERENCES 1. D. O'Kelly, "Flux Penetration in a Ferro-magnetic Material Including Hysteresis and Eddy Current Effects", J. Phys. D: Appl. Phys., Vol. 5, 1972 page 203, Great Brit ian 2. D. O'Kelly, "Fundamental and Harmonic Flux Estimation in a Steel Plate with a Sinusoidal Applied Field", J. Phys. D: Appl. Phys., Vol. 10, 1977 page 2107, Great Britian 3. D. O'Kelly, "Hysteresis and Eddy Current Losses in Steel Plates with Non-Linear Magnetisation Characteristics", Proc. lEE, Vol. 119, #11, Nov. 1972 page 1675 4. D. O'Kelly, "Losses in Cylindrical Ferro-magnetic Cores Including Hysteresis and Eddy Current Effects", J. Phys. D: Appl. Phys., Vol. 8, 1975 page 568, Great Britian 5. N. A. Demerdash and D. H. Gillott, "A New Approach for Determination of Eddy Current and Flux Penetration in Non-Linear Ferromagnetic Materials" IEEE, Trans. Magn. Vol. MAG 10, #3, Sep. 1974 page 682-685 6. K. Oberretl, "Magnetic Fields Eddy Currents and Losses Taking the Variable Permeability into Account", IEEE, Trans, PAS, Vol. 88, 1979 page 1646-1657 7. J. K. Stanley, "Metallurgy and Magnetism", ASM, 1949, Cleveland, Ohio TABLE 1l COMPARISON BETWEEN COIL vs PROBE EDDY CURRENT MATERIAL SORTING' OF CASE CARBURIZED CYLINDERSl CATEGORY TECHNIQUE ADVANTAGES DISADVANTAGES COIL GOOD SORT FOR MANY APPLICATIONS AMBIGUITIES EXIST FOR SOME HEAT TREAT SORTING CONDITIONS CAPABILITY GOOD SORT FOR MANY APPLICATIONS AMBIGUITIES EXIST FOR SOME HEAT TREAT PROBE CONDITIONS COIL INEXPENSIVE COMMERCIAL SYSTEMS AVAILABILITY AVAILABLE; AND EXPENSE INEXPENSIVE COMMERCIAL SYSTEMS PROBE AVAILfiBLE COIL WHOLE PART EXAMINED DOES NOT FOCUS IN AREA OF INTEREST AREA OF EXAMINATION PROBE FOCUSES ON AREA OF INTEREST RESTRICTED TO EXAMINING ONLY ONE AREA OF PART COIL NOT CRITICALLY SENSITIVE TO PART PART-PROBE POSITION IN COIL ORIENTATION LIFT OFF AND PART PROBE MUST BE PROBE CONTROLLED OR COMPENSATED FOR TAi!~E 1 (CONTINUED )j COMPARISON BETWEEN COIL vs PROBE EDDY CURRENT MATERIAL SORTING) . OF CASE CARBURIZED CYLINDERSl CATEGORY TECHNIQUE ADVANTAGES DISADVANTAGES CONTINUOUS. FAST, SIMPLE, COIL AMENABLE TO MANY HIGH SPEED HANDLING AND PRODUCTION LINES INSPECTION PROBE SOPHISTICATED HANDLING REQUIRED SPEED MULTI MODo/ COIL WOULD REQUIRE DISCONTINUOUS MULTI OPERATION, RESULTS IN REDUCED SPEED FREQUENCY CAPABILITY PROBE PRESENTLY EMPLOYED IN AVAILABLE SYSTEMS MASTERING COIL EASY CALIBRATION WITH REFERENCE REQUIRES SEPERATE MASTER FOR EVERY AND MASTER PART •.• INVENTORY NIGHTMARE CALIBRATION EASY CALIBRATION REDUCES MUST ACCOUNT FOR NEAR PROBE PROBE MASTERING INVENTORY SUBSTANTIALLY GEOMETRY VARIATIONS 551 w 150 "' z 0 a. ::l a: f-z w a: g; 100 u ,_ 0 il w > ;: :l w 50 a: ·o 20 40 NUMBER OF RESPONSES 2.5 3.0 DIAMETER 3.5 4.0 1.5 2.0 FIG. 1 EODY CURRENT RESPONSE FOR HOLLOW STEEL CYLINDERS LU Ul z 0 a. Ul LU a: 1-z LU a: a: :::J u >-Cl Cl LU LU > ;:: <t ..J LU a: 150 50 1.5 A AVERAGE (X) 2.0 2.5 3.0 3.5 4.0 DIAMETER (IN) fiG. 2 EDDY CURRENT RESPONSE FOR HOLLOW CONE (REGION OF VARIATION RESULTING FROM ECCENTRICITY IN CARBURIZED CONE CASE DUE TO DISTORTION IN QUENCHING ) UJ 150 APPROXIMATED ASYMPTOTE U) z 0 a. U) UJ UJ > a: i= 1-<( z UJ --' a: UJ a: a: :l 100 >- () >-0 0 w 50 1L.5 _____ 2 ___ o____ 2 ___ 5 _____ 3 __ ~o-----3-.5------4-.o-X• DIAMETER (IN) FIG. 3 PLOT OF ALGORITHM, Y = 117-178.6 exp ( -.95729X ). EMPIRICALLY DERIVED TO FIT X (.A.) 552 UJ U) z 0 a. w U) > UJ i= a: <( 1---' z w UJ a: a: >- a: :l (.) >-0 0 UJ 150 ----100 50 1.5 2.0 APPROXIMATED ASYMPTOTE 2.5 3.0 3.5 4.0 .X' DIAMETER (IN) FIG. 4 PLOT OF ALGORITHM,Y:135-163 exp (-.876G4x). EMPIRICALLY DERIVED TO FIT X+ 3 <Y (e) ~ z ~ w a: ... ~ 150 ~ 100 u ,_ 0 il w > ;: ~ a: 50-1.5 0 20 40 NUMOER OF RESPONSES 2.0 2.5 3.0 3.5 4.0 DIAMI:?:TEn FIG. 5 SUPEniMPOSITION OF CUnVES GENEnATED FnOM /\LGOniTIIMS ON EDDY CURnENT 0/\T/\ FIG. 6 IDEALIZED ELLIPTICAL REPRESENTATION OF HYSTERESIS) -, / I TYPICAL 8/H CURVE IDEALIZED ELLIPTICAL BIH CURVE ASSUMING SINUSOIDAL FLUX DENSITY B IDEALIZED ELLIPTICAL B/H CURVE ASSUMING SINUSOIDAL MAGNETIZATION FORCE H ( SEE REFS. 1-4) FIG. 7 COMPOSITIONAL AND MICROSTRUCTURAL EFFECTS IN CARBURIZATION z g tn t> w ::> "' c "' z ~ z Cl ~ tA "' .... ~ t: ... "' "' "' 24,0aO 22,000 20,000 18,000 16,aao " 0 2 0 CASE DEPTH FIG. 8 I Max. Permeability Vs. Corban 0-6% Silicon 20,000 1--1--+-0L_-L~--L-J-_L~~ 0 0.004 O.OOG 0.012 Carbon % CORE ( SEE REF. 7 ) FIG. 9 QUENCHED .4 .a 1.2 1.4 CARBON% 1.6 2.0 (sec ref. 7) FIG. 10 30.----.----.----.-----.----. Effect of Carbon on Coercive Force (l<oster) ~ ~ 20 1----t----:..t-<::::....--+---+---l 0<11 u.-;n ~ID ·-0 ~ Q> u I 0 1--IL---t---"""1----1~---+---l 8I 2500 .. ~ 2000 .... u ~0 ~ g 1500 w 0~ .,; . ~[!) 0 ~ ~ 1000 500 0 Carbon % ( SEE REF. 7) FIG. 11 I Carbon,w Effect of Small Pearlite Amounts of Carbon on Hysteresis I/ (Yensen) _ / ( Carbon as Cementite ( Fe,c) .:=:::Solid Solution of Corban in Ferri tc I Annealed 900-93?'C (1650-1715'F) 0 r. 0.0:> ,-0.1 0.1:> 0.2 Carbon% ( SEE REF. 7 ) FIG. 12 Steels Prepared From Electrolytic Iron and Graphite Deoxidized With Silicon 2~000 ~T~e~s,-s~~~la~d-e_o_n~N~o-rm-o~t1,--e~d~G~o~rb_o_n"s~r-ee~I~R~in_g_s+---------------~ 553 ~ Furnace-Cooled From 700'G (1290'F) After Machining ~ IG,OOO j-!-(~N::__o :.:' •_:L::o:.::g_:S::c:.:o~l e:_o::n.:_A:..:bfs:.:c_:i •:.:•::__o l:_ ________ -j-=====""":?:;;;::J ~ 0 "' ai c' 12,000 f---------------+-7""--,,..C.-.9 g ] 8,000 r------ ------r---1:t---------,."-0.1 10 Magnetizing Force, H,Ocrsteds 100 ( SEE REF. 7 ) 2o r2 ~ 18 " ~ ;:; 14 " ~ 10 > c .g ~ 6 0 Ul FIG. 14 -Co v --~~~- N; ' ~lAI~~ ~ ' E.c- Mn -£ ~~~ I' C(in Solution) _ 1--~ ii- j_ Jk 0- -gt-tit-1-~ FIG. 13 1380 60 Plain Carbon Steels, Water-Quenched c...----.1.75%C £ 20 1----1--;o.£:/-'-t---t--~ 1.33% ~ t.t6% I ~ =::::::::::.J:::::::-9· 9 7% <l: 0 0.57"/o '0.82% u "' c '.§ 40 "' a::1.33% Carbon Steel IOMM Diam. 800 900 1000 1150 1100 Quenching Temperoture°C {SEE REF. 7) FIG. 15 ........ Co ~ "' ' " ' v ~ N; '\ "' "' 0120 0 N 0 "'100 E u " o_ 80 E -C 0 -~ 60 2 I .. g ~ 40 "' -;;; " a:: 8 20 C(inSol) ~~ i .. ~ "'o c u _gUl u~ / 0 v / I ,..",.I SiJ7 -· - -I-//AI r '/ I v I -sn~~ ) r?/v v 'rr ~~~r 1/ Nl ~-~~ :::>- I:---f---- _ Mn (Quenched _ I-'J"m ?oo;ct-j -111-1---1--1- --- Seo~ Qetail -;: ~ 0 4 0 10 20 40 % Alloyinq Element 60 80 100 w 0 0 8 10 20 ~ ~~ ~1--1 ~ ~ AI ~~ -1\ - ..§i - -\ \ \ '\ \ w ' ~oboll lx ~ ~ Cu ---- -:::-40 60 80 100 % Allovinq Element (SEE REF. 7) (SEE REF. 7) 554 CORE-HARDENED AND TEMPERED B H CORE-OVERTEMPERED B H FIG. 16 CASE-HARDENED AND TEMPERED B 1-1 FIG. 17 CASE-OVERTEMPERED B H 555 

https://dr.lib.iastate.edu/handle/20.500.12876/58468

Industrial Problems Encountered in the Eddy Current Inspection of Steel

Abstract

Similar works

Full text

Available Versions

Digital Repository @ Iowa State University (ISU)

Digital Repository @ Iowa State University (ISU)