THESXS This is to certify that the thesis entitled A CORRELATION OF SOME OF THE MECHANICAL AND MAGNETIC PROPERTIES OF SAE 4140 STEEL LN TENSION presented by JEROME H. HEMMYE has been accepted towards fulfillment of the requirements for _bl_s___ degree in L 0.7: MM Major professor Date Ma re ‘1 8', M 9’5, 0-169 A CORRELATION OF SOME OF THE MEClit‘aNICAL AND MAGNETIC PROPERTIES OF SAE 1.11.0 STEEL IN TENSION By Jerome H. flemmye A THESIS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Mechanical Engineering 1955 Jerome H. Hemmye ABSTRACT There has been little work done to relate.the magnetic behavior of ferromagnetic materials with stress. This thesis is the record of the relationship found to exist between the mechanical and the magnetic be- havior of SAE 4140 steel. Standard ASTM tensile Specimens were loaded and measurements were hnflfi of the resistive and inductive components of the impedance of the test coil surrounding the Specimens. The final results were expressed in terms of the ratio of the initial value to the value under consider- ation because of the difficulty of reducing the measured values to ahso- lute units. The data indicate that there is a relationship between stress and the magnetic behavior of the steel. The relationship is determined by the heat treatment of the steel. 1. ACAN Ol-TLEILLMH ‘l' S The author wishes to express his sincere thanks to Doctor Rolhnui T. Hinkle for his encouragement and his advice. Professor Howard Vomochel has given freely of his time and knowledge of Metallurgy as well as providing a wealth of advice. The original idea and approach to the problem was due to Doctor Robert Jeffries. VITA The author was born May 14, 1927 in Burlington, Iowa. After graduation from high school he attended Burlington Junior College. He then transferred to Iowa State College at Ames, Iowa to study Electrical Engineering for a year before entering the United States Navy in April of 1945. He received his discharge from the navy in July of 1946 with the rating of Aviation Electronics Technician's Mate third class. He transferred to Michigan State College to study Mechanical Engi- neering in September of 1949 and received his Bachelor of Science degree in June of 1951. In September of 1951 he received a Graduate Assistant- ship at Michigan State College to continue his work in Mechanical Engi- neering. In September of 1953 he receiVed a full time appointment to the staff of the college as an instructor. i From January to June of 1949 the author worked in the engineering department of the Wood Brothers plant of The Dearborn Motor Corporation located in Des Moines, Iowa. His summer employment has included work with Capitol Steel Corporation, Atlas Drop Forge Company and Abrams Instru- ment Corporation, all of Lansing, Michigan. He is now a candidate for the degree of Master of Science in Mechanical Engineering. The author is a member of Pi Tau Sigma, associate member of The American Society of Mechanical Engineers and associate member of The Institute of Radio Engineers. He is Faculty Adviser to the Student Branch of The AmeriCan Society of Mechanical Engineers and to The Michigan State College Amateur Radio Club. II III IV Introduction Background. Apparatus and Methodology TABLE OF CONTENTS A. Bridge Equations. B. Accuracy and Error C. Specimens Presentation and Analysis Summary and Conclusions Appendix of Data 10 14 15 16 27 I Maxwe114Wein Bridge II_ Test Solenoid . . PLATES 11 13 II III IV FIGURES Specimens Drawn at 800 Degrees Fahrenheit 0 Specimens Drawn at 1,000 Degrees Fahrenheit. Specimens Drawn at 1,200 Degrees Fahrenheit. Specimens Stress Relieved Specimens Stress Relieved 0 17 18 19 23 INTRODUCTION The variation of the permeability of Steel when stressed has been recognized for years, having been investigated and described by Villari as early as 1864. Very little work has been done, however, on the effects of strain on the resistivity of steel. The use of magnetic methods for the determination of the mechanical preperties of steel has been suggested in the hope that a magnetic examp ination of the steel under stress would provide some information as to the condition or suitability of the material without destroying the part or specimen. Work done in this field, to date, has resulted in the development of metal comparators and allied devices which compare the magnetic properties of an unknown Specimen with the magnetic properties of a Specimen of known mechanical properties. The majority of these applications have been limited to conditions of no external stress. The work presented here has been done on standard ASTM tensile Specimens with known mechanical properties in an attempt to show the results of externally applied strain on the magnetic properties of SAE 4140 Steel. BACKGROUND The classical techniques used for the determination of such mag— netic properties of steel as permeability, magnetization, hysteresis loss and resistivity have been based on the use of direct current, 60 cps or 1,000 cps analysis. In most cases this work was done with thin laminations or very small sections with the result that the behavior of the complete cross section of the Specimen was determined. The net result to date, of the work done in the past, has been adequate for magnetic circuit design and for the development of metal comparators which compare the magnetic properties of one specimen directly with those of another of known mechanical properties. The majority of these applications have been limited to conditions of no external stress. In order to understand the behavior of a magnetic material under stress, an understanding of some of the basic theories was found to be vital. Regarding magnetostriction, Williams wrote; If a ferromagnetic rod shows an increase in length due to a magnetic field, that same rod will show an increase in mags netization when stretched or a decrease when compressed longitudinally. If the rod shortens in a magnetic field a corresponding Villari effect ensues. For substances which show an increase in magnetization for weak fields, and a de- crease for strong fields, there is a critical field strength where the intensity is the same whether the rod is stretched or not. This is known as the Villari reversal point. Burrows extended this statement somewhat; ”There is a certain value of tension for which the induction is a maximum for a given field."2 Many magnetic properties vary in a manner best described by the domain theory. The modern theory of ferromagnetism attributes the ferro- magnetic effects to groups of electrons called'domains'which consist of electrons spinning on their own axes. The magnetic axes of the Spinning electrons in any one domain are held parallel to each other by mutual forces known as exchange forces so that each domain behaves as a single unit. These domains account for the magneto-motive forces inherent in ferro-magnetic materials. When the material is unmagnetized, the domains are arranged in various orientations so that the total magnetic effect is zero in any direction. Under the influence of an external field the magnet axes of the domains are more or less oriented in the direction of this field so that their effect is added to that of the applied field.3 These Spinning electrons or Spinning nucleuses have definite mag- netic moments. The electron, in addition to its Spin moment has a mo- ment of momentum and a magnetic moment due to its movement in its orbit. The total magnet moment of the atom is the vector sum of all the come ponent magnetic moments, that is, the magnetic moment of the positive charge Spinning on its axis, that of the negative charge Spinning on its axis and that produced by the negative charge moving in its orbit. The magnetic effect of the nucleus is of such a magnitude as to be negligible. When a rearrangement of the magnetic vectors occurs, the balance between the electric and magnetic forces is disturbed and as a result the physical dimensions can be observed to change. These domains are assumed to be magnetically saturated areas in the ferromagnetic body. The ease or difficulty encountered in magnet- izing the body is dependent upon the ease or difficulty with which these saturated domains may be rotated into the preferred direction. Recapitulating the previous statements, the magnetic behavior of a ferromagnetic material depends on the behavior of the domains within that material. If the domain is subjected to a magnetic field it changes its dimensions in adjusting to that field and similarly if the dimensions of a domain are changed, its individual field is affected. Footnotes 1. S. R. Williams. .lgggnal Cf the Qgtjggl figci§;y_gf Engzica. Vol. 14, pp- 383~408. 2. C. W. Burrows. united fitpte§_gf Amegicaunatignal,Burefig_p§ spggggggg Scientific Papers. Numb. 272, pp. 130. 3. Eresent fitgtup of‘Egrrgmagnetig Ihegry. Transactions of the American Institute of Electrical Engineers. Vol. 54 (1935), pp. 1251-1261. lO. APPARATUS AND MLTHUDOLUGY The first equipment constructed to measure the magnetic properties of the steel utilized a dual winding pancake type coil with the windings at right angles to each other. The initial air coupling between the coils was adjusted to a minimum and the Specimen was placed in the area common to the axes of both coils. The magnetic properties were measured in relation to the effects on the mutual inductance between the two coils. Several difficulties became apparent soon after the equipment was set up. The initial air mutual was difficult to adjust to zero and beCause the two coils were interwound, the capacitive coupling between the windings became large enough to be prohibitive. The second piece of equipment prepared consisted of a solenoid with two co-axial windings with a fixed value of initial mutual inductance. Again it was found that the system was impractical. The measurement of mutual inductance required a multitude of bridge manipulations, making the system unwieldy. Since relatively small changes in mutual inductance were being measured, the percentage of probable error became prohibitive and the second system.was discarded. The equipment finally used consisted of a Single solenoid placed around the specimen. The inductance was measured with a Maxwell-Vein bridge. (pg.ll) With the Single winding supported rigidly on the Specimen, it was possible to measure the inductive and resistive components of the solenoid reactance with the Specimen under load. ll. R2 A C VTW TEST SOLENOID / II II MATCHING HANS l l —— L J "O VOLT AG '0’“; REGULATED OSCILLATOR MAXWELL‘WEIN BRIDGE C“ A. The test solenoid (ngJZ‘ was 0.5 inches long, had 50 turns of number 22 formvar wire layer wound on a bakelite Spool 2.5 inches long. The inside diameter of the coil was .9 inches. The bakelite Spool was designed so that it could be slid over the tensile Specimen and supported rigidly on the support bushings with the solenoid directly over the re- duced section of the Specimen. The inductance of the test solenoid in free Space was 107.29 microhenrys. The resistance at 10,000 cps was 0.30 ohms. The solenoid was connected to the bridge by a parallel rubber covered flexible cable. The low inpedance bridge and solenoid were used to prevent,as much as possible, the influence of stray 60 cps fields on the system since the 10,000 Cps field produced was itself of very small magnitude. The passive arms of the bridge consisted of two non-inductive 10.0 ohm resistances. The adjustable arm of the bridge was a capacity decade paralleled by a resistance decade. The interconnecting wires of the bridge were made of number 10 solid copper wire to prevent any variation of the distributed capacity. The bridge galvanometer was a high impedance vacuum tube audio frequency voltmeter. The source of the 10,000 cps voltage was an audio frequency generator with a low harmonic output. The audio frequency generator had an output impedance of approximately 6000 ohms, whereas the input impedance of the bridge was approximately 10 ohms, so it was necessary to use a matching transformer between them. The matching transformer used was of oversize proportions to insure operation with a minimum of distortion introduced by the transformer. 50 TURNS 322 FURMVAR L |~.. l—ua—j BAKELITE FORM AND SOLENOID ASSEMBLY L BAKELITE SOLENOID SUPPORT auswme ll. Bridge Equations Arn.impedances 1‘: R, I"? jQR'w 315' R: lhg==‘?4. For bridge balance the following equations must hold 3:. .. £1. 32 - 33 01' 2.23:2..2‘ R! o ’ Thati 5 (”ch'mXRwaI-ala 329+ RI R3 *3 WLIRI ' R: R, 0'4“” CIR0R3R1. Separating real and imaginary terms R | R 3 = R 2. R4 and l.3li.== ChfanHLIIQ So that the resistive component R=%ba (if ohms and the quadrature component 13‘0‘3 Roslooqhenrys. Therefore the reactance Ya GINO. 6: ohms Accuracy and Error In making the measurements of inductance and resistance of the Specimens, the capacitive bridge element had the largest calibration error and was assumed to be the only source of error since the magni- tude of its deviation was of the order of 3 percent, more than 10 times the possible error of any other element. However, since the results were 15. expressed in terms of ratios rather than in terms of absolute units, a large portion of the deviation could be expected to balance out and become negligible as was evident from the consistency of the results. Specimens The Specimens used in this work were first cut from a stock bar of one inch diameter SAE 4140 steel. The chemical analysis of this steel is carbon 0.373 percent, chromium 0.96 percent, molybdenum 0.21 percent and nickle a trace. The test sections were then heat treated in pairs to the desired mechanical properties, machined and the reduced sections were ground to a diameter of 0.505 inches for a distance of slightly over two inches, in accordance with the ASTM standards. In order to keep the inside diameter of the test solenoid as small as possible, the Spec— imens were prepared with threaded ends. when completed, the Specimens were aged at room temperature and away from magnetic fields for a year. In testing the Specimens, an approximate elastic limit for each was determined and the load divided into increments so that ten points on the magnetic property curves could be determined. Since the deformation beyond the elastic limit proceeded at a high rate of Speed, it was impos- sible to determine the magnetic prOperties after the elastic limit had been reached. PRESENTATION AND ANALYSIS OF DATA Analysis of the data for the specimens quenched from 1850 degrees FKpg.l7) indicated no significant difference from the behavior of the Specimens quenched from 1750 and 1550 degrees F. An examination of the fractured sections of the Specimen in which grain growth was expected showed that there had been little if any grain growth. Inasmuch as the steel used was an inherently fine grained material, this result should not have been unexpected. The influence of the draw temperature on the magnetic properties was found to be quite pronounced. For those Specimens drawn at 300 degrees F.(pg.l7), there was a point on both the resistivity and induct~ ance curves where the curves reached a maximum and then decreased in value as the load was increased. The load at which this maximum was reached seemed to lie between 50,000 and 60,000 psi. for the three quench temperatures used. The hardness on the Rockwell C scale for these Specimens was between 35 and 40. As the draw temperature was increased, the tendency was for the maximum to occur at lower loads and for the curve to flatten out slightly. The maximum, which may be considered to be the Villari reversal point, for the specimens drawn at 1200 degrees appeared at approximately 25,000 psi. The fact that the resistivity and inductance curves passed through a Villari reversal point at approximately the same load, indicated that the rearrangement of the domains affected the resistivity as well as the permeability of the steel. The appearance of an increase in the resis- F _ c n I u‘ . . a .ul . I’u lc.%1n-9O'Ilcit¢.' .— . . , . y--‘o--.-»‘—.— -r-“+-—J k~Ao+-- A ill. ”.5.“ -._., .._.- ~ . . . Q "[ l-5I4v9.x1- I’D"‘v5l ‘6: I I I‘ll ..‘I‘.I-1| -10.!- a | I I . . .. -- ~-._¢,_+_,-___- A T c D ‘I‘I‘.- a . . I-I.-.|'II|II . . . - n.‘¥\‘--|l1ll‘u-.. . p o l . . . ‘II I .‘z'tlir‘lo s‘f J 1 III . 6A., ‘1: ‘A u .1 I I. I . ..9 AA . :04! vl‘.-- . i .. . r . a. . a q M a l i . 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A L— "OW-«e» O 9 900 cl.{‘vl'.0l" fit . - - 4- - asslsnvnv m6 . .m... - -4- .... m-. i . w _ . .br . . — i 7 . . . , . . . .. o u I ll. 0,90! :0: '1"? 1 I 4.... d L a; tivity as the permeability increased led to the postulation that the preferred direction for conductance in SAE 4140 steel lies at a 90 degree angle to the direction of maximum permeability within the domain. This effect was first described by Lord Kelvin in 1857. The magnetizing forces present in the test solenoid were of small enough magnitude as to be considered negligible. They were of high enough frequency so that their effect could have had little or nothing to do with the rearrangement of the domains. The changes in the prop. erties measured could be said to be due to the effects of stress on the domains. Since the load at which the Villari reversal occurred varied with the hardness of the material and seemed to approach zero as the hardness decreased, it was felt that residual stresses in the Specimens might account for the variation. To determine the effects of initial stress on the Specimens, a set of Specimens were fully stress relieved and cycle loaded until there was no change in the properties from one cycle to the next. These stress relieved Specimens showed a steady decrease in both resistivity and permeability as the load was increased and as the tensile load was lowered, the properties increased to a max— imum value and then returned to the initial value. This would indiCete that the actual Villari reversal point for the material in the fully stress relieved condition was in the compressive quadrant. The reversal points on the hardened Specimens would seem to be due to the initial stress on the surface of the Specimen, a condition known to exist on martensite. Since the surface of the Specimens was in compressive stress initially, the application of tensile stress would relieve the stress. If the Villari reversal point was located in the compressive quadrant, the material would exhibit an increase in permeability and resistivity 21. as the compressive stress was relieved and the material passed through the Villari reversal point. Since the stress relieved Specimens were not martensite but were pearlitic in nature it must be assumed that the magnetostrictive behavior of martensite and pearlite are simil:r for this material, for the reasoning preSented to be valid. It was possible to approximate the depth of mersurement of the Specimen using empirical relations. Assuming a permeability of 100 and a resistivity of 10,000, the shell on which mersur ments were made was found to be 63 micro-inches thick. The finish of the Specimens was of the order of 32 micro-inches. The major effect on the magnetic behavior was due to the surface condition and its finish. The results arrived at in this work have been presented in terms of the ratio of the initial value measured at no external stress, to the value at the load being considered. Due to the complicated magnetic circuit, it was impossible in this case to present the absolute values of permeability and resistivity. However, since the actual values changed only a small percentage of the total value, the presentation in terms of ratios provided a very satisfactory picture of the behavior of the material. The resistivity and permeability ratios were plotted against strain up to the elastic limit in all but one case. From the graphical presenta- tion it was possible to analyse the effects of the mechanical properties on the magnetic properties. Mechanical hardness, chemical composition, grain size and stress were the factors considered in relation to their effect on the magnetic prop- erties. In the analysis, the chemical properties were controlled as closely as possible by utilizing one bar of steel from which all the spec- F“ “1 imens were cut. An attempt to control grain size was made by introducing conditions conducive to grain growth while heat treating several of the Specimens. The majority of the graphs presented were arranged in quench temperature groups, since the quench temperature was the major contrib- uting factor in grain growth in any given steel. Mechanical hardness was also controlled by proper heat treatment. The draw temperature was varied from that necessary to give the highest practical hardness to that value necessary to give a relatively soft condition. It was found that some factor was not being controlled in the Specimens and in an attempt to dis- cover if the initial stress due to machining or quenching was causing the variation from the results expected, a series of Specimens were stress relieved. 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I I I .. -..- 4--.---“ O + I F i i SUMMARX AND CUNCLUSlONS The influence of the mechanical properties on the magnetic properb ties of SAE 4140 steel were pronounced and to a certain degree predictable if compared with the properties of a similar material. The temperature from.which the material was guenched had little effect on the overall behavior so that it would seem possible in certain controlled Situations to utilize the magnetic behavior in the determination of the mechanical condition of a Specimen. The influence of initial stress on the Specimen was found to be quite significant. Since the magnetic examination was made to a depth of only 62 micro-inches, the surface stress determined the behavior of the magnetic properties. Having recognized the fact that martensite in formation tended to develop compressive stress on the surface, a set of VSpecimens fully stress relieved were examined and evaluated. The results of that evaluation indicated that the load at which the permeability and resistivity reached a maximum was actually a compressive load, in Spite of the fact that the reversal point occured in tension in the martensitic Specimens. (pg . (CL—24) The use of magnetic analysis methods for Specimens or materials under stress would seem to be feasible under certain circumstances. An accurate analysis of results obtained from a magnetic analysis would require considerable knowledge about the loading, making use of the method something short of practical for ordinary testing of materials. However, since the use of a high frequency technique limited the field 26. penetration, it would seem logical to apply the methods presented here for the measurement of superficial stresses on the surface of the material in question, a difficult process using any other method. It was assumed in this work that the magnetic properties of pearlitic steel were the same as the magnetic preperties of austenitic steel. This is not entirely the case. An investigation of the behavior of a series of pearlitic Specimens heat treated to the same hardness as the Specimens used in this work would be of considerable value. Further work in this field should be done since the effects of shear stress, direct compressive stress, and the effects of machining were not completely investigated. A series of tests made on torsion specimens would help establish the influence of shear stress on the magnetic behavior. Further verification of the location of the Villari reversal point should be obtained from.a series of compressive tests. The influence of mach- ining or surface finish could be determined while running the compressive tests. The specimens heat treated should be in a semi-finished condition to eliminate as much as possible the influence of machining stresses, and to allow closer correlation between the quenched condition and.mech- anical properties. APPENDIX 27. Loan“ 9":ch Q A I QM x A O O 3 5085 162.32 10.130 .3, 000 1.5000 ' 3.5401 153.72 10.107 . 5,000 950,0» 3.5714 mafia)”; 1.019 [0091 [0,000 50,000 3.5371 “'19 10. 3‘0 1.02! / 013 [5,000 80,000 3.57/4 l‘f.39 .0 329 1.0,; [.530 224000110400 3.5336 In.“ 10.274 1.007 1.004 @000 190,000 3.4“.) 151.44 “5.194 .3! 1 .392 “5°10” 3.4453 0965ch - If” 'F. 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R. ...... x. m m . 0| 0 3.1740 1:040 3.447 1.0 1.0' [2,000 10,000 3.1349 ISO-75' 9.472 1.06C 1.003 1 F5000 23,000 3.2154 107.0: 9.431 1.0/.9 1.00: ,7o°°° 3‘16“ 3.20.51 150.9: 9.484 1.010 1.004 [0,000 50,000 3.1917 ISQOffi478 1.003 1.0” _: _ 12.000 60,000 3.1746 ISO-75' 9.972, 1.000 1.00.) 14.000 70,000 3.1040 130.5: 9.4029 .997 1.001 3.1343 150.17%»!!! .387 .999 1495‘ 9.422 .984 .997 I7, 000 831°°° 005N011 - 1,750‘F , DRAW -/200’F 22,00 ° [/O)OO - LowL smug R A 1.),” 0 0 3.039: 147.75 ‘ 2000 10,000 3.05;] 4.5.13 5000 20,000 3.0804 140.57 7000 31,000 3.0709 143.37 9.354 1.012 1.007 10,000 $0,000 3.0.531 143.13 9.310 [006 1.403 12.00010501 148.00 9304 1.005 1.002 1.000-3.400 147.98 9.290 1.003 1.001 m 0 116016 H 147.48 9.200 -m - 13:0 '1’ , Duw -— Izaa'r LOAD L. 7,0°° [0,... 4,000 30, 000 6,000 ’.’... 9,000 ’0, bob 1!, 000 14,000 l‘,000 [6000 i [4000 ! 19,000 ~ 11,000 10,000 9,000 0,000 9.000 E 2,000. 40,000 80,000 (0,000 70, 000 ’0’... 30,... 00,000 70,000 (0,000 50,000 40, 000 ’0 00¢ 20, 000 10, 000 34'. Rh» Ly,” XALROH 3.20: 140.97 3.300 [.0 3.1!: 149.17 9.373 .934 3.146 144.97 3.300 .391 3.11: H“? 9.941 .972 9.100 140.07 9.941 91.4 9.090 144.17 3.941 410 3.0.90 141.7 333+? .91: 3.0.90 "(.9 9.334 .304 399 9.090 [40.7 9-39? .903 .999 9.077 149.07 9-341 .900 .990 3.100 149.47 9.191 .909 1.003 3.115 149.70 34" .97: 1.005- .119: 130-10 9.44! .914 1.009 1175’ 130.30 3-”? .931 1.60131. 3.1!: 156.33 9.999 .994 1.011 5.190- 150.“ 9.410 .997 1.011 3.20.!“ 1.;‘0-90 91934 1.000 1.010 9.19.:- 1.40.10 9.441 .937 1.003 3.170- 1499? 9.97: .991 1.003' 57:10:32-- Kean/{e 1:323:11. _ mm R“ Lywl xA Ro/R Xo/XJ -— 3.17.: 199.971 9.94: .991 1.00% mm 3.145 199.6'7] 3.9:! .991 .999 9.19: mm 9.911 .971 991 mm 9.11: 199.57 9.93,- .97: 99, mm 3.106 199.99 9.329 .919 a); -m 3.090 ”(.39 9.32, .915 Jasfi mm 3.0f‘ 141-29“ 3.3/7 .31.) 99: 3.077 149.1: 9.310 .950 .99: -m 9.047 199.11 9.310 .937 99: -m 3,057 19.01 9.904 9:7 .9” mm 9.0“ 199.47 9.929 .919 .9971; m- 9.100 149.07 9.9“ .919 1.001 mm 3.19: 199.“ 9.397 .979 1.000 3.1:: 156-; 3.4.29 .911 1.007 mm 3.175 750.20 9.44! .39! 1.00.9 mm ,J“ ”0.9: 9.449 .997 1.009 mm 3.19.9” 1.50.20 3.44! .997 I"°’ m- 3J9: 149.71 9.010 .937 1.00: —_ 0.17: 149.17 9.974.991 1.001 . $71099 (zeta/:3 ~ Pan “:2 mm 0.. -- 311: m [0,000 30,45 mm 2.1» mm 9...: mm 9-096 -m to" man so“ 9.017 am ......- mm ...... mm 1.100— mm ...... ma- 3-1“ —- 3-": Ly” X... 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