Magnetic Analysis 5 (2)

Roderic K. Stanley, NDE Information Consultants,Houston, Texas

Satish S. Udpa, Michigan State University, East Lansing,Michigan (Parts 1, 9, 10 and 11)

Rusty G. Waldrop, United States Coast Guard,Elizabeth City, North Carolina

2C H A P T E R

Fundamentals of MagneticTesting1,2

Magnetic testing is part of the widely usedfamily of electromagnetic nondestructivetesting. The theory and practice ofelectromagnetic techniques are discussedelsewhere in the NDT Handbook.1 Whenused with other methods, magnetic testscan provide a quick and relativelyinexpensive assessment of the integrity offerromagnetic materials.

Magnetic particle testing is in fact avariation of flux leakage testing that usesparticles to produce indications. Becauseof the prevalence of tests specifically usingparticles, magnetic particle testing isusually treated as a separate method byindustry and in the literature. The termflux leakage testing, also called diverted fluxtesting, is then meant to exclude tests withparticles.

The magnetic circuit and the means forproducing the magnetizing force thatcauses magnetic flux leakage are describedbelow. Theories developed for surface andsubsurface discontinuities are outlinedalong with some results that can beexpected.

Industrial Uses3

Magnetic testing is used in manyindustries to find a wide variety ofdiscontinuities. Much of the world’sproduction of ferromagnetic steel is testedby magnetic or electromagnetictechniques. Steel is tested many timesbefore it is used and some steel productsare tested during use for safety andreliability and to maximize their length ofservice.

Production TestingTypical applications of magnetic fluxleakage testing are by the steel producer,where blooms, billets, rods, bars, tubesand ropes are tested to establish theintegrity of the final product. In manyinstances, the end user will not acceptdelivery of steel product without testingby the mill and independent agencies.

Receiving TestingThe end user often uses magnetic fluxleakage tests before fabrication. This testensures the manufacturer’s claim that theproduct is within agreed specifications.Such tests are frequently performed by

independent testing companies or the enduser’s quality assurance department. Oilfield tubular goods are often tested at thisstage.

Inservice TestingGood examples of inservice applicationsare the testing of used wire rope, installedtubing, or retrieved oil field tubular goodsby independent facilities. Manylaboratories also use magnetic techniques(along with metallurgical sectioning andother techniques) for the assessment ofsteel products and prediction of failuremodes.

Steps in Magnetic TestingThere are four steps in magnetic testing:(1) magnetize the test object so thatdiscontinuities perturb the flux, (2) scanthe surface of the test object with amagnetic flux sensitive detector,(3) process the raw data from thesedetectors in a manner that bestaccentuates discontinuity signals and(4) present the test results clearly forinterpretation. The following discussiondeals with the first step, producing themagnetizing force.

Magnetic Particle Testing

PrinciplesMagnetic particle testing is anondestructive method of revealingsurface and subsurface discontinuities inmagnetizable materials. It may be appliedto raw materials such as billets, bars andshapes; during processes such as forming,machining, heat treating andelectroplating; and in testing for servicerelated discontinuities.

The testing method is based on theprinciple that magnetic flux in amagnetized object is locally distorted bythe presence of a discontinuity. Thisdistortion causes some of the magneticfield to exit and reenter the test object atthe discontinuity. This phenomenon iscalled magnetic flux leakage (MFL). Fluxleakage is capable of attracting finelydivided particles of magnetic materialsthat in turn form an outline or indicationof the discontinuity. The intensity or

42 Magnetic Testing

PART 1. Introduction to Magnetic Tests

curvature of magnetic flux leakage fields iscritical in causing particles to remain heldat an indication.

One of the objectives of magneticparticle testing is to detect discontinuitiesas early as possible in the processingsequence, thus avoiding the expenditureof effort on materials that will later berejected. Practically every process, fromthe original production of metal from itsore to the last finishing operation, mayintroduce discontinuities. Magneticparticle testing can reveal many of these,preventing flawed components fromentering service. Even though magneticparticle testing may be applied during andbetween processing operations, a final testis usually performed to ensure that alldetrimental discontinuities have beendetected. In welds with a tendency towarddelayed cracking, there may be a specifiedtime delay between the completion ofwelding and the final test.

The test itself consists of three basicoperations: (1)establish a suitablemagnetic flux in the test object; (2) applymagnetic particles in a dry powder or aliquid suspension; and (3) examine thetest object under suitable lightingconditions, interpreting and evaluatingthe test indications (as in Fig. 1).

Capabilities and LimitationsMagnetic particle testing can revealsurface discontinuities, including thosetoo small or too tight to be seen with theunaided eye. Magnetic particle indicationsform on an object’s surface above adiscontinuity and show the location andapproximate size of the discontinuity.Magnetic particle tests can also revealdiscontinuities that are slightly below thesurface, depending on their size.

There are limits on this ability to locatesubsurface discontinuities. These limits aredetermined by the intensity of the appliedfield and by the discontinuity’s depth,size, type and shape. In some cases,special techniques or equipment canimprove the test’s ability to detectsubsurface discontinuities.

Magnetic particle testing is forferromagnetic materials only: it cannot beused on nonmagnetic materials, includingglass, ceramics, plastics or such commonmetals as aluminum, magnesium, copperand austenitic stainless steel alloys.

In addition, there are certain positionallimitations: a magnetic field is directionaland for best results must be perpendicularto the discontinuity. This generallyrequires magnetizing operations indifferent directions to detectdiscontinuities. Objects with large crosssections may require a very high currentto generate a magnetic field adequate formagnetic particle tests. A final limitationis that a demagnetization procedure isusually required following the magneticparticle process.

Knowledge of limitations can behelpful to managers, supervisors andpersonnel outside nondestructive testingwho require general information on themagnetic particle testing process. It mayalso be helpful for introductory studies byindividuals already using magneticparticle testing or those preparing foradvanced training in the technique.

43Fundamentals of Magnetic Testing

FIGURE 1. Forging laps in piston rods.

Magnetic DomainsMaterials that can be magnetized possessatoms that group into magneticallysaturated regions called magnetic domains.These domains have a positive andnegative polarity at opposite ends. Inmacroscopically unmagnetized material,the domains are randomly oriented,usually parallel with the crystalline axesof the material, resulting in zero netmagnetization.

When the material is subjected to amagnetic field, the domains attempt toalign themselves parallel with the externalmagnetic field. The material then acts as amagnet. Figure 2 illustrates the domainalignment in nonmagnetized andmagnetized material.

Magnetic PolesA magnet has the property of attractingferromagnetic materials. The ability toattract (or repel) is not uniform over thesurface of a magnet but is concentrated atlocalized areas called poles. In everymagnet, there are two or more poles withopposite polarities. These poles areattracted to the Earth’s magnetic polesand therefore are called north and southpoles.

Figure 3 can be duplicated by placing asheet of paper over a bar magnet andsprinkling iron particles on the paper. Itshows the magnetic field leaving andentering the ends or poles of the magnet.This characteristic pattern illustrates theterm lines of force used to describe amagnetic flux field. There are a number ofimportant properties associated with linesof force.

1. They form continuous loops that arenever broken but must completethemselves through some path.

2. They do not cross one another.3. They are considered to have direction,

leaving from the north pole andtraveling through air to the southpole, where they reenter the magnetand return through the magnet to thenorth pole.

4. Their density decreases with increasingdistance from the poles.

5. They seek the path of least magneticresistance or reluctance in completingtheir loops.

When a bar magnet is broken into twoor more pieces, new magnetic poles areformed. The opposing poles attract oneanother as shown in Fig. 4.

If the center piece in Fig. 4 is reversedso that similar poles are adjacent, thelines of force repel one magnet from theother. If one of the bars is small enough,the lines of force can cause it to rotate sothat unlike poles are again adjacent. Thisillustrates the most basic rule ofmagnetism: unlike poles attract and likepoles repel.

44 Magnetic Testing

PART 2. Magnetic Field Theory

FIGURE 2. Orientation of magnetic domains:(a) in nonmagnetized material; (b) inmagnetized material.

(a)

(b)

FIGURE 3. Magnetic field surrounding barmagnet.

Types of MagneticMaterialsAll materials are affected to some degreeby magnetic fields. Matter is made up ofatoms with a positively charged nucleussurrounded by a field or cloud ofnegatively charged electrons. The electronfield is in continual motion, spinningaround the nucleus. When the material issubjected to a magnetic field, the electronorbits are distorted to some degree. Theamount of this distortion (or thecorresponding change in magneticcharacteristics) when subjected to anexternal magnetic field provides a meansof classifying materials into three maingroups: diamagnetic, paramagnetic orferromagnetic.

Diamagnetic MaterialsThe term diamagnetic refers to a substancewhose magnetic permeability is slightlyless than that of air. When a dimagneticobject is placed in an intense magneticfield, the induced magnetism is in adirection opposite to that of iron.Diamagnetic materials include mercury,gold, bismuth and zinc.

Paramagnetic MaterialsParamagnetic describes a substance whosepermeability is slightly greater than thatof air or unity. When such materials areplaced in an intense magnetic field, thereis a slight alignment of the electron spinin the direction of the magnetic flux flow.This alignment exists only as long as theparamagnetic material is in the externalmagnetic field.

Aluminum, platinum, copper andwood are paramagnetic materials

Ferromagnetic MaterialsFerromagnetic substances have apermeability that is much greater thanthat of air. When placed in an externalmagnetic field, the magnetic domainsalign parallel with the external field andremain aligned for some period of timeafter removal from the field.Ferromagnetism can only be explained viathe domain theory. Paramagnetic anddiamagnetic materials do not containsuch domains.

This continued alignment afterremoval from the external field is calledretentivity and can be an importantproperty in some magnetic particle testingprocedures.

Some examples of ferromagneticmaterials are iron, cobalt, nickel andgadolinium.

Sources of Magnetism

Permanent Magnets Permanent magnets are produced by heattreating specially formulated alloys in anintense magnetic field. During heattreatment, the magnetic domains becomealigned and remain aligned after removalof the external field. Permanent magnetsare essential to modern technology; theirapplications include magnetos, directcurrent motors, telephones, loud speakersand many electric instruments.

Common examples of permanentmagnetic materials include alloys ofaluminum, nickel and cobalt (alnico);copper, nickel and cobalt (cunico); copper,nickel and iron (cunife); and cobalt andmolybdenum (comol). Supermagnetmaterials such as neodymium-iron-boronand samarium-cobalt are now common.

Magnetic Field of the EarthThe planet Earth is itself a huge magnet,with north and south poles slightlydisplaced from the Earth’s axis. Thisdisplacement results in a slight deviationbetween geographic north and magneticnorth.

As a magnet, the Earth is surroundedby magnetic lines of force as shown inFig. 5. These lines of force make up whatis sometimes called the earth field andthey can cause problems in bothmagnetizing and demagnetizing offerromagnetic test objects. The earth fieldis weak, on the order of 0.03 mT (0.3 G).

Movement of ferromagnetic objectsthrough the earth field can induce slightmagnetization. This is a problem inaircraft where magnetized componentscan affect the compasses used innavigation. Similarly, demagnetizing canbe difficult if certain objects, usually longshafts, are not oriented from east to westduring the demagnetization process.

Mechanically Induced MagnetismCold working of some ferromagneticmaterials, either by forming operations orduring service, can magnetize the objects.


FIGURE 4. Broken bar magnet illustratinglocations of newly formed magnetic poles.

S NN

S

S

N

N

S

S

N

LegendN = northS = south

When mechanically inducedmagnetization occurs as a result offorming operations, it can be removed bysubjecting the magnetized object to aroutine demagnetization process.

It can be difficult to removemechanically induced magnetizationresulting from cold working. Disassemblyis usually impractical anddemagnetization must be accomplishedusing portable yokes or cable coils. Theoperation is complicated when otherferromagnetic components are near themagnetized object: the demagnetizingoperation can magnetize adjacent objectsand a sequence of demagnetizingoperations must then be performed.

46 Magnetic Testing

FIGURE 5. Magnetic field of Earth.

Circular Magnetic FieldsThe most familiar type of magnet is thehorseshoe shape shown in Fig. 6a. Itcontains both a north and south polewith the lines of magnetic flux leavingthe north pole and traveling through airto reenter the magnet at the south pole.Ferromagnetic materials are only attractedand held at or between the poles of ahorseshoe magnet.

If the ends of such a magnet are bentso that they are closer together (Fig. 6b),the poles still exist and the magnetic fluxstill leaves and reenters at the poles. Insuch a case, however, the lines of force arecloser and more dense. The number oflines of flux per unit area is calledmagnetic flux density and is measured intesla or gauss.

If the magnetic flux density is highenough, ferromagnetic particles arestrongly attracted and can even bridge thephysical gap between poles that are closeenough together. The area where the fluxlines leave the pole, travel through air andreenter the magnet is called a magneticflux leakage field.

When the ends of a magnet are benttogether and the poles are fused to form aring (Fig. 6c), the magnet no longerattracts or holds ferromagnetic materials(there are no magnetic poles and no fluxleakage field). The magnetic flux lines stillexist but they are completely containedwithin the magnet. In this condition, themagnet is said to contain a circularmagnetic field or to be circularlymagnetized.

If a crack crosses the magnetic fluxlines in a circularly magnetized object,north and south poles are immediatelycreated on either side of the discontinuity.This forces a portion of the magnetic fluxinto the surrounding air, creating a fluxleakage field that attracts magneticparticles (Fig. 6d) and forms a crackindication.

LongitudinalMagnetizationIf a horseshoe magnet is straightened, abar magnet is formed with north andsouth poles (Fig. 7a). Magnetic flux flowsthrough the magnet and exits or enters at

the poles. Magnetic poles can be thoughtof as occurring wherever field lines enterand leave a magnetized object.Ferromagnetic materials are attracted onlyto the poles and such an object is said tohave a longitudinal field or to belongitudinally magnetized.

If these magnetic flux lines areinterrupted by a discontinuity, additionalnorth and south poles are formed oneither side of the interruption (Fig. 7b).


PART 3. Magnetic Flux and Flux Leakage

FIGURE 6. Horseshoe magnet illustratingfundamental properties of magnetism:(a) direction of magnetic flux; (b) magneticflux in air around poles (moving poles closetogether raises magnetic flux density);(c) fusing poles, forming a circularlymagnetized object; (d) discontinuity incircularly magnetized object and its resultingflux leakage field.

Magnetic particles

Magnetic particles

N S

N S

S N

(a)

(b)

(c)

(d)

Such secondary poles and their associatedflux leakage fields can attract magneticparticles. Even if the discontinuity is avery narrow crack, it will still createmagnetic poles (Fig. 7c) that holdmagnetic materials (the magnetic fluxleakage field is still finite).

Magnetic Field IntensityThe intensity of a flux leakage field from adiscontinuity depends on several factors:(1) the number of magnetic flux lines,(2) the depth of the discontinuity and(3) the width of the discontinuity’s air gapat the surface (the distance between themagnetic poles).

The intensity and curvature of theleakage field determines the number ofmagnetic particles that can be attracted toform a test indication. The greater theleakage field intensity, the denser theindication, so long as the magnetic fluxleakage field is highly curved.

Subsurface DiscontinuitiesA slot such as a keyway on the backside ofan object creates new magnetic poles thatdistort the internal flux flow. If the slot isclose enough to the surface, somemagnetic flux lines may be forced to exitand reenter the magnetized object at thesurface. The resulting leakage field canform a magnetic particle test indication.

Size and intensity of the indicationdepends on: (1) the proximity of the slotto the top surface; (2) the size andorientation of the slot; and (3) theintensity and distribution of the magneticflux field. A similar effect occurs if thediscontinuity is completely internal to theobject. Figure 8 is an illustration of akeyway on the far side of a bar and Fig. 9illustrates a midwall discontinuity.

Effect of DiscontinuityOrientationThe orientation of a discontinuity in amagnetized object is a major factor in theintensity of the magnetic flux leakagefield that is formed. This applies to bothsurface and internal discontinuities. Themost intense magnetic flux leakage field isformed when the discontinuity isperpendicular to the magnetic flux flow. Ifthe discontinuity is not perpendicular, theintensity of the magnetic flux leakagefield is reduced and disappears entirelywhen the discontinuity is parallel to themagnetic flux flow.

48 Magnetic Testing

FIGURE 7. Bar magnet illustratinglongitudinal magnetization: (a) horseshoemagnet straightened into bar magnet withnorth and south poles; (b) bar magnetcontaining machined slot andcorresponding flux leakage field; (c) crack inlongitudinally magnetized object, producingpoles that attract and hold magneticparticles.

Magneticparticles

N S

S

N

N

S

S

N

N

S

N S

Magneticparticles

Crack

(a)

(b)

(c)

FIGURE 8. Slot or keyway on reverse side ofmagnetized bar.

FIGURE 9. Internal or midwall discontinuityin magnetized test object. There may ormay not be magnetic flux leakage,depending on value of flux in object.

Figure 10 illustrates the effect ofdiscontinuity orientation on the intensityof the magnetic flux leakage field.

Formation of IndicationsWhen magnetic particles collect at a fluxleakage site, they produce an indicationvisible to the unaided eye under theproper lighting conditions. Tightermagnetic flux leakage fields create theideal conditions for particle attraction, theforce on an individual particle being givenby the product of terms involving theparticle shape, the local value of the fieldintensity and a term involving the localcurvature of the magnetic flux leakagefield. Particles then tend to align end toend in this field. The stronger the abilityto hold particles, the larger the indication.


FIGURE 10. Flux leakage fields fromdiscontinuities with different orientations:(a) perpendicular to magnetic flux; (b) at45 degree angle to magnetic flux;(c) parallel to magnetic flux.

(a)

(b)

(c)

Circular MagnetizationWhen an electric current flows through aconductor such as a copper bar or wire, amagnetic field is formed around theconductor (Fig. 11a). The direction of themagnetic lines of force is always90 degrees from the direction of currentflow. When the conductor has a uniformshape, the flux density or number of linesof force per unit area is uniform along thelength of the conductor and uniformlydecreases as the distance from theconductor increases.

Because a ferromagnetized object is alarge conductor, electric current flowingthrough the object forms a circularmagnetic field. This magnetic field isknown as circumferential magnetizationbecause the magnetic flux lines formcomplete loops in the object (Fig. 11b).

A characteristic of circumferentialmagnetic fields is that the magnetic fluxlines form complete loops withoutmagnetic poles. Because magneticparticles are only attracted to and heldwhere flux lines exit and enter the objectsurface, indications do not occur unless adiscontinuity crosses the flux lines.

Inducing Circular Magnetizationin a Test ObjectFigure 12 illustrates a method forinducing a circular field using a magneticparticle testing unit. The test object isclamped between the contact plates sothat electric current passes through it.

When tubes are tested by passing acurrent through them, the magnetic fluxis zero at the inner surface or axis and isits maximum at the outside surface. Theinside surface is often equally importantwhen testing for discontinuities. Because amagnetic field surrounds a conductor, it ispossible to induce a satisfactory field inthe tube by inserting a copper bar or someother conductor through the tube andpassing the current through the bar.

This method is called internal conductormagnetization. Figure 13 indicates amethod employed for circular fieldinspection of short parts. For longer partssuch as steel tubes, an insulated metal rodis run along the bore of the tube, excitingit with a high current. The rod should nottouch the inside of the tube and the tubeshould be placed on wooden planks toisolate it from the ground. These measuresshould eliminate arc burns frommagnetizing current grounding.

Magnetic Field DirectionThe magnetic lines of force are always atright angles to the direction of themagnetizing current. One way to visualizethe direction of the magnetic flux is toimagine the conductor held in your righthand with the thumb extended in thedirection of the electric current flow. Yourcurved fingers then point in the directionof the magnetic flux flow. This is knownas the right hand rule (Fig. 14).

50 Magnetic Testing

PART 4. Electrically Induced Magnetism

FIGURE 11. Magnetic field generated:(a) around conductor carrying electriccurrent; (b) around ferromagnetic testobject used as conductor.

Magnetic field

Magnetizingcurrent

Conductor

Magnetic field

Magnetizingcurrent

Test object

(a)

(b) FIGURE 12. Inducing circumferentialmagnetic field in object used as conductor.

Magnetic fieldElectriccurrent

LongitudinalMagnetizationElectric current can induce longitudinalfields in ferromagnetic materials. Themagnetic field around a conductor isoriented lengthwise direction by formingthe conductor into a coil (Fig. 15). Theright hand rule shows that the magneticfield at any point within the coil is in alengthwise direction.

When a ferromagnetic object is placedinside a coil carrying an electric current(Fig. 16), the magnetic flux linesconcentrate themselves in a longitudinaldirection. An object that has beenlongitudinally magnetized is characterizedby poles close to each end where the fieldlines leave and enter the test object toform continuous loops around theassociated current.

When a longitudinally magnetizedobject contains a transverse discontinuity,a leakage field is produced that attractsmagnetic particles and forms anindication. Figure 17 illustrates a typical

coil found on magnetic particle testsystems used to locate transversediscontinuities.

MultidirectionalMagnetizationWhen testing for discontinuities indifferent directions, it is standard practiceto perform two tests, one with circularmagnetization and the one withlongitudinal. Two or more fields indifferent directions can be imposed on anobject in rapid succession.

When this is done, magnetic particleindications are formed whendiscontinuities are favorably oriented tothe direction of any field. Suchindications persist as long as the rapidalternations of current continue.


FIGURE 13. Inducing circumferentialmagnetic field using internal conductor:(a) for tube with inside and outside surfacediscontinuities; (b) for multiple ring shapeswith cracking on inside and outside surfaces.

Conductor

Cracks

Magnetizingcurrent

Magneticfield

(a)

Cracks

Magnetizingcurrent

Magnetic field

(b)

FIGURE 14. Right hand rule indicates direction of magneticflux flow based on direction of magnetizing current.

Magnetic field

Current

+

–

FIGURE 15. Formation of longitudinalmagnetic field using coiled conductor.

Magneticfield

FIGURE 16. Test object containinglongitudinal magnetic field induced by coil.

Path ofmagnetizing

current

Longitudinal crack(not detected)

Transversecrack(detected)

Magnetic linesof force

Permanent orflexible cable

FIGURE 17. Formation of transversediscontinuity indication during longitudinalmagnetization.

Current

Bath

Transversecracks

Magnetic field

Forty-five degreecrack (detected)

Current

Stationary MagneticParticle Test SystemsWet method horizontal magnetic particletest systems typically consist of (1) a highcurrent, low voltage magnetizing source;(2) head stock and tail stock for holdingtest objects and providing electricalcontact for circumferential magnetization;(3) a movable coil for longitudinalmagnetization; and (4) a particlesuspension tank with an agitation system.

The basic components, along withmagnetizing control indicators andampere meters, are enclosed within atabletop structural frame. Systems areavailable in a large number of sizes from a25 mm (1 in.) contact plate opening up tosystems that are 6 m (20 ft) long. Thesystems provide alternating current, directcurrent or a combination of the two, withmaximum magnetizing current outputfrom 1 to 10 kA. Figure 18 shows a typicalstationary or wet horizontal unit.

Stationary magnetic particle systemsinclude structures designed to beergonomically healthful for the user.Control functions are positionedconveniently for the human body.Instrumentation includes liquid crystaldisplays for digital readouts of amperesachieved and smart knobs for fine tuningof settings. Magnetic particle stationarysystems are designed for rugged durabilityand require practice to operate efficiently.

A common cause of misseddiscontinuities is the misuse of the systemparameters. The magnetic particle systemrequires an array of process controls andmaintenance. An organization using amagnetic particle system shouldincorporate an intensive maintenanceplan with a demanding training,qualification and certification program forindividuals using the magnetic particlesystem. The maintenance plan shouldinclude daily process validation checks.

Power PacksPower packs are the electrical sourcesneeded to produce high amperage, lowvoltage magnetizing current. They areused to magnetize test objects such ascastings and forgings that are too large tobe placed in a stationary testing unit. Thesize and weight of power packs preventmoving them and test objects areaccordingly transported to the test site.The rating or current output ofcommercial power packs varies widely butis typically from 6 to 20 kA ofmagnetizing current.

The current is applied by cable wraps,formed coils, clamps and prods. Mostpower pack units incorporate anadjustable current control, one or twoammeters and an automatic shot durationtimer.

Mobile and PortableTesting UnitsThere are many applications where it isnot possible to bring the test object to themagnetic particle system. Mobile units areone type of equipment that can betransported to the test site and stillprovide relatively high magnetizingcurrents. Traditional mobile units may beconsidered small versions of the powerpack systems. Some mobile units have amagnetizing current output of 6 kA butmost are limited by size considerations tobetween 3 and 4 kA. Transportability isalso improved by restricting the types ofmagnetizing current to alternating currentand half-wave direct current. Magnetizingcurrent is applied to the test object bycable wraps, formed coils, prods andclamps. Oil field portable magnetizing

52 Magnetic Testing

PART 5. Magnetic Particle Test Systems

FIGURE 18. Typical wet horizontal magneticparticle test system.

units can reach 15 kA by capacitordischarge through internal conductors orcable wraps.

The term portable equipment refers tocompact, lightweight units that can behand carried to the test site. Someportable units are mounted on wheeledcarts to facilitate portability. Likestationary and mobile equipment,portable units come in a variety of sizes,shapes, weights and amperage outputs.The most common method of applyingcurrent with a portable unit is with prodsor clamps. However, cable wraps andformed coils are also used in manyapplications. Reduced weight and size areachieved by omitting the step downtransformer needed for demagnetization.

Prods and YokesProds are magnetization accessories thatmay be used with stationary, power pack,mobile and portable units. They typicallyconsist of a pair of copper bars 12 to20 mm (0.5 to 0.75 in.) in diameter withhandles and connecting cables. One ofthe prod handles has a trigger to remotelyactivate the magnetizing current from theunit’s mainframe. Prods set up a circularmagnetic field that diminishes inintensity as the distance between prodsincreases (Fig. 19). Prods are avoided withcomponents that can be damaged byarcing.

Yokes are often cable connected to amobile or portable unit that provides themagnetizing current. A yoke designedwith a self-contained magnetizing sourceis often called a hand probe. Hand probescontain small transformers that generatelow voltage and high current. Yokesusually contain a magnetizing coil with acore of laminated transformer iron.Attached to the core are legs that may

either be fixed or articulated. Whenmagnetizing current is applied to the coil,a longitudinal magnetic field is created inthe core and transmitted to the legs.When coupled to a test object, alongitudinal magnetic field is generatedbetween the poles as shown in Fig. 20.

Yokes are often specified by their liftingability or the surface field they createmidway between their poles, as measuredwith a tesla meter.


FIGURE 19. Circular magnetic field generatedaround magnetizing prods.

Weld Lines of force

FIGURE 20. Longitudinal magnetic fieldgenerated by yoke.

Current

Magneticlines of force

Test object

Weld

Yoke

+

–

Magnetic Flux and Units ofMeasureA magnetic field is made up of flux lineswithin and surrounding a magnetizedobject or a conductor carrying an electriccurrent. The term magnetic flux is usedwhen referring to all of the lines of flux ina given area. Flux per unit area is calledmagnetic flux density (the number of linesof flux passing transversely through a unitarea). Flux density in a magnetized objectwill decrease with distance. Flux density isgreatest at the poles.

There can be some confusion about theunits of measure used to define thesemagnetic quantities. The unit of magneticflux was originally called a maxwell withone maxwell being one line of flux. Theunit of flux density was the gauss withone gauss equal to one maxwell persquare centimeter. In 1930, theInternational ElectrotechnicalCommission redefined and renamed thegauss as an oersted, or the intensity of amagnetic field in which a unit magneticpole experiences a force of one dyne.3

In 1960, the InternationalOrganization for Standardization releasedISO 1000: The International System of Units(SI). This document standardizes themetric units for magnetic flux. Fluxintensity is measured using the weber(Wb) with one weber equal to 108 lines offlux. The flux density unit is the tesla (T)or one weber per square meter (Wb·m–2);1 Wb·m–2 = 1 T = 10 000 gauss (10 kG).

Magnetic HysteresisAll ferromagnetic materials have certainmagnetic properties specific to thatmaterial. Most of these properties aredescribed by a magnetic hysteresis curve.The data for the hysteresis curve arecollected by placing a bar offerromagnetic material in a coil andapplying an alternating current. Byincreasing the magnetizing field intensityH in small increments and measuring theflux density B at each increment, therelationship between magnetic fieldintensity and flux density can be plotted.

The relationship between magneticfield intensity and flux density is notlinear for ferromagnetic materials. A

specific change in H may produce asmaller or larger change in B as shown inFig. 21, the initial curve for anunmagnetized piece of steel. Starting atpoint O (zero magnetic field intensity andzero magnetic flux) and increasing H insmall increments, the flux density in thematerial increases quite rapidly at first,then gradually slows until point P1 isreached. At point P1, the materialbecomes magnetically saturated. Beyondthe saturation point, increases in magneticfield intensity do not increase the fluxdensity in the material. In diagrams of fullhysteresis loops, the curve OP1 is oftendrawn as a dashed line because it occursonly during the initial magnetization ofan unmagnetized material. It is referred toas the virgin curve of the material(Fig. 21a).

When the magnetic field intensity isreduced to zero (Br in Fig. 21b), the fluxdensity slowly decreases. It lags the fieldintensity and does not reach zero. Theamount of flux density remaining in thematerial (line B1O) is called residualmagnetism or remanence. The ability offerromagnetic materials to retain a certainamount of magnetism is called retentivity.

Removal of residual magnetismrequires the application of a magneticfield intensity in an opposite or negativedirection, of force equal or greater thanused to saturate the test object (Fig. 21c).When the magnetic field intensity is firstreversed and only a small amount isapplied, the flux density slowly decreases.As additional reverse field intensity isapplied, the rate of reduction in fluxdensity (line –HcO) increases until it isalmost a straight line (point –Hc) where Bequals zero.

The amount of magnetic field intensitynecessary to reduce the flux density tozero is called coercive force. Coercive forceis a factor in demagnetization and is alsovery important in eddy current testing offerromagnetic materials.

As the reversed magnetic field intensityis increased beyond point –Hc, themagnetic flux changes its polarity andinitially increases quite rapidly. It thengradually slows until point P2 is reached(Fig. 21). This is the reverse polaritysaturation point and additional magneticfield intensity will not produce anincrease in flux density.

54 Magnetic Testing

PART 6. Ferromagnetic Material Characteristics

When the reversed magnetic fieldintensity is reduced to zero (point –Br inFig. 21), the flux density again lags themagnetic field intensity, leaving residualmagnetism in the material (line –BrO).The flux densities of the residualmagnetism from the straight and reversedpolarities are equal (line B1O is equal toline B1O).

Removal of the reversed polarityresidual magnetism requires application ofmagnetic field intensity in the originaldirection. Flux density drops to zero atpoint Hc in Fig. 21 with the application ofcoercive force HcO. Continuing to increasethe field intensity results in the magneticpolarity changing back to its originaldirection. This completes the hysteresisloop (note that the lower curve is a mirrorimage of the upper curve).

The hysteresis cycle repeats in this loopuntil the level of demagnetization is

acceptable. An initial cycle illustrates thephysical characteristics of permeability.

Magnetic DomainsWhen saturation is achieved, themagnetic domains are all aligned. Whenthe magnetizing force is removed and theferromagnetic component is at itsremanent field state, some of thedomains stay aligned while some returnto random positioning. When coerciveforce is applied and the remanent field isat B (Fig. 21b) and the magnetizing forceis at negative, the oriented domains rotate90 degrees from initial orientation. Whennegative saturation is accomplished, themagnetic domains are 180 degrees out oforiginal alignment. This effect continuesas long as demagnetization continuesusing a reversing polarity.


FIGURE 21. Hysteresis data for unmagnetized steel: (a) virgin curve of hysteresis loop;(b) hysteresis loop showing residual magnetism; (c) hysteresis loop showing coercive force;(d) hysteresis loop showing reverse saturation point; (e) hysteresis loop showing reverseresidual magnetism; (f) complete hysteresis loop.

B

OH

Zero flux density andzero magnetic

strength

Residualmagnetism

Coerciveforce

Reverseresidual

point

Reversemagnetization

point

Reversemagnetization

saturation point

Residualmagnetism

H H

H

B

B

B

B

(a) (d)

(b) (e)

(c) (f)

LegendB = magnetic flux densityH = magnetic field intensityO = origin before magnetizationP = saturation point

O

O

OO

O

–Br

P2

P2

–Hc

HC

Br

Br

P1

P1

–Br

–Hc

H

P2

Br

P1

–Hc

B

H

Br

P1

–Hc

Br

P1

P1

Magnetic PermeabilityOne of the most important properties ofmagnetic materials is permeability.Permeability can be thought of as the easewith which materials can be magnetized.More specifically, permeability is the ratioof the flux density to the magnetic fieldintensity (B divided by H). Figure 22a isthe virgin curve of a high permeabilitymaterial and Fig. 22b is the curve of a lowpermeability material.

The reciprocal of permeability isreluctance, defined as the resistance of amaterial to changes in magnetic fieldintensity.

Magnetic properties and hysteresisloops vary widely between materials andmaterial conditions. They are affected bytemperature, chemical composition,microstructure and grain size. Figure 23ais a hysteresis loop for hardened steel andthe loop is typical of a material with lowpermeability, high reluctance, highretentivity and high residual magnetismthat requires high coercive force forremoval. Figure 23b is the hysteresis loopfor an annealed low carbon steel. It istypical of a material with highpermeability, low reluctance, lowretentivity and low residual magnetismthat requires a low coercive force forremoval.

56 Magnetic Testing

FIGURE 23. Hysteresis loops: (a) hardenedsteel hysteresis loop; (b) annealed lowcarbon steel hysteresis loop.

Residualmagnetism

Coercive force

(a)

Flux

den

sity

B

Positive magnetic field intensity H

Residualmagnetism

Coercive force

(b)

Flux

den

sity

B


FIGURE 22. Magnetic permeability curves:(a) high permeability virgin curve; (b) lowpermeability virgin curve.

Saturation point(a)

Flux

den

sity

B


Saturation point(b)

Flux

den

sity

B


In the early days of magnetic particletesting, it was believed that the bestcurrent for magnetization was directcurrent from storage batteries. Asknowledge of the magnetic particleprocess expanded and electrical circuitrycontinued to advance, many types ofmagnetizing currents became available:alternating current, half-wave directcurrent and full-wave direct current. Theterms half-wave rectified direct current andfull-wave rectified direct current are used foralternating current rectified to producehalf-wave and full-wave direct current.

Alternating CurrentAlternating current is useful in manyapplications because it is commerciallyavailable in voltages ranging from 120 to440 V. Electrical circuitry to producealternating magnetizing current is simpleand relatively inexpensive because it onlyrequires transforming commercial powerinto low voltage, high amperagemagnetizing current.

In the United States and some othercountries, alternating current alternatessixty times in a second. Many othercountries have standardized fiftyalternations per second. The alternationsare called cycles. One hertz (Hz) equalsone cycle per second and 60 Hz is sixtycycles per second. Figure 24 shows thewaveform of alternating current. In onecycle, the current flows from zero to amaximum positive value and then dropsback to zero. At zero, it reverses directionand goes to a maximum negative peakand returns to zero. The curve issymmetrical with the positive andnegative lobes being mirror images.

There are three primary advantages tousing alternating current as a magnetizingsource. First, the current reversal causes aninductive effect that concentrates themagnetizing flux at the object surface(called the skin effect) and providesenhanced indications of surfacediscontinuities. This is especiallyimportant for inspection of irregularlyshaped components, such as crankshafts.Magnetic fields produced by alternatingcurrent are also much easier to removeduring demagnetization. A thirdadvantage is that the pulsing effect of theflux caused by the current reversalsagitates the particles applied to the testobject surface. By increasing particlemobility, this agitation allows moreparticles to collect at flux leakage pointsand so increases the size and visibility ofdiscontinuity indications.

Concentration of the flux at the testobject surface can be a disadvantage alsobecause most subsurface discontinuitiesare not detected. Another disadvantage isthat some specifications do not allowalternating current on plated componentswhen the coating is thicker than 0.08mm(0.003 in.). It is hard to determine whenthe flux in a test object is at peak; itdepends on when in the magnetizingcycle the current is turned off.

Alternating current is more effectivethan direct current on objects with thicknonmetallic coatings.

Half-Wave Direct CurrentWhen single-phase alternating current ispassed through a simple rectifier, thereversed flow of current is blocked orclipped. This produces a series of currentpulses that start at zero, reach a maximumpoint, drop back to zero and then pauseuntil the next positive cycle begins. Theresult is a varying current that flows onlyin one direction. Figure 25 shows thewaveform for half-wave direct current.

Half-wave direct current haspenetrating power comparable tosingle-phase full-wave direct current.Half-wave current has a flux density ofzero at the center of a test object and thedensity increases until it reaches amaximum at the object surface. Thepulsing effect of the rectified waveproduces maximum mobility for the


PART 7. Types of Magnetizing Current

FIGURE 24. Waveform of alternating current.

1 cycle+

0

–Flux

den

sity

Time

magnetic particles; dry method tests areenhanced by this effect. Another distinctadvantage of half-wave direct current isthe simplicity of its electrical components.It can be easily combined with portableand mobile alternating current equipmentfor weld, construction and casting tests.

One of the disadvantages of half-wavemagnetization is the problem indemagnetization: the current does notreverse so it cannot be used fordemagnetizing. Alternating current can beused to remove some residual magnetismbut the skin effect of alternating currentand the deeper penetration of half-wavedirect current cause incompletedemagnetization.

Full-Wave Direct CurrentElectrical circuits can rectify and invert sothat so that the number of positive pulsesis doubled. Figure 26 shows the waveformof single-phase full-wave rectifiedalternating current. The resulting currentis called single-phase full-wave direct current.

Single-phase full-wave direct currenthas essentially the same penetratingability as three-phase full-wave directcurrent. The current fluctuation causes askin effect that is not significant. It is alsopossible to incorporate switching devices

in the circuit that reverse the current flow.This permits built-in reversing directcurrent demagnetization. Because of itssimpler components, the initial cost ofsingle-phase full-wave direct currentequipment is much less than that ofthree-phase full-wave equipment.

One disadvantage of single-phase unitsis the input power requirement. Singlephase equipment requires 1.73 timesmore input current than three-phaseunits. This becomes very significant athigher magnetizing currents where inputvalues can exceed 600 A.

Three-Phase Full-WaveDirect CurrentCommercial electric power, especially at220 and 440 V, is provided as three-phasealternating current with each phaseproviding part of the total current.Figure 27 shows the time relation ofthree-phase alternating current.Three-phase full-wave magnetic particleequipment rectifies all three alternatingcurrent phases and inverts the negativeflow to a positive direction, producing anearly flat line direct current magnetizingcurrent. Figure 28 shows the waveform ofthree-phase full-wave direct current.

Three-phase full-wave direct currenthas all of the advantages of single-phasefull-wave direct current plus someadditional benefits. The current draw onthe power line is spread over three phases,reducing the demand by nearly half. Thedemand on the line is also balanced, with

58 Magnetic Testing

FIGURE 26. Single-phase full-wave directcurrent waveform.

+

0Cur

rent

Time

FIGURE 25. Half-wave direct currentwaveform.

Alternating current input

Half-wavedirect current output

+

0

–

+

0

–

FIGURE 27. Waveform of three-phasealternating current.

1/60 2/60

Seconds

+

0

Cur

rent

FIGURE 28. Three-phase full-wave directcurrent waveform.

Time

+

0Cur

rent

each leg providing part of the current(single-phase pulls all of the current fromone leg, resulting in an unbalanced line).Many power companies charge a higherrate to customers with unbalanced, highcurrent requirements. The three-phasedesign also permits incorporating a quickbreak circuit that improves the formationof indications at the ends oflongitudinally magnetized test objects.

DemagnetizationObjects that have been magnetic particletested retain some magnetism. Theamount of residual magnetism dependson the material and its condition. Lowcarbon steel in the annealed conditionretains little or no magnetism while

hardened alloy steels retain intensemagnetic fields for long periods of time.

A metal may be demagnetized either byheating it to the curie temperature or byreverse electromagnetization. Reverseelectromagnetization subjects amagnetized object to a magnetic forcethat is continually reversing its directionand gradually decreasing in intensity.

A common method of demagnetizingsmall objects is to pass them through acoil carrying alternating current or toplace the object in the coil and graduallyreduce the current to zero. The principleof demagnetizing with direct current isthe same with alternating current. Themagnetic field intensity or current mustreverse serially and reduce gradually.

Demagnetization is discussed in detailelsewhere in this volume.


The formation of reliably visiblediscontinuity indications is essential tothe magnetic particle testing method. Animportant factor in the formation andvisibility of indications is the use of theproper magnetic particles to obtain thebest indication from a particulardiscontinuity under the given conditions.Selection of the wrong particles can resultin (1) failure to form indications, (2) theformation of indications too faint fordetection or (3) a distorted pattern overthe discontinuity and the resultingmisinterpretations.

In magnetic particle tests, there are twoclasses of media that define the method:dry and wet. Dry method particles areapplied without the addition of a carriervehicle. Wet method particles aresuspended in a liquid vehicle. The liquidvehicle may be water or a light petroleumdistillate similar to kerosene.

Magnetic particles are also categorizedby the type of pigment bonded to themto improve visibility. Visible particles arecolored to produce a good contrast withthe test surface under white or visiblelight. Fluorescent particles are coated withpigments that fluoresce when exposed toultraviolet light. A third pigment categoryincludes particles coated with a materialthat is both color contrasting undervisible light and fluorescent whenexposed to ultraviolet light.

Magnetic ParticlePropertiesThe media used in magnetic particletesting consist of finely divided ironpowder ferromagnetic oxides. Theparticles can be irregularly shaped,spheroidal, flakes or rod shaped(elongated). The properties of differentmaterials, shapes and types vary widelyand some are discussed below.

The level of particles in suspensionshould be maintained consistently, from0.1 to 0.4 mL in a 100 mL settling test.For consistent results, the suspensionvehicle must be changed frequentlybecause of foreign materialcontamination.

Magnetic PermeabilityMagnetic particles should have thehighest possible permeability and thelowest possible retentivity. This allowstheir attraction only to low level leakagefields emanating from discontinuities. Asthe particles become magnetized, theythen attract additional particles to bridgeand outline the discontinuity, thusforming a visible indication.

Magnetic permeability alone does notproduce a highly sensitive particlematerial. For example, iron based drypowders have a higher permeability thanthe oxides used in wet methodsuspensions. Yet a typical dry powder doesnot produce indications of extremely finesurface fatigue cracks that are easilydetected with wet method suspensions.High permeability is desirable but is nomore important than size, shape or theother critical properties. All of thesecharacteristics are interrelated and mustoccur in appropriate ranges in order forhigh permeability to be of value.

Magnetic RetentivityMaterials used in dry method powdersand wet method suspensions should havea low coercive force and low retentivity. Ifthese properties were high in dry powders,the particles would become magnetizedduring manufacture or during their firstuse, reducing contrast and maskingrelevant discontinuity indications.

When wet method particles have ahigh coercive force, they are also easilymagnetized, producing the same highlevel of background. Magnetized particlesare attracted to any ferromagneticmaterial in the testing system (bath tank,plumbing system or rails) and this causesan extensive loss of particles from thesuspension. Particle depletion createsprocess control problems and requiresfrequent additions of new particles to thebath.

Another disadvantage of magneticallyretentive wet method particles is theirtendency to clump, forming large clusterson the test object surface if warmth makesthe pigment sticky.

Strongly magnetized particles formclusters and adhere to the test objectsurface as soon as bath agitation stops.Particles with low magnetic field intensitycluster more slowly while indications are

60 Magnetic Testing

PART 8. Media and Processes in Magnetic ParticleTesting

forming. The leakage field at thediscontinuity draws the particles toward itand the clusters are constantly enlargingdue to agglomeration. At the same time,the clusters sweep up nearby fine particlesas they move toward the discontinuity.

Effects of Particle SizeThe size and shape of magnetic particlesplay an important role in how theybehave when subjected to a weakmagnetic field such as that from adiscontinuity. Large, heavy particles arenot likely to be attracted and held by aweak leakage field as they move over theobject surface. However, very smallparticles may adhere to the surface wherethere is no leakage field and thus form anobjectionable background.

Dry Powder ParticlesWithin limits, sensitivity to very finediscontinuities typically increases asparticle size decreases. Extremely smallparticles, on the order of a fewmicrometers, behave like dust. They settleand adhere to the object surface eventhough it may be very smooth. Extremelyfine particles are very sensitive to lowlevel leakage fields but are not desirablefor production tests because of intensebackgrounds that obscure or maskrelevant indications.

Large particles are not as sensitive tofine discontinuities. However, inapplications where it is desired to detectlarge discontinuities, powders containingonly large particles may be used.

Many commercial dry powders are acarefully controlled mixture of particlescontaining a range of sizes. The smallerparticles provide sensitivity and mobilitywhile larger particles serve two purposes.They assist in building up indications atlarger discontinuities and help reducebackground by a sort of sweeping action,brushing finer particles from the testobject surface. A balanced mixturecontaining a range of sizes providessensitivity for both fine and largediscontinuities, without disruptivebackgrounds.

Wet Method Visible ParticlesParticles used in a liquid suspension areusually much smaller than those used indry powders. The smaller thediscontinuity, the smaller the particlesshould be.

Larger particles are difficult to hold insuspension. Even 20 µm (0.0008 in.)particles tend to settle out of suspensionrapidly and are stranded as the suspensiondrains off the test object. Stranded

particles may line up in drainage linesthat could be confused with discontinuityindications.

Wet Method Fluorescent ParticlesParticles treated with a fluorescentpigment or some of the visible pigmentsdiffer in size and behavior from black orred (uncoated) visible particles.Fluorescent particles must becompounded and structured to preventseparation of the pigment and magneticmaterial during use. A mixture of loosepigment and unpigmented magneticmaterial produces a dense backgroundand dim indications. In addition, theunpigmented magnetic particles may beattracted and held at leakage fields buttheir lack of contrasting color makes themdifficult to see.

Producing fluorescent magneticparticles involves bonding pigmentaround each magnetic particle. Thebonding must resist the solvent action ofpetroleum vehicles and surfactants andthe abrasive action occurring in pumpingand agitation systems. Somemanufacturers encapsulate the bondeddye particle in a layer of resin. As a resultof their processing, fluorescent particleshave a definite size range that ismaintained throughout the suspension’sservice cycle.

Effect of Particle ShapeMagnetic particles are available in avariety of shapes: spheres, elongatedneedles (or rods) and flakes. The shape ofthe particles affects how they formindications. When exposed to an externalmagnetic field, all particles tend to alignalong the flux lines. This tendency ismuch stronger with elongated particlessuch as the needle or rod shapes.Elongated shapes develop internal northand south poles more reliably thanspheroid or globe shaped particles,because they have a smaller internaldemagnetization field.

Because of the attraction of oppositepoles, the north and south poles of thesesmall magnets arrange the particles intostrings. The result is the formation ofmore intense patterns in weaker fluxleakage fields, as these magneticallyformed strings of particles bridge thediscontinuity. The superior effectivenessof elongated shapes over globular shapesis particularly noticeable in the detectionof wide, shallow discontinuities andsubsurface discontinuities. The leakagefields at such discontinuities are weakerand more diffuse. The formation ofparticle strings based on internal polesmakes more intense indications.


Dry Powder ShapesThe superiority of elongated particles indiffuse magnetic fields holds true for drypowder testing. However, there is anothereffect that must be considered. Drypowders are often applied to objectsurfaces by releasing them out ofmechanical or manual blowers. It isessential that the particles be dispersed asa uniform cloud that settles evenly overthe object surface. Magnetic powdercontaining only elongated particles tendsto become mechanically linked in itscontainer and is then expelled in unevenclumps.

Wet Method Particle ShapesThe performance of particles suspended ina liquid vehicle is not as shape dependentas that of dry particles. The suspendingliquid is much denser and more viscousthan air; the movement of particlesthrough the liquid is slowed so that theyaccumulate more reliably atdiscontinuities.

Because of this slower movement, wetmethod particles form minute elongatedaggregates. Even unfavorable shapes alignmagnetically into elongated aggregatesunder the influence of local, low levelleakage fields. In suspension, the particlesare kept dispersed by mechanical agitationuntil they flow over the surface of themagnetized object. There is no need toadd certain shapes to improve thedispersion of the particles.

Visibility and ContrastVisibility and contrast are properties thatmust be considered when selecting amagnetic particle material for a specifictesting application. Magnetic properties,size and shape may all be favorable forproducing the best indication, but if anindication is formed and the inspectorcannot see it, then the test procedure hasfailed.

Visibility and contrast are enhanced bychoosing a particle color that is easy tosee against the test object surface. Thenatural color of metallic powders is silvergray. The colors of iron oxides commonlyused in wet method powders are black orred. Manufacturers bond pigments to theparticles to produce a wide selection ofother colors: white, black, red, blue andyellow, all with comparable magneticproperties.

The white or yellow colors providegood contrast against mill surface objects.They are not effective against the silvergray of grit blasted or chemically etchedsurfaces or against bright, polishedmachine ground surfaces. For those

applications, black, red or blue is used.The choice of color depends on thesurface colors of the test objects and onthe prevailing test site lighting.

The ability to bond fluorescent dyes tomagnetic powders has produced a particlematerial that provides the best possiblevisibility and contrast under properlighting conditions. When test objects areexamined in ultraviolet light, it is difficultnot to see the light emitted by a fewparticles collected at a discontinuity.

Fluorescent particles are magneticallyless sensitive than visible particles but thereduction in magnetic sensitivity is morethan offset by the increase in visibilityand contrast.

Visibility and contrast of fluorescentparticles are directly related to thedarkness of the testing site. In a totallydarkened area, even a small amount ofultraviolet energy activates fluorescentdye to emit a noticeable amount of visiblelight. When the test site is partiallydarkened, the amount of requiredultraviolet energy increases dramaticallyyet the emitted visible light is only barelynoticeable, especially in the conventionalyellow-to-green range.

Most military and commercialspecifications require the test site to bedarkened to 20 lx (2 ftc) or less, with aminimum ultraviolet intensity of1000 µW·cm–2 at the test object surface.

Particle MobilityWhen magnetic particles are applied tothe surface of a magnetized object, theparticles must move and collect at theleakage field of a discontinuity in order toform a visible indication. Any interferencewith this movement has an effect on thesensitivity of the test. Conditionspromoting or interfering with particlemobility are different for dry and wetmethod particles.

Dry Powder MobilityDry particles should be applied in a waythat permits them to reach themagnetized object surface in a uniformcloud with minimum motion. When thisis properly done, the particles come underthe influence of leakage fields whilesuspended in air and are then said topossess three-dimensional mobility. Thiscondition can be approximated onsurfaces that are vertical or overhead.

When particles are applied tohorizontal surfaces, they settle directlyonto the surface and do not have mobilityin three dimensions. Some extension ofmobility can be achieved by tapping orvibrating the test object, agitating theparticles and allowing them to move

62 Magnetic Testing

toward leakage fields. Alternating currentand half-wave rectified alternating current(pulsed direct current) can give particlesexcellent mobility when compared todirect current magnetization.

Wet Method Particle MobilityThe suspension of particles in a liquidvehicle allows mobility for the particles intwo dimensions when the suspensionflows over the test object surface and inthree dimensions when the test object isimmersed in a magnetic particle bath.

Wet method particles have a tendencyto settle out of the suspension either inthe tank of the test system or on the testobject surface short of the discontinuity.To be effective, wet method particles mustmove with the vehicle and must reachevery surface that the vehicle contacts.The settling rate of particles is directlyproportional: (1) to their dimensions and(2) to the difference between their densityand the lower density of the liquidvehicle. Their settling rate is inverselyproportional to the liquid’s viscosity. As aresult, the mobility of wet methodparticles is never ideal and must bebalanced against the other factorsimportant to wet method test results.

Media SelectionThe choice between dry method and wetmethod techniques is influencedprincipally by the followingconsiderations:

1. Type of discontinuity (surface orsubsurface): for subsurfacediscontinuities, dry powder is usuallymore sensitive.

2. Size of surface discontinuity: wetmethod particles are usually best forfine or broad, shallow discontinuities.

3. Convenience: dry powder with portablehalf-wave equipment is easy to use fortests on site or in the field. Wetmethod particles packaged in aerosolspray cans are also effective for fieldspot tests.

The dry powder technique is superiorfor locating subsurface discontinuities,mainly because of the high permeabilityand favorable elongated shape of theparticles. Alternating current with drypowder is excellent for surface cracks thatare not too fine but this combination is oflittle value for cracks lying whollybeneath the surface.

When the requirement is to findextremely fine surface cracks, the wetmethod is superior, regardless of themagnetizing current in use. In some cases,direct current is considered advantageousbecause it also provides some indications

of subsurface discontinuities. The wetmethod also offers the advantage ofcomplete coverage of the object surfaceand good coverage of test objects withirregular shapes.

Visible or Fluorescent ParticlesThe decision between visible particles andfluorescent particles depends onconvenience and equipment. Testing withvisible particles can be accomplishedunder common shop lighting whilefluorescent particles require a darkenedarea and an ultraviolet light source.

Both wet method visible and wetmethod fluorescent tests have about thesame sensitivity, but under proper lightingconditions fluorescent indications aremuch easier to see.

Magnetic Particle TestingProcesses A test object may be magnetized first andparticles applied after the magnetizingcurrent has been stopped (called theresidual method) or the object may becovered with particles while themagnetizing current is present (known asthe continuous method). With test objectsthat have high magnetic retentivity, acombination of the residual andcontinuous methods is sometimes used.

Residual Test MethodIn the residual method, the test object ismagnetized, the magnetizing current isstopped and then the magnetic particlesare applied. This method can only be usedon materials having sufficient magneticremanence. The residual magnetic fieldmust be intense enough to producediscontinuity leakage fields sufficient forproducing visible test indications. As arule, the residual method is most reliablefor detection of surface discontinuities.

Hard materials with high remanenceare usually low in permeability, so higherthan usual magnetizing currents may benecessary to obtain an adequate level ofresidual magnetism. This differencebetween hard steels and soft steels isusually not critical if only surfacediscontinuities are to be detected.

Either dry or wet method particleapplication can be used in the residualmethod. With the wet method, themagnetized test object may be immersedin an agitated bath of suspended magneticparticles or may be flooded with particlesuspension in a curtain spray.

In the immersion technique, theintensity of discontinuity indications isdirectly affected by the object’s dwell timein the bath. By leaving the object in the


bath for extended periods, leakage fieldshave time enough to attract and hold themaximum number of particles, even atfine discontinuities. If the test object hashigh retentivity, longer dwell timeincreases the sensitivity over that of thewet continuous method. Note that thelocation of the discontinuity on theobject during immersion affects theaccumulation of particles. Indications aremore intense on upper horizontal surfacesand weaker on vertical or lower horizontalsurfaces.

Care must be exercised when removingthe test object from the bath or particlespray. Rapid movement can literally washoff indications held by weak discontinuityleakage fields.

Continuous Test MethodWhen a magnetizing current is applied toa ferromagnetic test object, the magneticfield rises to a maximum. Its value isderived from the magnetic field intensityand the magnetic permeability of the testobject. When the magnetizing current isremoved, the residual magnetic field inthe object is always less than the fieldproduced while the magnetizing currentwas applied. The amount of differencedepends on the BH curve of the material.For these reasons, the continuousmethod, for any specific value ofmagnetizing current, is always moresensitive than the residual method.

Continuous magnetization is the onlymethod possible for use on low carbonsteels or iron having little retentivity. It isfrequently used with alternating currenton these materials because of theexcellent mobility produced byalternating current.

With the wet method, the surface ofthe test object is flooded with particlesuspension. The bath application and themagnetizing current are simultaneouslystopped. The magnetic field intensitycontinues to affect particles in the bath asit drains. In some continuous methodprocedures, the magnetizing currentremains on during interpretations.

The wet continuous method requiresmore operator attention than the residualmethod. If bath application continues,even momentarily, after the current isstopped, particles held by a discontinuityleakage field can be washed away. If thereis a pause between stopping the bathapplication and applying the magnetizingcurrent, the suspension can drain off thetest object, leaving insufficient particlesfor producing discontinuity indications.Careless handling of the bath and currentsequence can seriously hinder theproduction of reliable test results.

The highest possible sensitivity for veryfine discontinuities is typically achievedby the following sequence: (1) immersethe test object in the bath, (2) passmagnetizing current through the objectfor a short time during immersion,(3) maintain the current during removalfrom the bath, (4) maintain the currentduring drainage of the suspension fromthe test object and (5) stop themagnetizing current.

ConclusionMagnetic particle tests are effectivenondestructive procedures for locatingmaterial discontinuities in ferromagneticobjects of all sizes and configurations. It isa flexible technique that can beperformed under a variety of conditions,using a broad range of supplementarycomponents.

Application of the magnetic particlemethod is deceptively simple — good testresults can sometimes be produced withlittle more than practical experience. Infact, the development of the techniquehas been almost entirely empirical ratherthan theoretical.

However, the method is founded onthe complex principles ofelectromagnetics and the magneticinteractions of at least three materialssimultaneously. In addition, there is thecritical consideration of the operator’sability to qualitatively and quantitativelyevaluate the results of the inspection.

64 Magnetic Testing

Successful testing requires the test objectto be magnetized properly. Themagnetization can be accomplished usingone of several approaches: (1) permanentmagnets, (2) electromagnets and(3) electric currents used to induce therequired magnetic field.

Excitation systems that use permanentmagnets offer the least flexibility. Themajor disadvantage with such systems liesin the fact that the excitation cannot beswitched off. Because the magnetization isalways turned on, it is difficult to insertand remove the test object from the testrig.

Electromagnets, as well as electriccurrents, are used extensively tomagnetize the test object. Figure 29 showsan excitation system where the test objectis part of a magnetic circuit energized bycurrent passing through an excitationcoil. The physics of magnetization aredescribed elsewhere in this volume.

To obtain maximum sensitivity, it isnecessary to ensure that the magnetic fluxis perpendicular to the discontinuity. Thisdirection is in contrast to the orientationin techniques that use an electric currentfor inspection of a test object, where itmay be more advantageous to orient thedirection of current so that adiscontinuity would impede the currentas much as possible. Because theorientation of the discontinuity isunknown, it is necessary to test twicewith a yoke, in two directions

perpendicular to each other. A grid isusually drawn on the test object tofacilitate the tests.

Magnetizing CoilA commonly used encircling coil is shownin Fig. 30. The field direction follows theright hand rule. (The right hand rulestates that, if someone grips a rod, holds itout and imagines an electric currentflowing down the thumb, the inducedcircular field in the rod would flow in thedirection that the fingers point.) With notest object present, the field lines formclosed loops that encircle the currentcarrying conductors. The value of the fieldat any point has been established for agreat many coil configurations. The valuedepends on the current in the coils, thenumber of turns and a geometrical factor.Calculation of the field from firstprinciples is generally unnecessary fornondestructive testing; a hall elementtesla meter will measure this field.

Two totally different situations,common in magnetic flux leakage testing,are described below.


PART 9. Magnetic Test Techniques

FIGURE 29. Electromagnetic yoke formagnetizing of test object.

Coil

Air gap where testobject is inserted

FIGURE 30. Encircling coil using directcurrent to produce magnetizing force.

R

SQP I

LegendI = electric current

P, Q = points of discontinuities in exampleR = point at which magnetic field intensity H is

measuredS = point at which magnetic flux density B is

measured

Testing in Active FieldIn this technique, the test object isscanned by probes near position R inFig. 30. Application of small fields issufficient to cause magnetic flux leakagefrom transversely oriented surfacebreaking discontinuities. For subsurfacediscontinuities or those on the insidesurface of tubes, larger fields are required.The inspector must experiment tooptimize the applied field for theparticular discontinuity.

Testing in Residual FieldTest objects are passed through the coilfield and tested in the resulting residualfield. Elongating the coil and placing thetest object next to the inside surface ofthe coil will expose the test object to thelargest field that the coil can produce.

This technique is often used inmagnetic particle testing. The mainproblem to avoid is the induction of somuch magnetic flux in the test object thatthe magnetic particles stand out like furalong the field lines that enter and leavethe test object, especially close to its ends.Optimum conditions require that the testobject be somewhat less than saturated.The inspector should experiment tooptimize the coil field requirements forthe test object because this field dependson test object geometry.

Applied Direct CurrentIf an electric current is used to magnetizethe test object, it may be moreadvantageous to orient the direction ofcurrent in a manner where the presenceof a discontinuity impedes the currentflow as much as possible. Bars, billets andtubes are often magnetized by applicationof a direct current I to their ends (Fig. 31).

Figure 32 shows a system where thecurrent I is passed directly through a

tubular test object to magnetize the testobject circularly. Figure 33 shows a centralconductor energized by a current sourceto establish a circular magnetic fieldintensity in a tubular test object.

Capacitor Discharge DevicesFor the circular magnetization of tubes orthe longitudinal magnetization of theends of elongated test objects, a capacitordischarge device is sometimes used.4,5 Thecapacitor discharge unit represents apractical advance over battery packs andconsists of a capacitor bank charged to avoltage V and then discharged through arod, a cable and a silicon controlledrectifier of total resistance R. Typicalconfigurations are shown in Fig. 34.

Larger capacitances at lower voltagesprovide better magnetization than smallercapacitances at higher voltages becauselarger capacitances at lower voltages leadto longer duration pulses and therefore tolower eddy currents. The lower voltage isan essential safety feature for outdoor use.A maximum of 50 V is recommended.6

66 Magnetic Testing

FIGURE 31. Circumferential magnetization by application ofdirect current: (a) rectilinear bar; (b) round bar; (c) tube.

(a)

(b)

(c)

H

H

H

I

I

I

LegendH = magnetic field intensityI = electric current

FIGURE 32. Current carrying clamp electrodes used fortesting ferromagnetic tubular objects with small diameters.

Magnetic flux lines

Current I in

Current I out

Clamp

FIGURE 33. Simple technique for circumferentialmagnetization of ferromagnetic tube.

r

H

Current sourceI

LegendH = magnetic field intensityI = electric currentr = tube radius

Magnitudes of MagneticFlux Leakage FieldsThe magnitude of the magnetic fluxleakage field under active direct currentexcitation naturally depends on theapplied field. An applied field of 3.2 to4.0 kA·m–1 (40 to 50 Oe) inside thematerial can cause leakage fields withpeak values of tens of millitesla (hundredsof gauss). However, in the case of residualinduction, the magnetic flux leakagefields may be only a few hundredmicrotesla (a few gauss). Furthermore,with residual field excitation, aninteresting field reversal may occur,depending on the value of the initialactive field excitation and the dimensionsof the discontinuity.

Optimal Operating PointConsider raising the magnetization levelin a block of steel containing adiscontinuity (Fig. 35). At low fluxdensity, the field lines tend to crowdtogether in the steel around thediscontinuity rather than go through thenonmagnetic region of the discontinuity.The field lines are therefore more crowdedabove and below the discontinuity thanthey are on the left or right. The materialcan hold more flux as the permeabilityrises, so there is no significant leakageflux at the surfaces (Fig. 35a).

However, an increase in the number oflines causes permeability to fall. At aboutthis point, magnetic flux leakage is firstnoticed at the surfaces. Although the linesare now closer together, representing ahigher magnetic flux density, they do nothave the ability to crowd closer togetheraround the discontinuity where thepermeability is low.

At higher values of applied field, thepermeability continues to fall. It is,however, still large compared to thepermeability of air, so the reluctance ofthe path through the discontinuity is stilllarger than through the metal. As a result,magnetic flux leakage at the outsidesurface helps provide a sufficiently highflux density in the material for theleakage of magnetic flux fromdiscontinuities (Fig. 35b) while partiallysuppressing long range surface noise.


FIGURE 34. Capacitor dischargeconfigurations causing magnetizationperpendicular to current direction:(a) conductor internal to test object createscircular field; (b) flexible cable around testobject creates longitudinal field.

(a)

C

(b)

SCR lc

le

le

Circular field

C

SCRlc

Longitudinal field

LegendC = capacitorIc = capacitor discharge currentIe = eddy current

SCR = silicon controlled rectifier

Capacitor discharge unit

FIGURE 35. Effects of induction on magneticflux lines at discontinuity: (a) no surface fluxleakage occurs where magnetic flux lines arecompressed at low levels of inductionaround discontinuity; (b) lack ofcompression at high magnetization results insurface magnetic flux leakage.

Flux leakage

(a)

(b)

For residual field testing, it is best toensure that the material is saturated. Themagnetic field starts to decay as soon asthe energizing current is removed.

Magnetic Test ProbesThe purpose of probes for magnetictesting is to detect and possibly quantifythe magnetic flux leakage field generatedby heterogeneities in the test object. Theleakage fields tend to be local andconcentrated near the discontinuities. Theleakage field can be divided into threeorthogonal components: normal(vertical), tangential (horizontal) and axialdirections. Probes are usually eitherdesigned or oriented to measure one ofthese components. Typical plots of thesecomponents near discontinuities areshown in this volume’s chapter on probes.

A variety of probes (or transducers) areused in industry for detecting andmeasuring leakage fields.

Pickup CoilsOne of the simplest and most popularmeans for detecting leakage fields is to usea pickup coil.7 Pickup coils consist of verysmall coils that are either air cored or usea small ferrite core. The voltage inducedin the coil is given by the rate of changeof flux linkages associated with the pickupcoil. Only the component of the fluxparallel to the axis of the coil (oralternately perpendicular to the plane ofthe coil) is instrumental in inducing thevoltage. This induction direction makes itpossible to orient the pickup coil so as tomeasure any of the three leakage fieldcomponents selectively (Fig. 36).

The output of the pickup coil isproportional to the spatial gradient of theflux along the direction of the coilmovement as well as the velocity of thecoil. Two issues arise as a result.

1. It is essential that the probe scanvelocity (relative to the test object)should be constant to avoidintroducing artifacts into the signalthrough probe velocity variations.

2. The output is proportional to thespatial gradient of the flux in thedirection of the coil.

The output of the pickup coil can beintegrated for measurement of the leakageflux density rather than of its gradient.Figure 37 shows the output of a pickupcoil and the signal obtained afterintegrating the output.8 The coil is used tomeasure, in units of tesla (or gauss), themagnetic flux density B leaking from arectangular slot.

The sensitivity of the pickup coil canbe improved by using a ferrite core. Toolsfor designing pickup coils, as well aspredicting their performance, aredescribed elsewhere in this volume.

MagnetodiodesThe magnetodiode is suitable for sensingleakage fields from discontinuities becauseof its small size and its high sensitivity.

68 Magnetic Testing

FIGURE 36. Effect of pickup coil orientationon sensitivity to components of magneticflux density: (a) coil sensitive to normalcomponent; (b) coil sensitive to tangentialcomponent.

(a) Pickup coil

(b) Pickup coil

Test object

Test object

FIGURE 37. Pickup coil and signal integrator (magnetic fluxleakage) output for rectangular discontinuity.7

30

20

10

0

–10

–20

–30

Out

put

fro

m s

earc

h co

il (m

V)

Coil position, mm (10–2 in.)

Search coil output

0 2 4 6 8 10(8) (16) (24) (32) (40)

0.3 (3)

0.2 (2)

0.1 (1)

0

Mag

netic

flu

x de

nsity

B,

T (k

G)

Magnetic fluxleakage

Discontinuitywidth

Because the coil probe is usually largerthan the magnetodiode, it is less sensitiveto longitudinally angled discontinuitiesthan the magnetodiode is. However, thecoil probe is better than themagnetodiode for large discontinuities,such as cavities.

Hall Effect DetectorsHall effect detector probes are usedextensively in industry for measuringmagnetic flux leakage fields in units oftesla. Hall effect detector probes aredescribed in this volume’s chapter onprobes for electromagnetic testing.

Giant Magnetoresistive ProbesMagnetic field sensitive devices calledgiant magnetoresistive probes,9,10 at themost basic level, consist of a nonmagneticlayer sandwiched between two magneticlayers. The apparent resistivity of thestructure varies depending on whether thedirection of the electron spin is parallel orantiparallel to the moments of themagnetic layers. When the momentsassociated with the magnetic layers arealigned antiparallel, the electrons withspin in one direction (up) that are notscattered in one layer will be scattered inthe other layer. This increases theresistance of the device. This is in contrastto the situation when the magneticmoments associated with the layers areparallel and the electrons that are notscattered in one layer are not scattered inthe other layer, either.

Giant magnetoresistive probes use abiasing current to push the magneticlayers into an antiparallel moment stateand the external field is used to overcome

the effect of the bias. The resistance of thedevice, therefore, decreases withincreasing field intensity values. Figure 38shows a typical response of a giantmagnetoresistive probe.

Magnetic TapeFor the testing of flat surfaces, magnetictape can be used. The tape is pressed tothe surface of the magnetized billet andthen scanned by small probes beforebeing erased. This technique is sometimescalled magnetography.

In automated systems, magnetic tapecan be fed from a spool. The signals canbe read and the tape can be erased andreused.

Unfortunately, the tangential leakagefield intensity at the surface of thematerial is not constant. To optimize theresponse, the amplification of the signalscan be varied.

Scabs or slivers projecting from the testsurface can easily tear the tape

Magnetic ParticlesMagnetic particles are the most popularmeans used in industry for makingmagnetic indications. The descriptionsbelow are cursory.

Magnetic particle testing involves theapplication of magnetic particles to thetest object before or after it is magnetized.The ferromagnetic particles preferentiallyadhere to the surface of the test object inareas where the flux is diverted, or leaksout. The magnetic flux leakage neardiscontinuities causes the magneticparticles to accumulate in the region andin some cases form an outline of thediscontinuity. Heterogeneities cantherefore be detected by looking forindications of magnetic particleaccumulations on the surface of the testobject either with the naked eye orthrough a camera. The indications areeasier to see if the particles are bright andreflective. Alternately, particles thatfluoresce under ultraviolet or visibleradiation may be used. The test object hasto be viewed under appropriate levels ofillumination with radiation of appropriatewavelength (visible, ultraviolet or other).

Application TechniquesMagnetic particles are applied to thesurface by two different techniques inindustry.Dry Testing. Dry techniques use particlesapplied in the form of a fine stream or acloud. They consist of high permeabilityferromagnetic particles coated with eitherreflective or fluorescent pigments. Theparticle size is chosen according to the


FIGURE 38. Resistance versus applied field for2 µm (8 × 10–5 in.) wide strip ofantiferromagnetically coupled, multilayertest object composed of 14 percent giantmagnetoresistive material.9

–32 –16 0 16 32(–0.4) (–0.2) (0.2) (0.4)

4.2

4.1

4.0

3.9

3.8

3.7

3.6

Resi

stan

ce (

kΩ)

Applied magnetic field, kA·m–1 (kOe)

dimensions of the discontinuity sought.Particle diameters range from �50 to180 µm (�0.002 to 0.007 in.). Finerparticles are used for detecting smallerdiscontinuities where the leakageintensity is low. Dry techniques are usedextensively for testing welds and castingswhere discontinuities of interest arerelatively large.Wet Testing. Wet techniques are used fordetecting relatively fine cracks. Themagnetic particles are suspended in aliquid (usually oil or water) that can besprayed on the test object. Particle sizesare significantly smaller than those usedwith dry techniques and vary in sizewithin a normal distribution, with mostparticles measuring from 5 to 20 µm(2 × 10–4 to 8 × 10–4 in.). As in the case ofdry powders, the ferromagnetic particlesare coated with either reflective orfluorescent pigments.

Imaging of Magnetic ParticleIndicationsThe magnetic particle distribution can beexamined visually after illuminating thesurface or the surface can be scanned witha flying spot system11,12 or imaged with acharge coupled device camera.Flying Spot Scanners. To illuminate thetest object, flying spot scanners use anarrow beam of radiation — visible lightfor nonfluorescent particles andultraviolet radiation for fluorescent ones.The source of the beam is usually a laser.The wavelength of the beam is chosencarefully to excite the pigment of themagnetic particles. The incidence of theradiation beam on the test object can bevaried by moving the scanning mirror.

The photocell does not sense any lightwhen the test object is scanned by thenarrow radiation beam until the beam isdirectly incident on the magnetic particlesadhering to the test object near adiscontinuity. When this occurs, a largeamount of light is emitted, calledfluorescence if excited by ultravioletradiation. The fluorescence is detected bya single phototube equipped with a filterthat renders the system blind to theradiation from the irradiating source. Theoutput of the photocell is suitablyamplified, digitized and processed by acomputer.Charge Coupled Devices. An alternativeapproach is to flood the test object withradiation whose wavelength is carefullychosen to excite the pigment of themagnetic particles. Charge coupled devicecameras,13,14 equipped with optical filtersthat render the camera blind to radiationfrom the source but are transparent tolight emitted by the magnetic particles,

can be used to image the surface veryrapidly.

In very simple terms, charge coupleddevices each consist of a two-dimensionalarray of tiny pixels that each accumulatesa charge corresponding to the number ofphotons incident on it. When a readoutpulse is applied to the device, theaccumulated charge is transferred fromthe pixel to a holding or charge transfercell. The charge transfer cells areconnected in a manner that allows themto function as a bucket brigade or shiftregister. The charges can, therefore, beserially clocked out through acharge-to-voltage amplifier that producesa video signal.

In practice, charge coupled devicecameras can be interfaced to a personalcomputer through frame grabbers, whichare commercially available. Vendors offrame grabbers usually provide softwarethat can be executed on the personalcomputer to process the image. Imageprocessing software can be used toimprove contrast, highlight the edges of adiscontinuity or to minimize noise in theimage.

Test CalculationsIn determining the magnetic flux leakagefrom a discontinuity, certain conditionsmust be known: (1) the discontinuity’slocation with respect to the surfaces fromwhich measurements are made, (2) therelative permeability of the materialcontaining the discontinuity and (3) thelevels of magnetic field intensity H andmagnetic flux density B in the vicinity ofthe discontinuity. Even with thisknowledge, the solution of the applicablefield equations (derived from Maxwell’sequations of electromagnetism) is difficultand is generally impossible in closedalgebraic form. Under certaincircumstances, such as those ofdiscontinuity shapes that are easy tohandle mathematically, relatively simpleequations can be derived for the magneticflux leakage if simplifying assumptions aremade. This simplification does not applyto subsurface inclusions.

Finite Element TechniquesAn advance in magnetic theory since1980 has been the introduction of finiteelement computer codes to the solutionof magnetostatic problems. Such codescame originally from a desire to minimizeelectrical losses from electromagneticmachinery but soon found application inmagnetic flux leakage theory. Theadvantage of such codes is that, once setup, discontinuity leakage fields can becalculated by computer for any size and

70 Magnetic Testing

shape of discontinuity, under anymagnetization condition, so long as theBH curve for the material is known.

In the models of magnetic flux leakagediscussed so far, the implicit assumptionsare (1) that the field within adiscontinuity is uniform and (2) that thenonlinear magnetization characteristic(BH curve) of the tested material can beignored. Much of the early pioneeringwork in magnetic flux leakage modelingused these assumptions to obtain closedform solutions for leakage fields.

The solutions of classical problems inelectrostatics have been well known tophysicists for almost a century and theirmagnetostatic analogs were used toapproximate discontinuity leakage fields.Such techniques work reasonably wellwhen the permeability around a

discontinuity is constant or whennonlinear permeability effects can beignored. The major problem that remainsis how to deal with real discontinuityshapes often impossible to handle byclassical techniques.

Such deficiencies are overcome by theuse of computer programs written toallow for nonlinear permeability effectsaround oddly shaped discontinuities.Specifically, computerized finite elementtechniques, originally developed forstudying magnetic flux distributions inelectromagnetic machinery, have alsobeen developed for nondestructivetesting. Both active and residualexcitation are discussed above. Theextension of the technique to includeeddy currents is detailed elsewhere.


Magnetic flux leakage testing is acommonly used technique. Signals fromprobes are processed electronically andpresented to indicate discontinuities.Although some techniques of magneticflux leakage testing may not be assophisticated as others, it is probable thatmore ferromagnetic material is tested withmagnetic flux leakage than with any othertechnique.

Magnetizing techniques have evolvedto suit the geometry of the test objects.The techniques include yokes, coils, theapplication of current to the test objectand conductors that carry current throughhollow test objects. Many situations existin which current cannot be applieddirectly to the test object because of thepossibility of arc burns. Designconsiderations for magnetization of testobjects often require minimizing thereluctance of the magnetic circuit,consisting of (1) the test object, (2) themagnetizing system and (3) any air gapsthat might be present.

Test Object Configurations

Short Asymmetrical ObjectsA short test object with little or nosymmetry may be magnetized tosaturation by passing current through itor by placing it in an encircling coil. Ifhollow, a conductor can be passedthrough the test object and magnetizationachieved by any of the standardtechniques (these include half-wave andfull-wave rectified alternating current,pure direct current from battery packs orpulses from capacitor discharge systems).For irregularly shaped test objects, testingby wet or dry magnetic particles is oftenperformed, especially if specificationsrequire that only surface breakingdiscontinuities be found.

Elongated ObjectsThe cylindrical symmetry of elongatedtest objects such as wire rope permits arelatively simple flux loop to magnetize arelatively short section of the rope.Encircling probes are placed at somedistance from the rope to permit thepassage of splices. Such systems are also

suited for pumping well sucker rods andother elongated oil field test objects.

After a well is drilled, the sides of thewell are lined with a relatively thin steelcasing material, which is then cementedin. This casing can be tested only fromthe inside surface. The cylindricalgeometry of the casing permits the fluxloop to be easily calculated so thatmagnetic saturation of the well casing isachieved.

As with inservice well casing, buriedpipelines are accessible only from theinside surface. The magnetic flux loop isthe same as for the well casing testsystem. In this case, a drive mechanismmust be provided to propel the testsystem through the pipeline.

Threaded Regions of PipeAn area that requires special attentionduring the inservice testing of drill pipe isthe threaded region of the pin and boxconnections. Common problems thatoccur in these regions include fatiguecracking at the roots of the threads andstretching of the thread metal. Automatedsystems that use both active and residualmagnetic flux techniques can be used fordetecting such discontinuities.

Ball Bearings and RacesSystems have been built for themagnetization of both steel ball bearingsand their races. One such system usesspecially fabricated hall elements asdetectors.

Relatively Flat SurfacesThe testing of welded regions between flator curved plates is often performed usinga magnetizing yoke. Probe systemsinclude coils, hall effect detectors,magnetic particles and magnetic tape.

Discontinuity MechanismsIn the metal forming industry,discontinuities commonly found bymagnetic flux leakage include overlaps,seams, quench cracks, gouges, rolled-inslugs and subsurface inclusions. In thecase of tubular goods, internal mandrelmarks (plug scores) can also be identifiedwhen they result in remaining wall

72 Magnetic Testing

PART 10. Techniques of Magnetic Testing15

thicknesses below some specifiedminimum. Small marks of the same typecan also act as stress raisers: cracking canoriginate from them during quench andtemper procedures. Depending on the useto which the material is put, subsurfacediscontinuities such as porosity andlaminations may also be detrimental.Such discontinuities may be acceptable inwelds where there are no cyclic stressesbut may cause injurious cracking whensuch stresses are present.

In the metal processing industries,grinding especially can lead to surfacecracking and to some changes in surfacemetallurgy. Such discontinuities ascracking have traditionally been found bymagnetic flux leakage techniques,especially wet magnetic particle testing.

Service induced discontinuities includecracks, corrosion pitting, stress inducedmetallurgy changes and erosion fromturbulent fluid flow or metal-to-metalcontact. In those materials placed intension and under torque, fatiguecracking is likely to occur. A discontinuitythat arises from metal-to-metal wear issucker rod wear in tubing from producingoil wells. Here, the pumping rod can rubagainst the inner surface of the tube andboth the rod and tube wear thin. In wirerope, the outer strands will break afterwearing thin and inner strands sometimesbreak at discontinuities present when therope was made. Railroad rails are subjectto cyclic stresses that can cause crackingto originate from otherwise benigninternal discontinuities.

Loss of metal caused by a conductingfluid near two slightly dissimilar metals isa very common form of corrosion. Thedissimilarity can be quite small, as forexample, at the heat treated end of a rodor tube. The result is preferentialcorrosion by electrolytic processes,compounded by erosion from a containedflowing fluid. Such loss mechanisms arecommon in subterranean pipelines,installed petroleum well casing and inrefinery and chemical plant tubing.

The stretching and cracking of threadsis a common problem. For example, whentubing, casing and drill pipe areovertorqued at the coupling, the threadsexist in their plastic region. This causesmetallurgical changes in the metal andcan create regions where stress corrosioncracking takes place in highly stressedareas at a faster rate than in areas of lessstress. Couplings between tubes are agood example of places where materialmay be highly stressed. Drill pipe threadsare a good example of places where suchstress causes plastic deformation andthread root cracking.

Typical MagneticTechniques

Short PartsFor many short test objects, the mostconvenient probe is magnetic particles.The test object can be inspected forsurface breaking discontinuities during orafter it has been magnetized to saturation.For active field testing, the test object canbe placed in a coil carrying alternatingcurrent and sprayed with magneticparticles. Or it can be magnetized tosaturation by a direct current coil and theresulting residual induction can be shownwith magnetic particles. In the latter case,the induction in the test object can bemeasured with a flux meter. Wet particlesperform better than dry ones becausethere is less tendency for the wet particlesto fur (that is, to stand up like short hairs)along the field lines that leave the testobject. These techniques will detecttransversely oriented, tightdiscontinuities.

The magnetic flux leakage fieldintensity from a tight crack is roughlyproportional to the magnetic fieldintensity Hg across the crack, multipliedby crack width Lg. If the test is performedin residual induction, the value of Hg(which depends on the local value of thedemagnetization field in the test object)will vary along the test object. Thus, thesensitivity of the technique todiscontinuities of the same geometryvaries along the length of the test object.

For longitudinally orienteddiscontinuities, the test object must bemagnetized circumferentially. If the testobject is solid, then current can be passedthrough the test object, the surface fieldintensity being given by Ampere’s law:

(1)

where d� is an element of length (meter),H is the magnetic field intensity (ampereper meter) and I is the current (ampere) inthe test object.

If the test object is a cylindrical bar, thesymmetry of the situation allows H to beconstant around the circumference, so theclosed integral reduces:

(2)

or:

(3) H =πIR2

2π =R IH

H∫ =d I�


where R is the radius (meter) of thecylindrical test object. A surface fieldintensity that creates an acceptablemagnetic flux leakage field from theminimum sized discontinuity must beused. Such fields are often created byspecifying the amperage per meter of thetest object’s outside diameter.

Transverse DiscontinuitiesBecause of the demagnetizing effect at theend of a tube, automated magnetic fluxleakage test systems do not generallyperform well when scanning fortransverse discontinuities at the ends oftubes. The normal component Hy of thefield outside the tube is large and canobscure discontinuity signals. Testspecifications for such regions ofteninclude the requirement of additionallongitudinal magnetization at the tubeends and subsequent magnetic particletests during residual induction. Thissituation is equivalent to themagnetization and testing of short testobjects as outlined above.

The flux lines must be continuous andmust therefore have a relatively shortpath in the metal. Large values of themagnetizing force at the center of the coilare usually specified. Such values dependon the weight per unit length of the testobject because this quantity affects theratio of length L to diameter D. Where thetest object is a tube, the L·D–1 ratio isgiven by the length between the polesdivided by twice the wall thickness of thetube. (The distance L from pole to polecan be longer or shorter than the actuallength of the test object and must beestimated by the operator.) As a roughexample, with L = 460 mm (18 in.) andD = 19 mm (0.75 in.), the L·D–1 ratio is24.

The effective permeability of the metalunder test is small because of the largedemagnetization field created in the testobject by the physical end of the testobject. An empirical formula is often usedto calculate approximately the effectivepermeability µ:

(4)

so effective permeability µ = 139 in theabove example.

For wet magnetic particle testing, thesurface tension of the fluids that carry theparticles is large enough to confine theparticles to the surface of the test object.This is not the case with dry particles,which have the tendency to stand up likefur along lines of magnetizing force.

In many instances, it may be better touse some other test technique for

transverse discontinuities, such asultrasonic or eddy current techniques.

Alternating Current versus DirectCurrent MagnetizationAlternating current magnetization is moresuitable for detection of outer surfacediscontinuities because it concentrates themagnetic flux at the surface. For equalmagnetizing forces, an alternating currentfield is better for detecting outside surfaceimperfections but a direct current field isbetter for detecting imperfections belowthe surface.

In practice, the ends of tubes are testedfor transverse discontinuities by thefollowing magnetic flux leakagetechniques.

1. Where there is a direct current activefield from an encircling coil, magneticparticles are applied to the testedmaterial while it is maintained at ahigh level of magnetic induction by adirect current field in the coil. Thistechnique is particularly effective forinternal cracks. Fatigue cracks in drillpipe are often found by thistechnique.

2. Where there is an alternating currentactive field from an encircling coil,magnetic particles are applied to thetested material while it lies inside acoil carrying alternating current. Using50 or 60 Hz alternating current, thepenetration of the magnetic field intothe material is small and thetechnique is good only for thedetection of outside surfacediscontinuities.

When tests for both outer surface andinner surface discontinuities are necessary,it may be best to test first for outer surfacediscontinuities with an alternating currentfield, then for inner surfacediscontinuities with a direct current field.

Liftoff Control of Scanning HeadTo obtain a stable detection ofdiscontinuities, liftoff between the probeand the surface of the material must bekept constant. Usually liftoff is keptconstant by contact of the probe with thesurface but the probe tends to wear withthis technique. A magnetic floatingtechnique has been used for noncontactscanning. In this technique, liftoff ismeasured by a gap probe and the probeholder is moved by a coil motor,controlled by the gap signal.16 This systemand related technology are described inthis volume’s chapter on primary metalsapplications.

μ = −6 5LD

74 Magnetic Testing

Discontinuities2

For magnetic testing, discontinuities maybe broadly categorized according to theirorigin in the stages of fabrication andservice.

1. Primary production and processing testsare used to inspect the stages ofprocessing from pouring andsolidification of the ingot toproduction of basic shapes, includingsheet, bar, pipe, tubing, forgings andcastings. These tests are typically usedto locate two discontinuity subgroups:(a) those formed during solidificationare called inherent discontinuities;(b) those formed during mill reductionare called primary processingdiscontinuities.

2. Secondary processing, or manufacturingand fabrication tests, are used to inspectthe results of processes that convertraw stock into finished components.Forming, machining, welding andheat treating discontinuities aredetected.

3. Service tests are widely used fordetecting overstress and fatiguecracking. Magnetic particle tests arenot used to detect corrosion,deformation or wear, three of the mostcommon service induced problems.Flux leakage testing, however, can beused to detect material loss resultingfrom corrosion or abrasion — in wirerope, for example.

Discontinuities caused duringmanufacture include cracks, seams,forging laps, laminations and inclusions.

1. Cracking occurs when quenched steelcools too rapidly.

2. Seams occur in several ways,depending on when they originateduring fabrication.

3. Discontinuities such as piping orinclusions within a bloom or billet canbe elongated until they emerge as longtight seams or gouges during initialforming processes. They may later beclosed with additional forming.

4. Their metallurgical structures are oftendifferent but the origin ofmanufactured discontinuities is notusually taken into account whenrejecting a part.

5. Forging laps occur when gouges or finscreated in one metal working processare rolled over at an angle to thesurface in subsequent processes.

6. Inclusions are pieces of nonmagneticor nonmetallic materials embeddedinside the metal during cooling.Inclusions are not necessarilydetrimental to the use of the material.

7. The pouring and cooling processes canalso result in lack of fusion within thesteel. Such regions may be workedinto internal laminations.

Discontinuities in used materialsinclude fatigue cracks, pitting, corrosion,erosion and abrasive wear.

Much steel is acceptable to theproducer’s quality assurance department ifno discontinuities are found or ifdiscontinuities are considered to be of adepth or size less than some prescribedmaximum. Specifications exist for theacceptance or rejection of such materialsand such specifications sometimes lead todebate between the producer and the enduser. Discontinuities can either remainbenign or can grow and cause prematurefailure of the part. Abrasive wear can turnbenign subsurface discontinuities intodetrimental surface breakingdiscontinuities.

For used materials, fatigue crackingcommonly occurs as the material iscyclically stressed. Fatigue cracks growrapidly under stress or in the presence ofcorrosive materials such as hydrogensulfide, chlorides, carbon dioxide andwater. For example, drill pipe failure fromfatigue often initiates at the bases of pits,at tong marks or in regions where thetube has been worn by abrasion. Pitting iscaused by corrosion and erosion betweenthe steel and a surrounding or containingfluid. Abrasive wear occurs in many steelstructures. Good examples are (1) thewear on drill pipe caused by hardformations when drilling crooked holes or(2) the wear on both the sucker rod andthe producing tubing in rod pumping oilwells. Specifications exist for themaximum permitted wear under theseand other circumstances. In manyinstances, such induced damage is firstfound by automated magnetic techniques.


PART 11. Magnetic Testing Applications

Basic Ferromagnetic MaterialsProductionIn the production of ferrous alloys, ironore is converted to steel in one or morefurnaces where it is melted and refinedand where alloying elements are added.While in the liquid state, the metal ispoured into a mold and allowed tosolidify into a shape typically called aningot or is continuously cast.

Ingots are quite large and must beformed into more manageable shapes byhot working through a series of rolls ormills. These semifinished shapes are calledblooms, billets or slabs, depending on sizeand shape. A bloom is an intermediateproduct, rectangular in shape with a crosssectional area typically larger than0.02 m2 (36 in.2). A billet can be round orsquare with a cross sectional area from1600 mm2 to 0.02 m2 (2.5 to 36 in.2). Aslab is an intermediate shape between aningot and a plate with a width at leasttwice its thickness.

Inherent DiscontinuitiesThis group of discontinuities occursduring the initial melting and refiningprocesses and during solidification fromthe molten state. Such discontinuities arepresent before rolling or forging isperformed to produce intermediateshapes.

PipeWhen molten metal is continuously castor poured into a mold, solidificationprogresses gradually, starting at the sidesand progressing upward and inward.There is progressive shrinkage duringsolidification. For ingots, the last metal tosolidify is at the top and center of themold. Because of the shrinkage, there istypically insufficient liquid metalremaining to fill the mold and adepression or cavity is formed.

In addition, impurities such as oxidesand entrapped gases tend to migrate tothe center and top of a mold and maybecome embedded in the last portions tosolidify. After solidification, the upperportion is cut off or cropped anddiscarded, removing most of theshrinkage cavity and impurities. However,if the cavity is deeper than normal or ifthe cropping is short, some of theunsound metal will show up in theintermediate shape as a void called pipe.Pipe is almost always centered in thesemifinished shape and is undesirable formost purposes.

Nonmetallic InclusionsAll steel contains nonmetallic matter thatmainly originates in deoxidizing materialsadded to the molten metal during therefining operation. These additives areeasily oxidized metals such as aluminum,silicon, manganese and others. The oxidesand sulfides of the metals make up themajority of nonmetallic inclusions. Whenfinely divided and well distributed, thesediscontinuities are often notobjectionable.

However, sometimes the additivescollect during solidification and formlarge clumps in an ingot. During primaryprocessing these large clumps are rolledout into long discontinuities calledstringers. In highly stressed components,stringers can act as nucleation points forfatigue cracking. In certain test objects,stringers are acceptable in a limitedamount. Government and industryspecifications on steel cleanliness definethe amount of inclusions or stringers thatmay be accepted.

The addition of lead or sulfur tomolten steel is a common practice for thealloys known as free machining steels.These alloys contain a large number ofnonmetallic inclusions that break or chipduring machining operations. Magneticparticle tests of free machining alloysoften indicate an alarming number ofdiscontinuities that are not considereddetrimental in service.

BlowholesAs molten steel is poured into an ingotand solidification commences, there is anevolution of gases. These gases risethrough the liquid in the form of bubblesand many escape or migrate to thecropped portion of the ingot.

However, some gases can be trapped inthe ingot, forming the discontinuitiesknown as blowholes. Most blowholes areclean and will weld or fuse shut duringprimary and secondary rolling. Those nearthe surface may have an oxidized skinand will not fuse, appearing as seams inthe rolled, forged or extruded product.Oxidized blowholes in the interior of slabsappear as laminations in plate products.

Ingot CracksContraction of the metal duringsolidification and cooling of the ingotgenerates significant surface stresses andinternal stresses that can result incracking. If the cracks are internal and noair reaches them, they are usually weldedshut during rolling and do not result indiscontinuities. If they are open to the airor otherwise become oxidized, they willnot seal but remain in the finishedproduct.

76 Magnetic Testing

During the rolling of an ingot into abillet, oxidized cracks form long seams. Itis common practice to use magneticparticle tests of billets before additionalprocessing. Such preprocessing testspermit the removal of seams by grinding,chipping or flame scarfing. If not removedbefore rolling or working, seams arefurther elongated in finished shapes andthis may make the final productunsuitable for many applications.

Primary ProcessingDiscontinuitiesWhen steel ingots are worked down toshapes such as billets, slabs and forgingblanks, some inherent discontinuities mayremain in the finished product. Inaddition, rolling or forming operationsmay themselves introduce otherdiscontinuities. The primary processesconsidered here include the hot workingand cold working methods of producingshapes such as plate, bar, rod, wire, tubingand pipe.

Forging and casting are also includedin this category because they typicallyrequire additional machining or othersubsequent processing. All the primaryprocesses have the potential forintroducing discontinuities into cleanmetal.

SeamsSeams in bar, rod, pipe, wire and tubingare usually objectionable. They originatefrom ingot cracks and despitepreprocessing tests, some cracks can beoverlooked or incompletely removed.

Rolling and drawing operations canalso produce seams in the finishedproduct. If the reduction on any of therolling passes is too great, an overfill maythen produce a projection from the billet.This projection can be folded or lappedon subsequent passes, producing a longdeep seam.

The reverse also occurs if the shapedoes not fill the rolls, resulting in adepression or surface groove. Onsubsequent rolling passes, this underfillproduces a seam running the full lengthof the shape. Seams originating fromoverfilled rolls usually emerge at an acuteangle to the surface. Seams caused byunderfilled rolls are likely to be normal toor perpendicular to the surface.

Seams or die marks can be introducedby defective or dirty dies during drawingoperations. Such seams are often fairlyshallow and may not be objectionable,especially when subsequent machiningremoves the seam. Seams are alwaysobjectionable in components thatexperience repeated or cyclic stresses in

service. These seams can initiate fatiguecracks.

LaminationsLaminations in plate, sheet and strip areformed when blowholes or internal cracksare not fused shut during rolling but areflattened and enlarged. Laminations arelarge and potentially troublesome areas ofhorizontal discontinuity.

Magnetic particle testing detectslamination only when it reaches andbreaks the edges of a plate. Laminationsthat are completely internal to the testobject typically lie parallel to its surfaceand cannot be detected by magneticparticle procedures.

CuppingCupping occurs during drawing orextruding operations when the interior ofthe shape does not flow as rapidly as thesurface. The result is a series of internalruptures that are serious whenever theyoccur. Cupping can be detected by themagnetic particle method only when it issevere and approaches the surface.

Cooling CracksBar stock is hot rolled and then placed ona bed or cooling table and allowed toreach room temperature. During cooling,thermal stresses may be set up by unevenrates of temperature change within thematerial. These stresses can be sufficientfor generating cracks.

Cooling cracks are generallylongitudinal but because they tend tocurve around the object shape, they arenot necessarily straight. Such cracks maybe long and often vary in depth alongtheir length. Magnetic particle indicationsof cooling cracks therefore can vary inintensity (heavier where the crack isdeepest).

Forging DiscontinuitiesForgings are produced from an ingot, abillet or forging blank that is heated tothe plastic flow temperature and thenpressed or hammered between dies intothe desired shape. This hot workingprocess can produce a number ofdiscontinuities, some of which aredescribed below.

FlakesFlakes are internal ruptures that somebelieve are caused by cooling too rapidly.Another theory is that flakes are caused bythe release of hydrogen gas duringcooling.


Flakes usually occur in fairly heavysections and some alloys are moresusceptible than others. These ruptures areusually well below the surface, typicallymore than halfway between the surfaceand the center. Because of theirpositioning, flakes are not detectable bymagnetic particle techniques unlessmachining brings the discontinuity closeto the surface.

Forging BurstsWhen steel is worked at impropertemperatures, it can crack or rupture.Reducing a cross section too rapidly canalso cause forging bursts or severecracking.

Forging bursts may be internal orsurface anomalies. When at or near thesurface, they can be detected by themagnetic particle method. Internal burstsare not generally detected with magneticparticles unless machining brings themnear the surface.

Forging LapsDuring the forging operation, there areseveral factors that can cause the surfaceof the object to fold or lap. Because this isa surface phenomenon, exposed to air,laps are oxidized and do not fuse whensqueezed into the object (Fig. 1).

Forging laps are difficult to detect byany nondestructive testing method. Theylie at only slight angles to the surface andmay be fairly shallow. Forging laps arealmost always objectionable because theyserve as fatigue crack initiation points.

Flash Line TearsAs the dies close in the final stage of theforging process, a small amount of metalis extruded between the dies. Thisextruded metal is called flash and must beremoved by trimming.

If the trimming is not done or notdone properly, cracks or tears can occuralong the flash line. Flash line tears arereliably detected by magnetic particletesting.

Casting DiscontinuitiesCastings are produced by pouring moltenmetal into molds. The combination ofhigh temperatures, complex shapes, liquidmetal flow and problematic moldmaterials can cause a number ofdiscontinuities peculiar to castings. Someof these are described below.

Cold ShutsCold shuts originate during pouring ofthe metal when a portion of the molten

liquid solidifies before joining with theremaining liquid. The presence of anoxidized surface, even though it is liquidor near liquid, prevents fusion when twosurfaces meet. This condition can resultfrom splashing, interrupted pouring orthe meeting of two streams of metalcoming from different directions.

Cold shuts can be shallow skin effectsor can extend quite deeply into thecasting. Shallow cold shuts called scabscan be removed by grinding. Deep coldshuts cannot be repaired.

Hot Tears and Shrinkage CracksHot tears are surface cracks that occurduring cooling after the metal hassolidified. They are caused by thermalstresses generated during uneven cooling.Hot tears usually originate at abruptchanges in cross section where thinsections cool more rapidly than adjacentheavier masses.

Shrinkage cracks are also surface cracksthat occur after the metal cools. They arecaused by the contraction or reduction involume that the casting experiencesduring solidification.

Weldment DiscontinuitiesWelding can be considered a localizedcasting process that involves the meltingof both base and filler metal. Welds aresubject to the same type of discontinuitiesas castings but on a slightly differentscale. In addition, other discontinuitiesmay be formed as a result of improperwelding practices. Some of thediscontinuities peculiar to weldments aredescribed below.

Lack of Fusion and Lack ofPenetrationFailure to melt the base metal results in avoid between the base and filler materials.This lack of fusion can be detected bymagnetic particle methods if it is closeenough to the weld surface.

With lack of penetration, the root areaof the weld is inadequately filled.Magnetic particle testing does notgenerally detect lack of penetration.

Heat Affected Zone Cracks andCrater CracksCracks in the base metal adjacent to theweld bead can be caused by the thermalstresses of both melting and cooling. Suchcracks are usually parallel to the weldbead. Heat affected zone cracking is easilydetected by magnetic particle testing.

Cracks in the weld bead caused bystresses from solidification or uneven

78 Magnetic Testing

cooling are called crater cracks. Crackscaused by solidification usually occur inthe final weld puddle. Cracks caused byuneven cooling occur in the thin portionat the junction of two beads. Magneticparticle testing is widely used to detectcrater cracks.

Manufacturing andFabrication DiscontinuitiesDiscontinuities associated with variousfinishing operations, e.g., machining, heattreating or grinding are described below.

Machining TearsMachining tears occur if a tool bit dragsmetal from the surface rather than cuttingit. The primary cause of this is improperlyshaped or dull cutting edges on the bit.

Soft or ductile metals such as lowcarbon steel are more susceptible tomachining tears than harder mediumcarbon and high carbon steels. Machiningtears are surface discontinuities and arereliably detected by magnetic particletesting.

Heat Treating CracksWhen steels are heated and quenched (orotherwise heat treated) to produceproperties for strength or wear, crackingmay occur if the operation is not suited tothe material or the shape of the object.The most common sort of such cracking isquench cracking, which occurs when themetal is heated above the criticaltransition point and is then rapidlycooled by immersing it in a cold mediumsuch as water, oil or air.

Cracks are likely to occur at locationswhere the object changes shape from athin to a thick cross section, at fillets ornotches. The edges of keyways and rootsof splines or threads are also susceptible toquench cracking.

Cracks can also originate if the metal isheated too rapidly, causing unevenexpansion at changes of cross section. Inaddition, rapidly increasing heat cancause cracking at corners, where heat isabsorbed from three surfaces and istherefore absorbed much more rapidlythan by the body of the object. Cornercracking can also occur during quenchingbecause of thermal stresses from unevencooling.

Straightening and GrindingCracksThe uneven stresses caused by heattreating frequently result in distortion orwarping and the metal forms must bestraightened into their intended shape. If

the distortion is too great or the objectsare very hard, cracking can occur duringthe straightening operation.

Surface cracks can also occur inhardened objects during impropergrinding operations. Such thermal cracksare created by stresses from localizedoverheating of the surface under thegrinding wheel. Overheating can becaused by using the wrong grindingwheel, a dull or glazed wheel, insufficientor poor coolant, feeding too rapidly orcutting too heavily. Grinding cracks areespecially detrimental because they areperpendicular to the object surface andhave sharp crack tips that propagateunder repeated or cyclic loading.

Another type of discontinuity that mayoccur during grinding is cracking causedby residual stresses. Hardened objects mayretain stresses that are not high enough tocause cracking. During grinding, localizedheating added to entrapped stresses cancause surface ruptures. The resultingcracks are usually more severe andextensive than typical grinding cracks.

Plating, Pickling and EtchingCracksHardened surfaces are susceptible tocracking from electroplating, acid picklingor etching processes.

Acid pickling can weaken surface fibersof the metal, allowing internal stressesfrom the quenching operation to berelieved by crack formation. Anothercracking mechanism is the interstitialabsorption of hydrogen released by theacid etching or electrodeposition process.Absorption of nascent hydrogen adds tothe internal stresses of the object andsubsequently may cause cracking. Thismechanism, called hydrogen embrittlement,can result in cracking during the etchingor plating operation or at some later timewhen additional service stresses areapplied.

Service DiscontinuitiesDiscontinuities also occur from serviceconditions. Some discontinuities such asdeformation and wear are not detected bythe magnetic particle test, but thetechnique is useful for indicating thediscontinuities listed below.

Overstress CrackingAll materials have load limits (calledultimate strength). When service stressingexceeds this limit, cracking occurs.Usually the failure is completed by surfacefracture of the object. In this case, thecrack is easy to detect and magneticparticle testing is not required. However,


there are instances where surfaces do notvisibly separate and magnetic particletesting is needed to detect and locate thecracking.

Fatigue CrackingObjects subjected to repeated alternatingor fluctuating stresses above a specificlevel eventually develop a crack. The crackcontinues to grow until the objectfractures. The stress level at which fatiguecracks develop is called the fatigue strengthof the material and is well below theultimate strength of the material. There isan inverse relationship between thenumber of stress applications (cycles) andthe stress level necessary to initiatecracking: low cycles and high stressproduce the same results as high cyclesand low stress.

Another factor contributing to fatiguecracking is the presence of surfaceanomalies such as copper penetrationsharp radii, nicks and tool marks. Theseact as stress raisers and lower both thenumber of cycles and the stress levelneeded to initiate cracking. Fatiguecracking typically occurs at the surfaceand is reliably detected by magneticparticle testing.

CorrosionMagnetic particle procedures are not usedto detect surface corrosion or pitting.However, there are secondarydiscontinuities that can be revealed by themagnetic particle method. When objectsare under sustained stress, either internalor external, and are at the same timeexposed to a corrosive atmosphere, aparticular kind of cracking results. Knownas stress corrosion cracking, thisdiscontinuity is easily detected bymagnetic particle testing.

Another occurrence related tocorrosion is pitting. Pitting itself does notusually produce magnetic particleindications (in some applications, sharpedged pits can hold particles). Pitting canserve as a stress raiser and often initiatesfatigue cracks. Fatigue cracks originatingat corrosion pits are reliably detected bythe magnetic particle method.

Applications of MagneticFlux Leakage Testing

Wire RopesAn interesting example of an elongatedsteel product inspected by magnetic fluxleakage testing is wire rope. Such ropes areused in the construction, marine and oilproduction industries, in mining

applications and elevators for personneland raw material transportation. Testing isperformed to determine cross sectionalloss caused by corrosion and wear and todetect internal and external broken wires.The type of flux loop used (electromagnetor permanent magnet) can depend on theaccessibility of the rope. Permanentmagnets might be used where takingpower to an electromagnet might causelogistic or safety problems.

By making suitable estimates of theparameters involved, a reasonably goodestimate of the flux in the rope can bemade. Because discontinuities can occurdeep inside the rope material, it isessential to maintain the rope at a highvalue of magnetic flux density, 1.6 to1.8 T (16 to 18 kG). Under theseconditions, breaks in the inner regions ofthe rope will produce magnetic fluxleakage at the surface of the rope.

The problem of detecting magneticflux leakage from inner discontinuities iscompounded by the need to maintain themagnetic probes far enough from the ropefor splices in the rope to pass through thetest head. Common probes include halleffect detectors and encircling coils.

The cross sectional area of the rope canbe measured by sensing changes in themagnetic flux loop that occur when therope gets thinner. The air gap becomeslarger and so the value of the fieldintensity falls. This change can easily besensed by placing hall effect probesanywhere within the magnetic circuit.

Internal Casing or PipelinesThe testing of inservice well casing orburied pipelines is often performed bymagnetic flux leakage techniques. Varioustypes of wall loss mechanisms occur,including internal and external pitting,erosion and corrosion caused by theproximity of dissimilar metals.

From the point of view of magnetizingthe pipe metal in the longitudinaldirection, the two methods are identical.The internal diameters and metal massesinvolved in the magnetic flux loopindicate that some form of active fieldexcitation must be used. Internaldiameters of typical production ortransportation tubes range from about100 mm (4 in.) to about 1.2 m (4 ft).

If the material is generally horizontal,some form of drive mechanism isrequired. Because the test device (a roboticcrawler) may move at differing speeds, themagnetic flux leakage probe should havea signal response independent of velocity.For devices that operate vertically, such aspetroleum well casing test systems, coilprobes can be used if the tool is pulledfrom the bottom of the well at a constantspeed. In both types of instrument, the

80 Magnetic Testing

probes are mounted in pads pressedagainst the inner wall of the pipe.

Because both line pipe and casing aremanufactured to outside diameter size,there is a range of inside diameters foreach pipe size. Such ranges may be foundin specifications. To make the air gap assmall as possible, soft iron attachmentscan be screwed to the pole pieces.

For the pipeline crawler, a recorderpackage is added and the signals fromdiscontinuities are tape recorded. Whenthe tapes are retrieved and played back,the areas of damage are located. Pipewelds provide convenient magneticmarkers. With the downhole tool, themagnetic flux leakage signals are sent upthe wire line and processed in the loggingtruck at the wellhead.

A common problem with this andother magnetic flux leakage equipment isthe need to determine whether the signalsoriginate from discontinuities on theinside or the outside surface of the pipe.Production and transmission companiesrequire this information because it letsthem determine which form of corrosioncontrol to use. The test shoes sometimescontain a high frequency eddy currentprobe system that responds only to insidesurface discontinuities. Thus, theoccurrence of both magnetic flux leakageand eddy current signals indicates aninside surface discontinuity whereas theoccurrence of a magnetic flux leakagesignal indicates only an outside surfacediscontinuity.

Problems with this form of testinginclude the following.

1. The magnetic flux leakage systemcannot measure elongated changes inwall thickness, such as might occurwith general erosion.

2. If there is a second string around thetested string, the additional metalcontributes to the flux loop, especiallyin areas where the two strings touch.

3. A relatively large current must be sentdown the wire line to raise the pipewall to saturation. Temperatures indeep wells can exceed 200 °C (325 °F).

4. The tool may stick downhole orunderground if external pressurescause the pipe to buckle.

Cannon TubesIn elongated tubing, the presence ofrifling affects the ability to perform agood test, especially for discontinuitiesthat occur in the roots of the rifling.Despite the presence of extraneous signalsfrom internal rifling, however, riflingcauses a regular magnetic flux leakagesignal that can be distinguished fromdiscontinuity signals. As a simulateddiscontinuity is made narrower and

shallower, the signal will eventually beindistinguishable from the rifle borenoise. In magnetic flux leakage testing,cannon tubes can be magnetized tosaturation and scanned with hall elementsto measure residual induction.

Round Bars and TubesIn some test systems, round bars andtubes have been magnetized by analternating current magnet and rotatedunder the magnet poles. Because theleakage flux from surface discontinuities isvery weak and confined to a small area,the probes must be very sensitive andextremely small. The system uses adifferential pair of magnetodiodes tosense leakage flux from the discontinuity.The differential output of these twinprobes is amplified to separate the leakageflux from the background flux. In thissystem, pipes are fed spirally under thescanning station, which has analternating current magnet and an arrayof probe pairs. The system usually hasthree scanning stations to increase thetest rate.

In one similar system, round billets arerotated by a set of rollers while the billetsurface is scanned by a transducer arraymoving straight along the billet axis.Seamless pipes and tubes are made fromthe round billets.

In another tube test system, thetransducers rotate around the pipe as thepipe is conveyed longitudinally.Overlapping elliptical printed circuit coilsare used instead of magnetodiodes and arecoupled to electronic circuits by slip rings.The system can separate seams intocategories according to crack depth.

BilletsA relatively common problem with squarebillets is elongated surface breakingcracks. By magnetizing the billetcircumferentially, magnetic flux leakagecan be induced in the resulting residualmagnetic field.

Magnetic flux leakage systems fortesting tubes exhibit the same generalability to classify seam depth. It isgenerally accepted that even with the lackof correlation between some of theinstrument readings and the actualdiscontinuity depths, the automaticreadout of these two systems stillrepresents an improvement over visual ormagnetic particle testing.

One technique, often calledmagnetography, for the detection ofdiscontinuities uses a belt of flux sensitivematerial, magnetic tape, to recordindications. Discontinuity fieldsmagnetize the tape, which is thenscanned with an array of microprobes or


hall effect detectors. Finally, the tapepasses through an erase head beforecontacting the billet again. Because thefield intensity at the corners is less than atthe center of the flat billet face, acompensation circuit is required for equalsensitivity across the entire surface.

Damage AssessmentIn most forms of magnetic flux leakagetesting, discontinuity dimensions cannotbe accurately measured by using thesignals they produce. The final signalresults from more than one dimensionand perhaps from changes in themagnetic properties of the metalsurrounding the discontinuity.

Signal shapes differ widely, dependingon location, dimensions andmagnetization level. It is thereforeimpossible to accurately assess the damagein the test object with existingequipment. Under special circumstances(for example, when surface breakingcracks can be assumed to share the samewidth and run normal to the material

surface), it may be possible to correlatemagnetic flux leakage signals anddiscontinuity depths. This correlation isnormally impossible.

Commercially available equipmentdoes not reconstruct all the desireddiscontinuity parameters from magneticflux leakage signals. For example, thesignal shape caused by a surface breakingforging lap is different from that causedby a perpendicular crack but noautomated equipment uses this differenceto distinguish between thesediscontinuities.

As with many forms of nondestructivetesting, the detection of a discontinuityand subsequent followup by eithernondestructive or destructive methodspose no serious problems for theinspector. Ultrasonic techniques,especially a combination of shear waveand compression wave techniques, workwell for discontinuity assessment aftermagnetic flux leakage has detected them.In some cases, however, the discontinuityis forever hidden. Such is very often thecase for corrosion in downhole andsubterranean pipes.

82 Magnetic Testing

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