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8/11/2019 MT Ref Matl

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MAGNETIC PARTICLE

EXAMINATION

BECHTEL

MATERIALS AND QUALITY SERVICES&

SUPPLIER QUALITY DEPARTMENT

SAN FRANCISCO, CALIFORNIA


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TABLE OF CONTENTS

PAGE

1.0 INTRODUCTION TO MAGNETIC PARTICLE EXAMINATION l

1.1 History of Magnetic Particle Examination 1

1.2 Purpose of Magnetic Particle Examination 1

3 Advantages and Limitations of Magnetic Particle Examination 2

2.0 BASIC PRINCIPLES OF MAGNETIC PARTICLE EXAMINATION

2.1 Theory of Magnetism

4

2.2 Magnetic Fields and Their Lines of Force 5

2.3 Magnetization Used in Magnetic Particle Examination 9

3.0 MATERIALS THAT CAN BE EXAMINED 12

3.1 Types of Materials 123.2 Examination Environment 12

4.0 EQUIPMENT AND SUPPLIES 13

4.1 Types of Power Supply 13

4.2 Types of Prods and Contact Tips 16

4.3 Magnetic Particles 16

4.4 Advantages and Disadvantages of the Dry Method 19

4.5 Advantages and Disadvantages of the Wet Method 19

4.6 Advantages and Disadvantages of the Fluorescent Particle Method 20

5.O SURFACE PREPARATION 21

5.1 Types of Surfaces 21

5.2 Acceptable and Unacceptable Examination Surfaces 21

5.3 Acceptable Methods of Surface Preparation 21

5.4 Preparation of Welds 21

6.0 EXAMINATION TECHNIQUE 23

6.1 Examination Sequence 23

6.2 Prod Placement 23

6.3 Current Requirements 23

6.4 Current and Magnetization 23

6.5 Magnetizing with Prod Contacts 24

6.6 Magnetizing with Yokes 25

6.7 Powder Application and Removal 25

6.8 Questionable Areas and Re-examination 26

7.0 OBSERVATION OF EXAMINATION RESULTS 27

7.l Inspection 27

7.2 Classification and Characteristics of Discontinuities 27

7.3 Types and Identification of Discontinuities 30


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TABLE OF CONTENTS (continued)

PAGE

8.0 RECORDING RESULTS OF EXAMINATION 35

8.1 Lacquer Transfer Technique 35

8.2 Daylight Photography 358.3 Transparent Tape 35

9.0 BIBLIOGRAPHY 36

10.0 APPLICABLE SPECIFICATIONS AND PROCEDURES 37

11.0 GLOSSARY 38

12.0 NOTES 45


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1.0 INTRODUCTION TO MAGNETIC PARTICLE EXAMINATION

1.1 History of Magnetic Particle Examination

Prior to World War II, use of nondestructive examination was almost nonexistent. The early

examination methods were limited to destructive testing. Destructive testing was considerednecessary to provide data for use in design, or to check the overall physical properties of the

material. These tests included; chemical analysis, tensile, compressive or impact test of steel

and other metals, and similar efforts intended to reassure the engineer that he was getting the

properties that were assumed in the design. Large factors of safety had to be used in the design

making the equipment large and heavy, and no way of knowing if the parts contained defects,

which would increase their failure rates. What was needed was a way to detect and classify

defects in the parts, without destroying the part under test.

Nondestructive examinations are inspection methods used to detect internal, surface, and

concealed defects or flaws in materials using methods that do not damage or destroy the item

being examined. Some of the more common nondestructive examination methods are liquid

penetrant, radiographic, ultrasonic and magnetic particle. Magnetic particle examination is today

one of the most widely used methods of nondestructive examination. It was first used on alarge scale in the years immediately preceding World War II. Since that time, techniques and

equipment have been developed and refined until today the speed and sensitivity of the

magnetic particle method makes it practically indispensable for many applications.

1.2 Purpose of Magnetic Particle Examination

The magnetic particle examination method is used to detect discontinuities at or slightly below

the surface of ferromagnetic materials. Under proper conditions - that is, with proper powder and

magnetization - exceedingly fine discontinuities can be detected. Even grain boundaries in steel

and the outlines of magnetic domains can be shown by using special techniques. The magnetic

particle method is the most sensitive means available for locating very fine and very shallow

surface cracks in ferromagnetic materials. The magnetic particle method will also produce

indications at cracks that are large enough to be seen by the naked eye. In this case, magnetic

particle examination is worth while because the presence of a prominent and easily seen powder

pattern makes for more rapid inspection, and assures that the crack will not be missed by the

examiner. Exceedingly wide surface cracks will not produce a powder pattern if the surface

opening is too wide for the particle to bridge.

Subsurface discontinuities are also indicated by magnetic particle examination although certain

limitations must be recognized and understood.

Fine and sharp discontinuities close to the surface normally produce a good sharp indication. For

subsurface discontinuities the indication will be fainter. Put another way, the deeper the

discontinuity lies below the surface, the larger it must be to give a readable indication.

A general statement can therefore be made that the magnetic particle method of nondestructive

examination is of importance for finding all sizes and depths of flaws that break the surface, but

runs into difficulty detecting internal discontinuities as they lie deeper and deeper below the

surface.


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1.3 Advantages and Limitations of Magnetic Particle Examination

1.3.1 Advantages - The magnetic particle method has a number of advantages when used on

ferromagnetic materials. Some of these are:

l. A sensitive means of locating small and near surface discontinuities in ferromagnetic

materials.

2. This method is rapid and the equipment is easy to operate.

3. The indications are produced directly on the surface of the part, and are a magnetic

picture of the actual discontinuity.

4. Operators can be trained to perform the examination easily without lengthy or highly

technical training.

5. There is little or no limitation due to size of the part being tested.

6. It will detect cracks fil led with foreign material.

7. Normally no elaborate precleaning is necessary.

8. It may work through thin coatings of paint or other nonmagnetic coatings. However,

under such tests, the prod contact areas must be cleaned to bare metal. This type of

examination is not a normal or recommended practice since it is used only under certain

specific conditions.

9. Skilled operators can estimate the depth of cracks quite accurately with suitable powders

and proper technique.

10. It lends itself well to automation.

11. It is relatively inexpensive to perform.

12. It utilizes electro-mechanical equipment that can be ruggedly built and adequately

maintained by existing plant personnel.

1.3.2 Limitations- although the method has many desirable and attractive advantages, it has certain

limitations. The examiner must be aware of and take into account these limitations. Some of

these are:

l. It will work only on ferromagnetic materials.

2. It is not, in all cases, reliable for locating discontinuities which lie wholly below the

surface.

3. The magnetic field must be in a direction that will intercept the principle plane of the

discontinuity. Sometimes this will require two or more sequential inspections with

different magnetizations.

4. A second step, demagnetization, following inspection, is often necessary.


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5. Post-cleaning, to remove remnants of the magnetic particles clinging to the surface, may

sometimes be required after examination and demagnetizing. 6. Odd-shaped parts

sometimes present a problem in how to apply the magnetizing forces to produce a field

in the proper direction.

7. Heavy currents are sometimes required for the examination of large castings andforgings.

8. Care is required to avoid local heating and burning of highly finished parts at the points

of electrical contact.

9. Individual handling of parts for magnetization is usually necessary, which is a

disadvantage, particularly with large numbers of small parts.

10. Although the indications are easily seen, experience and skill in interpreting their

significance is required. Compared to some other methods such as radiography,

ultrasonics, and eddy-currents, interpretation of magnetic particle indications is simpler.

2.0 BASIC PRINCIPLES OF MAGNETIC PARTICLE EXAMINATION

2.1 Theory of Magnetism

A body which possesses the ability to attract iron pieces is called a magnet. Magnets may be

permanent, retaining their magnetism more or less permanently; or they may be temporary,

retaining their magnetism only as long as a magnetizing force is being applied. Each magnet

has two opposite poles, which are attracted by the earths magnetic poles; hence, the poles are

respectively called the north and south poles, as illustrated in Figure 2-1.


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The two magnets will attract each other because unlike poles attract each other. A south pole of

one magnet is attracted by the north pole of the other magnet. Just as the earth itself is a large

magnet having a north and south pole, materials which can be magnetized are made up of

domains or regions which have a north and south-pole. The north and south poles are in direct

relationship with positive and negative charges. These charges can add to or cancel each other.

In an unmagnetized piece of iron the domains are arranged in a haphazard fashion as in Figure

2, thus we have a canceling effect. Now if all the domains in Figure 2-2 were lined up in anorganized manner, as in Figure 2-3, the piece of iron would be a magnet.

In Figure 2-3 all the north poles are

oriented in one direction and all the south

poles are oriented in one direction. With

all of the domains like this, the piece of

iron then has a north and south pole. The

force of the magnetic bar now develops a

total force equal to the sum of all the

domains. This alignment of domains

produces a magnetic field.

2.2 Magnetic Fields and Their Lines ofForce

A magnetic field exists within and around

a permanent magnet or around a

conductor carrying an electric current.

The magnetic field surrounding a

permanent bar magnet has polarity, but

the magnetic field surrounding a

conductor does not. As an example, the

earth itself can be thought of as a bar

magnet because of its two poles. Figure

2-4 shows the magnetic lines of force

which surround every magnet and have

definite characteristic patterns. In all

magnets, the lines of force flow from the

south pole to the north pole. The force

that attracts other magnetizable materials

to the magnetic poles is knows as

magnetic flux. Magnetic flux is made up

of all the lines of force. The magnetic

lines of force never cross; they seek the

path of least resistance; they are most densely packed at the poles of the magnet; and they flow

from north to south outside the magnet, and from south to north within the magnet.

A permanent bar magnet is the simplest example of longitudinal magnetization. Since thedirection of the magnetic flux is axial, it has two poles. Longitudinal magnetization is said to exist

in an object when the flux lines traverse in a direction essentially parallel to one of its axes. if a

straight bar magnet is bent, it becomes a horseshoe magnet. When the magnet is bent farther to

make a complete loop and the ends are fused together, the poles disappear; for example, a

closed magnetic circuit is formed. If the circle is cut, either partially or all the way through, the

poles reappear as shown in Figure 2-5.


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Figure 2-3 Magnetic Field Surrounding a Bar Magnet

If a bar magnet is broken in two, making

two shorter bars, each immediately

becomes a separate bar magnet with a

north and south pole. if the two bars are

again fused together, with opposite

magnetic poles adjacent, the poles will not

completely disappear. A small

concentrated leakage field will remain in

the fusion area. Similarly, if the bar is cut

only partially, two opposite poles will

appear, and a leakage field will exist in

the area of the cut. It can be seen thatleakage fields are actually magnetic lines

of force that leave the bar and pass

through the air from one pole to the other

of opposite polarity. Figure 2-5 Poles in a Straight Bar, Horseshoe, and Broken Magnet

Since the new opposite poles were created by the interruption of the paths of the lines of force

within the magnet, it follows that nonmetallic inclusions in a magnetized article, or changes in the

material of the article, will also cause the creation of two opposite poles and a resultant leakage

field. Magnetic particle examination is a process used to detect the presence of a leakage field

and thereby the presence of discontinuities, either voids or inclusions.

When an electric current passes through a conductor, a magnetic field is formed around the

conductor. If the conductor has a uniform shape, the density of the field (number of lines of forceper unit area) is uniform at any point along the conductor, and it uniformly decreases as the

distance from the electrical conductor increases. The direction of the magnetic field (lines of

force) is at a 90 degree angle to that of the current in the conductor. An easy method for finding

the direction of an electrically induced magnetic field is to imagine grasping the conductor in the

right hand with the thumb pointing in the direction of current flow. The fingers will then point in


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the direction of the lines of force. This is the right-hand rule. From Figure 2-6 it can be seen that

the current flow in the conductor creates circular lines of force.

When a current carrying

conductor is formed into a

loop, the lines of force circling

the conductor form amagnetic field inside and

outside the loop. inside the

loop the field is similar to that

of a bar magnet and is said to

be a longitudinal magnetic

field. when a coil consists of

several loops, the magnetic

field within the coil is

strengthened in proportion to

the number of loops,

therefore a current carrying

conductor can be used to

establish a magnetic field.

The effects of flux direction

can be seen in Figures 2-7 and 2-8. In Figure 2-7, the bar is said to be longitudinally

magnetized. A crack produced by partially breaking the bar transversely will lie at right angles to

the long axis of the bar. A crack oriented transversely offers the greatest interruption to the lines

of force and therefore forms a strong leakage field.

A linear crack-like

discontinuity lying parallel to

the axis of the bar and

parallel to the lines of force

will produce no leakage field

as no lines of force would be

cut.

Therefore, if a discontinuity is

to produce a leakage field and

a readable magnetic particle

pattern it must intercept the

lines of force at some angle.

The leakage field will be

strongest if the angle is 90

degrees and will become

weaker as the angle the

discontinuity makes with thelines of force becomes smaller. Figure 2-7

It is often true that a crack which has a general direction that is parallel to the axis of the bar, but

which deviates somewhat from & straight line, may give quite strong magnetic particle

indications. In such a case, the segments of the crack not exactly parallel to the axis do intercept

the lines of force.


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In Figure 2-8 the bar is said to

be circularly magnetized. In

this case, it is the crack lying

parallel to the longitudinal

axis of the bar or tube which

intercepts the flux lines at 90

degrees. Cracks at rightangles to the axis would be

parallel to the field and would

create no leakage field.

Cracks at intermediate angles

would create leakage fields of

strengths varying with the

angle; and the crack,

essentially transverse, which

deviates from a straight line

would probably give an

indication.

Figure 2-8

2.3.1 Longitudinal magnetization of a specimen is accomplishedby the use of longitudinal fields

set up in a coil or solenoid. When the length of a specimen is several times its diameter or cross

section, the specimen may be successfully magnetized by placing it lengthwise in the field of the

coil or solenoid. This is referred to as a "coil shot" which is shown in Figure 2-9.

The yoke method, as shown

in Figure 2-10, may be used

to magnetize a specimen

longitudinally. Essentially, it

is a temporary horseshoe

magnet made of soft, low

retentivity iron, which is

magnetized by a small coil

wound around its horizontal

bar. Yokes may be designed

to use either A.C or D.C.

current or both. With D.C.,

the flux density of the

magnetic field can be changed by varying the amount of current. Direct current will allow greater

penetration for the detection of some subsurface defects. The use of A.C. current when

employing the yoke method allows the detection of surface defects. When the energized yoke is

placed on a specimen, the flux flow from the yoke's south pole through the specimen to the north

pole induces a local longitudinal field in the specimen.


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2.3.2 A circular magnetic field is induced by

using direct or indirect magnetization or by the use of a conductor surrounded by a hollow

article.

Direct induction of a circular field into an article is accomplished by passing a current through the

article. This method, illustrated in Figure 2-11, is called a head shot. Specimen, is by the use of

prods. Prod magnetization is used where the size or location of an article does not permit the

use of a head shot. The prod method of magnetization is shown in Figure 2-12.

In the indirect method for inducing a circular field, the specimen is placed so that a current

carrying conductor induces a magnetic field into the specimen. This method, as shown in Figure 2-

13, is known as the central conductor technique.

2.9 Magnetization Used in Magnetic Particle Examination

3.O MATERIALS THAT CAN BE EXAMINED

3.1 Types of Materials

Figure 2.10

3.1.1 Some materials are attracted by a magnet, while others are repelled. From the definition of

magnetism it follows that magnetic materials are those which are attracted by magnetism. These

materials are known as ferromagnetic materials. diamagnetic materials are repelled by

magnetism. In the magnetic particle examination method, ferromagnetic material is of concern,

as only ferromagnetic materials can be strongly magnetized. Below is a table detailing the

characteristics of diamagnetic and ferromagnetic materials.

Figure 2.11


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Diamagnetic Materials

1. Cannot be magnetized.

2. Are repelled by magnetism.

3. Cannot be magnetic particleexamined.

Ferromagnetic Materials

1. Can be strongly magnetized.

2. Are strongly attracted to magnetic

fields.

3. Can be magnetic particle examined.

Figure 2.12


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3.1.2 The magnetic particle examination

method can be used on any ferromagnetic

material, though not in all cases with equal

effectiveness. It works best on steels and alloys

that have a high permeability. Permeability is the

term used to refer to the ease with which a

magnetic field or flux can be set up in a magneticcircuit. It is not a constant value for a given

material, but is a ratio. At any given value of

magnetizing force, permeability is BIN, the ratio

of flux density, B, to magnetizing force, H.

Subsurface discontinuities lying wholly below the

surface are more likely to be located in soft steels

having high permeability, than in hardened steels

and alloys which in nearly all cases have a lower

permeability. This difference is less critical if only

surface defects are being sought.

Figure 2.13

In the case of gray or malleable iron castings, surface discontinuities are easily located. Themethod also works quite well on metallic nickel and cobalt. Stainless steel and other alloys, which

are in the austenitic state, cannot be examined with magnetic particles at all, since iron in these

alloys are non-magnetic.

3.2 Examination Environment

1. At all times personal safety must be of the utmost concern. Instructions pertaining to the

operation of power supply and equipment should be read and understood before the

equipment is used.

2. The sample under examination should be protected from the elements, such as wind,

rain, snow, etc.

3. Good lighting is required. If natural lighting is not adequate, artificial lighting must be

provided.

4.0 EQUIPMENT AND SUPPLIES REQUIRED

4.1 Types of Power Supplies

There are basically two types of electric current in common use, and both are suitable for

magnetizing purposes of magnetic particle examination. These are direct current (DC) and

alternating current (AC). Direct current is a constant current flowing in one direction only.

Alternating current is considered to be commercial AC, which is current reversing its direction

completely at the rate of 50 or 60 cycles per second.


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The magnetic force fields produced by direct and by alternating current differ in many

characteristics. The difference that is of importance in magnetic particle examination is that

fields produced by direct current generally penetrate the entire cross section of the part, whereas

the fields produced by alternating current are confined to the metal at and near the surface of the

part. Straight DC available at 110, 220 or 440 volts is suitable for use with solenoids and yokes.

Since DC cannot be stepped down in voltage except by motor generators, it is not suitable for

circular magnetization usage where the current must be varied in accordance with the specimensize. Because direct current is constant and surge free, the DC power source does not motivate

the magnetic particles to pulsate or move. The magnetic particles are held fast and collect

heavily on irregular surfaces.

Alternating current is most often available in voltage ranging from 110 volts through 440 volts.

Commonly used single-phase alternating current usually alternates at 60 cycles per second. This

type of current creates a maximum flux at the surface of the magnetized article, and has

relatively little penetrating ability. Alternating current provides some pulsation for the movement

of magnetic particles due to the periodic reversing in polarity. The pulsating action caused by the

reversing polarity enables the magnetic particles to move over the surface freely.

The basic half-wave rectifier circuit

consists of a rectifier connected in seriesbetween the AC voltage source and the

circuit load resistance. The rectifier

permits current to flow only during the

positive half cycles of the applied AC

voltage; the circuit thus becomes a

pulsating DC as illustrated in Figure 4-1.

The most predominant application for

half-wave current is for weld inspection. In

this application, half-wave current is used

with dry powder and prod magnetization,

which provides the highest sensitivity for

discontinuities which lie wholly below the

surface. In many cases, suitable

amperages can be obtained from relatively Figure 3-1 Rectification of Alternating Currentsmall portable units. Half-wave current is to Half-Wave Direct Current

widely used for casting inspection when

sensitivity to sub-surface discontinuities is required.

The surges of current occurring at each cycle motivate the magnetic particles which forces them

to move and form indications. Even in the valleys of a weld ripple, the particles move and do not

become trapped.

It is generally accepted that the best types of magnetizing current for magnetic particleexaminations are alternating and half-wave rectified currents. Alternating current is best suited

for locating surface discontinuities (because of skin effect). Half-wave rectified current is best

suited for locating below-the- surface discontinuities.


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The strength of the magnetizing current plays an important role in detecting discontinuities. Too

strong a current will cause the particle to cling to the surface, especially with the dry powder

method and will not allow the particles free movement and allowing them to be attracted to areas

of flux leakage. On the other hand, when current is applied and no pattern is observed it could

be an indication that insufficient current is being applied. There are numerous mathematical

formulas that are useful in determining correct current, such as the coil method. Multiplication of

the numbers of coils times the current (amps) gives a magnetizing force.

c (a) = f

c Number of coil turns

a 1 current (amps)

f a magnetizing force

The influence of Length/Diameter ratio for DC coil magnetization method is an important formula

to remember. The part to be inspected should be at least two to three times as long as its

diameter. This formula works well in many cases but specific variables should first be met whichare the following:

l. The cross sectional area of a part should not be greater than one tenth of the area of the

coil opening.

2. The part or section to be inspected should not be greater than 18 inches.

3. The part should be held against the inside wall of the coil - not positioned in the center.

4. The part has a Length/Diameter ratio of between 2 and 15.

5. The part is positioned in the coil with its long axis parallel to the applied field.

To use the formula c(a) a f, the Length

Distance ratio is reduced to a number

and divided into 45,000. The result will

be the number of ampereturns

(magnetizing force) necessary to

magnetize the part. The number of

ampere-turns should then be divided by

the number of turns in the coil, this will

give the correct current amps to be

selected.

Current (amps) for the flexible cablemethod using prods, leeches, and

clamps, will vary depending upon the

code or specification you are working to.

A good rule of thumb is 100 to 125 amps for each inch of prod spacing. Figure 4-2 gives a

general range of amps for each inch of spacing but the governing code or procedure should be

checked for specific requirements.

PROD SPACING

(INCHES) AMPERES

3 300 - 375

4 400 - 500

5 500 - 625

6 600 - 750

7 700 - 875

8 800 -1000


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Figure 4-3

After completion of a magnetic particle examination, it may be necessary to demagnetize the

part. There are various methods which can be used. The AC coil method is the most common

method of demagnetization. The coil is designed to operate at line voltage and a frequency of 60

cycles per second. When a part is placed in the coil it is subjected to a reversing field due to the

alternating action of the current. The magnitude of the field is gradually reduced as the part is

slowly withdrawn from the coil. The current should not be shut off until the part is well out of

range of the magnetic field.

In the DC reversing method (as shown in Figure 4-3), demagnetization is accomplished by use of

a coil or passing the current directly through the part. The D.C. reversing method is similar to the

A.C. coil method, in that the direct current is alternately reversed in direction and amplitude. The

D.C. method is very good for large parts because of its deep penetrating ability.

Some portable magnetic particle examination equipment (prod and yoke) may be used to

demagnetize a part using the following techniques.

l. With the prods placed on the area to be demagnetized increase amperage to maximum and

decrease slowly to zero.

2. When utilizing yoke equipment, demagnetization may be accomplished on large piecesby placing the poles in the same position as used to magnetize the part, switch on the

equipment with the selector switch in the A.C. position and remove from the work piece

to a distance of two


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4.2 Types of Prods and Contact Tips

The magnetizing equipment should be capable of inducing into the examined object a magnetic

flux of sufficient intensity to reveal surface and near surface discontinuities The power source

should be capable of producing either direct or rectified alternating current. An ammeter should

be located in the "prod circuit" and be visible and readable by the operator when performing the

examination.

Prod type electrical contacts are used to pass current through the parts being examined. Copper

tipped prods may be used when the open circuit is 25 volts or less but steel or aluminum tipped

prods should be used to prevent copper penetration into the examined object when the open

circuit voltage exceeds 25 volts. The prod handles should be equipped with an on-off switch to

control the flow of current.

4.3 Magnetic Particles

4.3.1 The success of magnetic particle examination is dependent upon selection of material(medium) and the method used to conduct the examination. When the medium, whether dry or

liquid, is applied to the specimen while the magnetizing current is flowing, the procedure is known

as the continuous method. If the medium is applied after the magnetizing current is shut off, theprocedure is known as the residual method. The medium comes in either powder or paste form.

In the dry method, the powder in its dry form, is applied by sprinkling the specimen. When the

wet method is used the medium, which may be a paste or a powder, is first mixed with a liquid to

make a bath, which is then sprayed or brushed onto the surface of the specimen. No one

medium or method is best for detection of all conditions or types of discontinuities.

4.3.2 Great importance is attached to detecting mediums and their effect on the indicationsobtained. Four properties enter into the selection of a satisfactory medium: magnetic, geometric,

mobility, and visibility.

1. Magnetic Properties

It is desirable that the particles of the examination medium possess two important

properties: high permeability and low retentivity. Permeability is defined as the degree

of ease with which a particle is magnetized, retentivity, is that property which enables

particles to hold, to a greater or lesser degree, a certain amount of residual magnetism.

Particles incorporating high permeability and low retentivity have maximum response in

a leakage field, and at the same time do not remain magnetized when the pass out of

the influence of the magnetic field.

2. Geometric Properties

The spherical shaped particle offers a high degree of mobility but has low attractive

power. On the other hand, the long slender Jagged particle has a high degree of

attractive power and low mobility. A multi-facet nugget type particle is a goodcompromise in that it reasonably combines the optimum qualities of the other two types.

Particle size is an important consideration as it is desirable to have particles of various

sizes. Small particles are required to bridge a tight-lipped crack and larger sizes are

necessary for wider cracks. Also, a weak leakage field is unable to hold a large particle

but is able to fix and retain one of a smaller size. Dry powder magnetic particles are

made up in a wide range of sizes, though all will pass through a 100-mesh screen.


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a. In the wet method, magnetic oxides of iron are employed. While extremely fine

in size, they are of lower permeability than the metallic dry particles, having

neither the most desirable shape nor variety of size. The advantages of applying

these particles in the form of a suspension are numerous. When used, other

factors need to be considered; particularly, the ability to maintain a suspension.

b. Magnetic particles, unless extremely fine, cannot be maintained in suspension

without sacrificing mobility. Fine magnetic oxides are used because they can be

suspended in a liquid when a dispersing agent is employed.

3. Mobility

When the particles are brought into the influence of the leakage field of a discontinuity,

they are free to form a pattern or indication. This freedom is influenced by condition,

shape, and application of the particles.

a. In the dry method, mobility is obtained by dusting or blowing the particles over

the surface of the article. This permits the leakage field at the discontinuity to

catch and hold some particles as they move by. Mobility is also obtained byvibrating the article after the particles have been dusted on. This is why AC is

used advantageously since the influence of the alternating field causes the

particles to "dance and thus provides mobility.

b. The principal advantage of the wet method is the excellent mobility (freedom to

move in three dimensions) of the suspended particles. It is important to use a

low viscosity liquid so that the suspended particles are retarded as little as

possible by the liquid in which they are suspended. The most nearly ideal

condition from the point of mobility, is a cloud of particles floated with very low

velocity up to the surface being tested. This condition is obtained only with

special equipment.

4. Visibility

An indication must be readily visible. A good light source is essential. With various

types of surfaces, from highly polished articles to rough castings, no one color is always

satisfactory. The choice of color is entirely dependent on visibility. The most widely

used colors of particle are grey, red, and black. The grey powder has excellent contrast

against practically all surfaces, with exception of certain silver-gray sandblast surfaces.

Recent developments are the fluorescent powders and pastes, particles coated with a

dye which fluoresce brilliantly under an ultra- violet or black light, thereby increasing

visibility.

4.3.3 Methods of Application

Dry magnetic particles are commonly applied from shaker cans or bulbs. This is the simplest,

but not necessarily the best, method. Automatic particle blowing equipment is usually

economical in its use of particles and in most instances the most satisfactory way of floating the

dry particles to the examination surface.


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4.3.4 Particle Requirements

To properly function, the particles composing the medium in both the wet and dry method must

be:

a. Non-toxic.

b. Finely divided.

c. Ferromagnetic.

d. Free of contaminants.

e. High in permeability.

f. Low in retentivity.

g. High in color contrast (visibility).

h. Within correct size range.

4.3.5 Wet Suspensions (Bath)

The bath used with the wet method of magnetic particle examination consists of a liquid vehicle

in which the particles are suspended. The liquid vehicles used is usually light oil. Water treated

with anticorrosion, anti-foam, or wetting agents, may also be used. The vehicle must be

nonfluorescent, non-toxic, non-volatile, and with a high flashpoint. The particles used are

obtainable in a powder or paste form or in a highly concentrated liquid form and may either be

fluorescent or nonfluorescent. To achieve the required test sensitivity, the degree of particle

concentration in the bath must be correct. Too light a concentration leads to very light indication

of discontinuities; too heavy a concentration results in too much over-all surface coverage, which

may mask or cause incorrect interpretation of discontinuity indications. The wet suspension

method is good for shop work on small or consistent size items which can be tested in an

assembly line fashion, for mobil field work the dry method is usually used.

4.4 Advantages and Disadvantages of the Dry Method

4.4.1 Advantages:

l. Excellent for locating defects wholly below the surface and deeper than a few

thousandths of an inch.

2. Easy to use for large objects with portable equipment.

3. Easy to use for field inspection with portable equipment.

4. Good mobility when used with A.C. or Half Wave D.C.

5. Not as "messy as the wet method.

6. Equipment may be less expensive.


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4.4.2 Disadvantages:

1. Not as sensitive as the wet method for very fine and shallow cracks.

2. Not easy to cover all surfaces properly, especially of irregular shaped or large parts.

3. Slower than the wet method for large numbers of small parts.

4. Not readily useable for the short, timed 'shot" technique of the continuous method.

5. Difficult to adapt to a mechanized test system.

4.5 Advantages and Disadvantages of the Wet Method

4.5.1 Advantages:

l. It is the most sensitive method for very shallow and fine surface cracks.

2. It quickly and thoroughly covers all surfaces of irregular shaped parts, large or small,

with magnetic particles.

3. It is the fastest and most thorough method for examining large numbers of small parts.

4. The magnetic particles have excellent mobility in liquid suspension.

5. It is easy to measure and control the concentration of particles in the bath, which makes

for uniformity and accurate reproducibility of results. 6. It is easy to recover and re-use

the at

7. It is well adapted to the short, timed "shot" technique of magnetization for the continuous

method.

8. It is readily adaptable to automatic unit operation.


1. It is usually not capable of finding defects lying wholly below the surface if more than a

few thousandths deep.

2. It is "messy" to work with, especially when used for the expendable technique, and in

field testing.

3. When oil is used for a bath with direct contact for circular magnetization, it can present a

fire hazard.

4. A recirculating system is required to keep the particles in suspension.

5. Sometimes it presents a post-inspection cleaning problem to remove magnetic particles

clinging to the surface.


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4.6 Advantages and Disadvantages of the Fluorescent Particle Method

4.6.1 Advantages:

Fluorescent particles have one tremendous advantage over the untreated or "visible" particles.

This is their ability to give off a brilliant glow under "black light". This brilliant glow serves three

principal purposes:

1. In semi- or complete darkness even very minute amounts of the fluorescent oxide are

easily seen, having the effect of increasing the apparent sensitivity of the process

tremendously, even though, magnetically, the fluorescent particles are not superior to

the untreated oxides.

2. Even on discontinuities large enough to give good visible indications, fluorescent

indications are more easily seen so that missing an indication is greatly reduced even

when the speed of scanning parts is increased.

3. Inside drilled holes or cavities, or in sharp corners such as threads or keyways, the

fluorescent indications are clearly and readily seen, while visible color indications may

be obscured.

The fluorescent particle method is faster, more reliable, and more sensitive to very fine

defects than the visible colored particle method in most applications. Indications are

harder to overlook, especially in high volume testing. In addition, the fluorescent method

has all the other advantages possessed by the liquid suspension technique.


Fluorescent magnetic particles used in suspension in liquids have the same unfavorable

characteristics which go with the usual wet method techniques. There is also the additional

requirement for a source of "black light" and an inspection area from which at least most of the

white light can be excluded. Experience had shown however that these added special

requirements are more than justified by the gains

5.0 SURFACE PREPARATION BEFORE EXAMINATION

5.1 Types of Surfaces

In general, the smoother the surface of the part to be examined and the more uniform its color,

the more favorable are the conditions for the formation and the observation of the powder

pattern. This statement applies particularly to inspections being made on horizontal surfaces.

For sloping and vertical surfaces, the dry powder may not be held on a very smooth surface by a

weak leakage field. The surface should be clean, dry, and free of grease. The dry particles will

stick to wet or oily surfaces, and not be free to move around over the surface to form indications.

This may completely prevent the detection of significant discontinuities. An initial application ofthe dry magnetic powder, followed by wiping, often will give a surface over which a second

application of powder will move readily. If it is feasible to use it, vapor decreasing will give a dry,

oil-free surface.


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5.2 Acceptable and Unacceptable Examination Surfaces

Paint or plating on the surface of a part has the effect of making a surface defect behave like a

subsurface one. The relative thickness of the plating or paint film and the size of the defects

sought determine whether or not the coatings should be stripped. The dry method is more

effective in producing indications through nonmagnetic coatings than the wet method. If fine

cracks are expected, the surface should be stripped of the coating if its thickness exceeds 0.005inch. Most coatings of cadmium, nickel or chromium are usually thinner than this and the plating

makes an excellent background for viewing indications. Hot galvanized coatings are thicker and

in general should be removed before testing, unless only gross discontinuities are important.

Broken or patchy layers of heavy scale also tend to interfere by their tendency to hold powder

around the edges of the breaks or patches, and should be removed if they are extensive enough

to interfere seriously with the detection of genuine discontinuities.

5.3 Acceptable Methods of Surface Preparation

Any loose dirt, paint, rust, or scale should be removed with a wire brush, shot, grit blasting, or

other means. If cleaning is done with shot or grit blasting, there is a peening effect, especially on

softer steels, which may close up fine surface discontinuities. The effect is more pronounced

with shot than with grit, but if these cleaning methods are used the operator should be aware ofthe danger of missing very fine cracks. A thin, hard, uniform coating of rust or scale will not

usually interfere with the detection of any but the smallest defects. The examiner should know

the size of the smallest defect he is to consider significant, in order to judge whether or not such

a coating of rust or scale should be removed.

5.4 Preparation of Welds

The surface to be examined, including at least one inch on either side of the weld, should be free

of water, grease, oils, paint, rust, slag, loose scale, or other material which can produce

inaccurate examination results.

The surface finish of the welds, weld grooves, and adjacent base metal should be such that

proper interpretation can be accomplished. As-welded surfaces, following the removal of slag by

chipping and wire brushing, should be considered suitable without grinding if this does not

interfere with the interpretation of the examination results and if the weld contour blends into the

base metal without undercutting. When a weld is to be examined in the as-welded surface

condition, the weld should be free of sharp surface irregularities, such as deep valleys between

stringer beads. In some cases, surface preparation by grinding or machining may be necessary

when surface irregularities could mask unacceptable indications.

6.1 Examination Sequence

1. Position prods.

2. Switch on the magnetizing current.

3. Apply magnetic particles.4. Remove the excess magnetic particles.

5. Switch off current and remove prods.

6. Evaluate magnetic particle indications.


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6.2 Prod Placement

At least two separate examinations shall be carried out on each area so the lines of flux in one

examination are approximately perpendicular to the lines of flux in the other. A different means

of magnetizing may be used for the second examination.

Examinations shall be conducted with sufficient overlap to assure 100 percent coverage at theestablished test sensitivity. For example:

1. When positioning prods longitudinally along a weld, a minimum overlap of one inch shall

be maintained between subsequent positions.

2. When positioning prods transversely along the weld, spacing between subsequent

positions shall be a maximum of 1/2 of the prod or pole spacing or three inches,

whichever is smaller.

When examining welds, the adjacent base material within 1/2 inch on each side of the

weld shall be included in the examination.

6.3 Current Requirements

Direct or rectified current should generally be used at a minimum of 100 and a maximum of 125

amps per inch of prod spacing for material 3/4 inch thick or greater. For material less than 3/4

inch thick, amperage should be 90-110 amps per inch of prod spacing.

If arc burns occur on the examined object during magnetic particle examination, they should be

removed by grinding and the removal of arc burns verified by either the magnetic particle yoke

method or the liquid penetrant method.

6.4 Current and Magnetization

6.4.1 Residual and Continuous Current

The choice between the residual and the continuous method of applying current is a relatively

easy one. In the residual method, parts are magnetized and subsequently the magnetic particles

are applied. The method can be used only on parts having sufficient retentivity. The permanent

field they retain must be sufficiently strong to produce leakage fields at discontinuities which in

turn will produce readable indications. This method, in general, is reliable only for the detection

of Since hard materials which have high retentivity are usually low in permeability, higher than

usual magnetizing currents may be necessary to obtain a sufficiently high level of residual

magnetism.

The residual method, either wet or dry, has many attractive features and finds many applications,

even though the continuous method has the inherent advantage of greater sensitivity.

The advantage of the continuous method is basic. When the magnetizing force is applied to a

ferromagnetic part, the field rises to a maximum, its value or intensity deriving from the strength

of the magnetizing force and the material permeability of the part. But when the magnetizing

force is removed, the residual magnetism in the part is always less than the field present while

the magnetizing force was acting. The amount of difference depends on the retentivity of the

material. The continuous method, for a given value of magnetizing current, is therefore always


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more sensitive than the residual, at least so far as sensitivity is determined by the strength of

field in the part.

The continuous method is the only possible one to use on low carbon steels or iron having little or

no retentivity. It is frequently used with AC on such materials because the alternating current

field produces excellent mobility of the particles.

6.5 Magnetization with Prod Contacts

Magnetization by passing current directly through the part, or through a local portion of the part,

is often resorted to for the inspection of large and massive articles too bulky to be put into a unit

having clamping contact heads. Such local contacts do not always produce true circular fields,

but are very convenient and practical for many applications. Examinations of large castings and

weldments are frequently made in this manner.

The field produced by the prod method passes between the contact points and the field crosses

the area between the contacts at 90 degrees to the current. Cracks parallel to the line between

the prods will be shown by this method of magnetization.

This method is widely used and has many advantages. Easy portability makes it the mostconvenient method for field use for the examination of large tanks and welded structures.

Sensitivity to defects lying wholly below the surface is greater with this method of magnetizing

than with any other, especially when half wave current is used.

The use of prod contacts has some disadvantages of which the operator should be aware. They

are:

l. It is necessary to scan the surface of the part being examined in small sections, since

suitable fields exist only between and near the prod contact. Since the spacings are

seldom greater than 12 inches and usually much less, this means many separate

contacts and a time consuming job. Modern techniques such as the "overall" method

previously referred to, have in a great many cases replaced inspection with prods, and

have shortened the time for such inspections by very large percentages.

2. Interference of the external field that exists between the prods sometimes makes

observation of pertinent indications difficult. The strength of the current that can be used

is limited by this effect.

3. Great care must be used to avoid burning of the part under the contact points. Burning

may be caused by dirty contacts, insufficient contact pressure or excessive currents.

The chance of such damage is particularly great on steel with 0.30 to 0.40 percent

carbon or higher. The heat under the contact points produces local spots of very hard

material that can interfere with later operations, such as machining. Sometimes actual

cracks are produced by this heating affect. Contact heating is not likely to damage low

carbon steel such as used for structural purposes.

6.6 Magnetizing with Yokes

When using a yoke, the same requirements for surface preparation as for the prod method apply,

although with the yoke method no electrical contact is required.


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The direction of the magnetic field when employing a yoke is basically perpendicular to the

magnetic field produced by prods placed in the same location (see Figure 10 and Figure 12),

therefore positioning the yoke with respect to the direction of the defects must be exactly

opposite that when using the prod method.

6.7 Powder Application and Removal

A few rules for the application of the dry powders will, if followed, make the process of

examination easier and more effective. It should be remembered that the dry particles are

heavier and individually have a much greater mass than the very fine particles of the wet

method. Therefore if, they are applied to the surface of a part with any appreciable velocity, the

fields at the discontinuities may not be able to stop them and hold them. This is especially true

when vertical or overhead surfaces are being examined. The powder should reach the surface of

parts as a thin cloud with practically zero velocity, drifting to the surface so that leakage fields

have only to hold it in place. For vertical and overhead surfaces, the fields must overcome the

pull of gravity which tends to cause the particles to fall away. Since the dry particles have a wide

range of sizes, the finer particles will be held under these conditions unless the leakage fields are

extremely weak.

On horizontal surfaces this problem is minimized. The usual mistake is to apply too muchpowder. Once on the horizontal surface of a part, the powder has no mobility (unless AC or half

wave is being used) and too heavy an application tends to obscure indications. If the part can be

lifted and tapped, the excess powder will fall away and indications will be more readily visible, or

the excess powder can be gently blown away with an air stream not strong enough to blow off

magnetically held particles forming an indication.

Various devices have been used to make proper powder application easy. Two of the most

widely used are the "squeeze bottle" and powder gun. The "squeeze bottle" is light and easy to

use. With some practice, by a combination of shaking and a squeeze on the bottle, powder can

be ejected with minimum velocity. Practicing with the bottle on a sheet of white paper will enable

the operator to produce an even, gentle overall coverage. With the powder gun or blower a

better Job of application can be done, especially on vertical and overhead surfaces. The powder

gun throws a cloud of powder at low velocity, much like a very thin paint spray. Held about a foot

distant from the surface being inspected, a very light dusting of powder permits easy observation

of the formation of indications. On horizontal surfaces the excess of powder is blown away with a

gentle air stream from the blower.

6.8 Questionable Areas and Re-examination

Surface and near surface discontinuities will be revealed by the retention of the ferromagnetic

powder near discontinuities. All indications revealed by magnetic particle examination are not

necessarily defects. Nonrelevant indications can be caused by excessive surface roughness or

magnetic permeability variations which can occur in weld metal heat affected zones. Indications

that are Judged to be nonrelevant should be regarded as unacceptable until the indications are

either eliminate by surface conditioning or are re-examined by magnetic particle or another type

of non destructive examination and the indication is demonstrated to be nonrelavent.

Relevant indications are those which result from unacceptable mechanical discontinuities. Linear

indications are those indications in which the length is more than three times the width. Rounded

indications are circular or elliptical with the length less than three times the width.

Sufficient illumination should be provided to assure adequate sensitivity.


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7.0 Visual Observation of Examination Results


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7.1 Inspection

A good light and good eyesight are the principal requirements for observing the presence of

indications on the surface of parts. Choice of the best color powder for contrast against the

surface is an aid to visibility. On the large discontinuities, powder build-up is often very heavy,

making indications stand out clearly from the surface. For finer cracks the build-up is less, since

only the smaller particles are held by the leakage field in this case. For exceedingly fine cracks itmay sometimes be better to go to some form of the wet method, which is more sensitive to very

fine discontinuities.

In the case of discontinuities lying wholly and deep below the surface, experience and skill, or

carefully established and controlled practices for repetitive tests are required to secure the best

results. The depth below the surface and the size and shape of the discontinuity determines the

strength and spread of the leakage field. An experienced examiner will observe the surface as

the powder is allowed to drift onto it so he can see faint but significant tendencies of the powder

to gather. Often indications are seen under these conditions, but are no longer visible when more

powder has been applied, the excess blown off, and the surface then examined for indications.

Standardized techniques for careful and proper application of the powder can provide excellent

sensitivity where similar assemblies are repetitively tested.

In the case of steels having appreciable retentivity, indications are held at the defect by the

remanent field, making the examination somewhat easier. In low carbon steels, such as low to

medium strength plate and structural steels, the retentivity is very low. On these it is important to

make the examination while the magnetizing current is on and the powder is being applied, since

indications may not remain in place after the current is turned off. This is particularly true on

vertical and overhead surfaces, where gravity plays a part in causing particles to fall away if

loosely held. Harder new high strength steels are being used more and more for pipe, pressure

vessels, and structural shapes. Smaller discontinuities become more important in this type of

steel. Since it is used at higher stress levels, great care must be taken in the examination of

these steels, especially for fine cracks open to the surface.

7.2 Classification and Characteristics of Discontinuities

7.2.1 Surface DiscontinuitiesSurface cracks and other surface discontinuities make up by far the largest an most important

group which magnetic particle examination is used to locate. This is true for two principal

reasons. First, the surface crack is the type most effectively located with magnetic particles; and

second, surface cracks are much more important and dangerous to the service life of a part than

are defects lying wholly below the surface. They are more frequently the object of the

examination. Fiber stresses are usually highest at the surface of a part, and any break in the

surface constitutes a point of still higher concentration of stress. A surface crack, by its nature, is

very sharp at the bottom and is the most severe kind of stress-riser. For this reason surface

discontinuities are looked for with extreme care if expected stresses are even moderately high.

The surface discontinuities looked for with magnetic particle examination include all fatigue andservice cracks and serious sources of potential failure such as seams, laps, quenching, grinding

cracks, and surface ruptures occurring in castings, The magnetic particle examination method is

a sensitive and reliable method for locating surface cracks in ferromagnetic material. This is due

largely to the fact that in a great majority of cases, no critical conditions or techniques are

required for the detection of surface discontinuities by this method. Magnetizing and particle

application methods may be critical in certain special instances, but in most applications the


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requirements are relatively easily met because leakage fields tend to be strong and are highly

localized. A few simple but important principles are involved which must be observed, but there

is usually a fairly wide latitude in the selection of procedures and materials when surface cracks

only are being sought. If the discontinuities are of a size or character to be in the threshold area

of detect- ability, special techniques may be necessary. It is relatively easy to define the

characteristics of a surface discontinuity that make it favorable for detection. For successful

detection in any given case, it must be possible to set up a field of sufficient strength and in agenerally favorable direction to produce strong leakage fields. This is especially true if the

discontinuities are small and fine. Assuming that this has been done, the most favorable

characteristics of a discontinuity itself for its detection are:

l. That its depth be at right angles to the surface,

2. That its width at the surface be small so that the air gap it creates is small,

3. That its length at the surface be large with respect to its width,

4. That it be comparatively deep in proportion to its surface

opening.

The field set up in the part should be at right angles to the length of the defect.

7.2.2 Sub-Surface Discontinuities

The magnetic particle method is capable of finding many discontinuities which do not break the

surface of the part in which they occur. This is an important characteristic since there are

circumstances when radiography and ultrasound, methods whose primary field is locating such

discontinuities, cannot be used.

These two methods are inherently better adapted to the location of interior discontinuities than

magnetic particle, but sometimes the shape of the part, location of the defect, or the cost or

availability of the methods and the equipment needed, makes the magnetic particle method the

best one to use. As a group those discontinuities which lie wholly below the surface are less

dangerous from the point of view of potential failure than are surface cracks. This is because

they are usually


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lying defects, the wet method should be used for this type of discontinuity if its detection is

important. This is because these fine, non-metallic stringers are not really "deep-lying", for the

reason that, though sub-surface, they must be very close to the surface to be found at all, by any

method.

The second and much more important group of sub-surface discontinuities are those larger and

more serious conditions which may be quite deep in heavy sections - perhaps one quarter inch totwo inches or more below the surface. In weldments these may be lack of penetration, sub-

surface lack of fusion, or cracks in the beads. Such deep discontinuities in castings may be

internal shrinks, slag inclusions, or gas pockets.

7.2.3 Non-Relevant Discontinuities

Before an operator has progressed very far in his experience with magnetic particle examination

he will have become aware of non-relevant indication. They can sometimes be very puzzling,

since in many cases, no apparent reason exists for their occurrence.

Such indications are true particle patterns actually formed and held in place by magnetic leakage

fields; but the leakage fields responsible are not caused by conditions that are relevant to the

strength or useability of the part. The name non-relevant" has been given to this type of pattern.Obviously the magnetic partial operator must be acquainted with these nonrelevant indications

and be able to recognize them for what they are.

l. False Indications: The term "false" has sometimes been applied to all non-relevant

indications since such indications are in nearly all cases magnetic in origin. There is

perhaps one truly false indication, and that is the case of particles held mechanically or

by gravity in surface irregularities, with no relation whatever to leakage fields. When

using the wet method, a "drainage line" of particles will often form and in such cases it is

only necessary to shake or blow, or to rinse off the particles to prove that they are not

magnetically held.

2. Cold Working: The type of plastic deformation called cold working produce a hardening

in steel with a consequent change in permeability. When the cold working is very local,

the abrupt permeability change is often sufficient to cause a particle pattern. The

indication produced is, at times, similar in appearance to magnetic writing. On

demagnetizing and remagnetizing, however, the indication from cold work reappears,

whereas that due to magnetic writing does not. If the object is sectioned and etched, no

discontinuities are found to account for the pattern, but examination under the

microscope will usually show the typical grain distortion representing cold work.

3. Grain Boundaries: When grain size is very large, the macrostructure showing grain

outlines may be found by a magnetic particle pattern even though no metallic

discontinuity exists. The pattern is due to sharply different permeability between

significant differences in grain size.

4. Boundary Zones in Welds: In weld inspection, an indication is often obtained at the

boundary between the fused metal and the base metal. Other indications in the form of

lines may appear at the edges of decarburized zones. These occurrences actually

indicate an abrupt change in permeability in the path of the magnetic flux, but are not

necessarily indicative of an objectionable condition. Many sound welds will yield a

powder line at the Junction of base and weld metal. If in doubt, a metallurgical


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evaluation should be obtained.


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5. Joint Between Dissimilar Magnetic Materials: Sometimes a piece of hard steel is

butt-welded to a softer piece by any of several methods. At the point of welding there

will be a sharp change of permeability, the soft steel having a high permeability and the

hard steel a much lower one. If a magnetic field is set up to flow across this Joint there

will be a concentrated leakage field and consequently a magnetic particle pattern. This

pattern, however, does not give any information regarding the soundness of the welded

Joint.

6. Forced Fits: When two members of an assembly are very tightly fitted together, as in a

pressed fit between a shaft and pinion gear, a magnetic particle pattern of this Joint may

be formed. If the fit is tight enough no indication may be produced, since the air gap

between the two members may be almost zero. If an indication appears it is never

misleading, unless the Joint is hidden by paint or rust and the operator does not know

that the joint exists. Polishing with fine emery cloth will reveal the line between the two

members of the assembly.

7.3 Types and Identification of Discontinuities

7.3.1 Inherent Discontinuities

Inherent discontinuities are those which are related to the melting and original solidification of the

metal in the ingot. As the metal is poured, gas bubble and slag are entrapped in the ingot. The

ingot is then cropped, which removes most of the impurities gathered in the top; however, some

entrapped discontinuities may find their way into the finished product. Following is a list of the

more common inherent discontinuities which may occur.

1. Inclusions: These are nonmetallic impurities such as slag, oxides, and sulphides which

are present in the original ingot. In the rolling of billets and bar stock, these materials

are rolled out length wise to form long stringers, or lines of nonmagnetic foreign

materials. In bar stock and forgings, they are often spoken of as nonmetallic inclusions

or nonmetallic stringers. Inclusions in bar stock give straight indications parallel to the

longitudinal axis of the material, usually appearing as fine lines quite tightly adherent.

They are often short and are likely to occur in groups. They seldom appear on the

original bar surface, but are commonly found on machined surfaces. In forgings, they

parallel the grain flow lines. They are not objectional except when they occur in critical

areas, on highly stressed surfaces, or in unusual numbers.

2. Blowholes: Blowholes are formed by gas which is insoluble in the molten metal and is

trapped when the metal solidifies. As the ingot is worked into fabricated products, the

blowholes are elongated and their sides brought closer together. In finished articles,

they often appear as seams or laminations, depending on their location.

3. Pipe: Pipe is a discontinuity in the center of the ingot caused by internal shrinks or

cavities during solidification which have become elongated in the rolling operations. In

fabricated articles, they usually are found a considerable distance below the surface.

4. Segregations: When an ingot solidifies, the distribution of the various elements or

compounds generally is not uniform throughout the mass of the ingot and marked

segregations of some constituents may occur. As the ingot is forged and rolled, these

segregations are elongated and reduced in cross section. Upon subsequent processing,

they may appear as very thin parallel lines or bands, known as banding. Banding is not


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c. Laps: These are similar to seams and may result from improper rolling

practices. In working down the billet into a bar, corners may be folded over or

small fins of metal forced out between the rolls may be flattened into the bar and

form a lap. They are usually straight and parallel to the longitudinal axis, and are

similar to seams but extend into the bar at an angle not normal to the surface.

6. Grinding Cracks: Grinding of hardened surfaces frequently introduce cracks. Theseare actual thermal cracks similar to the heat treating and hardening cracks. The

overheating in this case occurs under the grinding wheel and may be due to the wheel

becoming glazed so that it rubs instead of cuts the surface. These types of cracks are

generally at right angles to the direction of grinding. Grinding cracks are usually shallow,

but as they are separations of the metal and are very sharp at their roots, they are often

potential sources of fatigue failures. Case-hardened articles are also sensitive to

improper grinding, and readily develop cracks unless proper precautions are taken.

7. Pickling and Etching Cracks: When removing scale by pickling, precleaning prior to

electroplating, or electroplating itself in which hydrogen is generated at the surface of the

article, cracks can be formed or minute cracks already present may be enlarged.

Cracking of this type is encountered in articles that are hardened by heat treatment or

cold-work, as in coldworked or heat-treated springs. The cracks usually develop atpoints where minute cracks or excessive internal stresses exist. Cracks which are

formed or enlarged by pickling, precleaning, or electroplating are usually not detected

before the treatment; therefore, it is important that the examination be performed both

prior to and subsequent to plating operations.

8. Machining Tears:When an article is worked with a dull tool or by cutting at too great a

depth, the metal may not break away clean and the tool may leave a rough, torn surface.

Close examination of this rough, torn surface may reveal numerous short

discontinuities. Machining tears may be too deep to be completely removed by later

finish machining or grinding operations. They may also be covered over and concealed

by the burnishing action of finishing or polishing operations.

9. Heat Treating: In this operation the metal is heated and cooled under controlled

conditions for the purpose of hardening or securing other desired characteristics of grain

structure or strength in the finished article. Cracking during this process usually results

from stresses set up by unequal heating or cooling of certain portions of the piece.

Cracks can occur either in the heating or cooling cycles. The cracks are usually deep,

seldom follow a definite pattern and may be in any direction on the article. Quenching

cracks often start at a thin cross section, or where thick and thin cross sections meet.

10. Welding: Welding, a process for Joining metals, involves heating the edges to be

Joined by means of a torch or electric arc until they reach the fusion point. Additional

weld metal is supplied by a welding rod which is melted by the torch flame or arc and

flowed into the space between the edges being Joined after which the whole is allowed

to cool and "freeze" into a single piece.

a. Surface shrink cracks are the most common discontinuities found in weldments.

They are frequently found at a crater where the welding rod is removed from the

work. Crater cracks often are star-shaped, running out from the center of the

crater, though they may be single cracks. Longitudinal shrink cracks usually

occur in the center of the weld as well as near the edges of the bead.


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b. Cracks often appear in the parent metal at a point adjacent to the weld,

particularly when hard steels are welded. These cracks are caused by stresses

induced in the material from expansion and contraction due to thermal changes.

c. Lack of fusion is found in welds where temperature

has not been high enough to melt the parent metal to the weld, or by improperpositioning of the welding torch, causing a lack of bond.

d. Lack of complete penetration is caused by failure

of the weld metal to penetrate completely through the Joint before solidifying.

e. Overlapping is a common condition found in

weldments. It is caused by leaving a toe of the weld overlapping the parent

metal instead of being fused to it. The indication appears along the edge of the

overlap.

f. Slag and gas inclusions are the result of impurities

being trapped as the metal solidifies.

7.3.3 Service Discontinuities

Service or fatigue discontinuities are by far the most important discontinuities to be considered.

The articles that are in service, which may develop defects due to metal fatigue, are considered

extremely critical and demand close attention of nondestructive examination personnel.

Fatigue cracks normally develop in or adjacent to areas of stress concentration. These may

include oil holes, fillets, keyways, splines, and threads. These areas are usually designed to

withstand the stresses imposed; however, faulty design, such as oil holes with sharp edges or

poorly finished or insufficient fillets often result in a concentrated stress much higher than

expected. Also, the presence of any discontinuities in an area of stress concentration greatly

increases the possibilities of fatigue failure.

A fatigue failure is progressive in that it starts as a fine submicroscopic crack or an accumulation

of such cracks, and spreads under the action of repeated stressing. This spreading action

continues until the cross-section of the article has been reduced to such an extent that the article

ultimately fractures statically under a low load. Once a crack has started, its ability to progress is

greatly increased by the stress concentration at the crack tip. It is interesting to note that the rate

of progress of fatigue cracks may vary, depending on the stress condition. In some instances the

progress of the crack may be slow. This has been observed in some types of articles, which

have appeared to operate many hours in a cracked condition. In other instances where high

stresses are continually applied, particularly to brittle materials, the progress of the crack may be

practically instantaneous.

Since cracked articles are a potential source of failure, their detection during examination is ofprime importance. The rapidly rotating and reciprocating parts of a pump or the vibration of a

structure produce many applications of repeated stress. Fatigue cracks start as fine

submicroscopic cracks and become detectable by examination as soon as they reach any

measurable length. Since a small fatigue crack may be confused with a much less significant

discontinuity, particular care is to be exercised in evaluating all indications. Fatigue cracks are

always cause for rejection unless the article can be salvaged by rework. A fatigue crack is often


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identified by its direction in relation to the applied stresses. For example, primary discontinuities,

such as seams and inclusions, normally run in the direction of the grain flow, while fatigue cracks

in most areas run transverse to the grain flow.

8.0 RECORDING RESULTS OF EXAMINATION SURFACE

8.1 Lacquer Transfer Technique

If it is desired to retain the indication in place on the specimen, a transparent lacquer is sprayed

over the discontinuity. Spraying or dipping are more effective that brushing because the latter,

no matter how carefully done, tends to disturb and mar the pattern. Stock lacquers are thinned at

least three to one before being used for this purpose.

1. Wet Method: When using the wet method, the surface is allowed to dry before the

lacquer is applied. Drying may be facilitated by the use of naptha.

2. Lacquer Mixtures: There is another use for lacquer which employs a colored lacquer

as a suspensoid for a powder of a different color. The magnetic field is applied before

the lacquer sets, and the pattern becomes permanently fixed after the lacquer dries. A

white lacquer with black paste in suspension gives a black pattern on a white backgroundand can be applied on practically any surface, or the lacquer can be applied first, allowed

to dry, and the powder applied afterwards. The resultant patterns are then

photographed.

8.2 Daylight Photography

Photography is one of the best ways to make permanent records of the appearance of

indications, but takes more time and equipment than the tape or lacquer techniques. Also, if

precise delineation of the contour of indications is important, the tape or lacquer records lifted

directly from the surface of the part can be more exact. A photograph, on the other hand, shows

the indication in its natural environment on the part in which the discontinuity occurs, and has this

advantage over the lifted records.

Black and white photographs of parts containing indications sometimes require some ingenuity in

"posing" the specimen and in securing lighting that sets off the indication so that it gives a clear

picture and be a faithful reproduction of the indication on the resulting photograph.

Use of the Polaroid camera and film process is a most convenient and quick method of making

photographic records. The immediate availability of the picture makes it possible to make

corrections in lighting or positioning quickly. Several successive shots can be made if duplicates

are wanted.

8.3 Transparent Tape Transfer Technique

Probably the most convenient and by far the most widely used method of preserving indicationsand patterns is the transparent tape method. If the dry magnetic particle method is used, the

excess powder is blown carefully away or otherwise removed. If the wet method is employed,

sufficient time is allowed for the solvent to evaporate from the particles composing the indication.

A strip of transparent tape is then carefully laid over the indication and gently pressed down with

the fingers or a rounded stick. The tape is then peeled off, bringing the indication with it. The

strip is then laid on white paper for photographing, on tracing paper for blueprinting, or on a page


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of a permanent record book. With care, the transferred pattern remains well-defined and

accurate in every detail, and may serve as well as the original pattern as a basis for Judging and

studying the indication.


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9.0 BIBLIOGRAPHY

l. GENERAL DYNAMICS (Convair Division) Classroom Training Handbook NDT

MAGNETIC PARTICLE TESTING (CT-6-3).

2. PRINCIPLES OF MAGNETIC TESTING, C. E. Betz, Feb. 1, 1967, Magnaflux Corp.

3. NDT HANDBOOK, R. C. McMaster, Ronald Press Company 1963.

4. ASM METALS HANDBOOK VOLUME 11, NONDESTRUCTIVE INSPECTION, Aug.

1976.

5. HANDBOOK FOR STANDARDIZATION OF NONDESTRUCTIVE TESTING

METHODS, MIL-HOBK-333 (USAF) 10 April, 1974.

6. THE STRUCTURE AND PROPERTIES OF MATERIALS, VOLUME IV ELECTRONIC

PROPERTIES, R. M. Rose, John Wiley & Sons, Inc., New York.

7. GLOSSARY OF TERMS FREQUENTLY USED IN NONDESTRUCTIVE TESTING,

MATERIALS EVALUATION, April, 1975.

8. INTRODUCTION TO MATERIALS SCIENCE, A. G. GUY, McGraw-Hill, 1972.

10.0 APPLICABLE SPECIFICATIONS AND PROCEDURES

This area has been left blank to give room for insertion, by the individual, of specifications and

procedures to which the individual will be working.


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11.0 GLOSSARY

air gap- When a magnetic circuit contains a small gap which the magnetic flux must cross, the space is

referred to as an air gap. Cracks produce small air gaps on the surface of a part.

alternating current (AC) - Alternating current is current that reverses its direction of flow at regular

intervals. Such current is frequently referred to as AC.

ampere - The unit of electrical current. One ampere is the current which flows through a conductor

having a resistance of one ohm, at a potential of one volt.

ampere turns - Refers to the product of the number of turns in a coil and the number of amperes of

current flowing through it. This is a measure of the magnetizing or demagnetizing strength of the coil.

For example: 800 amperes in a 6-turn coil 4800 ampere turns.

artifact- An indication that is not associated with the actual condition of the object under test.

bath- The suspension of iron oxide particles in a vehicle (usually a light oil).

blowhole- A hole in a casting or a weld caused by gas entrapped during solidification normally ventedto the surface.

central conductor- A conductor that is passed through the opening in a ring or tube, or any hole in a

part, for the purpose of creating a circular of circumferential field in the tube or ring, or around the hole.

circular magnetization - Circular magnetization involves the production of a magnetic field in a part

such that the magnetic lines of force are mostly contained within the part.

coercive force - The reverse magnetizing force necessary to remove remnant or residual magnetism

and demagnetize the part.

coil shot- A short pulse of magnetizing current passed through a solenoid or coil surrounding a part, for

the purpose of longitudinal magnetization is called a "coil shot." Duration of the passage of the current is

usually only a fraction of a second.

cold cracks - Appear as a straight line, usually continuous throughout its length and generally exist

singly. These cracks start at the surface.

cold shut- (1) A discontinuity that appears in cast metal as a result of two streams of liquid meeting and

failing to unite; (2) a portion of the surface of a forging that is separated, in part, from the main body of

metal by oxide.

conductivity- This is the inverse of resistance and refers to the ability of a material to carry current or

heat.

continuous method- Current and bath are applied together; that is, the indicating particles are on the

part while the magnetizing current is being applied.

crack- A material separation which has a relatively large cross section in one direction and a small or

negligible cross section when viewed in a direction perpendicular to the first.


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defect - A discontinuity whose size, shape, orientation, location of properties make it detrimental to the

useful service of the part in which it occurs or which exceeds the accept/reject criteria for this given

design.

demagnetization- The process of removing existing magnetism from within a part.

dendrite - A crystal that has a tree-like branching pattern, most evident in cast metals usually formedduring solidification.

direct current (dc) - As the name implies, this term refers to an electric current flowing continually in

one direc

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