ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control

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Fig. 16

Representative magnetization (hysteresis) curve for a ferromagnetic material. Dashed line in each of

the charts is the virgin hysteresis curve. See text for discussion.

Magnetic Particle Inspection

Revised by Art Lindgren, Magnaflux Corporation

Magnetic Hysteresis

Some ferromagnetic materials, when magnetized by being introduced into an external field, do not return to a completely

unmagnetized state when removed from that field. In fact, these materials must be subjected to a reversed field of a

certain strength to demagnetize them (discounting heating the material to a characteristic temperature, called the Curie

point, above which the ferromagnetic ordering of atomic moments is thermally destroyed, or mechanically working the

material to reduce the magnetization). If an external field that can be varied in a controlled manner is applied to a

completely demagnetized (virgin) specimen and if instrumentation for measuring the magnetic induction within the

specimen is at hand, the magnetization curve of the material can be determined. A representative magnetization

(hysteresis) curve for a ferromagnetic material is shown in Fig. 16.

As shown in Fig. 16(a), starting at the origin (O) with the specimen in the unmagnetized condition and increasing the

magnetizing force in small increments, the flux in the material increases quite rapidly at first, then more slowly until it

reaches a point beyond which any increase in the magnetizing force does not increase the flux density. This is shown by

the dashed (virgin) curve Oa. In this condition, the specimen is said to be magnetically saturated.

When the magnetizing force is gradually reduced to zero, curve ab results (Fig. 16b). The amount of magnetism that the

steel retains at point b is called residual magnetism, B

When the magnetizing current is reversed and gradually increased in value, the flux continues to diminish. The flux does

not become zero until point c is reached, at which time the magnetizing force is represented by Oc (Fig. 16c), which

graphically designates the coercive force, H

, in the material. Ferromagnetic materials retain a certain amount of residual

magnetism after being subjected to a magnetizing force. When the magnetic domains of a ferromagnetic material have

been oriented by a magnetizing force, some domains remain so oriented until an additional force in the opposite direction

causes them to return to their original random orientation. This force is commonly referred to as coercive force.

As the reversed field is increased beyond c, point d is reached (Fig. 16d). At this point, the specimen is again magnetically

saturated. The magnetizing force is now decreased to zero, and the de line is formed and retains reversed-polarity residual

magnetism, B

, in the specimen. Further increasing the magnetizing force in the original direction completes the curve efa.

The cycle is now complete, and the area within the loop abcdefa is called the hysteresis curve.

The definite lag throughout the cycle between the magnetization force and the flux is called hysteresis. If the hysteresis

loop is slender (Fig. 16e), the indication usually means that the material has low retentivity (low residual field) and is easy

to magnetize (has low reluctance). A wide loop (Fig. 16f) indicates that the material has high reluctance and is difficult to

magnetize.

Magnetic Particle Inspection

Revised by Art Lindgren, Magnaflux Corporation

Magnetic Particles and Suspending Liquids

Magnetic particles are classified according to the medium used to carry the particles to the part. The medium can be air

(dry-particle method) or a liquid (wet-particle method). Magnetic particles can be made of any low-retentivity

ferromagnetic material that is finely subdivided. The characteristics of this material, including magnetic properties, size,

shape, density, mobility, and degree of visibility and contrast, vary over wide ranges for different applications.

Magnetic Properties. The particles used for magnetic particle inspection should have high magnetic permeability so

that they can be readily magnetized by the low-level leakage fields that occur around discontinuities and can be drawn by

these fields to the discontinuities themselves to form readable indications. The fields at very fine discontinuities are

sometimes extremely weak.

Low coercive force and low retentivity are desirable for magnetic particles. If high in coercive force, wet particles

become strongly magnetized and form an objectionable background. In addition, the particles will adhere to any steel in

the tank or piping of the unit and cause heavy settling-out losses. Highly retentive wet particles tend to clump together

quickly in large aggregates on the test surface. Excessively large clumps of particles have low mobility, and indications

are distorted or obscured by the heavy, coarse-grain backgrounds.

Dry particles having coercive force and high retentivity would become magnetized during manufacture or in first use and

would therefore become small, strong permanent magnets. Once magnetized, their control by the weak fields at

discontinuities would be subdued by their tendency to stick magnetically to the test surface wherever they first touch. This

would reduce the mobility of the powder and would form a high level of background, which would reduce contrast and

make indications difficult to see.

Effect of Particle Size. Large, heavy particles are not likely to be arrested and held by weak fields when such particles

are moving over a part surface, but fine particles will be held by very weak fields. However, extremely fine particles may

also adhere to surface areas where there are no discontinuities (especially if the surface is rough) and form confusing

backgrounds. Coarse dry particles fall too fast and are likely to bounce off the part surface without being attracted by the

weak leakage fields at imperfections. Finer particles can adhere to fingerprints, rough surfaces, and soiled or damp areas,

thus obscuring indications.

Magnetic particles for the wet method are applied as a suspension in some liquid medium, and particles much smaller

than those for the dry method can be used. When such a suspension is applied over a surface, the liquid drains away, and

the film remaining on the surface becomes thinner. Coarse particles would quickly become stranded and immobilized.

The stranding of finer particles as a result of the draining away of the liquid occurs much later, giving these particles

mobility for a sufficient period of time to be attracted by leakage fields and to accumulate and thus form true indications.

Effect of Particle Shape. Long, slender particles develop stronger polarity than globular particles. Because of the

attraction exhibited by opposite poles, these tiny, slender particles, which have pronounced north and south poles, arrange

themselves into strings more readily than globular particles. The ability of dry particles to flow freely and to form

uniformly dispersed clouds of powder that will spread evenly over a surface is a necessary characteristic for rapid and

effective dry-powder testing. Elongated particles tend to mat in the container and to be ejected in uneven clumps, but

globular particles flow freely and smoothly under similar conditions. The greatest sensitivity for the formation of strong

indications is provided by a blend of elongated and globular shapes.

Wet particles, because they are suspended in a liquid, move more slowly than do dry particles to accumulate in leakage

fields. Although the wet particles themselves may be of any shape, they are fine and tend to agglomerate, or clump, into

unfavorable shapes. These unfavorable shapes will line up into magnetically held elongated aggregates even when the

leakage field is weak. This effect contributes to the relatively high sensitivity of fine wet particles.

Visibility and contrast are promoted by choosing particles with colors that are easy to see against the color of the

surface of the part being inspected. The natural color of the metallic powders used in the dry method is silver-gray, but

pigments are used to color the particles. The colors of particles for the wet method are limited to the black and red of the

iron oxides commonly used as the base for wet particles.

For increased visibility, particles are coated with fluorescent pigment by the manufacturer. The search for indications is

conducted in total or partial darkness, using ultraviolet light to activate the fluorescent dyes. Fluorescent magnetic

particles are available for both the wet and dry methods, but fluorescent particles are more commonly used with the wet

method.

Types of Magnetic Particles. The two primary types of particles for magnetic particle inspection are dry particles

and wet particles, and each type is available in various colors and as fluorescent particles. Particle selection is principally

influenced by:

• Location of the discontinuity, that is, on the surface or beneath the surface

• Size of the discontinuity, if on the surface

• Which type (wet or dry particles) is easier to apply

Dry particles, when used with direct current for magnetization, are superior for detecting discontinuities lying wholly

below the surface. The use of alternating current with dry particles is excellent for revealing surface cracks that are not

exceedingly fine, but is of little value for discontinuities even slightly beneath the surface (Fig. 17).

Fig. 17 Comparison of sensitivity of alternating current, direct current, direct current with surge, and half-

wave

current for locating defects at various distances below a surface by means of the dry-

particle continuous

magnetizing method. See text for description of test conditions.

Wet particles are better than dry particles for detecting very fine surface discontinuities regardless of which form of

magnetizing current is used. Wet particles are often used with direct current to detect discontinuities that lie just beneath

the surface. The surface of a part can easily be covered with a wet bath because the bath flows over and around surface

contours. This is not easily accomplished with dry powders.

Colored particles are used to obtain maximum contrast with the surface of the part being inspected. Black stands out

against most light-colored surfaces (Fig. 18), and gray against dark-colored surfaces. Red particles are more visible than

black or gray particles against silvery and polished surfaces. Fluorescent particles, viewed under ultraviolet light, provide

the highest contrast and visibility.

Fig. 18 Black magnetic-powder indications of a cold shut in a casting when seen under black light.

Courtesy of

Magnaflux Corporation

Dry particles are available with yellow, red, black, and gray pigmented coloring and with fluorescent coatings. The

magnetic properties and particle sizes are similar in all colors, making them equally efficient. Dry particles are most

sensitive for use on very rough surfaces and for detecting flaws beneath the surface. They are ordinarily used with

portable equipment. The reclamation and reuse of dry particles is not recommended.

Air is used to carry the particles to the surface of the part, and care must be taken to apply the particles correctly. Dry

powders should be applied in such a way that they reach the magnetized surface in a uniform cloud with a minimum of

motion. When this is done, the particles come under the influence of the leakage fields while suspended in air and have

three-dimensional mobility. This condition can be best achieved when the magnetized surface is vertical or overhead.

When particles are applied to a horizontal or sloping surface, they settle directly to the surface and do not have the same

degree of mobility.

Dry powders can be applied with small rubber spray bulbs or specially designed mechanical powder blowers. The air

stream of such a blower is of low velocity so that a cloud of powder is applied to the test area. Mechanical blowers can

also deliver a light stream of air for the gentle removal of excess powder. Powder that is forcibly applied is not free to be

attracted by leakage fields. Neither rolling a magnetized part in powder nor pouring the powder on the part is

recommended.

Wet particles are best suited for the detection of fine discontinuities such as fatigue cracks. Wet particles are commonly

used in stationary equipment where the bath can remain in use until contaminated. They are also used in field operations

with portable equipment, but care must be taken to agitate the bath constantly.

Wet particles are available in red and black colors or as fluorescent particles that fluoresce a blue-green or a bright

yellow-green color. The particles are supplied in the form of a paste or other type of concentrate that is suspended in a

liquid to produce the coating bath.

The liquid bath may be either water or a light petroleum distillate having specific properties. Both require conditioners to

maintain proper dispersion of the particles and to allow the particles the freedom of movement to form indications on the

surfaces of parts. These conditioners are usually incorporated in the powder concentrates.

Oil Suspending Liquid. The oil used as a suspending liquid for magnetic particles should be an odorless, well-refined

light petroleum distillate of low viscosity having a low sulfur content and a high flash point. The viscosity of the oil

should not exceed 3 × 10

-6

/s (3 cSt) as tested at 40 °C (100 °F) and must not exceed 5 × 10

-6

/s (5 cSt) when tested

at the bath temperature. Above 5 × 10

-6

/s (5 cSt), the movement of the magnetic particles in the bath is sufficiently

retarded to have a definite effect in reducing buildup, and therefore the visibility, of an indication of a small discontinuity.

Parts should be precleaned to remove oil and grease because oil from the surface accumulates in the bath and increases its

viscosity.

Water Suspending Liquid. The use of water instead of oil for magnetic particle wet-method baths reduces costs and

eliminates bath flammability. Figures 19, 20, 21, and 22 illustrate the effectiveness of water-base magnetic particle

testing. Water-suspendible particle concentrates include the necessary wetting agents, dispersing agents, rust inhibitors,

and antifoam agents.

Fig. 19 Spindle defects revealed with water-base magnetic particle testing.

Courtesy of Circle Chemical

Company

Fig. 20 Compressor vane microcracks revealed with water-base magnetic particle testing.

Courtesy of Circle

Chemical Company

Fig. 21 50 mm (2 in.) diam gear subjected to water-

base magnetic particle testing showing cracks 0.25 mm

(0.0098 in.) long by 0.1 mm (0.004 in.) wide. Courtesy of Circle Chemical Company

Fig. 22 Defects in the leading edge and fillet areas of a hydroplane propeller as revealed by water-bas

magnetic particle testing. (a) Overall view. (b) Close-up of fillet area. Courtesy of Circle Chemical Company

Water baths ordinarily should not be used where the temperature is below freezing. However, ethylene glycol can be

employed to protect against reasonably low temperatures. Care must be exercised, however, because a high percentage of

ethylene glycol can impede particle mobility. For the inspection of large billets, a solution containing three parts water

and one part anti-freeze has been successfully used.

Because water is a conductor of electricity, units in which water is to be used must be designed to isolate all high-voltage

circuits so as to avoid all possibility of operator shock, and the equipment must be thoroughly and positively grounded.

Also, electrolysis of parts of the unit can occur if preventive measures are not taken. Units specifically designed to be

used with water as a suspensoid are safe for the operator and minimize electrolytic corrosion.

The strength of the bath is a major factor in determining the quality of the indications obtained. The proportion of

magnetic particles in the bath must be maintained at a uniform level. If the concentration varies, the strength of the

indications will also vary, and the indications may be misinterpreted. Fine indications may be missed entirely with a weak

bath. Too heavy a concentration of particles gives a confusing background and excessive adherence of particles at

external poles, thus interfering with clean-cut indications of extremely fine discontinuities.

The best method for ensuring optimum bath concentration for any given combination of equipment, bath application, type

of part, and discontinuities sought is to test the bath using parts with known discontinuities. Bath strength can be adjusted

until satisfactory indications are obtained. This bath concentration can then be adopted as standard for those conditions.

The concentration of the bath can be measured with reasonable accuracy with the settling test. In this test, 100 mL (3.4

oz) of well-agitated bath is placed in a pear-shaped centrifuge tube. The volume of solid material that settles out after a

predetermined interval (usually 30 min) is read on the graduated cylindrical part of the tube. Dirt in the bath will also

settle and usually shows as a separate layer on top of the oxide. The layer of dirt is usually easily discernible because it is

different in color from the magnetic particles.

Magnetic Particle Inspection

Revised by Art Lindgren, Magnaflux Corporation

Ultraviolet Light

A mercury-arc lamp is a convenient source of ultraviolet light. This type of lamp emits light whose spectrum has several

intensity peaks within a wide band of wave-lengths. When used for a specific purpose, emitted light is passed through a

suitable filter so that only a relatively narrow band of ultraviolet wavelengths is available. For example, a band in the

long-wave ultraviolet spectrum is used for fluorescent liquid penetrant or magnetic particle inspection.

Fluorescence is the characteristic of an element or combination of elements to absorb the energy of light at one

frequency and emit light of a different frequency. The fluorescent materials used in liquid penetrant and magnetic particle

inspection are combinations of elements chosen to absorb light in the peak energy band of the mercury-arc lamp fitted

with a Kopp glass filter. This peak occurs at about 365 nm (3650 Å). The ability of fluorescent materials to emit light in

the greenish-yellow wavelengths of the visible spectrum depends on the intensity of ultraviolet light at the workpiece

surface. In contrast to the harmful ultraviolet light of shorter wavelengths, which damages organs such as the eyes and the

skin, the black light of 365 nm (3650 Å) wavelengths poses no such hazards to the operator and provides visible evidence

of defects in materials, as shown in Fig. 23, 24, 25, 26, 27, and 28.

Fig. 23

Casting viewed under black light showing strong magnetic particle indication with minimal background

fluorescence. Courtesy of Magnaflux Corporation

Fig. 24

Potentially dangerous cracks in a lawn mower blade revealed when fluorescent magnetic particles are

exposed to black light. Courtesy of Magnaflux Corporation

Fig. 25 Fluorescent magnetic particle indications of quenched-and-

drawn drill casing tubing that shows

rejectable, cracked spiral seams in the tubing when exposed to black light. Courtesy of Magnaflux Corporation

Fig. 26 Black light used to reveal magnetic particle indication of a crack in the centering stud of a ball-yoke-

type universal-joint section of a drive-shaft assembly. Courtesy of Magnaflux Corporation

Fig. 27

Black light used to reveal magnetic particle indication of a lap that could lead to potential failure of a

forged connecting rod. Courtesy of Magnaflux Corporation

Fig. 28

Black light used to reveal magnetic particle indication of a dangerous crack near a wheel stud. The

crack was detected during manufacture. Courtesy of Magnaflux Corporation

Early specifications required 970 lx (90 ftc) of illumination at the workpiece surface measured with a photographic-type

light meter, which responds to white light as well as ultraviolet light. Because the ultraviolet output of a mercury-arc lamp

decreases with age and with hours of service, the required intensity of the desired wavelength is often absent, although the

light meter indicates otherwise. Meters have been developed that measure the overall intensity of long-wave ultraviolet

light only in a band between 300 to 400 nm (3000 to 4000 Å) and that are most sensitive near the peak energy band of the

mercury-arc lamp used for fluorescent inspection. These meters read the intensity level in microwatts per square

centimeter (μW/cm

). For aircraft-quality fluorescent inspection, the minimum intensity level of ultraviolet illumination is

1000 μW/cm

. High-intensity 125-W ultraviolet bulbs are available that provide up to 5000 W/cm

at 380 mm (15 in.).

Magnetic Particle Inspection

Revised by Art Lindgren, Magnaflux Corporation

Nomenclature Used in Magnetic Particle Inspection

An indication is an accumulation of magnetic particles on the surface of the part that forms during inspection.

Relevant indications are the result of errors made during or after metal processing. They may or may not be

considered defects.

A nonrelevant indication is one that is caused by flux leakage. This type of indication is usually weak and has no

relation to a discontinuity that is considered to be a defect. Examples are magnetic writing, change in section due to part

design, or a heat affected zone line in welding.

False indications are those in which the particle patterns are held by gravity or surface roughness. No magnetic

attraction is involved.

A discontinuity is any interruption in the normal physical configuration or composition of a part. It may not be a defect.

A defect is any discontinuity that interferes with the utility or service of a part.

Interpretation consists of determining the probable cause of an indication, and assigning it a discontinuity name or

label.