magnetic particle testing by dipak chandiramani …

MAGNETIC PARTICLE TESTING BY

DIPAK CHANDIRAMANI LLOYD’S REGISTER ASIA

MUMBAI

Mechanical Engineering Books

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MAGNETIC PARTICLE INSPECTION is a method of locating surface and subsurface discontinuities in ferromagnetic materials. It depends on the fact that when the material or part under test is magnetised, magnetic discontinuities that lie in a direction generally transverse to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part. The presence of this leakage field and therefore the presence of the discontinuity, is detected by the use of fine ferromagnetic particles applied over the surface, with some of the particles being gathered and held by the leakage field. This magnetically held collection of particles forms an outline of the discontinuity and generally indicates its location, size, shape and extent. Magnetic particles are applied over a surface as dry particles or as wet particles in a liquid carrier such as water or oil.

Ferromagnetic materials include most of the iron, nickel and cobalt alloys. These materials lose their ferromagnetic properties above a characteristic temperature called Curie point which is approximately 7600C.

Method Advantages and Limitations Advantages

The magnetic particle method is a sensitive means of locating small and shallow surface cracks in ferromagnetic materials. Indications may be produced at cracks that are large enough to be seen with the naked eye, but exceedingly wide cracks will not produce a particle pattern if the surface opening is too wide for the particles to bridge.

Discontinuities that do not actually break through the surface are also indicated in many cases by this method, although certain limitations must be recognised and understood. If a discontinuity is fine, sharp and close to the surface, such as a long stringer of nonmetallic inclusions, a clear indication can be produced. If the discontinuity lies deeper, the indication will be less distinct. The deeper the discontinuity lies below the surface, the larger it must be to yield a readable indication and the more difficult the discontinuity is to find by this method.

Magnetic particle indications are produced directly on the surface of the part and constitute magnetic pictures of the actual discontinuities. There is no electrical circuitry or electronic readout to be calibrated or kept in proper operating condition. Occasional monitoring of field intensity in the part is needed to ensure adequate field strength

There is little or no limitation on the size or shape of the part being inspected. Ordinarily, no elaborate precleaning is necessary and cracks filled with foreign material can be detected.

Limitations

There are certain limitations to MT, for example, thin coatings of paint and other nonmagnetic coverings such as plating, adversely affect the sensitivity of magnetic particle inspection. Other limitations are:



• The method can only be used on ferromagnetic materials

• For best results, the magnetic field must be in a direction that will intercept the principal plane of the discontinuity; this sometimes requires two or more sequential inspections with different magnetisations.

• Demagnetisation following inspection is often necessary.

• Postcleaning to remove remnants of the magnetic particles clinging to the surface may sometimes be required after testing and demagnetisation.

• Exceedingly large currents are sometimes needed for very large parts.

• Care is necessary to avoid local heating and burning of finished parts or surfaces at the points of electrical contact.

• Although magnetic particle indications are easily seen, experience and skill are sometimes needed to judge their significance.

Description of Magnetic Fields Magnetic fields are used in MT to reveal discontinuities. When a magnetic material is placed across the poles of a horse shoe magnet having square ends, forming a closed or ring like assembly, the lines of force flow from the north pole through the magnetic material to the south pole as shown in Figure 1 below.

(Magnetic lines of force flow preferably through magnetic material rather than through nonmagnetic material or air.) The magnetic lines of force will be enclosed within the ring like assembly because no external poles exist and iron filings or magnetic particles dusted over the assembly are not attracted to the magnet even though there are lines of magnetic force flowing through it. A ring like part magnetised in this manner is said to contain a circular magnetic field that is wholly within the part.



If one end of the magnet is not square and an air gap exists between that end of the magnet and the magnetic material, the poles will still attract magnetic materials. Magnetic particles will cling to the poles and bridge the gap between them as shown in Figure 2 below.

Any radial crack in a circularly magnetised piece will create a north and a south pole at the edges of the crack. Magnetic particles will be attracted to the poles created by such a crack, forming an indication of the discontinuity in the piece. The fields set up at cracks or other physical or magnetic discontinuities in the surface are called leakage fields. The strength of a leakage field determines the number of magnetic particles that will gather to form indications.

Magnetised Bar

A straight piece of magnetised material (bar magnet) has a pole at each end. Magnetic lines of force flow through the bar from the south pole to the north pole. Since the magnetic lines of force within the bar magnet run the length of the bar, it is said to be longitudinally magnetised or to contain a longitudinal field.

If a bar magnet is broken, north and south poles are created between the pieces and a leakage field is produced. The field exists even if the fracture surfaces are brought together. If the magnet is cracked but not broken completely, a somewhat similar result occurs. This field attracts the iron particles that outline the crack. See Figure 3 below.



The direction of the magnetic field in an electromagnetic circuit is controlled by the direction of the flow of magnetising current through the part to be magnetised. The magnetic lines of force are always at right angles to the direction of current flow. To remember the direction taken by the magnetic lines of force around a conductor, consider that a conductor is grasped with the right hand so that the thumb points in the direction of the current flow. The fingers then point in the direction taken by the magnetic lines of force in the magnetic field surrounding the conductor. This is known as the right hand rule.

Circular Magnetisation

Electric current passing through any straight conductor such as a wire or bar creates a circular magnetic field around the conductor. When the conductor of electric current is a ferromagnetic material, the passage of current induces a magnetic field in the conductor as well as in the surrounding space. A part magnetised in this manner is said to have a circular field or to be circularly magnetised. See Figure 4 below.

Longitudinal Magnetisation

Electric current can also be used to create a longitudinal magnetic field in magnetic materials. When electric current is passed through a coil of one or more turns, a magnetic field is established lengthwise or longitudinally within the coil as shown in the figure above.

Effect of Flux Direction

To form an indication, the magnetic field must approach a discontinuity at an angle great enough to cause the magnetic lines of force to leave the part and return after bridging the discontinuity. For best results, an intersection approaching 900 is desirable. For this reason, the direction, size and shape of the discontinuity are



important. The direction of the magnetic field is also important for optimum results, as is the strength of the field in the area of the discontinuity.

Magnetisation Methods

In MT, the magnetic particles can be applied to the part while the magnetising current is flowing or after the current has ceased, depending largely upon the retentivity of the part. The first technique is known as the continuous method; the second, the residual method.

Magnetising Current Both DC and AC are suitable for magnetising parts for MT. The strength, direction and distribution of magnetic fields are greatly affected by the type of current used for magnetisation.

The fields produced by DC and AC differ in many respects. The important difference with regard to MT is that the fields produced by DC generally penetrate the cross section of the part, while the fields produced by AC are confined at or near the surface of the part, a phenomenon known as the skin effect. Therefore, AC should not be used in searching for subsurface discontinuities.

Direct Current

The best source of DC is the rectification of AC. See Figure 5 below. Both the single phase (Fig. 5(a)) and three phase types of alternating current (Fig. 5(b)) are furnished commercially. By using rectifiers, the reversing alternating current can be converted into unidirectional current and when three phase AC is rectified in this manner (Fig. 5(c)), the delivered DC is entirely the equivalent of straight DC for purposes of MT.

When single phase AC is passed through a simple rectifier, current is permitted to flow in one direction only. The reverse half of each cycle is completely blocked out (Fig. 5(d)). The result is unidirectional current that pulsates. A rectifier for AC can also be connected so that the reverse half of the cycle is turned around and fed into the circuit flowing in the same direction as the first half of the cycle (Fig. 5(e)). This produces pulsating direct current, but with no interval between the pulses. Such current is referred to as single phase full wave DC.

There is a slight skin effect from the pulsations of the current, but it is not pronounced enough to have a serious impact on the penetrations of the field. The pulsations of the current are useful because they impart some slight vibration to the magnetic particles, assisting them in arranging themselves to form indications.



Half wave current, used in magnetisation, with prods and dry magnetic particles, provides the highest sensitivity for discontinuities that are wholly below the surface, such as those in castings and weldments.

Methods of Generating Magnetic Fields One of the basic requirements of MT is that the part undergoing inspection be properly magnetised so that the leakage fields created by discontinuities will attract the magnetic particles. Permanent magnets serve some useful purpose in this respect but magnetisation is generally produced by electromagnets.

Yokes There are two basic types of yokes that are commonly used for magnetising purposes: permanent magnet and electromagnetic yokes. Both are hand held and therefore quite mobile.

Permanent magnet yokes are used for applications where a source of electric power is not available or where arcing is not permissible (as in an explosive atmosphere). The limitation of permanent magnet yokes include the following:

• Large areas or masses cannot be magnetised with enough strength to produce satisfactory indications.

• Flux density cannot be varied at will.



• If the magnet is very strong, it may be difficult to separate from a part.

• Particles may cling to the magnet, possibly obscuring indications.

Electromagnetic yokes (see Figure 6 below) consist of a coil wound around a U-shaped core of soft iron. The legs of the yoke can be either fixed or adjustable. Adjustable legs permit changing the contact spacing and the relative angle of contact to accommodate irregularly shaped parts. Unlike a permanent magnet yoke, an electromagnetic yoke can be readily switched on or off. This feature makes it convenient to apply and remove the yoke from the testpiece.

The design of an electromagnetic yoke can be based on the use of either DC or AC or both. The flux density of the magnetic field can be changed by varying the amount of current in the coil. The DC yoke has greater penetration while the AC type concentrates the magnetic field at the surface, providing good sensitivity for the disclosure of surface discontinuities over a relatively broad area. In general, discontinuities to be disclosed should be centrally located in the area between pole pieces and oriented perpendicular to an imaginary line connecting them as shown in the figure above. Extraneous leakage fields in the immediate vicinity of the poles causes an excessive buildup of magnetic particles.

In operation, the part completes the magnetic path for the flow of magnetic flux. The yoke is a source of magnetic flux and the part becomes the preferential path completing the magnetic circuit between the poles.

Coils Single loop and multiple loop coils (conductors) are used for the longitudinal magnetisation of components. The flux density passing through the interior of the coil is proportional to the product of the current, I, in amperes and the number of turns of the coil, N. Therefore the magnetising force of such a coil can be varied by changing either the current or the number of turns in the coil.



The relationship of the length of the part being inspected to the width of the coil must be considered. For a simple part, the effective overall distance that can be inspected is 150 to 230 mm on either side of the coil. Thus, a part 300 to 450 mm long can be inspected using a coil 25 mm wide. In testing longer parts, either the part must be moved at regular intervals through the coil, or the coil must be moved along the part.

The ease with which a part can be longitudinally magnetised in a coil is related to the length-to-diameter (L/D) ratio of the part. This is due to the demagnetising effect of the magnetic poles setup at the ends of the part. This demagnetising effect is considerable for L/D ratios of less than 10:1 and is very significant for ratios of less than 3:1.

The number of ampere turns required to produce sufficient magnetising force to magnetise a part adequately for inspection is given by:

NI = 45000/(L/D)

Where N is the number of turns in the coil, I is the current in amperes and L/D is the length-to-diameter ratio of the part.

When it is desirable to magnetise the part by centering it in the coil, the above equation becomes:

NI = 43000 r /µeff

Where r is the radius of the coil in inches

µeff = (6L/D) – 5

This equation is applicable to parts that are centered in the coil (coincident with the coil axis) and that have cross sections constituting a low fill factor, that is, with a cross sectional area less than 10% of the area encircled by the coil.

Central Conductors For many tubular or ring shaped parts, it is advantageous to use a separate conductor to carry the magnetising current rather than the part itself. Such a conductor, commonly referred to as a central conductor, is threaded through the inside of the part as shown in Figure 7 below and is a convenient means of magnetising a part without the need for making direct contact with the part itself. Central conductors are made of solid or tubular nonmagnetic or ferromagnetic materials that are good conductors of electricity.



The basic rules regarding magnetic fields around a circular conductor carrying direct current are as follows:

• The magnetic field outside a conductor of uniform cross section is uniform along the length of the conductor.

• The magnetic field is 900 to the path of the current through the conductor

• The flux density outside the conductor varies inversely with the radial distance from the centre of the conductor

Solid Nonmagnetic Conductor Carrying Direct Current

The distribution of the magnetic field inside a nonmagnetic conductor, such as a copper bar, when carrying DC is different from the distribution external to the bar. At any point inside the bar, the flux density is the result of only that portion of the current that is flowing in the metal between the point and the centre of the bar. Therefore, the flux density increases linearly, from zero at the centre of the bar to a maximum value at the surface. Outside the bar, the flux density decreases along a curve, as shown in the Figure 8(a) below.



Fig. 8

Solid Ferromagnetic Conductor Carrying Direct Current

If the conductor carrying direct current is a solid bar of steel or other ferromagnetic material, the same distribution of field exists as in a similar nonmagnetic conductor, but the flux density is much greater. Figure 8(b) above shows a conductor of the same diameter as that shown in Figure 8(a). The flux density at the centre is zero, but at the surface is µH, where µ is the permeability of the magnetic material. The actual flux density, therefore, may be many times that in a nonmagnetic bar. Just outside the surface, however, the flux density drops to exactly the same value as that for the nonmagnetic conductor and the decrease in flux density with increasing distance follows the same curve.

Solid Ferromagnetic Conductor Carrying AC

The distribution of the magnetic field in a solid ferromagnetic conductor carrying AC is shown in Figure 8(c). Outside the conductor, the flux density decreases along the same curve as if DC produced the magnetizing force; however, while the AC is flowing, the field is constantly varying in strength and direction. Inside the conductor, the flux density is zero at the center and increases toward the outside surface – slowly at first, then accelerating to a high maximum at the surface. The flux density at the surface is proportional to the permeability of the conductor material.

Central Conductor Enclosed Within Hollow Ferromagnetic Cylinder

When a central conductor is used to magnetise a hollow cylindrical part made of ferromagnetic material, the flux density is maximum at the inside surface of the part as shown in Figure 9 below. The flux density produced by the current in the central conductor is maximum at the surface of the conductor through the space between the conductor and the inside surface of the part. At this surface, however, the flux density is immediately increased by the permeability factor, µ, of the material of the part and then decreases to the outer surface. Here the flux density again drops to the same decreasing curve it was following inside the part.

This method, then, produces maximum flux density at the inside surface and therefore gives strong indications of discontinuities on that surface. Sometimes these indications may even appear on the outside surface of the part. The flux density in the wall of the cylindrical part is the same irrespective of whether the central



conductor is of magnetic or nonmagnetic material, because it is the field external to the conductor that constitutes the magnetising force for the part.

If the axis of a central conductor is placed along the axis of a hollow cylindrical part, the magnetic field in the part will be concentric with its cylindrical wall. However, if the central conductor is placed near one point on the inside circumference of the part, the flux density of the field in the cylindrical wall will be much stronger at this point and will be weaker at the diametrically opposite point.

In small hollow cylinders, it is desirable that the conductor be centrally placed so that a uniform field for the detection of discontinuities will exist at all points on the cylindrical surface. In large diameter tubes, rings, or pressure vessels, however, the current necessary in the centrally placed conductor to produce fields of adequate strength for proper inspection over the entire circumference becomes excessively large.

An offset central conductor should then be used. See Figure 10 below. The current used should be between 12 to 31 A/mm diameter where the diameter is the sum of diameter of the central conductor and twice the wall thickness of the part being inspected. The distance along the part circumference (interior or exterior) that is effectively magnetised will be taken as 4 times the diameter of the central conductor as shown in the figure below. The entire circumference can be inspected by rotating the part on the conductor, allowing for approximately a 10% magnetic field overlap.

The diameter of a central conductor is not related to the inside diameter or wall thickness of the cylindrical part. Conductor size is usually based on its current carrying capacity and ease of handling. In some applications, conductors larger than that required for current carrying capacity can be used to facilitate centralising the conductor within the part.



Direct Contact Method

For small parts, circular magnetic fields can be produced by current flow directly through the part. This is done by clamping the part between contact heads (head shot), generally on a bench unit. A similar unit can be used to provide current to a central conductor.

The contact heads must be constructed so that the surfaces of the part are not damaged – either physically by pressure or structurally by heat from arcing or from high resistance at the points of contact.

Prod Contacts For the inspection of large and massive parts too bulky to be put into a unit having clamping contact heads, magnetisation is often done by using prod contacts (see Figure 11 below) to pass the current directly through the part or through a local portion of it. Such local contacts do not always produce true circular fields, but they are very convenient and practical for many purposes. Prod contacts are often used in MT of large castings and weldments.

Advantages

Prod contacts are widely used and have many advantages. Easy portability makes them convenient to use for the field inspection of large tanks and welded structures. Sensitivity to defects lying wholly below the surface is greater with this method of magnetisation than with any other, especially when half wave current is used in conjunction with dry powder and the continuous method of magnetisation.



Fig. 11

Limitations

The use of prod contacts involves some disadvantages:

• Suitable magnetic fields exist only between and near the prod contact points. These points are seldom more than 300 mm apart and usually much less; therefore it is sometimes necessary to relocate the prods so that the entire surface of the part can be inspected.

• 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.

• Great care must be taken to avoid burning of the part under the contact points. Burning may be caused by dirty contacts, insufficient contact pressure or excessive currents.

Permeability of Magnetic Materials The term permeability is used to refer to the ease with which a magnetic field or flux can be set up in a magnetic circuit. For a given material, it is not a constant value but a ratio. At any given value of magnetising force, H creating a flux density B in the part, the permeability,

µ = B/H.



Magnetic Hysterisis Some ferromagnetic materials, when magnetised by being introduced into an external field, do not return to a completely unmagnetised state when removed from that field. In fact, these materials must be subjected to a reversed field of a certain strength to demagnetise 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 magnetisation). If an external field that can be varied in a controlled manner is applied to a completely demagnetised specimen and if instrumentation for measuring the magnetic field within the specimen is at hand, the magnetisation curve of the material can be determined. A representative magnetisation (hysterisis) curve for a ferromagnetic material is shown in Figure 12 below.

Fig. 12



As shown in Figure 12(a), starting at the origin O with the specimen in the unmagnetised condition and increasing the magnetising 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 magnetising 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 magnetising force is gradually reduced to zero, curve AB results (Fig. 12(b)). The amount of magnetism that the steel retains at point B is called residual magnetism.

When the magnetising 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 magnetising force is represented by OC, which graphically designates the coercive force in the material, Figure 12(c). Ferromagnetic materials retain a certain amount of residual magnetism after being subjected to a magnetising force. When the magnetic domains of a ferromagnetic material have been oriented by a magnetising 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, Figure 12(d). At this point, the specimen is again magnetically saturated. The magnetising force is now reduced to zero and the DE line is formed, Figure 12(e) and retains reversed polarity residual magnetism in the specimen. Further increasing the magnetising force in the original direction completes the curve EFA, Figure 12(f). The cycle is now complete and the area within the loop ABCDEFA is called the hysterisis curve.

The definite lag throughout the cycle between the magnetising force and the flux is called hysterisis. If the hysterisis loop is slender, it usually means that the material has low retentivity (low residual field) and is easy to magnetise (has low reluctance). A wide loop indicates that the material has high reluctance and is difficult to magnetise.

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 MT should have high magnetic permeability so that they can be readily magnetised 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.

Low coercive force and low retentivity are desirable for magnetic particles. If high in coercive force, wet particles become strongly magnetised and form an objectionable



background. In addition, the particles will adhere to the steel of the settling tank causing heavy losses. Highly retentive wet particles tend to form agglomerates on the test surface. These agglomerates have low mobility and indications are distorted or obscured.

Dry particles having coercive force and high retentivity would become magnetised during manufacture or in first use and would become small permanent magnets. Once magnetised, their control by the weak leakage fields would be subdued by their tendency to stick magnetically to the test surface wherever they first touch.

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 imperfections.

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 immobilised. The stranding of finer particles as a result of the draining away of the liquid occurs much later, giving these particles for a sufficient period of time to be attracted by leakage fields and to accumulate and form true indications.

Effect of Particle Shape

Long slender particles develop stronger polarity than globular particles. Due to 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 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 into unfavourable shapes. These unfavourable 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 colours that are easy to see against the colour of the surface of the part being inspected. The natural colour of the metallic powders used in the dry method is silver gray, but pigments are used to colour the particles. The colours of the 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 magnetic particles for MT are dry particles and wet particles and each type is available in various colours and as fluorescent particles. Particle selection is primarily influenced by:

• Location of the discontinuity, that is, on 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 DC for magnetisation, are superior for detecting discontinuities lying wholly below the surface. The use of AC with dry particles is excellent for revealing surface cracks that are not exceedingly fine, but is of little value for discontinuities even slightly below the surface as shown in the Figure 13 below.



Wet particles are better than dry particles for detecting very fine surface discontinuities regardless of which form of magnetising current is used. Wet particles are often used with DC 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.

Coloured particles are used to obtain maximum contrast with the surface of the part being inspected. Black stands out against most light coloured 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.

Dry particles are available with yellow, red, black and gray pigmented colouring and with fluorescent coatings. The magnetic properties and particle sizes are similar in all colours, 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 magnetised 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 magnetised 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 magnetised 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.

Wet particles are available in red and black colours or as fluorescent particles that fluoresce a blue-green or a bright yellow-green colour. The particles are supplied in the form of a paste or other type of concentrate that is suspended in a liquid.

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 the 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 odourless, well-refined light petroleum distillate of low viscosity having a low sulphur content and a high flash point. 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 inflammability. Water suspendible particle concentrates include the necessary wetting agents, dispersing agents, rust inhibitors and antifoam agents.

Since 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 grounded. Also, electrolysis of the parts of the unit can occur if preventive measures are not taken.

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.

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 light is available.

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 MT are combinations of elements chosen to absorb light in the peak energy band of the mercury arc lamp fitted with a Kopp glass filter. The peak occurs at about 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 wave lengths which damage organs such as eyes and skin, the black light of 365 nm wavelengths poses no such hazards to the operator and provides visible evidence of defects in materials.

Detectable Discontinuities Surface Discontinuities

The largest and most important category of discontinuity consists of those that are exposed to the surface. Surface cracks or discontinuities are effectively located with MT. Surface cracks are also more detrimental to the service life of a component than



are subsurface discontinuities and as a result they are more frequently the object of inspection.

MT is capable of locating seams, laps, quenching and grinding cracks and surface ruptures in castings, forgings and weldments. The method will also detect surface fatigue cracks developed during service.

Many incipient fatigue cracks and fine grinding cracks are less than 0.025 mm deep and have surface openings of perhaps one-tenth that or less. Such cracks are readily located using wet MT. The depth of a crack has a pronounced effect on its detectability; the deeper a crack, the stronger the indication for a given level of magnetisation.

Detectability generally involves a relationship between surface opening and depth. A surface scrath, which may be as wide at the surface as it is deep, usually does not produce an indication. Due to many variables, it is not possible to establish any exact values for this relationship, but in general a surface discontinuity whose depth is at least five times its opening at the surface, will be detectable.

Internal Discontinuities

The magnetic particle method is capable of indicating the presence of many discontinuities that do not break the surface. Although RT and UT are inherently better for locating internal discontinuities, sometimes the shape of the part, the location of the discontinuity, or the cost or availability of the equipment needed makes MT more suitable.

Nonrelevant Indications Nonrelevant indications are true patterns caused by leakage fields that do not result from the presence of flaws. Nonrelevant indications have several possible causes and therefore require evaluation, but they should not be interpreted as flaws.

Sources of Nonrelevant Indications Particle patterns that yield nonrelevant indications can be the result of design, fabrication, or other causes and do not imply a condition that reduces the strength or utility of the part. Since nonrelevant indications are true particle buildups, they are difficult to distinguish from buildups caused by flaws. Therefore, the investigator must be aware of design and fabrication conditions that would contribute to or cause nonrelevant indications.

Particle Adherence Due to Excessive Magnetising Force

One type of nonrelevant indication is that caused by particle adherence at leakage fields around sharp corners, ridges, or other surface irregularities when magnetised longitudinally with too strong a magnetising force. The use of too strong a current with circular magnetisation can produce indications of the flux lines of the external field. Both the above phenomena (excessive magnetising force or excessive current) are clearly recognised by experienced operators and can be eliminated by a reduction in the applied magnetising force.



Mill Scale

Tightly adhering mill scale will cause particle build up, not only because of mechanical adherence but also because of the steel and the scale. In most cases, this can be detected visually and additional cleaning followed by retesting will confirm the absence of a true discontinuity.

Configurations that result in a restriction of the magnetic field are a cause of nonrelevant indications. Typical restrictive configurations are internal notches such as splines, threads, grooves for indexing, or keyways.

Abrupt changes in magnetic properties, such as those between weld metal and base metal or between dissimilar base metals, result in nonrelevant indications. Depending on the degree of change in the magnetic property, the particle pattern may consist of loosely adhering particles or may be strong and well defined. Again, it is necessary for the investigator to be aware of such conditions.

Magnetised writing is another form of nonrelevant indication. Magnetic writing is usually associated with parts displaying good residual characteristics in the magnetised state. If such a part is contacted with a sharp edge of another (preferably magnetically soft) part, the residual field is locally reoriented, giving rise to a leakage field and consequently a magnetic particle indication. For example, the point of a common nail can be used to write on a part susceptible to magnetic writing. Magnetic writing is not always easy to interpret, because the particles are loosely held and are fuzzy or intermittent in appearance. If magnetic writing is suspected, it is only necessary to demagnetise the parts and retest. If the indication was magnetic writing, it will not reappear.

Additional Sources

Some other conditions that cause nonrelevant indications are brazed joints, voids in filled parts and large grains.

Distinguishing Relevant From Nonrelevant Indications There are several techniques for differentiating between relevant and nonrelevant indications:

• Where mill scale or surface roughness is the probable cause, close visual inspection of the surface in the area of the discontinuity and use of magnification upto 10 diameters.

• Study of a sketch or drawing of the part being tested to assist in locating welds, changes in section, or shape constrictions

• Demagnetisation and retesting

• Careful analysis of the particle pattern. The particle pattern typical of nonrelevant indications is usually, wide, loose and lightly adhering and is easily removable even during continuous magnetisation.

• Use of another method of NDE, such as UT or RT, to verify the presence of subsurface defect.



Inspection of Casting and Forgings Castings and forgings may be difficult to inspect because of their size and shape. External surfaces can usually be inspected with prods; however, on large parts this can be time consuming and inspection of interior surfaces may not be adequate.

High amperage power supplies, in conjunction with flexible cable used with clamps (as contact heads), central conductors, or wrapping, can effectively reduce inspection time because relatively large areas can be inspected with each processing cycle. The Figure 14 below illustrates the direct contact, cable wrap and central conductor techniques that can be used to inspect relatively large parts.

Crane Hooks

The inspection of crane hooks is generally carried out with electromagnetic yokes having flexible legs using AC and HWDC. Stress areas in a crane hook are:

• The bight (in tension) on both sides in the throat (area A, Figure 15 below)

• The area below the shank (in compression and tension) on four sides (area B)

• The shank (in tension), mainly in threads and fillet (area C).



Inspection of Weldments Many weld defects are open to the surface and are readily detectable by MT using prods or yokes. For the detection of subsurface discontinuities, such as slag inclusions, voids and inadequate joint penetration at the root of the weld, prod magnetisation is the best, using HWDC and dry powder. Yokes, using AC, DC or HWDC, are suitable for detecting surface discontinuities in weldments.

The position of the yoke with respect to the direction of the discontinuity sought is different from the corresponding position of the prods. As the field traverses a path between the poles of the yoke, the poles must be placed on opposite sides of the weld bead to locate cracks parallel to the weld and adjacent to the weld bead to locate transverse cracks. Prods are placed adjacent to the weld bead for locating parallel cracks and on opposite sides of the bead for transverse cracks.

Demagnetisation After Inspection All ferromagnetic materials, after having been magnetised, will retain a residual magnetic field to some degree. The field may be negligible in magnetically soft metals, but in harder metals, it may be comparable to the intense fields associated with the special alloys used for permanent magnets.

Although it is time consuming and represents an additional expense, the demagnetisation of parts after MT is necessary in many cases. Demagnetisation may be easy or difficult depending on the type of metal. Metals having high coercive force are the most difficult to demagnetise. High retentivity is not necessarily related directly to high coercive force, so that the strength of the retained magnetic field is not always an accurate indicator of the ease of demagnetisation.

Reasons for Demagnetising

There are many reasons for demagnetising a part after MT. Demagnetisation may be necessary for the following reasons:



• The part will be used in an area where a residual magnetic field will interfere with the operation of instruments that are sensitive to magnetic fields or may affect the accuracy of instrumentation incorporated in an assembly that contains the magnetised part.

• During subsequent machining, chips may adhere to the surface being machined and adversely affect surface finish, dimensions and tool life.

• During cleaning operations, chips may adhere to the surface and interfere with subsequent operations such as painting or plating.

• Abrasive particles may be attracted to magnetised parts such as bearing surfaces, bearing raceways, or gear teeth, resulting in abrasion or galling, or may obstruct oil holes or grooves.

• During some electric arc welding operations, strong residual magnetic fields may deflect the arc away from the point at which it should be applied.

• A residual magnetic field in a part may interfere with remagnetisation of the part at a field intensity too low to overcome the remnant field in the part.

Methods of Demagnetising

There are a number of ways of demagnetising a part, all based on the principle of subjecting the part to a field continually reversing its direction and at the same time gradually decreasing its strength to zero as shown in Figure 16 below. The sine wave or curve of a reversing current at the bottom of the figure is used to generate the hysterisis loops. As the current diminishes in value with each reversal, the loop traces a smaller and smaller path. The curve at the upper right of the figure represents the flux density in the part as indicated on the diminishing hysterisis loops. Both current and flux density curves are plotted against time and when the current reaches zero, the field remaining in the part will also have approached zero.

In using this principle, the magnetising force must be high enough at the start to overcome the coercive force and to reverse the residual field initially in the part. Also, the incremental decrease between successive reductions in current must be small enough so that the reverse magnetising force will be able, on each cycle, to reverse the field in the part from the last previous reversal.



Figure 16

Demagnetising with Alternating Current

A common method of demagnetising small to moderate size parts is by passing them through a coil through which AC is passing. Alternatively, the AC is passed through a coil with the part inside the coil and the current is gradually reduced to zero. In the first method, the strength of the reversing field is reduced by axially withdrawing the part from the coil (or the coil from the part). In the second method, gradual decay of the current in the coil accomplishes the same result.

Demagnetisation with Direct Current

Methods of demagnetisation with DC are essentially identical in principle to the methods for AC. By using reversing and decreasing DC, low frequency reversals are possible, resulting in more complete penetration of even large cross sections.



magnetic particle testing by dipak chandiramani …

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