Non-destructive testing—Magnetic particle testingof ferromagnetic products,components and structures
Non-destructive testing—Magnetic particle testingof ferromagnetic products, components and structures
S E C T I O N 1 S C O P E A N D G E N E R A L
1.1 SCOPE This Standard specifies requirements for magnetic particle testing for the detection of surface and near-surface discontinuities in all types of ferromagnetic products, components and structures. It also specifies requirements for magnetic particle testing process control. Requirements for materials (media) are specified in ASTM E 1444.
1 Advice and recommendations on information to be supplied by the purchaser at the time of enquiry and order are given in Appendix A.
2 This Standard does not indicate the method to be used for the testing of any particular product. The method and the appropriate acceptance/rejection criteria should be specified in the relevant product Standard or application code.
3 The unit symbols used in this Standard are defined in AS 1000.
1.2 REFERENCED DOCUMENTS The following documents are referred to in this Standard:
1000 The International System of Units (SI) and its application
1239 Steel—Schedule of tool steel compositions
1442 Carbon steels and carbon-manganese steels—Hot-rolled bars and semifinished products
1929 Non-destructive testing—Glossary of terms
2536 Surface texture
3000 Electrical installations—Buildings, structures and premises (known as the SAA Wiring Rules)
3669 Non-destructive testing—Qualification and registration of personnel—Aerospace
3998 Non-destructive testing—Qualification and certification of personnel—General engineering
3100 Approval and test specification—General requirements for electrical equipment
E 1444 Practice for magnetic particle examination
5044 Specification for contrast aid paints used in magnetic particle flaw detection
1.3 DEFINITIONS For the purpose of this Standard, the definitions given in AS 1929 apply.
NOTE: The term ‘product’ in this Standard is taken to include component, test piece, work piece or structure.
1.4 PRINCIPLE OF TEST METHOD Ferromagnetic materials are magnetized, using electrical currents or permanent magnets, to a level where the magnetic field becomes distorted by surface and near-surface discontinuities, causing local flux leakage fields.Finely divided particles of ferro- or ferri-magnetic materials, applied as a powder or in acarrier fluid, are attracted to these flux leakage fields and accumulate to indicate thepresence of discontinuities.
1.5 WRITTEN PROCEDURE REQUIREMENTS All magnetic particle inspections
shall be performed to a specific written procedure that implements the requirements of this Standard for the components under test. A master written procedure may be utilized to cover the requirements common to a variety of similar components. As a minimum, the following information, where relevant, shall be included in individual procedures, a master procedure, or a combination thereof:
(a) Company name and address.
(b) Description and identity of the component.
(c) Written procedure identification number and date of issue.
(d) Material specification or type.
(e) A sketch of the component showing its main dimensions and the area to be tested.
(f) Surface condition at the time of testing, including the thickness and uniformity of any coating present.
(g) Purpose of the test.
(h) The manufacturing or overhaul stage at which the component is to be tested.
(i) Magnetizing technique to be used, including waveform and current value.
(j) The method of application of the indicating medium.
(k) Technique, i.e. whether continuous or residual.
(l) Equipment to be used.
(m) Distance between contact areas, or coil dimensions.
(n) Detecting media to be used.
(o) Contrast aids to be used.
(p) Viewing conditions.
(q) Demagnetization procedure.
(r) Method of reporting results.
(s) Acceptance/rejection criteria, if applicable.
(t) Minimum qualification of test operator.
(u) Identity of person responsible for the procedure.
(v) Reporting requirements.
1.6.1 General As magnetic particle testing methods may require the use of toxic, flammable and volatile materials, safety precautions shall be observed and testing shall be carried out in well-ventilated areas remote from heat and naked flame.
NOTE: Safety data sheets are available on request from suppliers of consumable materials and should be consulted to determine the hazards to personnel.
1.6.2 Fire hazards As the current flow methods described in this Standard require the use of high levels of current, it is important to ensure that any occurrence of overheating and arcing will not ignite flammable vapours that may be present when a carrier fluid containing magnetic particles is used.
CAUTION: ENSURE THAT ALL ELECTRICAL EQUIPMENT IS PROPERLY MAINTAINED AND THAT FIRE SAFETY PRECAUTIONS ARE OBSERVED. KEEP MATERIALS AND AUXILIARY EQUIPMENT AWAY FROM HEAT AND NAKED FLAMES. THE USE OF HIGH INTENSITY LAMPS OF BOTH BLACK AND WHITE LIGHT CAN IGNITE EXPLOSIVE GAS MIXTURES. ENSURE HAZARDOUS AREAS ARE WELL VENTILATED.
1.6.3 Electrical safety Electrical equipment used shall comply with the requirements of AS/NZS 3100 and shall be wired to comply with the requirements of AS 3000.
NOTE: Magnetic fields resulting from the use of high current may affect the functioning of heart pacemakers.
1.6.4 Toxic materials Magnetic particle testing media shall be used with caution and always in accordance with the manufacturer’s printed instructions.
WARNING: PROVIDE ADEQUATE VENTILATION TO ENSURE THE REMOVAL OF VAPOURS AND AIRBORNE PARTICLES GENERATED WHERE TOXIC MATERIALS ARE USED IN CONFINED SPACES.
Testing personnel shall use appropriate protective equipment to prevent skin contact and eye exposure to toxic liquids and the inhalation of fumes.
1.6.5 The use of black lights A black light should never be used when a filter glass is cracked or broken, as ultraviolet radiation harmful to the eyes may be emitted.
1.7 TESTING PERSONNEL The effectiveness of magnetic particle testing depends on the technical competence of the personnel performing the tests and on their ability to interpret indications. The responsibility for the nomination of acceptance/rejection criteria does not lie with the testing authority. Personnel who perform testing to this Standard shall have appropriate qualifications in the specific area of test and shall meet the visual acuity requirements of a relevant national Standard.
1 The Australian Standards for qualification of personnel who perform non-destructive testing are AS 3669 and AS 3998. Organizations responsible for personnel certification are the Australian Institute for Non-Destructive Testing (AINDT) and the Certification Board for Inspection Personnel (CBIP), New Zealand.
2 The accreditation of testing laboratories is carried out in Australia by the National Association of Testing Authorities (NATA), and in New Zealand, by International Accreditation New Zealand (IANZ).
S E C T I O N 2 E Q U I P M E N T A N D M A T E R I A L S
2.1 GENERAL The test equipment shall be capable of inducing magnetic flux in the product under test, in accordance with the requirements of the applicable test procedure. The magnetic field employed for this purpose may be generated from any source that is capable of producing the required flux density at the surface under test.
2.2 REQUIREMENTS FOR MAGNETIZING, DEMAGNETIZING AND AUXILIARY EQUIPMENT
2.2.1 Bench-type magnetizing equipment (incorporating a reservoir containing magnetic ink) Bench-type current flow, magnetizing, demagnetizing and associated equipment shall—
(a) when designed either for continuous or stepped current adjustment, be capable of producing current values that are within ±10% of the current setting or within ±50 A, whichever is the greater, over the range from zero to the maximum current
(b) include a calibrated ammeter to indicate the current output;
(c) be capable of meeting the expected current demand;
(d) have, indicated on or near the ammeter, the nominal waveform of the magnetizing current together with the current parameter, e.g. peak, r.m.s., mean, or mean of the conducting half cycle; and
(e) have a means to ensure that current-carrying contacts make firm electrical contact with the test part to prevent arcing or mechanical damage.
2.2.2 Permanent magnets and d.c. electromagnets Unless otherwise specified, permanent magnets and d.c. electromagnets (d.c. yokes) shall be capable of lifting not less than 18 kg of mild steel at a pole spacing of between 75 mm and 300 mm.
2.2.3 a.c. electromagnets Unless otherwise specified, a.c. electromagnets (a.c. yokes)shall be capable of lifting not less than 4.5 kg of mild steel at a pole spacing of between 75 mm and 300 mm.
1 As the strength of magnetization resulting from the use of d.c. electromagnets and especially permanent magnets is relatively low, this type of equipment should only be used where other methods of magnetization are not available or are unsuitable.
2 The lifting test pieces applicable to Clauses 2.2.2 and 2.2.3 are manufactured from a rectangular bar with one face at least 30 mm wide. The steel should be similar in chemical composition to grade 1020 of AS 1442 and be in the annealed condition.
2.2.4 Auxiliary equipment
184.108.40.206 Light sources for viewing The following light sources are required for the inspection of work pieces during testing:
(a) For white light viewing when using non-fluorescent magnetic particles—unless suitable natural light is available, an artificial light source capable of illuminating a test area to a level of not less than 1000 lx.
(b) For black light viewing when using fluorescent magnetic particles—a black light source capable of giving an irradiance of not less than 10 W/m2 at a distance of 380 mm.
NOTE: For black light viewing it may be necessary to provide an inspection booth to restrict the ambient white light illuminance (see Clause 3.5.3).
220.127.116.11 Standard test pieces Test pieces that are used for checking test equipment, or that contain known discontinuities, shall be prepared in accordance with Appendix B.
18.104.22.168 Magnetic field strength meters and field indicators Magnetic field strength meters and field indicators are required to measure the flux density near a magnetized component and to prove the efficacy of demagnetizing procedures.
2.3 REQUIREMENTS FOR MAGNETIC POWDERS AND INKS
2.3.1 Composition The composition and properties of magnetic powders and inks shall be in accordance with the requirements of ASTM E 1444.
2.3.2 Concentration of inks The percentage volume of magnetic particles in wet suspension shall be in the following ranges:
(a) Fluorescent particles . . . . . . . . . . 0.1% to 0.5% (preferred range 0.15% to 0.25%).
(b) Non-fluorescent particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.0% to 3.0%.
2.3.3 Labelling The following information shall be legibly and durably marked on each container of magnetic powder or ink, or on a label fixed securely to each container:
(a) The name of the manufacturer.
(b) A description of the contents.
(c) The contents by mass or by volume.
(d) The batch number.
(e) Information for the user.
NOTE: Manufacturers making a statement of compliance with this Australian Standard on a product, packaging or promotional material related to that product are advised to ensure that such compliance is capable of being verified.
S E C T I O N 3 M E T H O D S O F T E S T
3.1 SCOPE This Section gives details of magnetizing methods that are designed to ensure that discontinuities oriented in any direction in the test component can be revealed.
3.2 PREPARATION OF TEST SURFACE All areas to be tested shall be free from any foreign matter which could interfere with the interpretation of the test results, e.g. scale, dirt, grease, loose or flaking paint. The processes of surface preparation and cleaning shall not be detrimental to the product, its dimensional tolerances and surface finish, or to the testing media. To improve contrast when using methods employing dry powder or magnetic inks, the product prior to test may be painted with a strippable lacquer, a titanium-based white paint or some other suitable material, provided that the coating chosen is compatible with the test medium. Care shall be taken to ensure that the thickness of the contrast aid does
not adversely affect the sensitivity of the test. Where a current flow method (see Clause 3.4.3 and 3.4.4) is being used, any paint or other non-conductive coatings present on the contact areas of the test component shall be removed to prevent arcing. Where magnetic particle testing is to be carried out on surfaces that have a non-magnetic coating, pre-qualification testing shall be undertaken to ensure the efficacy of the method.
If the surface being tested has a non-magnetic coating that cannot be removed, the thickness and uniformity of the coating and its likely effect on the test results shall be reported. Paint shall be removed from the area under test unless a degree of cracking in the test object is permitted by the specification. Requirements and advice on conducting magnetic particle inspections on painted surfaces are given in Appendix C.
NOTE: Information on contrast aid paints is given in BS 5044.
3.3.1 General During magnetization, the flux density and flux direction within the test part shall be such that the leakage flux from all discontinuities that require detection is capable of attracting and holding enough magnetic particles from the indicating medium to ensure that the discontinuities are detectable.
1 Whereas a.c. magnetization should be used for the detection of surface discontinuities, d.c. magnetization is the preferred method for the detection of sub-surface discontinuities.
2 Information on test methods for determining flux density and magnetizing current levels is contained in Appendix D.
3.3.2 Magnetic field directions Discontinuities are difficult to detect by the magnetic particle method when they make an angle of less than 45° to the direction of magnetization. To ensure their detection, regardless of their orientation, each part shall be magnetized in at least two directions at right angles to each other. Depending on part geometry, this may consist of—
(a) circular magnetization in two or more directions;
(b) both circular and longitudinal magnetization; or
(c) longitudinal magnetization in two or more directions. Exceptions, necessitated by part geometry, size, or other factors, require specific approval of the contracting agency.
3.3.3 Peak current The effective current value used in magnetic particle testing is the peak current. Appendix E gives wave forms and correction factors for sinusoidal-based waves to be applied should a test meter be calibrated in a convention other than peak value.
NOTE: Most magnetic particle machines using solid state controls employ non-sinusoidal wave forms in their magnetizing current circuits.
3.3.4 Application of indicating media To achieve the greatest test sensitivity, magnetic powder or ink should be applied immediately prior to magnetization, or during the magnetization of the product under test. Application should cease before the source of magnetization is removed.
The time of magnetization shall comply with the requirements of the product Standard or the written procedure, and shall be long enough to allow indications produced by the test to resolve, but shall be not less than 0.5 s.
NOTE: A good practice is to apply two successive magnetizing pulses of at least 0.5 s duration, such that the second follows the first in rapid succession.
3.4 MAGNETIZING METHODS
3.4.1 General The methods commonly employed to magnetize the work piece are tabulated in Figure 3.1 and listed as follows:
(a) Magnetic flow methods These methods employ magnetic flow through the work piece, and require the use of either of the following equipment:
(i) Permanent magnets of various suitable configurations.
(ii) Electromagnetic yokes (either a.c. or d.c.).
(b) Current flow methods These methods employ the passage of current through the work piece and require the use of either of the following equipment:
(i) Contact heads.
(ii) Prods or clamps.
(c) Coil methods Coil methods employ magnetizing current flowing around the work piece by means of insulated coils, and comprise the following techniques:
(i) Low fill factor techniques.
(ii) High fill factor techniques.
(iii) Flat, spiral, astride and adjacent coil techniques.
(d) Threading bar and threading cable methods These methods employ current flow through bars or flexible cables that are threaded through the work piece, for testing hollow products and holes.
(e) Induced current methods These methods employ induced current and are used for magnetizing ring-shaped components.
Examples of these magnetizing methods are shown in Figures 3.2 to 3.7.
3.4.2 Magnetic flow methods
22.214.171.124 Application Magnetic flow methods are suitable for the general location of discontinuities in work pieces and employ an electromagnet or a permanent magnet. They are suitable for detecting discontinuities oriented transverse to a line joining the poles of the magnet (see Figures 3.2 (a) and (b)).
126.96.36.199 Procedure The following procedure shall be used to detect discontinuities whose orientation is not known:
(a) Test with the magnet positioned in one direction, then retest in this same direction as many times as necessary, until the whole test area is covered.
NOTE: When using permanent magnets or d.c. or a.c. electromagnets, ensure that the width of the inspected area is not greater than 50 mm either side of the axis of the yoke pole pieces.
(b) During each test, apply magnetic media, in accordance with Clause 3.3.4.
(c) Inspect the work piece during and after each magnetization stage.
(d) Repeat Steps (a), (b) and (c) using a test direction at right angles to the original direction.
3.4.3 Current flow through work piece using contact heads method
188.8.131.52 Application The current flow method is suitable for the detection of discontinuities where the plane or axis of the discontinuity is essentially parallel to the direction of current flow.
184.108.40.206 Procedure The following test procedure shall be used:
(a) Clamp the work piece firmly between the contact pads of the testing machine and pass a magnetizing current through the work piece taking precautions to prevent burning or arcing of the work piece (see Figure 3.3(a)). The value of current should be between 12 A/mm and 32 A/mm of part diameter (normally up to 20 A/mm).
NOTE: The diameter of the part is taken as the greatest distance between any two points on the outside circumference, at the same cross-section.
(b) Apply magnetic ink or powder in accordance with Clause 3.3.4.
(c) Inspect the work piece using the appropriate method given in Clause 3.5.
3.4.4 Current flow through work piece using prods or clamps method
220.127.116.11 Application This method is suitable for the detection of discontinuities in work pieces where the plane or axis of each discontinuity is essentially parallel to the direction of current flow.
18.104.22.168 Procedure The following procedure shall be used:
(a) Position the prods on the surface and pass a magnetizing current through the work piece (see Figure 3.3(b)). The current spread and relative intensity are complex when using prods, especially if the component has changes of form. Take precautions to prevent burning or arcing.
NOTE: When inspecting flat surfaces and those with radii of curvature greater than half the prod spacing, and provided that the peak current value is not less than 7.5 A/mm of prod spacing, the inspected area is a circle inscribed between the prods.
(b) When testing large areas, ensure that the circles inscribed between the prods overlap each other. This is shown in Figure 3.4(a), which designates successive prod positions as CF1—CF1, CF2—CF2, and so on.
(c) Apply magnetic media in accordance with Clause 3.3.4.
(d) Inspect the work piece using the appropriate method (see Clause 3.5).
(e) Repeat the test with the test pattern rotated through 90° to ensure that discontinuities oriented in all directions are detected.
1 Multiplication factors to convert indicated current values to peak current values are given in Appendix E.
2 As an alternative to the procedure in Step (a) for testing flat surfaces and surfaces with radii of curvature greater than half the prod spacing, an elliptical inspection area inscribed between the prods with the minor axis equal to one half of the prod spacing can be used
(see Figure 3.4(b)), provided that the peak current value is at least 4.70 A/mm of prod spacing. For the inspection of a narrow region similar to the prod width, the peak current should be not less than 3.75 A/mm of prod spacing. These methods of assessment do not apply to the inspection of surfaces having a radius of curvature between the prods of less than half the prod spacing, as measured along the surface. In such cases, the effective area shall be determined and an appropriate search pattern following the general principles illustrated in Figure 3.4(b) shall be used.
3.4.5 Coil methods
22.214.171.124 Application Coil methods are suitable for testing hollow and solid products to locate transverse discontinuities (parallel to the direction of the coil windings). Either high fill factor or low fill factor coils may be employed to magnetize the product. High fill factor coils comprising parallel conductors may be rigid or flexible. Flexible coils are particularly useful for in situ testing of structural steelwork, off-shore structures, nozzles in boilers, pressure vessels, shafts, gears and pipework. Parallel conductors have
the ability to examine large areas with the application of one magnetization and are used effectively for the detection of longitudinally oriented defects in tubular structures. Examples of a coil with a low fill factor and a coil with a high fill factor are given in Figure 3.5 (a) and (b) respectively. Examples of astride coils, an adjacent coil and a flat coil are given in Figure 3.5 (c), (d) and (e) respectively.
NOTE: Guidance on the use of these procedures is given in Appendix F.
126.96.36.199 Determination of number of coil turns and current The number of coil turns and the value of current shall be determined as follows:
(a) For low fill factor coils Where the area inscribed by the coil is 10 or more times the cross-sectional area of the part being inspected, the product of the number of coil turns (N) and the current in amperes through the coil (I) is as follows:
(i) For parts positioned to the side of the coil—
NI=K/L/D (±10%) . ．．．．．．．．．. . 3.4(1)
K = 45 000 ampere turns
L = length of the part
D = diameter of the part (measured in the same units as the length)
(ii) For parts positioned in the centre of the coil—
NI= KR/(6 L/D) − 5 (±10%). ．．．．. . 3.4(2)
K = 1690 ampere turns
R = radius of coil, in millimetres
L = length of part
D = diameter of the part (measured in the same units as the length)
If the part has hollow portions, replace D with Deff as given in Clause 188.8.131.52.
(b) For high fill factor coils or cable wrap Where the area inscribed by the coil is less than twice the cross-sectional area (including hollow portions) of the part under test, the product of the number of coil turns (N) and the current in amperes through the coil (I) is as follows:
NI= K/L/D+ 2 (±10%). . .．．．．．．． 3.4(3)
K = 35 000 ampere turns
L = length of part
D = diameter of part (measured in the same units as the length)
If the part has hollow portions, replace D with Deff as given in Clause 184.108.40.206.
NOTE: Equations 3.4(1), 3.4(2) and 3.4(3) apply only if the L/D ratio is greater than 2 and less than 15. If the L/D ratio is less than 2, pole pieces (pieces of ferromagnetic material with the same diameter as the part being tested) can be placed on each end of the part to effectively
increase the L/D ratio to 2 or greater. If the L/D ratio is greater than 15, substitute the value of 15 for L/D. Alternatively, use the equation for astride coils (Equation F(1)) given in Appendix F.
220.127.116.11 Calculating the L/D ratio for a hollow or a cylindrical part When calculating the L/D ratio for a hollow or a cylindrical part, D is replaced by an effective diameter (Deff
), calculated using the following equation:
2[At − Ah/π]½ . ．．．．．．．．． 3.4(4)
At = total cross-sectional area of the part
Ah = cross-sectional area of the hollow portions of the part
＝ [(OD)2 − (ID)2]½. .．．．．．． . 3.4(５）
OD = outside diameter of the cylinder
ID = inside diameter of the cylinder
18.104.22.168 Procedure The following procedure shall be used:
(a) Locate the product in preformed coils or wrap insulated cable around the product to produce a magnetic field at 90° to the current flow (see Figure 3.5 for examples of coil methods). For the flat coil method the cable is located adjacent to the part. Where the product is long in relation to the coil, move the coil sequentially along the product until complete coverage is achieved.
(b) Apply magnetic media and pass current through the coil in accordance with Clause 3.3.4
(c) Inspect the work piece using the appropriate method.
3.4.6 Threading conductor methods
22.214.171.124 Application Threading conductor methods are suitable for locating discontinuities in hollow products and in areas adjacent to holes where the plane or axis of the discontinuity is essentially parallel to the current flow direction.
126.96.36.199 Procedure The following procedure shall be used:
(a) Place the insulated threading conductor centrally inside the hollow product or hole (see Figure 3.6(a) and (b)), or wrap insulated cable around and through the product (see Figure 3.6(c)), and then pass a current through the bar or cable to induce a magnetic flux in the work piece. The value of current should be between 12 A/mm and 32 A/mm of the internal diameter of the work piece and be sufficient to
encompass the test area. Where insufficient field strength is obtained with the conductor placed centrally through the work piece, reposition the conductor close to the inner surface of the product and make successive tests at a number of equally spaced fixed positions.
NOTE: To prevent arcing, the central conductor should be properly insulated.
(b) Apply magnetic media in accordance with Clause 3.3.4.
(c) Inspect the work piece using the appropriate method.
3.4.7 Induced current flow method
188.8.131.52 Application The induced current flow method is suitable for locating circumferential discontinuities in ring-shaped products, especially if the products have low rigidity or the use of a current flow method is made difficult by the contour of the crosssection.
184.108.40.206 Principle The induced current flow method is derived from the principle of a transformer. The test part acts as the secondary winding (see Figure 3.7(a) and (b)).
3.5.1 General The area under test shall be inspected in accordance with the conditions specified in Clauses 3.5.2 and 3.5.3.
3.5.2 Non-fluorescent dry powders and magnetic inks When inspecting work pieces that have been tested using non-fluorescent dry powders or magnetic inks, the surface area under test shall be illuminated to an intensity of not less than 1000 lx by natural or artificial light. The test area shall be free from unnecessary glare. As an exception, where it is deemed that the heat generated by the use of white lights could become an ignition source in designated hazardous areas, the tester may use a lower light intensity for the inspection, subject to the agreement of the purchaser. The use of a lower light intensity shall be recorded both in the record of test and in the test report.
NOTE: If necessary, contrast aids may be used to improve the contrast between the testing media and the area under test (see Clause 3.2).
3.5.3 Fluorescent magnetic inks When inspecting work pieces that have been tested using fluorescent magnetic inks, the irradiance of black light required to detect indications depends on the fluorescent brightness of the particles, the size of the indications required to be detected, the visual acuity of the inspector and the ambient lighting condition. The requirements for viewing conditions are as follows:
(a) Black light shall be used to view the test surface for indications given by the test.
(b) The irradiance of black light on the test surface shall be not less than 10 W/m2.
(c) Unless otherwise specified, the ambient visible light level measured at the test surface shall not exceed 20 lx. As an exception, where it is deemed that the heat generated by the use of black lights could become an ignition source in designated hazardous areas, the tester may use a lower light intensity for the inspection, subject to the agreement of the purchaser. The use of a lower black light intensity shall be recorded both in the record of test and in the test report. Final inspection shall be performed after the eyesight of the test operator has adapted to
the darkened viewing conditions.
1 The period required for eye adaptation depends on the lighting conditions to which the inspector is exposed before entering the darkened area, and on the illuminance in this area. Typically 2 min to 5 min are required; however, personnel coming from full sun light to an inspection area should allow at least 10 min in subdued lighting before commencing final inspection. If the eyes are exposed to bright white light during final inspection, for example through the use of white light to aid interpretation, a readaptation period is required.
2 Ultraviolet goggles may be used to increase contrast. Spectacles worn to correct eyesight should not reduce visual acuity under black light viewing conditions.
3 Personnel using black light lamps should not look directly at the black light source as this will cause temporary loss of visual acuity.
4 Black light sources should be regularly checked to ensure that the filter is correctly fitted and is not cracked. A cracked or incorrectly fitted filter can allow transmission of hazardous radiation.
5 It should be noted that a black light lamp may require up to 15 min warm up time, to achieve its maximum output.
3.6 MARKING THE LOCATION OF DISCONTINUITIES Unless otherwise specified in the product Standard, the location of any discontinuities detected shall be identified by an appropriate method such as with coloured marking ink. The method of marking used shall not cause corrosion or otherwise affect the end use of the product or interfere with any additional testing requirement.
3.7.1 General Most ferromagnetic materials will retain some residual magnetism after testing. Residual magnetism may cause problems such as abrasion from retained particles, retention of swarf during machining operations, malfunction of instruments sensitive to magnetism, or interference with welding. Demagnetization is required unless otherwise specified, and is achieved when the residual field does not exceed 0.3 mT. Because residual magnetism can cause spurious indications and prevent adequate magnetization, demagnetization prior to magnetic particle testing, using fields of different orientation, may be necessary.
NOTE: Residual magnetization may be checked using a magnetic field indicator.
3.7.2 Demagnetization procedures The following procedures are commonly used to demagnetize materials:
(a) The aperture coil steady alternating current method To carry out this method apply an a.c. field which initially has sufficient magnitude to saturate the component, and then diminish the field to zero in a finite period. This decay time is approximately 100 to 200 times the period of the a.c. field.
1 In practice this may be achieved by passing the component to be demagnetized at asteady rate through an energized coil carrying an alternating current to a distance from the coil where the magnetic field of the coil is no longer detectable.
2 For components of substantial mass and cross-section, complete demagnetization of the core of the component may not occur due to the ‘skin effect’ phenomena. In this case, either the reversing d.c. method or the field cancellation method should be used.
(b) The reversing d.c. method For this method the demagnetizing procedure is essentially the same as for Procedure (a), except that the period or frequency is reduced to approximately 1 Hz and the demagnetization time is typically 30 s to 40 s.
NOTE: This method can generally only be used with specialized proprietary equipment that contains sophisticated control systems and large reversing rectifiers.
(c) The field cancellation method This method is conducted as follows:
(i) Wrap a cable, of an appropriate cross-sectional area, around the component, making sure the turns are uniformly distributed.
(ii) Pass a direct current through the cable for approximately 1 s to magnetize the component to a value of about 0.7 T. Note the value of the current.
(iii) Check the magnitude and the direction of the residual field with a dial-type field indicator.
(iv) Reverse the connections of the coil so that when the current is passed again it will produce a field of the opposite polarity.
(v) Set the value of current to 10% of the value in Step (ii) and apply for 1 s.
(vi) Check the residual field as in Step (iii). If the field is less but of the same polarity, increase the current by 25% and re-apply.
(vii) Continue the process of reversing connections and applying decreasing values of current until the residual field is zero.
3.8 POST-TEST CLEANING After the completion of testing, it may be necessary to clean the work pieces to free them of all detecting media. The method used should not cause corrosion of the product surface.
S E C T I O N 4 P R O C E S S C O N T R O LP R O C E D U R E S A N D R E Q U I R E M E N T S
4.1 GENERAL The sensitivity of a magnetic particle system to indicate discontinuities is dependent on many factors associated with the testing materials and the amount of magnetic flux generated. Once in use, a system requires regular monitoring to ensure that its performance does not
deteriorate. In general, the frequency at which the materials, the electrical system and viewing lamps require checking is dependent on the frequency and conditions of their use (see Clause 4.2.3).
4.2 CONTROL PROCEDURES
4.2.1 Magnetic inks
220.127.116.11 General The requirements of this Clause apply to materials other than pre-packaged certified aerosol consumables.
18.104.22.168 Determination of particle concentration The particle concentration shall be assessed as follows:
(a) Agitate the magnetic ink to ensure an even distribution of particles. For a pump and reservoir system a minimum agitation time of 30 min is required.
(b) Pour a 100 mL sample from the ink media reservoir into a pear-shaped graduated centrifugal flask.
NOTE: The stem of the flask should be marked with graduations to facilitate the assessment of the percentage volumes.
(c) Demagnetize the sample and allow the flask to stand for 60 min to allow the particles in suspension to completely settle.
(d) Determine the particle concentration from the percentage volume of particles in the flask.
NOTE: If the concentration is out of tolerance adjust the bath concentration by making calculated additions of particles or suspension vehicle and then redetermine the particle concentration in accordance with Steps (a) to (d).
(e) Examine the settled particles and if they appear to be loose agglomerates rather than in the form of a compacted layer, take a second sample and repeat Steps (a) to (d). If at the end of retesting, the settled particles also appear agglomerated, the entire suspension shall be discarded and replaced.
22.214.171.124 Assessment of magnetic ink condition The assessment of the ink condition is carried out after completion of the particle concentration test (see Clause 126.96.36.199), as follows:
(a) For fluorescent suspensions, examine the liquid above the settled particles for fluorescence with black light. Replace the bath if the liquid is noticeably fluorescent.
(b) Examine the settled particles under suitable lighting (black light for fluorescent particles and white light for both fluorescent and non-fluorescent particles) for striations or bands different in colour or appearance, which indicate the presence of contaminants. Replace the bath if these contaminants appear to the naked eye to exceed 30% of the volume of the particles.
188.8.131.52 Water break test This test is applicable when the liquid vehicle for the particles is water, and is carried out as follows:
(a) Flood a clean part having a surface finish typical of that of the parts to be tested with the suspension, and note the appearance of the surface after the flooding has stopped.
(b) Assess the adequacy of cleaning or added wetting agent as follows:
(i) If a continuous even film forms over the entire part, sufficient wetting agent is present and the part is clean.
(ii) If the film of suspension breaks, exposing a bare surface, insufficient wetting agent is present or the part has not been adequately cleaned.
4.2.2 Equipment verification
184.108.40.206 General Magnetic particle testing equipment shall be checked for performance and accuracy when first installed and at intervals thereafter, and whenever any electrical maintenance, which may affect equipment accuracy, has been performed.
220.127.116.11 Ammeter accuracy Check the ammeter accuracy using the following procedure:
(a) Connect a calibrated ammeter in series with the output circuit of the test equipment.
(b) Take a minimum of three comparative readings ensuring that they cover the useable range of the equipment.
The equipment meter reading shall not deviate by more than ±10% of full scale from the current value indicated on the calibrated ammeter.
NOTE: When performing this test for the first time, due account needs to be taken of the waveform of the current being measured (see Appendix E) and the nature of the measurement recorded by the meter on the magnetic particle equipment. The waveform should be indicated
on or near the meter.
18.104.22.168 Field indicator accuracy Magnetic field indicators shall be checked for accuracy against a calibrated master magnetic field indicator.
22.214.171.124 Dead weight check Electromagnets (yokes) and permanent magnets shall be dead-weight tested and shall meet the requirements specified in Clauses 2.2.2 and 2.2.3.
126.96.36.199 Inspection of lighting intensity The intensity of the white light and black light used for inspection shall meet the requirements given in Clause 188.8.131.52. Measurements shall be made using calibrated meters suitable for the measurement of the wavelengths concerned.
184.108.40.206 System performance test The overall performance of the magnetic particle
inspection system shall be verified using the standard test pieces applicable to the test
system, shown in Appendix B, or similar test pieces. These test pieces shall be tested to a
written procedure. If the magnetic particle indications obtained from the discontinuities
are of undiminished clarity, the overall system performance is verified. Samples used for
verification shall be demagnetized and thoroughly cleaned before and after use, and
checked under white or black light, as appropriate, to ensure that residual indications do
4.2.3 Frequency of checks The performance of equipment and testing media for plant shall be verified at appropriate intervals, on each day the process is used, or whenever there is a change in operating conditions. For equipment in continuous operation, the intervals shall be in accordance with Table 4.1.
NOTE: For equipment/materials not in continuous use, checks may be less frequent. For equipment/materials only in occasional use, tests may be performed before use.
4.3 PROCESS CONTROL RECORDS A permanent record of all test results shall be maintained for reference purposes and shall contain the following information:
(a) Name of the laboratory or testing authority.
(b) Details of the test and identification of test equipment.
(d) Date of test.
(e) Identification of person carrying out test.
(f) Reference to this Australian Standard, i.e. AS 1171.
S E C T I O N 5 T E S T R E C O R D S A N D R E P O R T S
5.1 TEST RECORD The test record for each test shall include at least the following information:
(a) Name of laboratory or testing authority.
(b) Identification of the component.
(c) The product Standard.
(d) The material specification, or type.
(e) The number of this Australian Standard, i.e. AS 1171, identification of the test procedure used and details of any departure from that procedure.
(f) Areas tested.
(g) Surface condition including the thickness and uniformity of any coating present.
(h) Identification or description of equipment and test materials used (the name of the manufacturer and the manufacturer’s identification of the test materials).
(i) Method(s) of magnetization.
(j) Whether the product has been demagnetized.
(k) Results of the test and descriptions and positions of all discontinuities detected.
(l) Any other information the purchaser requires for assessment of test results.
(m) Date and place of test.
(n) Report number or other means of identifying the report.
(o) Identification and signatures of testing personnel.
5.2 TEST REPORT The test report for each test shall include at least the following information:
(a) The name of the laboratory or the testing authority.
(b) Identification of the component.
(c) The product Standard.
(d) The material specification, or type.
(e) The number of this Australian Standard, i.e. AS 1171, identification of the test procedure used and details of any departure from that procedure.
(f) Areas tested.
(g) Surface condition including the thickness and uniformity of any coating present.
(h) Method(s) of magnetization.
(i) Whether the product has been demagnetized.
(j) The test results and descriptions and positions of all discontinuities detected.
(k) Any other information the purchaser requires for assessment of test results.
(l) Date and place of test.
(m) Report number and date of issue.
(n) Identification and signature of the officer responsible for the test report.
PURCHASING GUIDELINES (Informative)
A1 GENERAL Australian Standards are intended to include the technical requirements for relevant products, but do not purport to comprise all the necessary provisions of a contract. This Appendix contains information to be supplied by the purchaser at the time of enquiry or order.
A2 INFORMATION TO BE SUPPLIED BY THE PURCHASER The purchaser should supply the following information at the time of enquiry or order:
(a) Identification, job reference number or order number.
(b) The description and identity of the component including the product Standard and the application standard reference number and material specification, as appropriate.
NOTE: For the purpose of this Standard, ‘product Standard’ is synonymous with ‘job specification’, ‘application standard’ and with ‘construction standard’.
(c) Manufacturing history.
(d) Surface condition and thickness of any coating present. If coatings are present, whether the components are subject to high stresses or cyclic loading (see Appendix C).
(e) Drawing or sketch showing the relevant test areas.
(f) Description of discontinuities being sought.
(g) Test method designation code or number and the number of this Australian Standard, i.e. AS 1171, if not specified in Item (b).
(h) Acceptance level for discontinuities, if not specified in Item (b).
(i) Any necessary departures from the test methods described in this Standard.
(j) Whether demagnetization is required.
(k) Type of temporary corrosion preventative to be applied, if applicable.
(l) Whether a test report is required (see Clause 5.2).
PREPARATION AND USE OF STANDARD TEST PIECES FOR CHECKING TEST EQUIPMENT AND SYSTEM OPERATION (Normative)
B1 SCOPE This Appendix specifies the requirements for the construction and use of standard test pieces for checking test equipment and the consistency of testing systems.
B2 FOR THE CURRENT FLOW METHOD
B2.1 Preparation of test piece The standard Ketos tool steel ring described in Figure B1 is used for checking current flow test equipment and magnetic particle powders and inks. The ring shall be kept free of corrosion.
B2.2 Procedure for use of standard test piece The Ketos ring shall be used in accordance with the following procedure to establish conditions of magnetization:
(a) Clean and demagnetize the ring. Place a conducting rod 25 mm to 31 mm in diameter, of convenient length, through the centre of the ring. Ensure that the ring is positioned midway along the rod.
(b) Clamp the rod between the heads of the testing machine, apply magnetic ink to the ring and energize the equipment to generate a magnetic field in the ring.
(c) Examine the ring within 1 min after current application.
(d) Note and record the value of current required to make the indication of the hole nearest to the outer surface of the ring visible, when viewed from the outer surface.
(e) Continue to increase the current to establish indications of the other holes in the ring.
(f) Note and record the current value required to establish each hole indication.
(g) Repeat the test for each current waveform that can be generated by the magnetizing
B3 FOR MAGNETIC FLOW AND COIL METHODS
B3.1 Preparation of test piece The standard steel test piece for checking magnetic flow equipment and test coils is 20 mm square in cross-section and 375 mm long. It contains a 1 mm diameter transverse hole, positioned approximately midway along the test piece with its centreline 1.5 mm below one of the side faces. The test piece shall comply with the requirements of Figure B2 and shall be kept free of corrosion.
B3.2 Procedure for the use of test piece The standard test piece shall be used in accordance with the following procedure to establish the conditions of magnetization:
(a) Clean and demagnetize the test piece.
(b) Clamp the test piece between the heads of the testing machine (for magnetic flow), or alternatively, position the test piece centrally in a coil and align it parallel to the axis of the coil (for coil magnetization).
(c) Energize the equipment, apply magnetic ink and establish that the transverse hole in the middle of the test piece shows a clearly visible indication. When using a coil, the magnetizing current should be the minimum required to obtain a clearly visible indication, as previously determined.
B4 OTHER TEST PIECES Test pieces comprising products with known discontinuities may be used if required by the product specification.
1 Hole numbers 1 to 12 are all 1.8 ±0.1 mm in diameter.
2 Tolerance on distance D = ±0.1 mm; all other length tolerances are ±0.8 mm.
3 The ring is made from O1A tool steel (see AS 1239), from annealed round stock.
4 The ring may be heat treated as follows: Heat to 760°C to 790°C, hold for 1 hour, cool at a maximum rate of 22°C/hour to below 540°C. Furnace or air cool to room temperature. Finish the ring to, achieve a s, , , , , , , , urface texture of 3.2 Ra (see AS 2536).
DIMENSIONS IN MILLIMETRES
FIGURE B1 KETOS TOOL STEEL RING
1 Roughness grade limits 0.8/1.6 Ra (see AS 2536).
2 Machine all surfaces.
3 The test piece material is grade 1020, or similar, of low carbon steel (see AS 1442).
DIMENSIONS IN MILLIMETRES
FIGURE B2 STANDARD STEEL BAR TEST PIECE APPENDIX C
REQUIREMENTS FOR CONDUCTING MAGNETIC PARTICLE EXAMINATIONS ON PAINTED SURFACES (Normative)
Magnetic particle testing on painted or other inadequately prepared surfaces may result in the non-detection of defects, the size of which may exceed application code acceptance criteria. Such surfaces typically reduce the effectiveness of the test and in extreme circumstances, can completely negate the test. Magnetic particle examination over painted surfaces shall not be performed on components that are subject to high stresses or cyclic loading. Magnetic particle examination over paint shall only be conducted if the following requirements are met:
(a) The test is carried out using a.c. magnetization.
(b) Welds and weld toes have been ground smooth prior to painting.
(c) The paint thickness is uniform.
1 The sensitivity of the magnetic particle method decreases as the paint thickness increases.
2 Measurements of paint thickness made on a flat surface may not accurately reflect the thickness of paint on a radius or corner, or on weld undercuts or laps, and other sites liable to accumulate paint and at which cracks are more likely to occur.
(d) The examination is carried out by a person who has appropriate qualifications and experience in the testing of painted surfaces (see Clause 1.7).
(e) Test pieces of similar configuration and steel composition as the component under examination are used. These test pieces shall contain defects under paint films of known thickness. If a discontinuity is detected, the paint shall be removed in the vicinity of the discontinuity to enable evaluation.
NOTE: Eddy current testing should also be considered when examining articles where the paint cannot be removed.
GENERAL INFORMATION ON TEST METHODS FOR DETERMINING FLUX DENSITY AND MAGNETIZING CURRENT LEVELS (Informative)
D1 GENERAL The flux density in the test part during magnetization needs to be sufficient to ensure that the leakage flux, from all discontinuities that are required to be detected, is able to attract and hold enough magnetic particles from the indicating medium
to form detectable indications. The flux density, however, should not be so great as to cause the background of magnetic particles to interfere with the visibility and interpretation of indications. Because other variables, including discontinuity size and surface roundness, affect the formation and detectability of indications, it is not always possible to state a value of flux density that will result in the reliable detection of all discontinuities.
D2 FACTORS AFFECTING THE VALUE OF LEAKAGE FLUX AND THE SIZE AND DETECTABILITY OF AN INDICATION
D2.1 Factors affecting the leakage flux produced at the surface The leakage flux produced at the surface of the test part is dependent on the following factors:
(a) The size (length, depth and width) and shape of the discontinuity.
(b) The location of the discontinuity including its depth below the surface and nearness to features of the part which could cause local variations in flux density.
(c) The permeability of the discontinuity.
(d) The orientation of the plane of the discontinuity relative to the direction of magnetization.
NOTE: The maximum leakage flux results when the plane of the discontinuity is at right angles to the direction of magnetization; the leakage flux decreases progressively as this angle is reduced. Consequently, when discontinuities, that may lie in any direction, are required to be detected, at least two magnetizations, essentially at 90° to each other, are required. In selecting magnetizing procedures it should be kept in mind that a flux density that will reveal a discontinuity at 90° to the direction of magnetization may not reveal a similar discontinuity located at a smaller angle. For example, when testing for forging laps that may lie at an acute angle to the surface, the flux density should be increased above that required for the detection of similar discontinuities at 90° to the direction of magnetization.
(e) The type and thickness of any surface coatings present.
D2.2 Factors affecting the size of an indication For a given level of leakage flux at the surface of a part, the size of the indication formed is dependent on the following factors:
(a) The type of magnetic particles used and the carrier fluid in which they are suspended.
(b) The manner of application of the indicating medium.
(c) The part configuration and surface roughness, both of which may affect the access and mobility of the magnetic particles and hence the formation of indications.
D2.3 Factors affecting the detectability of an indication For a given size of indication, its detectability is dependent on the following factors:
(a) Particle pigmentation.
(b) The colour of the test part (for colour contrast particles).
(c) The intensity of light reflected from the test surface.
(d) The level of background particles, which is dependent on—
(i) the surface roughness;
(ii) the magnetizing technique; and
(iii) the part configuration.
(e) The lighting conditions.
(f) The operator’s visual acuity, experience and alertness.
D3 METHODS FOR ENSURING THAT THE FLUX DENSITY IS IN THE RANGE REQUIRED FOR THE DETECTION OF DISCONTINUITIES
D3.1 The use of parts with known discontinuities The most reliable method of ensuring that the flux density is in the required range is to empirically determine the flux density required to reveal known discontinuities in a part using the same indicating medium and lighting conditions as those to be used during testing. For the results to be meaningful, the part with known discontinuities should be of the same size, configuration and material, and should have the same surface roughness and thermal and mechanical history as the parts to be tested. In addition, the discontinuities should be of the same type and minimum size and be located and oriented similarly to the discontinuities required to be detected. Test parts with naturally occurring discontinuities of the minimum size required to be detected, and which are located where the least magnetic flux is expected, and oriented at an angle of approximately 45° to the directions of magnetization, would also be suitable
for use as reference test pieces.
D3.2 The use of parts with artificial discontinuities An alternative method of ensuring that the flux density is in the required range is to use parts identical to those to be tested, but containing artificial discontinuities of the minimum size required to be detected and located and oriented as indicated in Paragraph D3.1. To adequately simulate actual discontinuities, the artificial discontinuities should be of the same order of width as those to be detected. Artificial discontinuities greater than 0.1 mm wide should not be used, unless they are representative of the discontinuities sought in the part under test. This method is normally feasible only with surface breaking discontinuities and is limited due to the uncertainty as to how effectively can an artificial discontinuity simulate material flaws, particularly tight cracks.
D3.3 The use of decreasing levels of magnetization A third and related method is to test a sample part at decreasing levels of magnetization, commencing with a level that produces excessive background; the part is demagnetized between successive magnetizations. After each test, the part should be closely observed to evaluate any indications of surface roughness, its edges and the general level of the background of magnetic particles. With experience, it is possible to select an adequate level of magnetization just below that which results in background particles which could interfere with the detection of indications from the discontinuities sought, yet sufficient to just reveal some of the features contributing to surface roughness. This method is based on the assumption that if such features are detectable, then material flaws at the surface will also be revealed. In each case, the background expected at various locations with the magnetizing technique used, the degree of surface roughness and the size of discontinuities required to be detected, should be taken into account. In some parts, different levels of magnetization may be required for testing different locations on the surface, e.g. at the thread roots and at other locations on a bolt.
D4 EQUIPMENT FOR THE MEASUREMENT OF FLUX DENSITY
D4.1 Hall effect instruments Hall effect instruments may be used to measure the tangential flux density in air at the part surface. If the permeability of the material at the value of the applied field strength is known, or is assumed to be above a minimum value (see Paragraph D5), the tangential flux density within the material at the surface of the test part can be determined from this measurement. Hall effect instruments should not be used for applications where magnetic flux emerges from the part surface at the location where the measurement is made, and when permanent magnets are used to magnetize the test part, as the measured tangential flux density in air at the part surface does not have a readily predictable relationship to the tangential flux density in the material at the part surface. However, when permanent magnets are used to magnetize the material around a hole, the use of Hall effect instruments to measure the flux density in the hole, for the purpose of obtaining a repeatable degree of magnetization, is a different application which does not suffer from this limitation, and valid results may be obtained using this procedure. A further limitation to this method is that it is necessary to make an assumption concerning the flux density required to detect the discontinuities and, since the permeability of the test part is known only rarely, a further assumption concerning this permeability is required (see Paragraph D5).
D4.2 Field indicators Field indicators such as attachable strips of magnetic material with artificial discontinuities, or the Berthold gauge, are used to indicate the level of magnetization in test parts. Such devices respond to the magnetic field within the material and to a lesser extent to the magnetic field in air at the part surface, by shunting flux from the material. They have similar limitations to those of Hall effect instruments and are designed to respond to a single value, or to a limited number of values of flux density. These indicators should be used with caution as it may be incorrectly assumed that their response will indicate the presence of sufficient flux to form indications from
discontinuities in the part under test.
D4.3 Eddy current instruments Eddy current instruments are recommended for the determination of the magnetic flux density in the test part, since they respond only to the state of the material and not to the magnetic flux in the air. Eddy current instruments dedicated to this purpose are available and should be used to determine the adequacy of magnetization during the development of test procedures. Their limitation is that they indicate only the level of magnetization of the material and not whether this level is adequate for the detection of a particular discontinuity. However,this limitation can be overcome by the comparison of the levels of magnetization required to detect similar discontinuities in other parts.
D5 THE DETERMINATION OF MAGNETIZING CURRENT One of the most common methods of determining the current required for magnetic particle testing, when using equipment incorporating an ammeter, is to perform a calculation using an appropriate equation. Equations for some magnetizing techniques are applicable only to parts of simple configuration. For this reason, as well as because the permeability of the material being tested is, normally, not known, the adequacy of the current values calculated by these equations should always be verified by one of the methods indicated in Paragraphs D3.1, D3.2, and D3.3. Such equations require that assumptions be made of the value of the minimum flux density required to form detectable indications, and of a minimum value of the permeability of the material being tested.
The minimum current values required are based on the need to achieve a minimum flux density of 0.72 T, which should at least allow the detection of surface breaking planar discontinuities of the order of 1 mm deep or greater, when alternating current and fluorescent magnetic particles in a liquid suspension are used. The recommended current values should be based on achieving a minimum flux density of 1.1 T and should result, as a minimum, in the detection of surface-breaking planar discontinuities of the order of 0.5 mm deep, or greater, using the same procedures. (For procedures requiring direct current and colour-contrast magnetic particles, surface-breaking planar discontinuities of the order of twice these depths should be detected.) Where shallower surface-breaking discontinuities or sub-surface discontinuities are sought, higher levels of flux density will normally be required. As a guide, a factor of three times the minimum current value given in this Standard should be used. However, verification of the adequacy of the magnetization should be regarded as essential in these applications. A common assumption for the minimum value of the permeability of materials being tested is 240 H/m. However, low alloy steels may have lower permeabilities when in the normalized condition. Because of the wide permeability range of materials that are magnetic particle tested (from approximately 80 H/m to greater than 1000 H/m, with the additional complexity that the permeability is not a constant, but varies with the applied field), it is considered inappropriate to base an equation for the calculation of magnetizing current on a single assumed minimum value of permeability. A single assumed value which is low enough to ensure reliable testing of low permeability materials will most likely cause excessive background with high permeability materials. Consequently material can be classified into three groups as follows:
(a) High permeability steels (μ > 360) High permeability steels include structural steels, and plain carbon steels of low carbon content.
(b) Medium permeability steels (μ > 120) Medium permeability steels include low alloy steels with low to medium carbon content.
(c) Low permeability steels (μ > 60) Low permeability steels include martensitic stainless steels, magnetic precipitation-hardenable stainless steels, and low alloy steels with high carbon content. Low alloy steels with a chromium content greater than 2% should also be included in this group. Equations for the calculation of magnetizing current therefore require the appropriate permeability group to be taken into account.
GUIDANCE ON THE USE OF COIL MAGNETIZING METHODS EMPLOYING PARALLEL CONDUCTORS
Flexible coils may be formed from any cable capable of transmitting high currents. A cable having a cross-sectional area of 95 mm2 is suitable for most parallel conductor flexible coil applications; however, cables of cross-sectional area smaller than 95 mm2 are prone to premature overheating. Two basic arrangements of parallel conductors are—
(a) astride the inspection area (see Figure 3.5(c)); and
(b) adjacent to the inspection area (see Figure 3.5(d)).
An advantage of the astride arrangement is that, within wide limits, the field between the coils is essentially constant for a given current. The current required is obtained from the following equation:
IN = 4pDH . . . F(1)
I = current, in amperes
N = the number of turns
D = the gap between each coil, in metres
H = the field, in amperes per square metre
For high permeability steels (see Appendix D), this requires a current of 20 amperes per millimetre of the coil separation distance when each coil consists of one turn. The effective area which can be examined between the astride coils is limited by the ability of the power source to overcome the resistive and inductive load and provide the required current. In practice, substantial areas can be examined using one magnetization. The adjacent arrangement (see Figure 3.5(d)) is suitable for the inspection of nozzles and flanges. When weld-testing using alternating current, the field coincides with the weld profile. The required current, the magnetizing level and the effectiveness of the test method should be evaluated by trial and error using test pieces with known defects. The current flow direction in the two centre parallel conductors of a flat coil described in Figure 3.5(e), is required to be the same. The return cables should have sufficient separation to avoid interference. Although in situ testing usually requires the use of flexible cables, there is scope for the development of tailor-made jigs when repetitive inspection of structures of similar geometry is required.