ASTM A966/A966M-96(2006)
(Test Method)Standard Test Method for Magnetic Particle Examination of Steel Forgings Using Alternating Current
Standard Test Method for Magnetic Particle Examination of Steel Forgings Using Alternating Current
SCOPE
1.1 This test method covers a procedure for the magnetic particle examination of steel forgings using alternating current as the power source. The procedure will produce consistent results upon which acceptance standards can be based. This test method does not contain acceptance limits or recommended quality levels.
1.2 Only alternating 50-60 cycle current shall be used as the electric power source for any of the magnetizing methods. The ability to detect subsurface discontinuities is very limited when using an alternating power source, and therefore the test surfaces should be in the final thermally treated and finish machined condition.
1.3 When subsurface indications are sought in forgings, then dc magnetization in accordance with Test Method A 275/A 275M should be used.
1.4 The values stated in either inch-pound units or SI units are to regarded separately as standard. Within the text, the SI units are shown in brackets. The values stated in each system are not exact equivalents; therefore each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the specification. Unless the order specifies the applicable "M" specification designation [SI units], the inch-pound units shall be used.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:A 966/A 966M–96 (Reapproved 2006)
Standard Test Method for
Magnetic Particle Examination of Steel Forgings Using
Alternating Current
This standard is issued under the fixed designationA966/A966M; the number immediately following the designation indicates the year
of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval.
A superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope A 275/A 275M Test Method for Magnetic Particle Exami-
nation of Steel Forgings
1.1 This test method covers a procedure for the magnetic
2.2 Other Document:
particle examination of steel forgings using alternating current
Practice No. SNT-TC-1A, Supplement B—Magnetic Par-
as the power source. The procedure will produce consistent
ticle Method
resultsuponwhichacceptancestandardscanbebased.Thistest
method does not contain acceptance limits or recommended
3. Terminology
quality levels.
3.1 Definitions:
1.2 Onlyalternating50–60cyclecurrentshallbeusedasthe
3.1.1 (ac) magnetic particle method of examination—a
electric power source for any of the magnetizing methods. The
method for detecting discontinuities on the surface in suitably
ability to detect subsurface discontinuities is very limited when
magnetized materials, which employs finely divided magnetic
using an alternating power source, and therefore the test
particles that tend to congregate in regions of leakage fields.
surfaces should be in the final thermally treated and finish
3.1.2 indication—the visual magnetic particle buildup re-
machined condition.
sulting from leakage fields in the magnetic field.
1.3 When subsurface indications are sought in forgings,
3.1.3 linear indication—an indication in which the length is
then dc magnetization in accordance with Test Method A 275/
at least three times the width. The minimum length of
A 275M should be used.
indications to be considered linear shall be ⁄16 in. [1.5 mm].
1.4 The values stated in either inch-pound units or SI units
3.1.4 magnetic flux—the product of the magnetic induction
are to regarded separately as standard. Within the text, the SI
and the area of a surface (or cross section) when the magnetic
units are shown in brackets. The values stated in each system
induction is uniformly distributed and normal to the plane of
are not exact equivalents; therefore each system shall be used
the surface. The concept that the magnetic field is flowing
independently of the other. Combining values from the two
along the lines of force suggests that these lines are therefore
systems may result in nonconformance with the specification.
“flux” lines, and they are called magnetic flux.
Unless the order specifies the applicable “M” specification
3.1.5 nonrelevant indications—indications produced by
designation [SI units], the inch-pound units shall be used.
leakagefieldswheretheconditionscausingthemarepresentby
1.5 This standard does not purport to address all of the
accident or part design, or other features of the part having no
safety concerns, if any, associated with its use. It is the
relationtothedamagingflawsbeingsought.Thistermsignifies
responsibility of the user of this standard to establish appro-
thatsuchanindicationhasnorelationtothediscontinuitiesthat
priate safety and health practices and determine the applica-
might constitute defects.
bility of regulatory limitations prior to use.
4. Basis of Application
2. Referenced Documents
4.1 When in accordance with the requirements of the
2.1 ASTM Standards:
inquiry, contract, order, or specifications, forgings are to be
examined by the magnetic particle method using alternating
This test method is under the jurisdiction of ASTM Committee A01 on Steel, current; the manufacturer and the supplier shall be in agree-
Stainless Steel and Related Alloys and is the direct responsibility of Subcommittee
ment concerning the following:
A01.06 on Steel Forgings and Billets.
4.1.1 The locations on the forging that are to be subjected to
Current edition approved March 1, 2006. Published April 2006. Originally
magnetic particle examination.
approved in 1996. Last previous edition approved in 2001 as A 966/A 966M – 96
(2001).
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Available from American Society for NonDestructive Testing, 1711 Arlingate
the ASTM website. Lane, P.O. Box 28518, Columbus, OH 43228-0518.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
A 966/A 966M–96 (2006)
4.1.2 The type, size, number, location, and orientation of 10.1.2 The surge and residual methods are not applicable to
indications that are to be considered injurious. this test method.
4.1.3 The method of application and type of magnetic 10.2 At least two separate examinations shall be carried out
particles, demagnetization requirements, and magnetic field on each area.The second examination shall be with the lines of
strengths. magnetic flux approximately perpendicular to those used for
4.1.4 Acceptance standards. the first examination in that area. A different means of
magnetization may be used for the second examination. Mag-
5. Personnel Requirements
netizing in more than one direction cannot normally be
5.1 Personnel performing the magnetic particle examination
accomplishedsimultaneously.Anexceptiontotheaboveruleis
inthistestmethodshallbequalifiedandcertifiedinaccordance
overall sequential multivector magnetization whereby several
with a written procedure conforming to Practice No. SNT-
magnetizing circuits are provided for sequentially magnetizing
TC-1A or another national standard that is acceptable to both
a part in multiple directions depending on the locations of the
the purchaser and supplier.
current connectors. By this technique, indications of any
orientation can be detected with a single application of
6. Stage of Examination
magnetic particles.
6.1 Unless otherwise specified by the purchaser, acceptance
10.3 The two general types of magnetization with regard to
examination shall be performed on a forging in the final
direction are longitudinal and circular as follows:
machined surface condition.
10.3.1 Longitudinal—When a forging is magnetized longi-
tudinally, the magnetic flux lines are usually parallel to the axis
7. Magnetizing Apparatus
of the piece. A longitudinally magnetized piece always has
7.1 A50 or 60 cycle alternating current shall be used.When
definite poles, readily detectable by compass or magnetometer.
current is passed through the forging itself, the equipment shall
Longitudinalmagnetizationisusuallyaccomplishedbyplacing
consist of contacting or clamping elements with sufficient
the forging within a solenoid, frequently formed by wrapping
surface area and clamping pressure to allow the required
cable around the piece (Fig. 1). For special applications,
current to flow without damaging (burning) the part being
magnetic yokes can be used (Fig. 2).
examined.
10.3.2 Circular—Circular magnetization is obtained by
7.2 Portable electromagnetic alternating current yokes may
passing a current through the piece (Fig. 3) or by induced by
be used as a magnetizing apparatus.
passing current through a conductor or conductors threaded
8. Magnetic Particles
through an opening in the piece (Fig. 4 or Fig. 5). By
agreement with the purchaser (see 10.5.3) localized circular
8.1 The inspection medium shall consist of finely divided
magnetization may be obtained by passing current through
ferromagnetic particles (which may be coated with a fluores-
local areas by the use of prod-type contacts (Fig. 6).
cent material) suspended in a suitable liquid medium or used in
10.4 The magnetic field is confined almost entirely to the
dry powder form.
piece and there may be no external manifestation of the
9. Surface Preparation
magnetized condition. Indications will appear strongest in the
direction perpendicular to the direction of the magnetic field.
9.1 Thesensitivityofthemagneticparticleexaminationwill
10.5 Field Strength—The minimum field strength that will
depend to a considerable extent upon the condition of the
surface being examined. While defects may be satisfactorily reveal and permit classification of all objectionable defects
shall be used. The maximum field strengths practical are the
revealed on shot-blasted or otherwise cleaned forged surfaces,
without any further surface treatment, all heat treatment or ones just below the point at which excessive adherence of the
particles begins to occur over the surface being inspected.
forging scale must be removed. However, to reveal fine defects
of ⁄8 in. [3 mm] or less in length, the surfaces to be examined 10.5.1 Coil Magnetization—When coil magnetization is
used, the magnetic field strength is directly proportional to the
shall be smooth machined to at least a 250-µin. [6.35-µm]
finish.
9.2 The surfaces shall be free from grease, oils, or other
substances to which the particles may adhere.
10. Methods of Magnetization
10.1 The forging may be magnetized either by passing
current through the piece or by inducing a magnetic field by
means of a central conductor, by coils, or by yoke.
10.1.1 Continuous Method—In the continuous method the
inspection medium is applied to the surface under examination
while the current is still flowing.The alternating current source
generateshighamperagecurrentinpulsesofupto1sduration.
The duration of this flow shall allow at least three pulses of
current, or in the case where machines supply continuous
1 1
current flow, a minimum shot of ⁄5 to ⁄2 s duration should be
applied. FIG. 1 Longitudinal Magnetization
A 966/A 966M–96 (2006)
FIG. 6 Circular Magnetization with “Prod” Type Contact
Electrodes
coils,theadequacyofthefieldshallbedemonstratedbytheuse
of field indicators (see 10.5.6).
FIG. 2 Longitudinal Magnetization, with Yoke (1) Small Forgings—Magnetizing force shall be 35 000
ampere-turns divided by the sum of 2 plus the “length over
diameter” ratio of the test part. For example, a part 10 in.
[250 mm]longby2in.[50mm]inoutsidediameterhasanL/D
ratio of 5. Therefore, 35 000/(2 + 5) = 5000 ampere-turns; if a
5-turn coil is used, the current required is 5000/5 or 1000 A.
This formula provides an adequate field strength on small parts
having an L/D ratio of 4 or greater. For parts having a smaller
L/D ratio, adequate field strengths shall be demonstrated by the
use of a field indicator (see 10.5.6).The graph in Fig. 7 may be
used to determine the ampere-turns required for each L/D
relationship.
FIG. 3 Circular Magnetization, Current Directly Through Forging
(2) Large Forgings—For large forgings the magnetizing
force shall be in the range from 1200 to 4500 ampere-turns.A
field indicator (see 10.5.6) shall be used to demonstrate the
presence of an adequate field strength over the area to be
inspected.
10.5.1.2 Circular Magnetization (Fig. 5)—For circular
magnetization with through coils, use the current with amper-
age as specified in 10.5.2 divided by the number of turns in the
coil.
10.5.2 Direct Magnetization—When current is passed di-
FIG. 4 Circular Magnetization, Current Through a Conductor rectly through the part to be examined, the current shall be
between 100 and 900A[4 and 35Aper millimetre] per inch of
diameter or cross section (per inch or millimetre of greatest
width in a plane at right angles to current flow). For hollow
FIG. 5 Circular Magnetization, Current Through Conductors
Threaded Through Forging
current (ampere-turns if a coil or solenoid is used) and
inversely proportional to the thickness of the section being
inspected.
10.5.1.1 Longitudinal Magnetization—For encircling coils
(Fig. 1), the turns of the coil shall be kept closely together.The
field strength decreases as distance from the coil increases and
long parts must be magnetized in sections. If the area to be
inspected extends beyond 6 in. [150 mm] on either side of the FIG. 7 Longitudinal Magnetization
A 966/A 966M–96 (2006)
parts this would be wall thickness when cables are clamped to shall be taken to prevent local overheating or burning of the
the wall. Suggested current for diameters or sections up to 5 in. surface being examined. Steel- or aluminum-tipped prods or
[125 mm] are 600 to 900A/inches [25 to 35Aper millimetre]; copper-brush-type prods rather than solid copper-tipped prods
for diameters or sections between 5 and 10 in. [125 to are recommended where the magnetizing voltage is over 25 V
250 mm], 400 to 600A/inches [15 to 25Aper millimetre]; and open circuit (bad contact) in order to avoid copper penetration.
100 to 400 A/inches [4 to 15 A per millimetre] for outside Permanent magnetic leeches may be used as a pair or in
diameters or sections over 10 in. [250 mm]. If it is not practical conjunction with a prod. Leeches should not be used in excess
to obtain these current levels for diameters over 10 in. [250 of 1500 A because loss of magnetization occurs.
mm], the presence of an adequate field strength shall be
10.5.3.3 A remote control switch, which may be built into
demonstrated using a field indicator. In all other instances the
the prod handles, shall be provided to permit the current to be
adequacy of the magnetizing force shall be demonstrated by
turned on after the prods have been properly positioned and to
means of a field indicator (see 10.5.6). When large parts have
turn off before the prods are removed in order to prevent
been examined by clamping contacts to the wall thickness, the
arcing.
adequacy of the field in the circumferential direction shall also
10.5.3.4 Examination Coverage—Examinations shall be
be determined by the field indicator.
conducted with sufficient overlap to ensure 100 % coverage at
10.5.3 Prod Magnetization—Since this method may induce
the established sensitivity.
arcing or burning at the contact areas, and the inspection is
10.5.3.5 Direction of Magnetization—At least two separate
intended to be performed on finished surfaces, the use of prod
examinations shall be carried out on each area. The prods shall
magnetization is not permissible without the prior approval of
be placed so that the lines of flux during one examination are
the purchaser. For the same reason magnetic leaches may not
approximately perpendicular to the lines of flux during the
be used to introduce current into the part without the prior
other.
approval of the purchaser. If the use of prods or magnetic
10.5.4 Indirect circular magnetization of forgings may be
leaches should be per
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