ASTM A275 – Standard Practice for Magnetic Particle Examination of Steel Forgings
Significance and Use
4.1 For ferromagnetic materials, magnetic particle examination is widely specified for the detection of surface and near surface discontinuities such as cracks, laps, seams, and linearly oriented nonmetallic inclusions. Such examinations are included as mandatory requirements in some forging standards such as Specification A508/A508M.
4.2 Use of direct current or rectified alternating (full or half wave) current as the power source for magnetic particle examination allows detection of subsurface discontinuities.
This test method covers the procedures for the standard practice of performing magnetic particle examination on steel forgings. The inspection medium shall consist of finely divided ferromagnetic particles, whose size, shape and magnetic properties, both individually and collectively, shall be taken into account. Forgings may be magnetized in the longitudinal or circular direction by employing the surge or continuous current flow methods. Magnetization may be applied by passing current through the piece or by inducing a magnetic field by means of a central conductor, such as a prod or yoke, or by coils. While the material is properly magnetized, the magnetic particles may be applied by either the dry method, wet method, or fluorescent method. The parts shall also be sufficiently demagnetized after inspection so that residual or leakage fields will not interfere with future operations to which the steel forgings shall be used for. Indications to be evaluated are grouped into three broad classes, namely: surface defects, which include laminar defects, forging laps and folds, flakes (thermal ruptures caused by entrapped hydrogen), heat-treating cracks, shrinkage cracks, grinding cracks, and etching or plating cracks; subsurface defects, which include stringers of nonmetallic inclusions, large nonmetallics, cracks in underbeads of welds, and forging bursts; and nonrelevant or false indications, which include magnetic writing, changes in section, edge of weld, and flow lines.
1.1 This practice covers a procedure for magnetic particle examination of steel forgings. The procedure will produce consistent results upon which acceptance standards can be based. This practice does not contain acceptance standards or recommended quality levels.
1.2 Only direct current or rectified alternating (full or half wave) current shall be used as the electric power source for any of the magnetizing methods. Alternating current is not permitted because its capability to detect subsurface discontinuities is very limited and therefore unsuitable.
1.2.1 Portable battery powered electromagnetic yokes are outside the scope of this practice.
NOTE 1: Guide E709 may be utilized for magnetic particle examination in the field for machinery components originally manufactured from steel forgings.
1.3 The minimum requirements for magnetic particle examination shall conform to practice standards of Practice E1444/E1444M. If the requirements of this practice are in conflict with the requirements of Practice E1444/E1444M, the requirements of this practice shall prevail.
1.4 This practice and the applicable material specifications are expressed in both inch-pound units and SI units. However, unless the order specifies the applicable “M” specification designation [SI units], the material shall be furnished to inch-pound units.
1.5 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.