Magnetic Particle Test Discontinuities
There are three general classes of discontinuities.
1. Inherent discontinuities, formed during the basic formation of the test object.
2. Processing discontinuities, formed during operations like machining or welding.
3. Inservice discontinuities, caused during use.
INHERENT DISCONTINUITIES
Inherent discontinuities are caused by the primary processing of the test object. For ferromagnetic metals, this includes the initial melting and refining processes and during solidification from the molten state. These discontinuities are present before rolling, forging and casting into the final shape. They will be found in the final production object and are typically below the surface. These discontinuities include the following.
1 . Ingot pipe.
2. Ingot cracks.
3. Nonmetallic inclusions.
4. Blow holes.
Direct current and half wave direct current magnetization is
effective in detection of inherent discontinuities.
PROCESSING DISCONTINUITIES
Additional processing of metals, such as forging, casting, rolling and welding, can result in processing induced discontinuities that include the following.
1. Seams.
2. Laminations.
3. Cupping.
4. Cooling cracks.
5. Forging flakes.
6. Forging bursts.
7. Forging laps.
8. Forging flash line tears.
9. Casting cold shuts.
10. Casting hot tears and shrinkages.
11. Weld lack of fusion and weld lack of penetration.
12. Weld heat affected zone cracks.
13. Weld crater cracks.
14. Machine grinding cracks.
15. Machine tears.
16. Plating, pickling and etching cracks.
17. Liquid metal embrittlement cracks.
18. Galvanization related weld cracks.
A combination of alternating current and half wave direct current or indirect magnetization will prove useful in the detection of processing discontinuities.
lNSERVICE DISCONTINUITIES
Inservice stresses result in cracking and corrosion discontinuities. This can happen in a single cycle and is called single cycle fatigue cracking. This type of cracking typically starts on or near the surface. Alternately, an acceptable inherent internal discontinuity may be locally overloaded and start a crack that propagates from the interior of the test object to the exterior. These internal cracks may propagate on a single or multiple overload cycle.
Multiple cycles of near overload conditions will lead to cracking typically originating on the surface of objects inservice. These cracks, if not from an inherent discontinuity, will originate on or near the surface of the test object.
Corrosion and underbead weld cracking may generate surf ace breaking stress cracks that are detectable with magnetic particle testing techniques.
Alternating current magnetization (direct, induced or by yokes) combined with an applicable magnetic particle is used for the detection of inservice discontinuities.
SUMMARY of Magnetic Particle Test Discontinuities
Magnetic particle testing is an enhanced form of visual testing that is applicable to a select group of metals that are said to be ferromagnetic. This magnetic technology enables the visual detection of tiny surface discontinuities and even some subsurface discontinuities.