1、MAGNETIC PARTICLE TESTINGIntroductionThis module is intended to present information on the widely used method of magnetic particle inspection. Magnetic particle inspection can detect both production discontinuities (seams, laps, grinding cracks and quenching cracks) and in-service damage (fatigue an
2、d overload cracks). OutlineMagnetism and Ferromagnetic MaterialsIntroduction of Magnetic Particle InspectionBasic Procedure and Important ConsiderationsComponent pre-cleaningIntroduction of magnetic fieldApplication of magnetic mediaInterpretation of magnetic particle indicationsExamples of MPI Indi
3、cationsMagnetic lines of force around a bar magnetOpposite poles attractingSimilar poles repellingIntroduction to MagnetismMagnetism is the ability of matter to attract other matter to itself. Objects that possess the property of magnetism are said to be magnetic or magnetized and magnetic lines of
4、force can be found in and around the objects. A magnetic pole is a point where the a magnetic line of force exits or enters a material.Magnetic field lines: Form complete loops. Do not cross. Follow the path of least resistance. All have the same strength. Have a direction such that they cause poles
5、 to attract or repel.Ferromagnetic Materials A material is considered ferromagnetic if it can be magnetized. Materials with a significant Iron, nickel or cobalt content are generally ferromagnetic. Ferromagnetic materials are made up of many regions in which the magnetic fields of atoms are aligned.
6、 These regions are call magnetic domains. Magnetic domains point randomly in demagnetized material, but can be aligned using electrical current or an external magnetic field to magnetize the material.DemagnetizedMagnetizedSNHow Does Magnetic Particle Inspection Work?A ferromagnetic test specimen is
7、magnetized with a strong magnetic field created by a magnet or special equipment. If the specimen has a discontinuity, the discontinuity will interrupt the magnetic field flowing through the specimen and a leakage field will occur.How Does Magnetic Particle Inspection Work? (Cont.)Finely milled iron
8、 particles coated with a dye pigment are applied to the test specimen. These particles are attracted to leakage fields and will cluster to form an indication directly over the discontinuity. This indication can be visually detected under proper lighting conditions. Basic Procedure Basic steps involv
9、ed:Component pre-cleaningIntroduction of magnetic fieldApplication of magnetic mediaInterpretation of magnetic particle indicationsPre-cleaning When inspecting a test part with the magnetic particle method it is essential for the particles to have an unimpeded path for migration to both strong and w
10、eak leakage fields alike. The parts surface should be clean and dry before inspection.Contaminants such as oil, grease, or scale may not only prevent particles from being attracted to leakage fields, they may also interfere with interpretation of indications.Introduction of the Magnetic Field The re
11、quired magnetic field can be introduced into a component in a number of different ways. Using a permanent magnet or an electromagnet that contacts the test piece Flowing an electrical current through the specimenFlowing an electrical current through a coil of wire around the part or through a centra
12、l conductor running near the part.Direction of the Magnetic FieldTwo general types of magnetic fields (longitudinal and circular) may be established within the specimen. The type of magnetic field established is determined by the method used to magnetize the specimen. A longitudinal magnetic field h
13、as magnetic lines of force that run parallel to the long axis of the part. A circular magnetic field has magnetic lines of force that run circumferentially around the perimeter of a part.Importance of Magnetic Field DirectionBeing able to magnetize the part in two directions is important because the
14、 best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimension of the defect. This orientation creates the largest disruption of the magnetic field within the part and the greatest flux leakage at the surface of the part. An orientation of
15、 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication.Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other.Flux LeakageNo Flux LeakageQuestion? ? From the previous slide re
16、garding the optimum test sensitivity, which kinds of defect are easily found in the images below?Longitudinal (along the axis)Transverse (perpendicular the axis)Producing a Longitudinal Magnetic Field Using a CoilA longitudinal magnetic field is usually established by placing the part near the insid
17、e or a coils annulus. This produces magnetic lines of force that are parallel to the long axis of the test part. Coil on Wet Horizontal Inspection Unit Portable Coil Producing a Longitudinal Field Using Permanent or Electromagnetic MagnetsPermanent magnets and electromagnetic yokes are also often us
18、ed to produce a longitudinal magnetic field. The magnetic lines of force run from one pole to the other, and the poles are positioned such that any flaws present run normal to these lines of force. Circular Magnetic FieldsCircular magnetic fields are produced by passing current through the part or b
19、y placing the part in a strong circular magnet field. A headshot on a wet horizontal test unit and the use of prods are several common methods of injecting current in a part to produce a circular magnetic field. Placing parts on a central conductors carrying high current is another way to produce th
20、e field.Magnetic FieldElectricCurrentApplication of Magnetic Media (Wet Versus Dry)MPI can be performed using either dry particles, or particles suspended in a liquid. With the dry method, the particles are lightly dusted on to the surface. With the wet method, the part is flooded with a solution ca
21、rrying the particles. The dry method is more portable. The wet method is generally more sensitive since the liquid carrier gives the magnetic particles additional mobility. Dry Magnetic Particles Magnetic particles come in a variety of colors. A color that produces a high level of contrast against t
22、he background should be used.Wet Magnetic ParticlesWet particles are typically supplied as visible or fluorescent. Visible particles are viewed under normal white light and fluorescent particles are viewed under black light.Interpretation of Indications After applying the magnetic field, indications
23、 that form must interpreted. This process requires that the inspector distinguish between relevant and non-relevant indications.The following series of images depict relevant indications produced from a variety of components inspected with the magnetic particle method.Crane Hook with Service Induced
24、 Crack Fluorescent, Wet Particle Method Gear with Service Induced Crack Fluorescent, Wet Particle Method Drive Shaft with Heat Treatment Induced Cracks Fluorescent, Wet Particle Method Splined Shaft with Service Induced CracksFluorescent, Wet Particle Method Threaded Shaft withService Induced CrackF
25、luorescent, Wet Particle Method Large Bolt with Service Induced CrackFluorescent, Wet Particle Method Crank Shaft with Service Induced Crack Near Lube HoleFluorescent, Wet Particle Method Lack of Fusion in SMAW WeldVisible, Dry Powder MethodIndicationToe Crack in SMAW WeldVisible, Dry Powder MethodT
26、hroat and Toe Cracks in Partially Ground WeldVisible, Dry Powder MethodDemagnetizationParts inspected by the magnetic particle method may sometimes have an objectionable residual magnetic field that may interfere with subsequent manufacturing operations or service of the component.Possible reasons f
27、or demagnetization include: May interfere with welding and/or machining operations Can effect gauges that are sensitive to magnetic fields if placed in close proximity. Abrasive particles may adhere to components surface and cause and increase in wear to engines components, gears, bearings etc. Dema
28、gnetization (Cont.)Demagnetization requires that the residual magnetic field is reversed and reduced by the inspector.This process will scramble the magnetic domains and reduce the strength of the residual field to an acceptable level.DemagnetizedMagnetizedAdvantages of Magnetic Particle Inspection
29、Can detect both surface and near sub-surface defects.Can inspect parts with irregular shapes easily.Precleaning of components is not as critical as it is for some other inspection methods. Most contaminants within a flaw will not hinder flaw detectability.Fast method of inspection and indications ar
30、e visible directly on the specimen surface.Considered low cost compared to many other NDT methods.Is a very portable inspection method especially when used with battery powered equipment.Limitations ofMagnetic Particle InspectionCannot inspect non-ferrous materials such as aluminum, magnesium or mos
31、t stainless steels.Inspection of large parts may require use of equipment with special power requirements.Some parts may require removal of coating or plating to achieve desired inspection sensitivity.Limited subsurface discontinuity detection capabilities. Maximum depth sensitivity is approximately
32、 0.6” (under ideal conditions).Post cleaning, and post demagnetization is often necessary.Alignment between magnetic flux and defect is importantGlossary of TermsBlack Light: ultraviolet light which is filtered to produce a wavelength of approximately 365 nanometers. Black light will cause certain m
33、aterials to fluoresce.Central conductor: an electrically conductive bar usually made of copper used to introduce a circular magnetic field in to a test specimen.Coil: an electrical conductor such a copper wire or cable that is wrapped in several or many loops that are brought close to one another to
34、 form a strong longitudinal magnetic field.Glossary of TermsDiscontinuity: an interruption in the structure of the material such as a crack.Ferromagnetic: a material such as iron, nickel and cobalt or one of its alloys that is strongly attracted to a magnetic field.Heads: electrical contact pads on
35、a wet horizontal magnetic particle inspection machine. The part to be inspected is clamped and held in place between the heads and shot of current is sent through the part from the heads to create a circular magnetic field in the part.Leakage field: a disruption in the magnetic field. This disruptio
36、n must extend to the surface of the part for particles to be attracted.Glossary of TermsNon-relevant indications: indications produced due to some intended design feature of a specimen such a keyways, splines or press fits.Prods: two electrodes usually made of copper or aluminum that are used to int
37、roduce current in to a test part. This current in turn creates a circular magnetic field where each prod touches the part. (Similar in principal to a welding electrode and ground clamp).Relevant indications: indications produced from something other than a design feature of a test specimen. Cracks, stringers, or laps are examples of relevant indications.Glossary of TermsSuspension: a bath created by mixing particles with either oil or water.Yoke: a horseshoe magnet used to create a longitudinal magnetic field. Yokes may be made from permanent magnets or electromagnets.
