Magnetic Particle Testing And Inspection

Published Date: 02 Nov 2017

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Introduction

Magnetic particle inspection deals with non destructive testing of subsurface and surface discontinuities in material that are ferroelectric e.g. nickel, iron, cobalt and alloys. Magnetic testing of particles is largely used since it relatively easy to apply and is fast. Raj reckons that this process entail using magnetic fields and iron filling as tiny magnetic particles to detect components flaws (Raj, 36). From a standpoint of inspectability, the component under inspection should contain ferromagnetic material such as cobalt, iron, nickel, among others.

Magnetism

The magnetism concept revolves around the field of magnetism and dipoles. The magnetic field term describes a space volume where a change in energy occurs in this volume. According to Paul, magnetic pole refers to the location of entry or exit of the magnetic field (Paul, 247). These poles have never been indentified in indentified in isolation but in pairs hence the use of the term dipole. A dipole refers to an object with a pole of magnetism on one side or ends and an opposite pole of magnetism on the other end. A magnetic bar is a dipole which consists of a southern and northern pole on each end.

Magnetism source is contained in matter’s basic block of building called an atom. Atom comprises of neutrons, electrons and protons. Neutrons and protons are found the nucleus of the atom and electrons constantly move in the region of the nucleus. Electrons have electrical charges that are negative and hence produce a field of magnetism in space movements. Whenever there is a motion of electrical charge, there is the production of a field of magnetism. Magnetic moment is a term that refers to this field’s strength. Mix reckons that whenever there are flows of current electricity in a conductor, electrons movement in the conductor results to the formation of a field of magnetism around this conductor (Mix, 249). Matter consists of atoms thus materials are affected by the field of magnetism in some way. Materials however do not respond in the same way to the field of magnetism.

Material reaction to the field of magnetism

When materials are placed in the field of magnetism, the material forces of magnetism are affected. This effect is referred to as Faradays Magnetic Law of Induction. The presence of external fields of magnetism leads to different reactions from materials. Moments of magnetism mentioned earlier some from; motion of electrons, electrons spin and motion changes caused by external fields of magnetism. In the majority of atoms, electrons are found in pairs which spin in directions opposite to each other. According to Paul, this opposite movement by fields of magnetism result to cancellation of the field of magnetism (Paul, 252). In regard to material interaction with the field of magnetism, material may be grouped as:

Diamagnetic materials- they have a weak negative susceptibility to the fields of magnetism and are repelled slightly by the field of magnetism. This material lack retention of the properties of magnetic fields and the electrons in diamagnetic materials are paired. Diamagnetic materials include gold, silver and copper.

Paramagnetic- they have positive small susceptibility to fields of magnetism and materials are attracted slightly to the field of magnetism hence do not retain properties of magnetism. Examples of paramagnetic materials include lithium, magnesium and molybdenum.

Ferromagnetic- they have positive large susceptibility to external fields of magnetism. They portray an attraction to fields of magnetism that is strong and are capable of retaining properties of magnetism after removal of the external field. Materials that are ferromagnetic have some electrons that are unpaired hence atoms have a net moment of magnetism. Ferromagnetic materials include cobalt, nickel and iron.

Magnetic testing of particles is applied in testing a variety of product forms such as forgings, castings and weldment. Mix reckons that, many distinct industries use magnetic method of particle inspection and they include aerospace, power generation, automotive, structural and petrochemical industries (Mix, 254). Inspection of underwater is another section where inspection of magnetic particle may be applied to test items e.g. underwater pipelines and structures of offshore.

Basic principles

Theoretically, testing of magnetic particles has a concept that is relatively simple. It can be viewed as a combination of two methods of nondestructive testing (NDT) i.e. visual testing and leakage testing of magnetic flux. In a bar magnets case, the field of magnetism is in the magnet and also around it. Any area that magnetic force line enters or exits the magnet is known as a pole. Magnetic force lines enter the magnetic from the southern pole and exit from the northern pole. When a magnetic bar is broken at the center, this results to the formation of two magnetic bars that are complete with southern and northern pole on each end. If the magnetic bar for instance cracks a south and north pole forms at each cracks end. Mix reckons that the magnetic field in this case reenters the southern pole and exits in the northern pole (Mix, 255). The field of magnetism spreads out in case it encounters a tiny gap of air. Air cannot hold up magnetism as a can of magnetism can support. When spreading of the magnetic field occurs a leakage occurs which is referred to as a field flux leakage.

How magnetic inspection of particles operates?

Iron particles are attracted to a magnet when they are sprinkled on a magnet that is cracked. They cling or are attracted at the poles and the edges of the magnets crack. The clustering of particles of iron is easily visible than a crack on a magnetic bar and hence inspection of magnetic particles arises through this concept.

Step one in magnetic testing of particles is the magnetization of the component due for inspection. Paul reckons that if there are defects near or on the magnetic surface, a field leakage occurs due to the defect (Paul, 264). After magnetization of the component occurs wet or dry suspended iron filling are applied on the magnetized part surface. These particles cluster and are attracted at the section of the fields flux leakage hence forming a clear indication that is detectable by the inspector.

Basic procedure

Components pre-cleaning- when a part is being inspected using the method of magnetic particle, it vital for the presence of a path that is unimpeded for particles to migrate to weak and strong fields of leakage. Surfaces of parts should be dry and clean before the inspection takes place. Contaminants may prevent attraction of particles and indicator interpretation hence should be removed. These contaminants include scale, grease and oil and should be wiped off to ensure accurate outcomes.

Magnetic field introduction- the field of magnetism that is required may be brought into a component through different methods.

Using electromagnets or permanent magnets that can contact the part under inspection

Flowing current of electricity through specimen under inspection

Flowing current of electricity through wire coils a or conductor operating closely to the part

Magnetic field direction and detection of flaws and orientation of magnetic fields

According to mix, circular and longitudinal field of magnetism may be created around a specimen (Mix, 257). The field of magnetism created is determined by the specimen method of magnetization. To inspect properly cracks or defect in a component, it’s necessary to appreciate that crack’s orientation in relations to magnetic force lines determine whether a crack is identifiable. Two types of magnetic fields can be detected in a component.

Longitudinal field of magnetism- they have magnetic force line running parallel to the part’s long axis. Longitudinal approach in magnetizing a component can be achieved by using a solenoid or coil formed by longitudinal field. Longitudinal magnetization can also be attained by using electromagnets of permanent magnets.

Circular field of magnetism- it has magnetic force lines that run around the part’s perimeter in a circumference. A circular field of magnetism is created in particles by the passage of current in a component.

Importance of the direction of magnetic field

The type of field of magnetism formed is influenced by the method of magnetization used. Magnetizing parts using two directions is necessary since best defects detection occurs when magnetic force lines are formed at angles right to the defects dimensions. Paul reckons that this orientation leads to the creation of the biggest disruption of the field of magnetism and the greatest leakage of flux on the parts surface (Paul, 257). If the defect is parallel to the field of magnetism, less disruption occurs hence there is no leakage of flux.

Application of dry versus wet particles- MPI may be carried using out wet or dry particles. When using particles that are dry, these particles are sprinkled lightly on the part surface. Magnetic inspection of particles involves flooding the surface part with a solution containing these particles. The dry method of magnetic inspection and testing is more portable as opposed to the wet method. According to Mix, the wet approach of testing is more sensitive since magnetic particles gain extra mobility due to the solution (Mix, 263). When using the dry method of testing and inspecting magnetic particles, a color that yields the highest contrast in comparison to its background should be applied in the testing process. When using the wet method particles that are wet are supplied as fluorescent or visible. Particles that are visible are observes under natural light while particles that are fluorescent are observed under light that is black.

Interpretation of indications of magnetic particle- after application of the field of magnetism the formed indications or results must be interpreted. Paul reckons that the inspector in this process is expected to distinguish between indicators that are non-relevant and relevant (Paul, 270). Images that exemplify the indicators of magnetic method of inspection and testing include; a hook crane which is crack induced, gear whose cracks are service induced, shaft drive cracks induced by heat treatment, splined and threaded shaft, large bolt whose cracks are service induced, crank shaft whose cracks are induced close to the lube hole, toe crack among others.

Demagnetization

According to Mix, parts tested using the particle method of magnetic testing may be result to a residual field of magnetism that is objectionable, and may hamper preceding manufacturing services or operations in the component (Mix, 290). Reasons for carrying out demagnetization include:

May result to interference in machining operations or welding.

Can result to sensitivity of gauges to fields of magnetism if closely placed.

Particles that are abrasive may affect surface component and result in high levels if wear in the components of the engine i.e. bearings and gears.

Demagnetization necessitates the inspector to reverse the residual field of magnetism.

Demagnetization ensures reduction of the residual’s field of magnetism strength to a level that is acceptable.

Advantages of magnetic inspection of particles and testing

High sensitivity i.e. tiny discontinuities are detected

Indications are relayed on the part surface directly and account to a visual flaw depiction

Preparation for the surface is minimal as one does not need to remove the paint

Material are available in spray cans of aerosol hence are portable

Equipment and materials required for this process are generally inexpensive

This method is relatively fast in inspection of magnetic particles.

Limitations of magnetic testing and inspection of particles

Only defects in surfaces and surfaces that are near are detected

Its only applicable to materials that are ferromagnetic

Material with surfaces that are non porous are the only ones that are inspectable

There must be direct access by the inspector for the surface under inspection

It’s necessary for an alignment between the flux of magnetism

Post demagnetization and cleaning is often vital

Large part inspection may necessitate the presence of special powered equipment

In conclusion, magnetic method of particle inspection and testing provides a useful method of examination that is non destructive in inspection of services and constructions. It can be applied only on ferromagnetic materials hence it’s not the most suitable or convenient method for some applications. For quick inspection using low costs, magnetic testing and inspection of particles is arguably the best non destructive testing approach for subsurface and surface discontinuities detection.