Inspection of magnetic particles (MT / MPI); Principles, applications and requirements of non-destructive testing
Quality control and ensuring the safety of equipment, structures and industrial parts is one of the most important parts of production, repair and maintenance processes.. Meanwhile, non-destructive tests (NDT) They play an important role in identifying possible defects and discontinuities without damaging the part.
Inspection of magnetic particles (Magnetic Particle Testing – MT یا MPI) One of the widely used methods is non-destructive testing, which is especially used to identify surface and near-surface discontinuities in ferromagnetic materials..This method is widely used in welding inspection, quality control of steel parts, inspection of industrial equipment, crack detection, control of machined parts and evaluation of parts under stress..
What is a magnetic particle test?
MT test is a non-destructive test method (NDT) which, by creating a magnetic field in the piece, allows the identification of cracks and discontinuities on the surface and close to the surface.The basis of the work is very simple but physically accurate:First, the test piece is magnetized.
Then, very fine ferromagnetic particles, usually as a dry powder or a wet suspension, are applied to the surface of the part..If there is a suitable crack or discontinuity in the path of the magnetic flux, the magnetic flux will change at the defect location and part of it will leave the surface of the part.. This phenomenon that Magnetic Flux Leakage (MFL) or it is called magnetic flux leakage, it causes the accumulation of magnetic particles in the place of discontinuity.As a result, the defect appears as a visible mark on the surface.
In simple language:Magnetic field → flux passing through the part → presence of cracks → flux deflection → flux leakage → accumulation of magnetic particles → visible mark formationThis mechanism is the basis of the operation of the Magnetic Particle Inspection method.
Principles of performance Inspection of MTTo better understand the magnetic particle test, one should pay attention to the behavior of the magnetic field in a ferromagnetic piece.When the piece is magnetized, lines of magnetic flux move through it.
If the structure of the part is uniform, the flux will also have a relatively uniform path.But the existence of such a discontinuity:surface crackSubsurface crackLack of FusionCracks caused by heat treatmentLeave fatiguestop grindingCracks caused by weldingIt can disrupt the flow path.At the place of discontinuity, due to the change of magnetic permeability, part of the flux leaves the surface of the piece and a magnetic leakage field is created..
The magnetic particles that are placed on the surface are affected by this field and accumulate in the place of flux leakage.Therefore, a visible indication is created that can indicate the existence of a discontinuity.Important point: Any symptom observed in the MT test does not necessarily mean the presence of a failing crack. The sign must be interpreted and, if necessary, evaluated and classified by a qualified inspector.
Magnetic particle test proceduresA standard MT inspection process typically includes the following steps:
1. Surface preparationThe surface to be tested should be as clean as possible and free from contamination such as:Oil and greaseRust and surface crustsPaint and thick coatingsdustPollution caused by the manufacturing processbe.
The quality of surface preparation can directly affect the ability to detect and interpret signs.
2. Magnetize the pieceAt this stage, a suitable magnetic field is created in the part.The method of creating the field can be different based on the geometry of the part and the possible direction of the defect.
3. Application of magnetic particlesMagnetic particles are applied on the surface as Dry Powder or Wet Suspension.Particles are possible:be visible in white light; orFluorescent and visible under UV/UV-A light.
4. Observation and interpretation of signsAfter applying the particles, the surface of the piece is checked and the marks created are recorded and interpreted.The shape, length, direction, location, and intensity of the sign can provide important information about the nature of the discontinuity.
5. EvaluationThe inspector must specify that the observed sign:RelevantNon-relevantor in some circumstances False Indicationis.Then relevant indications based on standard, code, project specifications or acceptance criteria (Acceptance Criteria) are evaluated.
6. Demagnetization and cleaningIf necessary, after inspection, the part is demagnetized to reduce residual magnetism.This stage is especially important for the parts that will enter sensitive industrial processes.The importance of direction Magnetic field in MT testOne of the most important technical points in the inspection of magnetic particles is the direction of the magnetic field relative to the direction of the defect.
To achieve the best sensitivity, the magnetic field should be approximately perpendicular to the probable direction of the discontinuity.For example, if the crack is along the length of the part, creating a transverse field can give a better indication.For this reason, bidirectional magnetization is commonly used in sensitive inspections to reduce the chance of missing cracks with different orientations..This issue is one of the key points in the design and correct implementation of the MT test.
Methods of creating a magnetic field in MTDepending on the type of part, equipment and geometry under test, there are different ways to create a magnetic field.Magnetic yoke method (Magnetic Yoke)The magnetic yoke is one of the common equipment in the inspection of welds and steel parts.By creating a local magnetic field, this equipment provides the possibility of inspecting different areas of the part, and it is especially useful in inspecting corner welds, joints and relatively large parts..
Direct current method (Direct Contact / Head Shot)In this method, electric current is passed through the part and the magnetic field created around the current path is used to identify discontinuities..
Induction current method and central conductorFor some specific geometric parts, such as circular or cylindrical parts, suitable field induction methods or the use of a central conductor can be used..Choosing the right method depends on the geometry of the part, the material, the type of expected defect, the direction of the defect and the level of sensitivity required.dry particles or wet particles; Which is better?Magnetic particles used in MT are usually divided into two main groups:Dry Magnetic ParticlesIn the dry powder method, the particles are applied directly to the surface.This method can be suitable for situations where the surface is relatively rough or when the inspection is done on site.Wet Magnetic ParticlesIn the particulate method, magnetic particles are suspended in a liquid carrier and applied to the surface.The Wet Fluorescent method usually has a high sensitivity for detecting subtle signs and is used in many industrial quality control processes..
What is the fluorescent MT test?
In Fluorescent Magnetic Particle Testing, fluorescent magnetic particles are used.These particles are exposed to UV-A radiation under suitable conditions and the resulting signs are visible with high contrast..This method can be very effective for detecting subtle discontinuities, especially in controlled inspection environments..Of course, its correct performance depends on factors such as ambient light conditions, appropriate intensity of UV-A light, surface condition, particle concentration, and correct equipment adjustment..Application of magnetic particle test in welding inspectionOne of the most important applications of MT is the inspection of welds and welded joints.After welding, some surface and near-surface cracks or discontinuities may develop in the weld zone, HAZ, or base metal..MT test can check things like:Weld surface cracksLongitudinal cracksTransverse cracksShrinkage cracksGrinding cracksSome inconsistencies related to Lack of FusionDiscontinuities near the surfacebe used.
However, MT is not suitable for all weld defects and the choice of NDT method should be based on material type, defect type, part geometry and project standard requirements..
Advantages of magnetic particle inspectionAmong the most important advantages of MT, the following can be mentioned:
Fast detection of defects: The inspection process is relatively quick in many applications.Good sensitivity to cracks: Especially the cracks that are on the surface or near the surface.
The possibility of inspecting different geometries: By choosing the right equipment and magnetizing method.Relatively reasonable cost: Compared to some more advanced NDT methods.No damage to the piece: As with other non-destructive methods, if performed correctly, the part can be reused.Suitable for welding inspection: One of the main applications of this method is in various industries.MT test limitationsDespite its significant advantages, MT is not a general method for all materials and all types of defects.Its most important limitation is that it is only suitable for ferromagnetic materials.
So usually for materials like:Carbon steelsMany low alloy steelsIron and some ferromagnetic alloyscan be used.But for materials such as aluminum, copper and many austenitic stainless steels, MT cannot be used in the usual way..
In such cases, methods such as PT, UT or RT may be more suitable options.Also, MT is not an ideal method for deep defects inside the part that are far from the surface.MT compared to PTOne of the common questions In the technical inspection, is the test MT or PT better?The answer depends on the material of the part and the type of defect.PT (Liquid Penetrant Testing) It can be used for a very wide range of materials, including non-magnetic materials, and mainly reveals defects that are exposed to the surface..In contrast, MT is only applicable to ferromagnetic materials, but it can reveal near-surface discontinuities in addition to some surface defects..Therefore, the choice between MT and PT should be made based on the actual condition of the part and the technical requirements of the project.
TestMT in inspection and quality control systemMagnetic particle testing should not be considered a separate process from other technical inspection and quality control activities.In industrial projects, MT can be part of a comprehensive NDT Inspection program that, depending on project requirements, includes methods such as:Visual Testing (VT)Liquid Penetrant Testing (PT)Magnetic Particle Testing (MT/MPI)Ultrasonic Testing (UT)Radiographic Testing (RT)and in specific applications of more advanced NDT methodsis.Choosing the right combination of NDT methods depends on the material, welding procedure, type of equipment, risk level, design standard and acceptance criteria..
Importance of inspector qualification and equipment in MTThe quality of the test result does not depend only on the equipment itself. Qualification and experience of the inspector, implementation method, surface preparation, correct choice of technique, field intensity, field direction, type of particles and correct interpretation of signs all affect the result..For this reason, the implementation of MT should be done according to the relevant guidelines and standards and by qualified people.Also, inspection equipment should be kept in proper conditions and calibrated and verified if needed.
Standards related to magnetic particle testingThe standard used should be selected according to the type of project, industry and contract. from Standardand technical documents known in this field can include things like:ASME Section VASTM E709ASTM E1444/E1444MISO 9934pointed out.
Of course, simply using a standard is not enough; Defect acceptance criteria may be specified in the design standard, welding code, project specifications or contractual documents.
Inspection of magnetic particles (MT/MPI) One of the important and practical methods in the family Non-destructive tests (NDT) It is used to identify cracks and other surface and near-surface discontinuities in ferromagnetic materials..The basis of this method is the creation of a magnetic field, the creation of flux leakage at the discontinuity location and the accumulation of magnetic particles at the leakage location.
Relative simplicity, appropriate speed, favorable sensitivity to cracks and the ability to be used in the inspection of welds and steel parts have made MT one of the important methods in quality control programs, technical inspection and ensuring the safety of industrial equipment..However, obtaining a reliable result requires the selection of the right technique, the correct direction of the magnetic field, the proper preparation of the surface, the control of the test conditions, the reliable equipment, the correct implementation of the instructions and the expert interpretation of the results..Ultimately, MT creates the most value when used as part of a comprehensive NDT and technical inspection program tailored to the risk and requirements of each project..