Magnetic Particle Testing

Magnetic penetrant testing

Magnetic particle inspection (MPI) is a widely used to detect flaws in ferromagnetic materials. It is used to locate surface and near surface defects such as cracks, laps, seams and inclusions.

MPI is fast and relatively easy to apply, and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods.

MPI uses magnetic fields and small magnetic particles (i.e. Iron filings) to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective.

The method is used to inspect a variety of product forms including castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use.

Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines.

Magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing.

Magnetic flux leakage

Consider the case of a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole.

When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack.

Magnetic flux leakage

The magnetic field exits the north pole and reenters at the south pole. The magnetic field spreads out when it encounters the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus is called a flux leakage field.

If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.


The first step in a magnetic particle inspection is to magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, either in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect.

Types of Magnetism :

There are two types of magnetism used in magnetic particle testing these are :

  • Longitudinal magnetism
  • Circular magnetization

Longitudinal Magnetization

A longitudinal magnetic field has magnetic lines of force that run parallel to the long axis of the part. Longitudinal magnetization of a component can be accomplished using the longitudinal field set up by a coil or solenoid. It can also be accomplished using permanent magnets or electromagnets. Longitudinal magnetic cannot detect the defects which are parallel to the magnetic field .There are 4 methods of conducting longitudinal magnetism they are:

  • Coil Inspection Stationary Unit (AC or DC)
  • Wrapped Coil
  • AC Coil – Portable
  • Yoke

Circular Magnetization

Circular magnetization, Part with no discontinuity, no poles are created and no flux lines contained in the part. If the part is with surface discontinuity, poles are formed and flux leakage occurs. Circular magnetization will show longitudinally orientated flaws.

Circular magnetization can be conducted any one of method by direct or indirect.

    Direct :

    • Head shot
    • Probs

    Indirect :

    • Central conductor
Circular magnetization

Advantages of Magnetic Particle Testing :

  • It does not need very stringent pre-cleaning operation.
  • Best method for the detection of fine, shallow surface cracks in ferromagnetic material.
  • Fast and relatively simple NDT method.
  • Generally inexpensive.
  • Will work through thin coating.
  • Few limitations regarding the size/shape of test specimens.
  • Highly portable NDT method.
  • It is quicker.

Limitations of Magnetic Particle Testing :

  • Material must be ferromagnetic.
  • Orientation and strength of magnetic field is critical.
  • Detects surface and near-to- surface discontinuities only.
  • Large currents sometimes required.
  • ‘Burning’ of test parts a possibility.
  • Parts must often be demagnetized, which may be difficult.

Field Indicator (Pie Gauge) :

Field Indicator / Pie Gauge

Device determines the approximate orientation and, to limited extent, indicates the adequacy of field strength. Presence of multiple gaps at different orientations helps reveal the approximate orientation of the magnetic flux (field).

Demagnetization

After conducting a magnetic particle inspection, it is usually necessary to demagnetize the component.

Dowload Sample Report and Procedure of Magnetic Particle Testing

Magnetic Particle Testing NDT Sample Procedure
Magnetic Particle Testing NDT Sample Test Report Format

This course covers the principles of Magnetic Particle Testing and prepares a candidate to

  • Setup and calibrate equipment
  • Interpret and Evaluate Results with respect to Applicable Codes, Standards and Specifications.
  • Familiar with the scope and limitations of the Methods
  • Write test reports

Responsibilities of ASNT Level II Magnetic Particle Testing (MPT) Personnel

  • A certified ASNT Level II Magnetic Particle Testing (MPT) personnel is qualified to select proper Magnetic Particle Testing (MPT) technique and equipment as per established procedure.
  • A certified ASNT Level II Magnetic Particle Testing (MPT) personnel is qualified to Calibrate the equipment and carry out test.
  • A certified ASNT Level II Magnetic Particle Testing (MPT) personnel is qualified to Perform Magnetic testing based on the standard procedure followed by ASNT.
  • A certified ASNT Level II Magnetic Particle Testing (MPT) personnel is qualified to Interpret the ultrasonic test results as per applicable standards.
  • A certified ASNT Level II Magnetic Particle Testing (MPT) personnel is qualified to Prepare Magnetic Particle Testing (MPT) report for acceptance/rejection Criteria.
  • A certified ASNT Level II Magnetic Particle Testing (MPT) personnel is qualified to Know about the merits and demerits of Magnetic Particle Testing (MPT) and other common NDT methods.
  • A certified ASNT Level II Magnetic Particle Testing (MPT) personnel is qualified to Prepare written instruction for Level I personnel.
  • A certified ASNT Level II Magnetic Particle Testing (MPT) personnel is qualified to Maintain and handle the Magnetic Particle Testing (MPT) equipment carefully.

Module 1: Manufacturing Discontinuities

  • Types of Discontinuities: Inherent, Processing and Service.
  • Primary Processing Discontinuities including discontinuities in Rolling, Forging, Drawing, Extruding.
  • Secondary Processing Discontinuities including discontinuities in Grinding, Heat Treating, Machining, Welding, Plating.
  • Service Discontinuities: Erosion, Wear, Fatigue, Corrosion, Creep, Hydrogen Attack.

Module 2: Theory of Magnetism

  • Magnetic field, Lines of force, Flux density.
  • Definitions of Permeability, Reluctance, Retentivity, Residual Magnetism and CoerciveForce.
  • Diamagnetic, Paramagnetic and Ferromagnetic materials.
  • Leakage flux.
  • Fleming’s Right Hand and Left Hand Rule ,Types of Magnetic Fields: Circular, Longitudinal, Vector.
  • Hysteresis Curve.

Module 3: Methods of Magnetization

  • Magnetization By Means of Electric Current.
  • Types of current AC, HWDC.
  • Circular field: Head Shot (Direct Contact), Prods and Central Conductor Techniques, offset Central Conductor.
  • Advantages and disadvantages of circular field.
  • Longitudinal field: Coils and Yoke.
  • Advantages and disadvantages of Longitudinal Field.
  • AC and DC Field Distribution in a Magnetic and a Nonmagnetic Conductor.
  • Demagnetization.

Module 4: Equipment

  • Equipment consideration.
  • Wet Horizontal, Mobile and Portable Equipments.
  • Fluorescent testing, Black Light.
  • Accessories.
  • Advantages and disadvantages of Longitudinal Field.
  • AC and DC Field Distribution in a Magnetic and a Nonmagnetic Conductor.
  • Demagnetization.

Module 5: Mediums and their Preparation

  • Dry and Wet method
  • Particles: Dry and Wet
  • Properties of particles
  • Visibility of particles
  • Methods of Application
  • Contamination of Magnetic Particles
  • Settling Test Procedure
  • Settling Test Procedure
  • Bath Maintenance

Module 6: Applications

  • Residual and Continuous Method
  • Magnetic Particle Inspection of Solid Cylindrical Parts, Gears, Multiple diameter Articles, Discs, Hollow Cylindrical Articles.
  • Selection of proper method of magnetization
  • Verification of magnetic fields
  • Checking the adequacy of field using the Pie gauge, shims
  • Magnetic Rubber Inspection

Module 7: Types of indications

Interpretation including Relevant, False, Non-relevant indications

Module 8: Codes and Standards (Specific Training)

MT Inspection Procedures

  • ASME Section V Article 7 2004
  • ASME Section VIII (Accept/Reject Criteria)
  • ASME B 31.1 – Power Piping
  • ASME B 31.3 – Petrochemical Piping
  • ASTM E-709
  • ASTM E-1444
  • Other codes and standards can be discussed if prearranged with the instructor at the time of registration.