Magnetic Particle Testing (MT)

Magnetic Particle Testing (MT) is a widely used non-destructive testing (NDT) method for detecting surface and near-surface discontinuities in ferromagnetic materials. It is quick, reliable, and especially effective for finding cracks and other linear defects.

Overview

MT works by magnetizing a ferromagnetic component and then applying fine magnetic particles (dry powder or suspended in liquid). If a surface or near-surface flaw is present, it disturbs the magnetic field and creates a leakage field. The particles are attracted to this leakage, forming a visible indication of the defect.

Apparatus and Working

Apparatus

  • Magnetizing equipment: Yokes, prods, coils, or central conductors to induce magnetism.
  • Magnetic particles: Dry powder or wet suspension (visible or fluorescent).
  • Carrier medium: Water or oil for wet suspensions.
  • Light source: White light for visible particles; UV light for fluorescent particles.
  • Accessories: Demagnetizers, field indicators, and measuring gauges.

Working Steps

  1. Surface preparation: Clean the surface of oil, grease, and scale.
  2. Magnetization: Apply magnetic field using yoke, coil, or current flow.
  3. Application of particles: Apply dry powder or wet suspension while magnetized.
  4. Indication formation: Particles gather at leakage fields caused by defects.
  5. Inspection: Examine under proper lighting; indications reveal defect locations.
  6. Demagnetization: Remove residual magnetism after testing.

Principle

MT is based on the principle that when a ferromagnetic material is magnetized, discontinuities such as cracks or voids interrupt the magnetic flux. This causes a leakage field at the defect site. Finely divided magnetic particles applied to the surface are attracted to these leakage fields, forming visible indications of the defect’s size, shape, and location.

Advantages and Disadvantages

Advantages

  • Detects both surface and near-surface defects.
  • Quick, simple, and relatively inexpensive.
  • Provides immediate results.
  • High sensitivity to fine cracks and linear defects.
  • Portable equipment available for field use.

Disadvantages

  • Only applicable to ferromagnetic materials (iron, steel, nickel, cobalt alloys).
  • Surface preparation is required for reliable results.
  • Depth of detection is limited (best for surface and shallow subsurface flaws).
  • Requires demagnetization after testing in many cases.
  • Interpretation depends on operator skill and experience.

Applications

  • Weld inspection: Detecting cracks, lack of fusion, and other surface-breaking flaws.
  • Automotive: Testing crankshafts, gears, axles, and engine components.
  • Aerospace: Inspection of landing gear, turbine shafts, and structural parts.
  • Power plants: Examination of rotors, shafts, and pressure parts.
  • General engineering: Quality control of forgings, castings, and machined parts.

Welding Defects Detectable by MT

  • Surface cracks: Hot cracks, cold cracks, and stress-induced cracks.
  • Lack of fusion (surface-breaking): Areas where weld metal has not fused with base metal.
  • Incomplete penetration (surface-reaching): Root defects open to the surface.
  • Undercut: Groove-like defects along the weld toe.
  • Overlap: Excess weld metal lying on the surface without fusion.
  • Seams and laps: Rolling or forging defects extending to the surface.
  • Grinding cracks: Cracks formed during finishing operations.