Magnetic Particle Inspection

Introduction

Magnetic particle inspection (MPI) is a nondestructive testing method used for defect detection. 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.

Wet Fluorescent Magnetic Particle (MT) Inspection Services

Wet Fluorescent magnetic particle testing is more accurate than dry powder testing for detecting very small discontinuities, because the particles are smaller. Pretect uses wet fluorescent magnetic particle inspections to locate surface and slightly subsurface discontinuities.

An advantage of wet fluorescent MT is its ease and speed of application, given that technicians can quickly spray the chosen magnetic particle fluid over large surface areas. It is extremely sensitive to even extremely small and fine indications, increasing indication detection with consistent and reliable results while minimizing inspection time. This method can be used for in-service inspections, making it possible to detect asset damage without halting production.

Wet Fluorescent MT Inspection Applications

• Fatigue Cracks
• Flakes
• Grinding Cracks
• Inclusions in aerospace blooms, billets, and bars
• Laps
• Quenching Cracks
• Seams
• Shrink Cracks
• Stress Corrosion Cracking
• Tears
• Welding Defects

Dry Powder Magnetic Particle (MT) Inspection Services

Dry particles are dusted onto the surface of the test object as the item is magnetized. The clustering of the magnetic particles is efficient for detecting defects on rough surfaces, including shallow and subsurface cracks, along with root penetration. Dry powder methods are more portable than wet fluorescent MT.

Particles of a variety of different sizes can be used, with small particles being more sensitive to small discontinuities, while larger particles offer a stronger resistance to contamination from dirt or surface debris. This method is particularly suited for detecting leakage fields from small flaws.

Dry Powder MT Inspection Applications

• Defects in Rough Surfaces
• Lack of Fusion in Welds
• Penetration
• Shallow Cracks

Magnetic Particle Inspection Benefits

• Can detect both surface and near-surface indications
• Can inspect irregularly-shaped parts
• Pre-cleaning of the asset to be inspected is less essential than in other inspection methods, as existing contaminants should not affect flaw detection
• Quick to be employed and lower cost than other NDT methods
• Real-time results, as indications are visible directly on the material’s surface
• Portable inspection method makes it ideal for field inspection services