How does direct magnetization differ from indirect magnetization?

Direct magnetization and indirect magnetization are two distinct methods used in the magnetic particle testing process to detect surface and slightly subsurface defects in ferromagnetic materials.

In direct magnetization, a current is passed directly through the specimen, which generates a magnetic field around the area of interest. This method creates a magnetic field that aligns with the material itself—enabling defects such as cracks or inclusions to become apparent when magnetic particles are applied. The use of direct magnetization allows for a strong and easily controlled magnetic field, making it effective for detecting defects in the immediate vicinity of the surface.

Conversely, indirect magnetization utilizes coils placed around the specimen to create a magnetic field. Instead of applying current through the material, this method relies on electromagnetic induction to magnetize the part. The magnetic field generated is similar in effect to that of direct magnetization, but it operates without direct electrical contact, which can be beneficial under specific circumstances or for certain geometries.

Understanding the distinction between these two methods is crucial for selecting the appropriate testing technique based on the material and defect types being examined.