What are the advantages and disadvantages of eddy current testing and magnetic particle testing when

Advantages and disadvantages of eddy current testing and magnetic particle testing for stainless steel seamless pipes: 

I. Advantages and Disadvantages of Eddy Current Testing (ET) Advantages

Non-contact, no need for coupling agent, facilitating high-speed and automated online inspection, suitable for large-scale pipe screening; applicable to conductive metals (including ferromagnetic and non-ferromagnetic stainless steels).

Highly sensitive to surface and near-surface defects (such as cracks, folds, inclusions, holes, etc.), can provide defect location and relative size, and can achieve digital recording and storage.

Can be used for conductivity measurement, evaluation of film/coating thickness and sheet thickness, with strong functional expansion capability.

Can be detected in high-temperature or narrow areas, with good adaptability.

Limitations

Only applicable to conductive materials; insensitive to deep internal defects, usually used for surface/near-surface detection.

Sensitive to factors such as material conductivity/magnetic conductivity, geometric shape, separation, temperature, etc., qualitative/quantitative interpretation is relatively difficult, and reasonable setting of frequency and filtering is required.

The detection consistency decreases for complex shapes/areas with large curvature changes; the results are mainly in the form of electrical signals, and the intuitiveness is not as good as MT. 

II. Advantages and Disadvantages of Magnetic Particle Inspection (MT) Advantages

It has extremely high sensitivity to surface and near-surface cracks and other defects in ferromagnetic materials. The magnetic traces are clearly visible, making it easy to locate, determine the shape/size/location.

The operation is simple and the cost is low. It is suitable for rapid on-site screening and large-scale component inspection; it can also be applied to complex geometric components.

It can be combined with fluorescent magnetic powder and ultraviolet to improve the detection rate of tiny cracks. The application range is wide (pipes, rods, welds, forgings, etc.).

Limitations

It is only applicable to ferromagnetic materials (such as carbon steel, low alloy steel, some martensitic/precipitation hardening stainless steel); it is not applicable to austenitic stainless steel (such as 304, 316).

It can only detect surface/near-surface defects and is powerless against internal deep defects; it is sensitive to the direction of the defect (too small angle with the magnetic lines makes it difficult to detect).

It has a high requirement for surface conditions (pre-treatment of oil stains, rust, coatings is necessary); false defects may occur (scratches, magnetic writing, etc.); some workpieces need demagnetization and post-cleaning. 

III. Recommendations for Selection on Stainless Steel Seamless Pipes

Austenitic stainless steel seamless pipes (such as 304/316): Preferentially use ET for online/offline surface and near-surface defect screening; for internal defect assessment, switch to UT.

Ferritic stainless steel or martensitic/ferritic stainless steel seamless pipes: Surface/near-surface cracks are prioritized with MT (intuitive and efficient); for high-speed 100% online screening or material/electrical conductivity sorting, ET can be combined.

Confirmation of surface cracks at stress concentration areas such as pipe ends and threaded zones: MT has a more intuitive advantage; ET can be used for rapid preliminary screening and process control.

In scenarios where only the tightness (not strength) is verified, water pressure testing or ET can be considered as alternatives; if strength verification is involved, water pressure testing cannot be substituted.