The differences and applications of ultrasonic testing, eddy current testing and magnetic particle

This question is very well-targeted. The core difference among the three flaw detection techniques lies in their detection principles, which directly determines the types of defects they can detect and the applicable scenarios. It is of crucial importance for the quality control of stainless steel seamless pipes.

The core conclusion is: Ultrasonic flaw detection focuses on internal defect detection, eddy current flaw detection is good at rapid screening of surface / near-surface defects, and magnetic particle flaw detection is only applicable to surface / near-surface defect detection of ferromagnetic stainless steel.

1. Core Difference Comparison (Table)

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II. Core purposes of each inspection method

1. Ultrasonic Testing (UT): Internal Quality "Lens"

Mainly used for detecting deep internal defects in stainless steel seamless pipes, which are inaccessible to eddy current and magnetic particle testing.

Typical application scenarios:

Factory inspection of pressure-bearing pipes such as high-pressure boiler tubes and oil and gas transportation pipes, ensuring no internal cracks or inclusions to prevent rupture during pressure application.

Quality inspection of thick-walled stainless steel pipes, focusing on checking internal stratification and pores generated during rolling or forging processes.

For suspicious signals detected by eddy current and magnetic particle testing, further use ultrasonic testing to confirm the depth and nature of the defects.

2. Eddy Current Testing (ET): Surface Quality "Quick Screening"

Core advantages are non-contact and high efficiency, suitable for surface/near-surface defect screening in mass production.

Typical application scenarios:

Continuous online inspection of stainless steel seamless pipe production lines, capable of detecting tens of meters per minute, quickly eliminating substandard products with surface cracks or scratches.

Full inspection of thin-walled stainless steel pipes, as surface defects of thin-walled pipes have a significant impact on strength and magnetic particle testing does not require damaging the pipe material.

Detection of internal or external wall corrosion of pipelines after installation (requires special probes), without disassembling the pipeline.

3. Magnetic Particle Testing (MT): Ferromagnetic Steel Parts "Magnifying Glass"

Only applicable to ferromagnetic stainless steel, the advantage is that it can directly observe the defect shape, suitable for fine inspection of key parts.

Typical application scenarios:

Detection of fatigue cracks (such as stress cracks after long-term use) on surface or near-surface of valve, flange, etc. structural components made of martensitic stainless steel.

Detection of welding joints of ferritic stainless steel pipes, checking surface cracks generated during welding (needs to magnetize the welding area first).

Detection of surface micro-cracks on ferromagnetic stainless steel pipes that have been in use for a period of time, caused by wear or corrosion. 

III. Selection Suggestions

If you need to detect internal defects (such as thick-walled pipes, pressure-bearing pipes): Ultrasonic testing should be the first choice.

If you need to conduct rapid and batch screening of surface defects (such as full inspection on the production line): Eddy current testing should be the first choice.

If you need to detect surface / near-surface defects of ferromagnetic stainless steel (such as structural components, welding joints): Magnetic particle testing should be the first choice; austenitic stainless steel should be excluded from this option.