High-Frequency MFL vs Traditional Low-Frequency: How to Choose for Thick-Wall Steel Pipes

Can I use magnetic flux leakage testing on my thick walled steel pipe? Is the sensitivity sufficient? Can the speed keep up with the production line? ”
High frequency leakage magnetic technology achieves a key breakthrough in the detection of defects on the inner wall of thick walled pipes and high-speed flaw detection by increasing the excitation frequency and focusing the magnetic field energy on the near surface of the workpiece using the skin effect.
II. Technical Principles – Starting with the "Skin Effect"

2.1 What Is the Skin Effect?

The distribution of an alternating magnetic field in ferromagnetic materials is non‑uniform: the higher the frequency, the more the field tends to concentrate near the workpiece surface; the lower the frequency, the deeper the penetration. This is the most fundamental difference between high‑frequency and low‑frequency MFL.

The physical foundation of high‑frequency MFL (with operating frequencies reaching 10 kHz–100 kHz) is precisely the skin effect. Under a 100 kHz excitation field, the near‑surface region of the workpiece exists in a magneto‑quasistatic state. Above the crack, there is not only an AC leakage magnetic field but also a perturbing field induced by eddy currents, resulting in richer defect signal characteristics. In contrast, conventional AC MFL inspection typically operates within a frequency range of 1 kHz–10 kHz.

In one sentence: low‑frequency "sees deep but not clearly," while high‑frequency "sees clearly but not as deep"—yet through technical optimization, high‑frequency has already found a breakthrough in inspecting the internal walls of thick‑walled pipes.

2.2 Differences in Detection Mechanisms: High‑Frequency vs. Low‑Frequency MFL

Studies have shown that under high‑frequency excitation, electromagnetic field scattering is suppressed, which reduces background noise caused by surface roughness. This enables reliable detection of micro‑cracks on ground surfaces with Ra ≈ 100 μm and flatness tolerance of approximately 1 mm.

III. Performance Comparison Data: One Table Shows the Gap

The following data are based on actual comparative tests between a domestically produced high‑frequency MFL system and mainstream imported low‑frequency solutions (represented by Tuboscope and Foerster):

Key Takeaways:

• Inspection wall thickness doubled. The high‑frequency MFL system can inspect wall thicknesses up to 25 mm—nearly 1.7 times that of imported equipment (15 mm). For steel pipes with wall thickness exceeding 15 mm, conventional low‑frequency solutions reach their limit, while the high‑frequency solution delivers stable detection.

• Internal crack detection in thick‑wall pipes achieves a "zero‑to‑one" breakthrough. For 25 mm‑wall pipes, neither Tuboscope nor Foerster can detect internal cracks, while the high‑frequency MFL system achieves a sensitivity of 15% of wall thickness (approximately 3.75 mm crack depth). This is a qualitative leap.

• Inspection speed doubled. 3.0 m/s vs. 1.5 m/s—for domestic oil‑well pipe manufacturers whose production lines already run at 8 m/s, a doubled inspection speed means that the inspection step no longer bottlenecks the entire production line.
  

  IV. Another Key Advantage of High‑Frequency MFL: Resistance to Rough Surface Interference

  In actual steel pipe production scenarios, workpiece surfaces often exhibit varying degrees of roughness and flatness fluctuations—a primary source of noise in conventional MFL inspection.

  High‑frequency MFL technology offers a distinct advantage in this regard: under high‑frequency excitation of 100 kHz and above, the scattering effect of the electromagnetic field on the workpiece surface is effectively suppressed, reducing background noise caused by roughness and improving the signal‑to‑noise ratio. Experiments have demonstrated that a flexible probe using array‑wire magnetization for high‑frequency AC MFL inspection can suppress lift‑off fluctuation interference during inspection, enabling stable detection of 0.3 mm‑deep and 0.3 mm‑wide cracks on rough ground surfaces (Ra ≈ 100 μm).

  For scenarios demanding even higher sensitivity, a high‑sensitivity probe configuration using dual‑layer array‑wire magnetization and tangential magnetic flux density measurement can reduce background magnetic field noise by 50% and increase detection sensitivity by 2.4 times—making it possible to effectively detect micro‑cracks as shallow as 15 μm.

  V. High‑Frequency Is Not a Panacea – Selection Recommendations

  High‑frequency MFL technology also has its applicable boundaries. Scientific research has confirmed that high‑frequency excitation generates significant eddy‑current effects, which under certain conditions may lead to attenuation or even reversal of the leakage magnetic field signal from internal surface defects.

  Core Recommendations for Customers:

  Product Characteristics	Recommended Solution	Rationale
  Wall thickness < 10 mm	Conventional low‑frequency MFL	Low cost, mature technology
  Wall thickness 10–15 mm, routine inspection	Either low‑ or high‑frequency	Both options can meet the requirements
  Wall thickness 15–25 mm, internal defects need to be detected	High‑frequency MFL recommended	Low‑frequency solutions lack sufficient sensitivity
  Wall thickness > 25 mm	UT + high‑frequency MFL combined	A single MFL solution cannot fully cover all requirements
  Production line speed > 2 m/s	High‑frequency MFL recommended	Low‑frequency inspection speed cannot keep up
  Inspection of rough‑surface workpieces	High‑frequency MFL recommended	High frequency suppresses surface scattering noise