Actual engineering data confirm this: for steel pipes with a wall thickness of 15–20 mm, the detection sensitivity for internal wall defects only reaches 15–20% of the wall thickness. If your products fall outside this thickness range, low‑frequency MFL may no longer be an adequate solution.
High‑Frequency vs. Low‑Frequency MFL: What Are the Technical Differences?
| Comparison Dimension | Conventional Low‑Frequency MFL | High‑Frequency MFL |
|---|---|---|
| Operating Frequency | 50–200 Hz | 400 Hz and above, up to 10 kHz–100 kHz |
| Magnetic Field Distribution | Deep penetration, full‑wall‑thickness magnetization | Energy concentrated in the near‑surface region |
| Sensitivity to Surface Defects | Relatively low | High (magnetic field is focused on the surface) |
| Detection of Internal Wall Defects | Suitable for thin‑wall pipes | Capable of detecting internal defects in thick‑wall pipes (with appropriate parameter settings) |
Actual Comparative Data Between Domestic High‑Frequency MFL System and Mainstream Imported Low‑Frequency Solutions
| Parameter | Conventional Low‑Frequency Solution (Imported Equipment) | High‑Frequency MFL Inspection System |
|---|---|---|
| Maximum Inspectable Wall Thickness | 15–20 mm | 25 mm |
| Surface Crack Sensitivity | 5% of wall thickness | 5% of wall thickness |
| Internal Crack Sensitivity (15 mm wall) | 12.5%–15% | 10% |
| Internal Crack Sensitivity (25 mm wall) | Not detectable | 15% |
| Maximum Inspection Speed | 1.5 m/s | 3.0 m/s |
For steel pipes with a wall thickness of 25 mm, the conventional low‑frequency solution fails to detect internal cracks entirely. In contrast, the high‑frequency MFL system achieves a detection sensitivity of 15% of wall thickness for internal cracks. Moreover, the inspection speed is doubled (3.0 m/s vs. 1.5 m/s), which means that for high‑speed production lines, the inspection step no longer constitutes a bottleneck.
High‑Frequency MFL Is Not a "One‑Size‑Fits‑All" Solution – Selection Should Be Based on Application Scenarios
High‑frequency MFL technology also has its applicable boundaries. According to academic research, high‑frequency excitation produces 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. This implies:
High‑frequency solutions are better suited for surface / near‑surface defect detection and specialized inspection of internal walls in thick‑wall pipes.
Low‑frequency solutions still hold irreplaceable value for full‑wall‑thickness inspection and thin‑wall pipe testing.
Wall thickness < 15 mm: Conventional low‑frequency solutions can meet most inspection requirements.
Wall thickness 15–25 mm: High‑frequency solutions offer clear advantages, especially in scenarios requiring precise detection of internal wall defects.
High‑speed production line inspection: High‑frequency solutions are the preferred choice for online inspection at elevated line speeds.
