If you’ve ever worked with steel and wondered why your eddy current signals drift, vanish, or don’t match your calibration standard, the answer might be hiding in plain sight: magnetic memory.

In technical terms, it’s called magnetic hysteresis—but in practical terms, it’s a lot like muscle memory. And just like your muscles, steel “remembers” what it’s been through.

Let’s break it down.

💪 Muscle Memory Meets Magnetic Hysteresis

When you go to the gym and train a specific movement, your nervous system and muscle fibers start to adapt. The more you do it, the easier it becomes to repeat—even after a break.

Steel does something similar with magnetic fields.

When a magnetic field is applied, the material’s internal domains (tiny regions that act like magnetic dipoles) align.
When the field is removed, they don’t all go back to zero.
The result: residual magnetism or remanence—a form of magnetic “muscle memory.”

This retention of past magnetic alignment is what we call magnetic hysteresis.

🔁 The Hysteresis Loop: A Replay of Magnetic History

The B-H curve (magnetic induction vs. applied field) is a loop, not a line. That loop records the material’s previous magnetic exposure, including:

How strong the last field was
Whether it was reversed or cycled
Whether the material was driven into magnetic saturation

Just like a well-trained athlete returns to good form quickly, steel that has been previously magnetized responds differently the next time you test it. And that’s where the eddy current challenges begin.

⚠ Why It Messes With Eddy Current Testing

In ECT, we rely on stable electromagnetic conditions—particularly in materials like carbon steel and ferritic stainless steel. But when a tube or bar has magnetic memory, your test results are no longer clean or predictable.

Here’s how magnetic memory can interfere:

Shifted baselines: The residual magnetism alters the coil’s impedance, making it hard to detect small flaws.
Variable responses: Two identical tubes may give different signals due to differing magnetic histories.
Inconsistent lift-off effects: The field coupling is influenced by prior magnetization, not just geometry.
Weld or forming zones with residual stress/magnetism behave unpredictably on the signal display.

🧲 The Fix: Resetting the Magnetic Memory

To ensure repeatable ECT on steel, professionals often need to reset or override this magnetic muscle memory.

Here’s how:

1. Degaussing (Demagnetization):

Apply an alternating magnetic field that slowly decays in amplitude.
This randomizes the internal domains and reduces remanence to near zero.
Often used before ECT or magnetic particle testing.

2. DC or AC Magnetic Saturation:

Apply a strong, steady field (DC) or cyclic peak field (AC) to drive the steel into saturation.
When fully saturated, the material operates in a region where its incremental permeability is stable.
This allows consistent eddy current response, even in magnetically complex materials.

3. Probe Design Compensation:

Some systems use phase gating, signal subtraction, or specialized coil geometry to minimize effects of hysteresis.

🧪 What Are You Actually Seeing in Steel?

When you perform ECT on steel, you’re not just measuring conductivity or crack signals.

You’re interacting with the material’s magnetic past—a memory etched into its structure by every field, bend, weld, or heat cycle it has experienced.

Just like an athlete carries the scars and gains of every training session, steel carries the electromagnetic imprint of its history.

🧰 Final Takeaway

If you’re testing steel and your signals are misbehaving, ask yourself:

Has this tube been cold worked?
Was it previously magnetized?
Is there a weld seam nearby?
Could hysteresis be skewing my signal?

Understanding magnetic memory isn’t optional—it’s essential.

Bonus Tip:

Want to see this in action? Try comparing ECT signals from two “identical” steel samples—one freshly annealed, the other cold drawn or formed. Watch how the signals shift. You’re not seeing flaws—you’re seeing memory.

And now, you know what to do about it.

🧭 For more deep dives into eddy current theory and history, visit eddycurrent.com—the best place to train your mind and your magnetism.

🧠 Magnetic Muscle Memory: Why Steel Remembers—and How It Affects Your Eddy Current Test