Why Steel Acts Weird: Understanding Magnetic Permeability, Hysteresis, and Saturation in Eddy Current Testing

By Ed Korkowski, eddycurrent.com

Ever wonder why some eddy current signals go haywire when testing steel tubing? Or why some flaw signals vanish entirely when you switch to ferromagnetic materials? Blame it on magnetic permeability—a property that sounds simple but hides a world of complexity, especially when ferromagnetism is involved.

Today, let’s break down a few terms that every electromagnetic testing professional should understand—but many never learned properly:

🧲 What Is Magnetic Permeability, Really?

At its core, magnetic permeability (μ) describes how easily a material allows magnetic fields to pass through it. In non-magnetic metals like aluminum or copper, μ is almost the same as that of free space. But in ferromagnetic metals like steel, μ can be hundreds or thousands of times higher—and wildly inconsistent.

What’s worse? It’s not even a single number.

🔍 Five Faces of Permeability

According to classic magnetic materials research, permeability comes in at least five flavors—and each one matters differently in ECT:

1. Initial Permeability (μᵢ):How easily the material magnetizes from rest. This governs probe coupling in low-field tests.

2. Normal Permeability (μₙ):The average slope of the B-H curve from the origin to any point. It’s a rough guide—but not very predictive during cycling.

3. Maximum Permeability (μₘ):The steepest part of the B-H curve—where the material most readily accepts field. But blink, and it’s gone.

4. Incremental Permeability (μᵢₙc):The slope between two close points on the hysteresis curve. This is what ECT systems actually “see” as your test signal oscillates in a narrow window.

5. Reversible Permeability (μʳ):A tiny slice of μᵢₙc at low amplitudes. Think of it as how magnetically “elastic” the material is under tiny field changes.

   
   

📉 Why Permeability Kills Your Signal

In ferromagnetic tubing, permeability isn’t fixed. It shifts with:

• Cold work and strain

• Weld zones

• Heat treatment

• Internal stress

• Prior magnetic history (sort of like muscle memory).

That means two identical-looking tubes might have wildly different incremental μ values—causing your eddy current bridge to wobble, your flaw signals to shift baseline, or your entire inspection to fail.

🔁 Enter Hysteresis

Hysteresis is the magnetic memory effect. As you cycle magnetic fields, the material lags behind—just like a spring that never quite returns.

This looped behavior means:

• Residual magnetism remains after the field is removed

• Every field reversal drags the signal across a different path

• Tiny changes in material history (e.g., from forming or machining) can cause massive signal variation.

🧲 The Fix: Magnetic Saturation

Smart engineers like Henry Nerwin (1966) discovered that if you drive the material into magnetic saturation, you can flatten out and control its behavior.

There are two main approaches to magnetic saturation:

1. AC Saturation (less common): Use a high-amplitude 60 Hz waveform to push the steel to peak magnetization on every cycle. Gating the ECT readout around that peak gives you a "quiet" window to detect flaws.

2. DC Saturation (more common): Overlay a strong DC field with magnets or coils. This biases the material onto a flat portion of the B-H curve, where μ is stable. Your ECT signal now rides on a calm magnetic sea instead of turbulent waters.

   
   

🔧 Why This Matters to You

If you’ve ever:

• Lost a flaw signal mid-inspection

• Seen false calls from seams or stress zones

• Found your calibration tube gives a totally different response than your production material

…then you may dealing with magnetic permeability issues.

Knowing when to use magnetic saturation—or switch to ultrasonic or remote-field methods—can save you hours of frustration and thousands of dollars in missed flaws.

⚙️ Final Takeaway

Understanding magnetic permeability and hysteresis isn’t just for physicists. It’s what separates an eddy current hack from a true ET guru!

ECT isn't just about coils and frequencies. It’s about controlling the environment where your signal lives. And in ferromagnetic materials, that environment is as much magnetic as it is conductive.

Want to go deeper? We’ve got classic papers by Nerwin, Hochschild, and Libby at eddycurrent.com that show how these principles evolved—and how they’re still used today.

Stay curious. Stay calibrated.

—Ed