Coercive Force and Magnetic Analyzer (Coercimeter) MA: The Key to Quality in the Industry.
Coercive force is an important physical parameter that characterizes the ability of a material to retain magnetization intensity when subjected to external magnetic fields. It is measured in amperes per meter (A/m) or amperes per centimeter (A/cm) and plays a key role in quality control of materials, especially in the automotive and mechanical engineering industries. A specialized device, a coervife force meter (coercimeter), is used for accurate coercive force measurement.
This article will explore in detail what coercive force is and how a coercimeter is operated using MA-412 as an example, as well as its application in the industry.
What is coercive force?
Coercivity, also called coercive force (Hc), is the magnetic field strength required to completely demagnetize a material. It depends on the structure of the material, its chemical composition and mechanical processing. For example, the same material subjected to different heat treatments (hardened to different depths) will have different values of coercivity.
A coercimeter, in turn, does not simply measure the coercive force (Hc), but actually determines the point on the hysteresis loop where the material loses residual magnetization. In order to understand the principles of operation of a coercimeter and what it measures exactly, it is important to figure out what magnetic hysteresis is.
Hysteresis (from the Greek *hysteresis* meaning “deficiency” or “lagging behind”) is a phenomenon when the magnetization of a material depends not only on the current magnetic field, but also on its previous states. This is most clearly shown in ferromagnets (iron, nickel, cobalt, their alloys). In case of a graph of dependence of magnetic induction (B) on external field strength (H), a closed hysteresis loop is formed.
How is the hysteresis loop formed?
Let us consider the process of magnetization of a ferromagnet:
1. Initial magnetization (curve 1 → 2)
- As H increases, domains (area with uniform magnetization) are guided by the field.
- At point 2, saturation is reached, which means that almost all domains are aligned.
2. Field reduction (2 → 3)
- When H decreases to zero, the magnetization does not fully disappear: residual induction (Bᵣ) remains at point 3.
This is the operating principle of permanent magnets!
3. Demagnetization (3 → 4)
- To reduce B to zero, a reverse field (−Hс) shall be applied. Hс is the coercive force.
4. Reverse saturation
- With further increase of −H, the material is saturated in the opposite direction (−Bs).
- The cycle repeats, forming a closed curve.
Coercive force arises due to:
- Domain structure: obstacles to movement of domain walls (defects, impurities).
- Irreversible processes: energy barriers during magnetization reversal.
- Effects of heat treatment: quenching increases Hc due to the formation of stress-assisted martensite and internal stresses.
MA-412 coercimeter reproduces the key section of the hysteresis loop:
1. Magnetization to saturation (reaching Bs value).
2. Gradual reduction of the field to zero (recording of Bᵣ).
3. Application of a reverse field until complete demagnetization (determination of Hc).
The advantage of the method is its speed: instead of a full hysteresis cycle, only the critical section Bᵣ → (Hc) is analyzed.
Measurement of the coercive force allows:
1. To evaluate the magnetic properties of materials.
2. To control the quality of materials. For example, when producing steel parts, it is important that their properties correspond to specified parameters, and using the coercive force, hardness or internal stress can be measured.
3. To determine the hardened layer thickness.
Let us review the hardening process in detail.
Hardened layer thickness measurement using coercive force
Coercive force will differ for hardened and unhardened sections of the material. Hardened sections have a higher coercive force compared to unhardened ones, and the method shows a virtually linear dependence of the coercive force value on the depth of the hardened layer when measurements are carried out on magnetic steels.
The method is also suitable for a material such as cast iron. Magnetic properties of cast iron are affected by the shape and size of graphite inclusions, chemical composition is highly variable. In addition, graphite inclusions contribute to the carbon saturation of the metal base of cast iron during hardening and to an increase in the coercive force. Thus, during magnetic inspection of cast iron hardening, a greater error shall be expected compared to steel.
The depth of the hardened layer is usually tested after rough turning by milling, as a result of which a very rough ribbed surface is formed on the tested object.
If measurements are started on products of high roughness, gaps will inevitably form between the sensor poles and the surface.
Gaps can also form if sensor is installed incorrectly.
Gaps in the sensor-part magnetic circuit lead to a decrease in the range of readings (sensitivity), and the larger the area of the coercimeter sensor pole, the more significant it is, as well as to a significant increase in error.
Therefore, when measuring the coercive force, it is necessary to select sensors with magnetic poles fitting as close to the test object as possible. For flat surfaces, sensors with flat poles are suitable, and for radial surfaces, the best option is to select or manufacture special sensors based on the radii of the measured surfaces.
Example of a tight fit of the magnetic cores of sensor MA-412 to the test object.
Mashproject develops sensors specifically designed for the customer’s test objects. For over 20 years, the company has been creating non-destructive testing devices, including coercimeters, and thanks to the accumulated experience, our coercimeter MA-412 is capable of measuring the depth of the hardened layer up to 7 mm.
One of the key applications of the MA coercimeter is the control of the hardening of high-frequency currents (HF) of crankshaft journals and flanges. Crankshafts are subject to significant mechanical loads, so their surface shall be hard enough to withstand wear.
Special sensors for coercimeter MA-412 are designed for parts of various radii
MA-412 as part of special sensors provides accurate measurements at depths of up to 7 mm, due to the high accuracy of coercive force measurement, which makes it ideal for quality control of the hardened layer. The device allows to detect even minor deviations in the thickness of a hardened layer. The device is equipped with an intuitive interface, which makes it easy to understand even for beginner users. The compact size of coercimeter MA provides for its convenient use in laboratories and on production lines.
Coercimeter MA-412
A coercimeter is not just a measuring device, it is a tool for “dialogue” with the microstructure of a material through a hysteresis loop. Its application covers everything from shop floor control to fundamental research.
Coercive force measurement by means of hysteresis loop analysis allows:
- To optimize production and to reduce defects by 20-30%;
- To predict the behavior of materials under operating conditions (e.g. heating in electric motors);
- To create innovative alloys with specified magnetic properties.
In addition, for laboratory purposes, employees of Research and Production Company Mashproject LLC developed a coercimeter STELS-MMA providing a full hysteresis cycle and compliant with GOST 8.268-77 “The Method of Carrying Out Measurements for Determination of Statistic Magnetic Characteristics of Hard Magnetic Materials”.