Introduction
Carbon steels, whether low, mild, or high in carbon content, are often viewed simply as structural materials, yet their magnetic behavior is equally important in many engineering applications. From transformer frames to magnetic clamps, loudspeaker components, and basic motor components, the magnetic behavior of these steels depends strongly on their chemistry and microstructure. The primary factor shaping their magnetic properties is carbon content, but the presence of other elements and the thermal history of the steel also play decisive roles.
In their purest form, iron-based materials are strongly ferromagnetic. Pure iron allows magnetic domains to align easily under an applied field, resulting in high permeability, low coercivity, and low iron losses, ideal characteristics for magnetic applications. However, the introduction of carbon progressively disrupts this magnetic behavior. Carbon atoms dissolve in the iron lattice or transform into carbides during heat treatment, and these microstructural features act as obstacles to the motion of magnetic domain walls. As carbon content increases, these obstacles multiply, causing magnetic permeability to decrease and coercivity to rise. The result is a material that becomes mechanically stronger but magnetically weaker.
Another important aspect affected by carbon content is the maximum magnetic saturation (Ms), which is the highest magnetization a material can achieve under an externally applied strong magnetic field. Saturation depends primarily on the amount of magnetizable iron still available in the microstructure. As carbon increases and more cementite (Fe₃C) or carbide phases form, both non-magnetic, the proportion of ferromagnetic iron decreases. For this reason, low-carbon steels reach magnetic saturation values close to pure iron, typically around 1.9–2.1 T, while steels containing more carbon show progressively lower saturation even under strong fields.
Carbon Steel Types
Low-carbon steels, typically containing up to 0.15% carbon, retain magnetic properties closest to pure iron. Their maximum magnetic saturation remains high—usually in the range of 1.9 to 2.1 Tesla, depending on purity and processing—because very little iron is locked into carbides. This makes low-carbon steels suitable for applications where magnetic response must be consistent and relatively strong.
Mild steels, with around 0.15–0.25% carbon, show reduced permeability and higher coercivity compared to low-carbon steels. The growing presence of pearlite introduces thin layers of cementite that do not contribute to magnetic alignment. As a result, the maximum magnetic saturation decreases slightly. Typical saturation values for mild steels fall in the range of about 1.7 to 1.9 Tesla, generally 5–10% lower than low-carbon steels. Although the drop is modest, it becomes relevant in magnetic circuits that must operate near their saturation limits, such as actuators or magnetic clamps designed for high flux densities.
As carbon content increases further, the magnetic nature changes drastically. High-carbon steels, containing more than 0.6% carbon, are dominated by pearlite and carbide formations. These steels exhibit a much poorer magnetic performance not only because their permeability drops, but also because a significant portion of iron is bound in the non-magnetic Fe₃C phase. Depending on carbon level and heat treatment (especially if martensite is present), their magnetic saturation typically falls to around 1.3 to 1.6 Tesla. In hardened tool steels, with very fine carbide distributions and high internal stresses, saturation can sometimes drop even further. These steels therefore exhibit both lower saturation and “stiff” magnetic behavior, making them unsuitable for efficient flux-carrying components.
Beyond carbon, alloying elements and thermal processing further influence magnetism. Manganese, sulfur, and phosphorus tend to reduce magnetic quality, while silicon in small amounts can help reduce eddy-current losses. Heat treatment alters the grain size and phase distribution, affecting both domain-wall mobility and usable saturation.
Ultimately, magnetic saturation in carbon steels decreases steadily as carbon increases: low-carbon steels remain near the ideal saturation of iron, mild steels experience a slight drop, and high-carbon steels show significantly reduced saturation due to their carbide-rich microstructures. Understanding how chemistry influences these values is essential when selecting steels for magnetic components that must operate close to their flux limits.
Popular Carbon Steel Grades
Low-Carbon Steels (≤ 0.15% C):
Mild Steels (≈ 0.15–0.25% C):
Medium-Carbon Steels (0.25–0.60% C):
High-Carbon Steels (≥ 0.60% C):
Measuring Magnetic Properties of Carbon Steels
Understanding the magnetic properties of carbon steels requires not only knowledge of their composition and microstructure but also rigorous laboratory testing using standardized methodologies. In practice, the magnetic behavior of any carbon steel, whether low-carbon, mild, or high-carbon, is established experimentally by measuring its B–H response, permeability, coercivity, remanence, and core losses under conditions that represent real industrial environments. Because these steels exhibit strong shape-dependent magnetic behavior and can be affected by residual stresses, surface conditions, and texture, laboratory procedures rely on carefully prepared specimens and well-defined instrumentation.
Most magnetic measurements begin with precise sample preparation. For steels supplied as sheets or laminations, technicians commonly cut the material into standardized strips and test them in an Epstein frame, a well-established configuration used worldwide to measure AC magnetic properties such as core loss and permeability. The Epstein method averages the anisotropy of the material by arranging multiple strips into a square frame with uniform magnetic path length and tightly controlled winding geometry. When a single sheet must be evaluated independently, for example, to study the directional behavior of grain-oriented steels, a single-sheet tester is used instead. This setup allows the laboratory to obtain BH curves and AC losses from one sheet under well-regulated flux and field conditions.
When highly accurate DC BH curves or magnetization saturation values are needed, ring-shaped specimens are preferred. Toroidal samples, whether machined from steel or assembled as wound rings, eliminate the demagnetizing fields created by rectangular or irregular geometries. Their closed-loop geometry means that almost all applied magnetic field contributes to magnetization, enabling precise measurement of saturation induction, initial magnetization, remanence, and coercivity. These rings are typically tested using a hysteresisgraph or B-H loop tracer, which slowly cycles the magnetic field while monitoring flux through a secondary coil. This technique provides high-fidelity hysteresis loops and is the standard method for DC characterization of soft and semi-soft magnetic steels.
For research-level characterization or when only small pieces of steel are available, vibrating sample magnetometers (VSMs) or SQUID magnetometers are used. These instruments directly measure the magnetic moment of a specimen under an applied field and can reach very high magnetic field strengths, sufficient to determine the true saturation magnetization of steels with high accuracy. VSMs are particularly useful for investigating microstructural effects, such as carbide precipitation, grain refinement, or the influence of heat treatments on magnetic response. SQUID magnetometers, though more common in physics laboratories, provide even higher sensitivity and can detect minute changes in magnetic phase composition or domain behavior.
In all these measurements, attention to experimental conditions is essential. Temperature must be controlled because permeability and magnetic losses are temperature - dependent. Residual stresses from machining or rolling can artificially increase coercivity or distort B–H shapes, so samples are often stress-relieved prior to testing. Moreover, because the geometry of a sample strongly affects its magnetic response, demagnetizing factor corrections must be applied for non-toroidal samples to ensure that results reflect intrinsic material properties rather than geometric artifacts. Calibration procedures using known reference materials also ensure traceability and repeatability.
An important extension of these techniques is high-temperature measurement. In fact, researchers have developed ring-specimen (closed magnetic circuit) methods that enable BH curve measurements from room temperature all the way up to the Curie point of steel. For example, in the study “High‑temperature Magnetization Characteristics of Steels” the authors successfully measured BH curves at temperatures beyond 200 °C, showing how saturation, permeability, and coercivity evolve with temperature. This makes it possible to characterize how carbon steels behave magnetically under elevated-temperature conditions, an important capability for applications where steels operate under thermal stress or for processes involving heating. Such high-temperature testing can reveal, for example, reductions in magnetic saturation or changes in coercivity as temperature rises, especially in steels containing carbides or other non-ferromagnetic phases.
Once measurements are collected (whether at room temperature or elevated temperature), engineers frequently fit the results to established constitutive models such as the Jiles–Atherton or Preisach models. These models allow the measured properties—including hysteresis shape, coercivity, saturation, and temperature dependence—to be incorporated into finite-element simulations of motors, transformers, actuators, or magnetic sensors. Modern research regularly develops improved identification methods for extracting these model parameters from measured B–H loops, especially for steels whose properties change through manufacturing steps like cold working, welding, or annealing.
A large body of standards, textbooks, and technical papers underpins these testing methodologies. Core procedures for sheet steels are defined in IEC 60404-2 for Epstein frames and IEC 60404-3 for single-sheet testers. ASTM standards such as A927/A927M and A773 provide guidance for testing DC and AC magnetic properties in toroidal or laminated cores. Foundational texts such as Introduction to Magnetic Materials by Cullity and Graham describe the physics of measurement techniques and the interpretation of hysteresis loops. Numerous research papers describe practical implementations of VSM measurements, high-temperature magnetic measurement rigs, laboratory construction of magnetometers, and numerical identification of hysteresis parameters. Together, these resources form the reference framework that laboratories and industry rely on when evaluating the magnetic characteristics of carbon steels.
References
[1] D. A. Aragon-Verduzco, J. C. Olivares-Galvan, R. Escarela-Perez, E. Campero-Littlewood, R. Ocon-Valdez and S. Magdaleno-Adame, "Experimental procedure to obtain electromagnetic properties of A-36 low carbon steel plates utilized in transformers," 2016 IEEE PES Transmission & Distribution Conference and Exposition-Latin America (PES T&D-LA), Morelia, Mexico, 2016, pp. 1-5.
[2] Phamella Reinert Tamanini Piccoli, Sérgio Henrique Lopes Cabral, Luiz Fernando de Oliveira, Odirlan Iaronka, Diogo Fernando Harmel, João Paulo Vieira, João Egídio Sapeli, "Experimental evaluation of electric and magnetic properties of structural steel", COMPEL - The international journal for computation and mathematics in electrical and electronic engineering, vol.37, no.3, pp.1029, 2018.
[3] Hirohisa Takeuchi, Yasuhiro Yogo, Tsuyoshi Hattori, Tomonori Tajima, Takashi Ishikawa, "High-temperature Magnetization Characteristics of Steels", ISIJ International, 2017, Volume 57, Issue 10, Pages 1883-1886, Released on J-STAGE October 17, 2017.
[4] IEC 60404-2, Magnetic Materials — "Part 2: Methods of Measurement of the Magnetic Properties of Electrical Steel Strip and Sheet by Means of an Epstein Frame"
[5] IEC 60404-3, Magnetic Materials — "Part 3: Methods of Measurement of the Magnetic Properties of Electrical Steel Strip and Sheet by Means of a Single Sheet Tester"
[6] ASTM A927/A927M, "Standard Test Method for Alternating-Current Magnetic Properties of Laminated or Toroidal Cores and Tape-Wound Cores"
[7] ASTM A773/A773M, "Standard Test Method for Direct-Current Magnetic Properties Using the Hysteresigraph (B–H Loop Methods)"
[8] Cullity, B. D., and Graham, C. D., Introduction to Magnetic Materials, 2nd ed., Wiley–IEEE Press.
[9] López-Domínguez, V., et al., “A Simple Vibrating Sample Magnetometer for Macroscopic Samples,” Review of Scientific Instruments, 2018.
[10] Regan, A., et al., “Extension to the Jiles–Atherton Hysteresis Model…,” Sensors, 2025.
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Citation: S. Magdaleno, "Magnetic Properties of Carbon Steels: Measuring Magnetic Properties of Carbon Steels," Salvador Consultant.