2017 Volume 57 Issue 10 Pages 1883-1886
A method was developed for the measurement of BH curves for steels from room temperature to the Curie point (Tc). A closed magnetic circuit measurement was applied to obtain accurate temperature-dependent magnetic characteristics. BH curves were successfully measured at temperatures over 473 K (200°C), which was not possible before the development of this method.
In addition, at room temperature BH curves measured by the proposed method and a conventional method correspond well, which indicates the validity of the proposed method.
In the measurement for a medium-carbon steel and a electromagnetic soft iron, the difference in the magnetic flux density at 8000 A·m−1 (B8k) came from the low saturation magnetic flux density of cementite in the medium-carbon steel. B8k for the medium-carbon steel clearly decreased as the temperature increased from room temperature to 473 K (200°C), which corresponds to Tc for cementite.
In addition, B8k for the electromagnetic soft iron decreased almost linearly from 973 K (700°C) to 1048 K (775°C), whereas B8k for the medium-carbon steel decreased significantly from 998 K (725°C) to 1023 K (750°C). This difference indicates that magnetic transformation occurs only in the electromagnetic soft iron at around Tc, while magnetic and phase transformations, from ferrite to austenite, occur simultaneously in the medium-carbon steel. The differences in the temperature-dependent coercive force for electromagnetic soft iron, and for the low and medium-carbon steels were observed in detail.
The proposed method provided exact BH curves at high temperature, which were reflected by the complex and detailed magnetic behavior.
A BH curve illustrates the relationship between the magnetic field and magnetic flux density of a material. The BH curve of a particular material is a temperature-dependent characteristic of magnetization that can be measured using a vibrating sample magnetometer (VSM).1,2) However, the measurement results are affected by diamagnetic fields because the VSM performs an open magnetic circuit measurement.3) The temperature dependence of BH curves can also be measured using a superconducting quantum interference device (SQUID) and other methods up to a maximum measurable temperature of approximately 473 K (200°C); however, this is far from the Curie temperature (Tc) of metallic materials,4,5) e.g., Tc for pure iron is 1043 K (770°C).
Carbon steels, especially low-carbon steels are applied as core materials for electronically controlled parts such as solenoid valves.6) Therefore, the temperature-dependent coercive force measured by BH curves is important from the viewpoint of loss reduction.7) These accurate magnetization characteristics will also be useful as input data for the simulation of electromagnetic phenomena.
It is possible to conduct closed magnetic circuit measurements and obtain accurate BH curves with a DC recording fluxmeter and a ring specimen.8,9) In this method, two wires are wound onto the specimen for magnetic field excitation and magnetic flux detection. The two wires must have high electrical conductivity, surface insulation, and flexibility to be wound around the specimen. At room temperature, an enamel-coated copper wire is typically used because it satisfies these requirements. However, it is not applicable for the measurement of BH curves at high temperatures, because the insulation properties are lost around 473 K (200°C) and the copper wire tends to be oxidized, which decreases the electrical conductivity. There is thus, presently, no conventional method for the measurement of BH curves at high temperature.
The purposes of this study was to measure temperature-dependent BH curves from room temperature to the Tc of iron and steels, and confirm the validity of the measured data.
To improve the insulation property and oxidation resistance of the wires used for BH curve measurements with a DC recording fluxmeter, an alternative wire with the following features was applied.
• A silver wire with a diameter of 0.8 mm was used as an inner conductor because it has higher electrical conductivity and higher oxidation-resistance than copper. (Silver cannot be oxidized under atmospheric pressure, even at high temperatures.10))
• A ceramic braided tube with high electrical insulation that does not breakdown below 1473 K was used to cover the inner conductor, silver.
A schematic diagram of the measurement is shown in Fig. 1. The specimens were electromagnetic soft iron (JIS SUY-111)), which is close to pure iron, and low and medium-carbon steels (JIS S25C and S45C,12) which are equivalent to ISO C25 and ISO C45 respectively). Table 1 shows the standard compositions of the electromagnetic soft iron, low carbon steel, and medium carbon steel specimens. The dimensions of the specimens are shown in Fig. 2. As-received steels were applied for the measurements. Therefore, initial microstructures of both steels were ferrite-pearlite. The wires were wound onto the specimens for magnetic flux detection (40 turns) and magnetic field excitation (175 turns). A ceramic paste was coated onto the specimen and wires to prevent oxidation of a specimen. Figure 3 shows the external appearance of a specimen with wound wires and the ceramic paste. Both ends of the wire were connected to the DC recording fluxmeter (TRF-5A, Toei Industry Co., Ltd.).
Schematic diagram for measurements of temperature dependant BH curves.
| Steel grade | Chemical composition (mass%) | ||||
|---|---|---|---|---|---|
| C | Si | Mn | P | S | |
| SUY-1 | ≦0.030 | ≦0.200 | ≦0.50 | ≦0.030 | ≦0.030 |
| S25C | 0.22–0.28 | 0.15–0.35 | 0.30–0.60 | ≦0.030 | ≦0.035 |
| S45C | 0.42–0.48 | 0.15–0.35 | 0.60–0.90 | ≦0.030 | ≦0.035 |
Dimensions of a specimen for BH curves measurement.
An external appearance of a specimen.
The temperature of the specimens was controlled with an electrical furnace. Two specimens (A and B) were placed in the furnace: specimen A for BH curve measurements and specimen B for measurement of the actual specimen temperature. Specimen B was placed next to specimen A. The end of the thermocouple wire for temperature measurement was welded to the surface of specimen B. The wires were also wound onto the specimen B, but the ends of the wound wire on specimen B were not connected to the DC recording fluxmeter. The reason for the preparation of two specimens was to prevent the influence of the thermocouple wire on magnetic flux detection and magnetic field excitation during the measurements. Before each measurement, the temperature of the furnace was maintained for 10 minutes as the temperature of specimen B was within ±2 K of the target temperatures. Each measurement took approximately 20 seconds.
Using this method, there is a possibility of insulation failure due to spreading of the ceramic braided tube fibers. Insulation failure would lead to failure of the measurement. Therefore, to ensure sufficient insulation of the wires, the electrical resistance between the wire for magnetic field excitation and that for magnetic flux detection was measured before each BH curve measurement.
In addition, BH curve measurement using the conventional technique was also conducted for comparison to evaluate the validity of the developed method. In this measurement, enamel-coated copper wires were applied as winding wires for magnetic field excitation and magnetic flux detection. The number of turns and the DC recording fluxmeter were the same as those used for the developed method. This conventional technique measurement was conducted only at room temperature.
All measurements in this study were performed within the magnetic field range of ±8000 A·m−1, which is the maximum magnetic field range for these experimental conditions.
Figure 4 shows BH curves for the medium-carbon steel obtained using both the developed and conventional methods. The results correspond well, which indicates that the wire used for the developed method has the same performance as the conventionally used enamel-coated copper wire. The temperature history for the BH curve measurement is shown in Fig. 5. Figure 6 shows BH curves (initial magnetization curves) for the electromagnetic soft iron specimen from room temperature (289 K, 16°C) to 1048 K (775°C). A slight change in the BH curves was observed from 289 K (16°C) to 673 K (400°C). In contrast, the magnetic flux density at 8000 A·m−1 (B8k) rapidly decreased over 873 K (600°C). These results correspond to typical magnetic characteristics, where the curve rapidly changes as the material temperature approaches Tc.1) The transition from ferromagnetism to paramagnetism was observed at 1048 K (775°C) when the temperature of the specimen exceeded Tc (1043 K, 770°C).
Comparison for BH curves of the medium carbon steel measured by the developed and the conventional method at room temperature (289 K).
Thermal history for BH curves measurement.
Temperature dependent BH curves (initial magnetization) of the electromagnetic soft iron.
Figure 7 shows B8k for the electromagnetic soft iron and the medium-carbon steel from room temperature to 1048 K (775°C) measured using the developed method. B8k for the medium-carbon steel decreased as the temperature was increased from room temperature to 473 K (200°C). This is a reasonable result because Tc for cementite is 493 K (220°C).13) The difference in the paramagnetization behavior of the electromagnetic soft iron and medium-carbon steel specimens was also elucidated. B8k for the electromagnetic soft iron decreased almost linearly from 973 K (700°C) to 1048 K (775°C), whereas that for the medium-carbon steel specimen decreased rapidly from 998 K (725°C) to 1023 K (750°C). This difference of paramagnetization behavior indicates that magnetic transformation only occurs in the electromagnetic soft iron specimen near Tc, while both magnetic and phase transformations from ferrite (ferromagnetism) to austenite (paramagnetism) occur simultaneously in the medium-carbon steel specimen.
Temperature dependence for magnetic flux density at 8000 A·m−1 of the electromagnetic soft iron and the medium carbon steel.
The coercive force of the electromagnetic soft iron, low and medium-carbon steel specimens from 289 K (16°C) to 1048 K (775°C) is shown in Fig. 8. The coercive force of the electromagnetic soft iron decreased with an increase in temperature. In contrast, the coercive force of the low-carbon steel increased from room temperature to 373 K (100°C), and then decreased at temperatures over 373 K (100°C). The coercive force of the medium-carbon steel was similar to that for the low-carbon steel; however, the temperature where the coercive force stops increasing and then decreases was higher, at 473 K (200°C). These different results are due to the different amounts of cementite present in the low and medium-carbon steels. Paramagnetization of cementite proceeds from room temperature to 493 K (220°C), which is Tc for cementite,13) and paramagnetized cementite causes an increase of coercive force by the pinning effect at domain walls. In a matrix of iron or steel, the permeability increases with the temperature. Therefore, the coercive force of the electromagnetic soft iron decreased with the temperature. It is considered that the medium-carbon steel is more strongly affected by the paramagnetization of cementite than the low-carbon steel because the medium-carbon steel contains larger amounts of cementite. Therefore, the increase of coercive force continued to higher temperature for the medium-carbon steel.
Temperature dependence for coercive force of the electromagnetic soft iron, the low and medium carbon steel.
There is no other method presently available that enables the measurement of BH curves from room temperature to the Tc of steels. Therefore, it is not possible to compare the BH curves measured in the study with those in other studies. However, the changes in the BH curves observed in this study can be supported in terms of the metallurgy of steels. Thus, it is confirmed that the developed method and the measured BH curves are valid.
A method for the measurement of BH curves from room temperature to the Tc of steel was developed. BH curves measured using the developed method and the conventional method corresponded well at room temperature. These results showed the validity of the developed method. Detailed BH curves, permeability and coercive force were successfully measured at temperatures over 473 K (200°C) by the developed method. The magnetization characteristics of the medium-carbon steel specimen varied around 493 K (220°C) and 1043 K (770°C). The variation around 493 K (220°C) corresponds to the Tc of cementite, and the variation around 1043 K (770°C) corresponds to magnetic and phase transformations of steel. Detailed differences for the coercive force of the electromagnetic soft iron and the low and medium–carbon steels were also observed at several temperatures.
The measurement results obtained with this developed method are very useful to clarify the behaviors of magnetic materials.
