ˇTable of Contents
Barkhausen noise at deep drawing steel after various degrees of cold plastic deformationJ. Grum, B. Pecnik,
Faculty of Mechanica Engineering, Akerceva 6, 1000 Ljubljana, Slovenia
Faculty of Electrical Engineering, Trzaka 25, 1000 Ljubljana, Slovenia
2.1 Preparation of a specimen material
Specimens were cold rolled from different initial thicknesses of a common structural steel Fe360-B in accordance with ISO standard to three equal final thicknesses. The initial dimensions of the specimens were 300 mm x 40 mm with different thickness which was obtained by means of preliminary machining. The rolled pieces were rolled in the longitudinal direction. By cold rolling they were thinned to uniform final thicknesses, i.e., of 2.0 mm, 3.5 mm, and 5.0 mm respectively, and thus attained different degrees of cold deformation, i.e., of 20 %, 40 %, and 60 % respectively. The initial material in a soft state shows fine-grained pearlitic-ferritic normalised microstructure.
3.1 Hardness analysis of the specimen material
Fig. 1 shows measured Vickers hardness of the specimen material as a function of the final specimen thickness and the degree of cold deformation of the specimen material. It shows hardness variations for the final sheet thicknesses, i.e., 2.0 mm, 3.5 mm, and 5.0 mm, with 0 %, 20 %, 40 %, and 60 % degrees of cold deformation. The values of the average hardness of the individual specimen material are in the range expected with regard to the chemical composition of the steel and the given microstructure with the individual degrees of cold deformation of the common structural steel Fe360-B.
|| Fig 1: Variations of HV1.0 hardness as a function of the degree of cold deformation of steel with different final material thicknesses.
Prior to cold deformation, i.e., in the soft state, individual specimens showed hardness ranging from 152 HV1.0 to 164 HV1.0, which, with an increase in the degree of cold deformation, increased to 277 HV1.0. Fig. 1 indicates that the highest hardness values were obtained with the final specimen thickness of 2.0 mm, a bit lower values with the final thickness of 3.5 mm, and the lowest values with the material thickness of 5.0 mm. This is to say that homogeneity of cold deformation decreases with an increase in the specimen thickness. With smaller material thicknesses the material becomes more cold hardened at the upper and lower specimen surfaces than in the central part.
3.2 Experimental set-up with a sensor unit
The experimental set-up used to capture the induced voltage consists of four basic units:
The laboratory sensor unit consists of a magnetising yoke with 1250 turns used to apply an alternating magnetic field to the ferromagnetic specimen material. Good contact between the magnetising yoke and the specimen surface may be achieved by an appropriate preliminary preparation of the specimens by grinding and calibration rolling. Fig. 2 shows the sensor unit consisting of three major parts, i.e., the magnetic yoke with turns, the detecting coil and components for connecting a power amplifier and BNC terminal for transmission of the captured BN voltage signal to the amplifier, the band-pass filter and finally the unit for display of the captured voltage signal.
|Fig 2: Sensor unit with yoke for the specimen magnetisation and capturing of voltage signals|
The sensor unit consists of the magnetising yoke with turns and with the detecting coil designed for capturing the BN voltage signal. The detecting (measuring) coil is the most important element of the experimental set-up since it is on the detecting coil that the quality of the captured voltage signal depends, which may have a decisive influence on preliminary processing and processing of the voltage signals. The detecting coil consists of 40 turns with a thickness of 0.08 mm arranged in one winding as a spiral coil.
|Fig 3: Size of the slice of BN voltage signal on time axis.|
For an efficient analysis of BN voltage signals, the most suitable magnetising-current intensity and frequency for external magnetisation should be ensured; therefore, very extensive preliminary studies were performed. The results indicated that the most distinctive BN voltage signals are obtained with a current intensity of 0.5 A and an excitation frequency of 10 Hz. A sampling frequency of 50 kHz was used. Analyses of the captured voltage signals were made in terms of four characteristics, i.e.:
4.1 Comparison of measurements of the BN voltage signal with a constant magnetising current and a constant magnetic flux
Fig. 4 shows calibration curves during the phase of cold deformation of the material as a function of time delay of the voltage signal, the calculated signal power, the signal variance, and the value of Vrms of the BN voltage signal in the case of magnetisation of the specimen material with the constant magnetising current and the constant magnetising flux.
Fig 4: Calibration curves for degrees of cold deformation with constant magnetising current and constant magnetising flow for a sheet with a final thickness of 3.5 mm.
The calibration curves in Fig. 4 show the dependence of the degree of cold deformation from the time delay of the voltage signal, the calculated signal power, the signal variance and value of Vrms of the BN voltage signal captured at the final material thickness of 3.5 mm. The emphasis is laid on the plotted calibration curves as well as their comparison with the constant magnetising current or constant magnetising flux. With the curves of linear approximation of the calibration curves, also an equation is given which defines the relationship between the degree of cold deformation and, indirectly, the specimen material hardness. The equation of a calibration curve is determined directly as a result of the characteristic searched for from the captured BN voltage signal. A deficiency of the calibration curves determined in this way is that they are valid only for the given material and the given state of the same specimen material.
The experimental results shown in Fig. 4 permit a conclusion that the voltage signals captured with the constant magnetising current provide more useful data than those captured with the constant magnetising flux. That is to say that a strong enough change of the measured as well as calculated parameters of the voltage signal was sensed with the selected degrees of cold deformation of the specimen material. The experimental results indicate that the time delays give the highest value of correlation coefficient. Then follow the calculated value of Vrms, the variance, and the voltage-signal power. If the results of Vrms are compared with the results of variance and the voltage-signal power, then the inclination of the straight line and the value of correlation coefficient are to be considered in the selection of a suitable comparative curve. The data obtained from the voltage signal indicate that it is better to select the variance or the voltage-signal power as a feature due to a steeper inclination of the straight line in spite of a poorer correlation. It turned out anyway that magnetisation with the constant current is more reliable for the prediction of the degree of cold deformation. The calibration curves between the degree of cold deformation and the calculated feature of Vrms indicate that the state of the stressed structural steel Fe360-B is satisfactory.
The repeatability of measurements of the BN voltage signal was statistically verified by means of the single-factor analysis of the variance. Based on the single-factor analysis of the time delay variance, the variance, and the calculated voltage-signal power and the value of Vrms of the BN voltage signal, it can be confirmed that the differences are significant and that by means of the selected procedure and the selected characteristics of the voltage signal, the degree of cold deformation may efficiently be predicted or the microstructure, i.e. the material hardness attained, may be assessed.
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