The magnetic properties of plastically deformed steels.
This study concentrates on low carbon pearlitic steels. Two sets of experiments are carried out, the first on a section of semi-killed gas pipe and the second on specially prepared alloys of iron and carbon with pearlite fractions varying from 0.19% to 100%. Their magnetic properties are studied both in the as received state and after tensile plastic deformation. In addition, four different heat treatments are applied to the low carbon steel. Standard magnetisation and fluxmeter techniques are used to determine the bulk magnetic properties, with further use of a vibrating sample magnetometer for coercivity measurements. The Barkhausen noise of the samples is also recorded and High Voltage Lorentz Electron Microscopy used to directly observe the domain configurations and the interaction of the domain walls with dislocation tangles. The changes in the magnetic properties after tensile deformation are similar to those due to elastic compressive stress, with an additional increase in the coercivity. For the initial magnetisation curve initial permeability (J-Li) and maximum relative permeability both decrease, while the field at which the latter occurs (Hm) increases. The hysteresis curve shears over reducing the maximum differential permeability and the remanence and also increasing the coercivity. These results and the change in the shape of the hysteresis curve, most noticeable in the low carbon steels, are explained in terms of the reduction in easy domain wall movement due to the dislocation tangles, as observed under the electron microscope, and to the magnetostrictive effect of the compressive residual stress. Inter-relationships are found between coercivity and both J-Li and Hm. The coercivity io also found to vary linearly with both Vickers Hardness and Yield Stress. The Kneppo formula for the initial magnetisation curve is found to hold better for the higher carbon content steels with the fit deteriorating with increasing plastic deformation.