Magnetometric Method in Analysis of Ferritic-Martensitic Steels
The possibility of using magnetometric analysis based on measuring the magnetic permeability of samples for determination of the structural characteristics of ferritic-martensitic steels possessing high stability of undercooled austenite is substantiated. The structure of those steels at room temperature consists of martensite and high-temperature 5-ferrite. A facility has been designed and manufactured that provides measuring the magnetic permeability and, using the data thus obtained, determine the Curie temperature of ferritic-martensitic steel samples upon cooling them from the temperatures of the gamma region to room temperature. The inductance coil connected in parallel with the capacitor is the heart of the installation. The oscillatory circuit is tuned to a resonant frequency in the range 40 - 80 kHz. The test piece is placed in the coil preheated to a temperature of 1100°C and in fact is the inductor core. It is shown that the relative change in the inductance of the coil together with the sample during continuous cooling depends on the structural class of the steel under test. For ferritic-martensitic steel the change is two-staged: the first abrupt change (at a higher temperature) corresponds to the onset of ferromagnetism in the ferrite, the second is smoother occurs upon further cooling due to martensite formation. For ferritic steel samples there is only one sharp change at the Curie temperature of the ferrite. When testing samples of austenitic steel, there is no abrupt change in the magnetic permeability up to room temperature. It is shown that the magnitude of the effect corresponding to the onset of ferromagnetism in ferrite is proportional to the content of 5-ferrite in the structure. This makes it possible to separate the contribution of two ferromagnetic structural components, martensite and 5-ferrite. The developed technique also makes it possible to determine the temperatures of the beginning (Mn) and the end (Mk) of the martensitic transformation. The implementation and experimental errors of the developed installation and measuring technique are estimated.