For the characterization of different steel types in particular tensile strength andyield strength as well as hardness are being used. These quantities are normallydetermined by destructive tests, which are time consuming and costly. This paper dealswith a non destructive method which is able to characterize mechanical and technologicalproperties, like tensile and yield strength, by the magnetoinductive analysis of higherharmonics. The working principle of the technique is the relation between physicalproperties (electric conductivity, magnetic permeability) and mechanical properties offerromagnetic materials. The characteristic properties of materials depend, amongstothers, on alloy composition, grain size, texture and dislocation density. As aconsequence of this the electrical and magnetic properties are formed. For the NonLinear Harmonic Analysis (NLHA) is of particular interest the shape of the magneticHysteresis Loop, which reflects all the magnetic properties, as well as its variationdue to changes in material properties. Due to the fact that the physical properties arestrongly related to the mechanical properties (yield strength, tensile strength,consolidation, anisotropy, hardness etc.) these mechanical properties can be determinedby NLHA. The measurement of the complete hysteresis loop, or parts of it, is not a veryefficient way to determine mechanical properties. NLHA, instead, uses a sinusoidalsignal from a coil for excitation of the hysteresis loop within the material andanalyses the output signal of a receiving coil modulated by the hysteresis loop. Thehigher harmonics determined from the analysis contain the complete information about theshape of the hysteresis loop. These magnetic data (amplitude and phase of the harmonics)are then correlated with the mechanical data, mentioned before, which are determinedfrom destructive test in order to calibrate the technique.Results have been obtained from various types of steels, ranging from low strength tohigh strength steels, which will be presented in the paper. The accuracy for themechanical properties is in the order of 3%. For mechanical properties more difficult todetermine, like anisotropy etc., in some cases less than 3%. Regarding hardness, corehardness and surface hardness can be determined, adjusting the measuring frequency inaccordance with the Skin-Effect. By variation of the measuring frequency also thehardness penetration depth can be found.Due to its electromagnetic character the NLHA-method is very fast. Measuring times,including evaluation, are in the order of seconds. Furthermore it works without contactwith the material. Distances between the sensor and the material to be tested of up toseveral mm can be used. In conclusion the NLHA is a reliable and fast test technique forthe determination of mechanical properties of ferritic steel types. It is also suitedfor in-line application to control the steel production process.
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