Dynamic magnetostriction for material characterization of micro structure states of degraded structural steel

The magnetostriction effect has been known for over 100 years. The first observation was made by Joule in 1842; while applying a magnetic field to a nickel rod specimen the length changed. Magnetostriction is the change of the dimensions and the Young modulus of a magnetic material caused by a change in its magnetic state. Magnetostriction is classified as linear or longitudinal; transverse and volume magnetostriction. Linear magnetostriction is also referred to as Joule magnetostriction, where deformation occurs in the direction of the applied field while the volume of the distorted magnetic domains remain constant. During transverse magnetostriction the dimension change is perpendicular to the applied field. Volume magnetostriction is mainly caused by the increase of distance between the atoms by the applied magnetic field. If the volume of an iron-based material expands with an increase in magnetisation the volume magnetostriction is defined as positive. The magnetisation state of a material can also be changed by temperature variations or by applied mechanical stress. Spins in excited state can transfer their excess energy to the lattice so a disturbance of the coupled magnetic and crystal lattice affects the magnetostriction. Moreover, in Joule magnetostriction the dimensional change is associated with the distribution of distorted magnetic domains so it could be expected that domain movement hindrance would affect magnetostriction. Magnetisation state changes can cause a strain just as strain can cause a change in the magnetisation state. Thus magnetostriction effects are reciprocal causing a magnetostrictive material to behave as piezoelectric materials due to magneto elastic interaction. A sound wave is generated by the change in the magnetic state while the sound wave strains change the magnetisation state of the material. This principle of the reciprocity of magnetic state - strain was used as the theoretical background of the dynamic magnetostriction test. By applying the dynamic magnetostriction test the effect of the precipitates on the crystalline of a ferromagnetic polycrystalline can be recognised by its interaction on the magnetisation state. Dynamic magnetostriction tests have been further developed at the Fraunhofer Institute for NDT, Saarbrücken, in order to define material properties. A 15NiCuMoNb5 copper content ferritic steel material was submitted to isothermal thermal ageing and the material properties changing because of copper precipitation were studied. Mechanical and physical properties, e.g. hardness, electrical conductivity were tested. The objective was to provide a basis for developing a non-destructive testing technique for this type of steel, which is widely used in the energy industry as pipeline and vessel material. This contribution discusses the physical background, the NDE-approach and the results.