Surface crack detection in ferritic and austenitic steel components using inductive heated thermography

Active dynamic thermography using inductive heating was used for investigations on different steel components with a high allotment of carbon. The set of test specimens are semi-finished products from a continuous casting process. The cross sectional area of the products is 50×50 mm to 250×250 mm. Defects resulting from the production process have to be detected to avouch the desired quality to the customers. The probed surface is rudely brushed to lift off large oxide parts. Anomalies in the surface temperature during and after inductive heating correspond to inhomogeneities in the material. The defects are perpendicular and slanted surface cracks. The penetration depth of the induced eddy current depends on the induction frequency and on the material properties for example the relative permeability value. In the case of magnetic materials, like ferritic steel and high excitation frequency, the penetration depth of the current is very small in comparison to austenitic steel. Thus, the eddy current is much closer to the 0.1-1mm deep surface cracks. By contrast, the eddy current in austenitic steel at the same excitation frequency has a much larger penetration depth and therefore leads to a different behavior around a crack of similar depth. Thus, lower temperatures around the crack are observed. It is shown, that depending on the material properties, the excitation frequency and the duration of the heating pulse higher or lower temperatures around the surface crack are achieved. To homogenize the emissivity of the oxidized surface, a very thin water film superimposed on the surface is tested.