Nondestructive Evaluation of Ageing Steel Structures

Usually, materials are inspected by nondestructive methods to detect and evaluate defects that may cause failure under the designed operation conditions. However, the structure may also fail due to uncertainties of material properties like strength or hardness, and may also suffer from unexpected degradation during operation. A reliable NDE technique for detecting the inception of failure during early stages of embrittlement is presently not available. For the characterization of material degradation phenomena, nondestructive methods that are sensitive to the microstructure of the material are required and useful. However, they must be applied in the field. The difference between Microscopy and NDE poses a challenge when dealing with the scanning of macroscopic objects. The presented results were tested under laboratory conditions only and have to be certified through comprehensive field trials. Appropriate techniques for the assessment of material degradation should be able to detect and identify micromagnetic properties of ferritic steel, for example; magnetization phenomena are affected by microstructure and stress states. Other techniques with similar potential are Eddy Current, Ultrasonic and Thermal testing methods. We present results of a research project funded within the German Nuclear Safety Program. Using different micromagnetic methods we were able to analyze the neutron induced embrittlement of a 20 MnMoNi 55 steel material. Magnetic Barkhausen noise data, the upper harmonics analysis of magnetization, and eddy current data allowed a nondestructive prediction of Vickers hardness and the shift of the ductile-to-brittle transition temperature (DBTT) that characterizes embrittlement. As a second example, we present results that allow the assessment of the precipitation induced embrittlement in WB 36 (15 NiCuMoNb 5) steel material. Both examples demonstrate the complexity of the problem and the viability of the NDT approach.