Micromagnetic Characterization of Operation-Induced Damage in Charpy Specimens of RPV Steels

The embrittlement of two types of nuclear pressure vessel steel, 15Kh2NMFA and A508 Cl.2, was studied using two different methods of magnetic nondestructive testing: micromagnetic multiparameter microstructure and stress analysis (3MA-X8) and magnetic adaptive testing (MAT). The microstructure and mechanical properties of reactor pressure vessel (RPV) materials are modified due to neutron irradiation; this material degradation can be characterized using magnetic methods. For the first time, the progressive change in material properties due to neutron irradiation was investigated on the same specimens, before and after neutron irradiation. A correlation was found between magnetic characteristics and neutron-irradiation-induced damage, regardless of the type of material or the applied measurement technique. The results of the individual micromagnetic measurements proved their suitability for characterizing the degradation of RPV steel caused by simulated operating conditions. A calibration/training procedure was applied on the merged outcome of both testing methods, producing excellent results in predicting transition temperature, yield strength, and mechanical hardness for both materials.

Inspection of Reactor Steel Degradation by Magnetic Adaptive Testing

Degradation of nuclear pressure vessel steel materials, 15Kh2NMFA type and A508 Cl2 type (definition is given in the text) were investigated by a novel magnetic nondestructive testing method, so-called Magnetic Adaptive Testing (MAT), which is based on systematic measurement and evaluation of minor magnetic hysteresis loops. The measured samples were thermally treated by a special step cooling procedure, which generated structural changes in the material. It was found that this type of degradation can be easily followed by magnetic measurements. Charpy impact test were also performed and the results were compared with the magnetic parameters. In case of 15Kh2NMFA steel, a good, reliable and closely linear correlation was found between magnetic descriptors and transition temperature.

Influence of magnetizing current speed in magnetic adaptive testing

Influence of changing rate of magnetizing current in magnetic adaptive testing was studied. Sensitivity of magnetic descriptors, which characterize the samples degradation, was found to depend on this speed. Low speed results in a better sensitivity, but the effect depends on permeability of the investigated specimens. Recommendation was made for the optimal choice of the applied speed of the magnetizing current.

Adaptive Testing for Nondestructive Evaluation of Degraded Materials

Method of Adaptive testing (AT) is described, as a way of optimization of experimental evaluation of degraded materials, which are measured nondestructively, indirectly via a convenient physical process. The general AT is illustrated by a well-explored special case of magnetic adaptive testing (MAT) of strained steel. Degradation curves obtained from AT are regularly more sensitive and experimentally friendly than traditional descriptors of the used physical process itself. Any of the traditional descriptors of the physical process, being optimized for description of the process itself, is very probably not optimized for description of the investigated material degradation.