Estimation of Structural Degradation of the Heat Affected Zone of the Welded Joint Under Fatigue

We consider the features of using modern nondestructive methods for assessing structural degradation in the process of elastic-plastic cyclic loading of normalizing (average grain size 10 μm) and overheating (average grain size 45 μm) zones of the heat affected zone of St3 steel. In this case, the approaches of digital image processing of microstructures, fractal analysis, and ultrasonic and magnetic testing were used. The features of the stages of degradation of the structure under fatigue and the corresponding behavior of the numerical microstructure parameters such as fractal dimension and the concentration and fraction of defects and slip bands are analyzed, as well as changes in the elastic properties of the material under study (determining an acoustic parameter that takes into account the complex change in the velocities of longitudinal and transverse waves) and the coercive force. It has been established that cyclic loading of samples from the sections of the heat affected zone leads to the formation of a large number of structural defects in the form of deformation bands, discontinuities, and microcracks, the occurrence of which is accompanied by a considerable change in the numerical microstructure characteristics (a decrease in the value of fractal dimension) as well as elastic (a decrease in the values of acoustic parameters) and magnetic (an increase in coercive force) properties. It is also shown that cyclic loading of the material under study leads to an increase in microhardness. However, the nature of such an increase for overheating and normalizing zones is different. The results of the research can be applied to the problems of diagnosing premature destruction and monitoring the most loaded zones of St3 steel structures to determine the residual resource.