The paper is devoted to investigation of S235J2 steel quality criteria variation during thirty years’ service life as a part of construction of power-generation unit #1 of Zaporizhia Nuclear Power Plant (NPP). The microstructure of the steel has been evaluated by fractal methods. Fractal size of the steel structure has higher correlation to mechanical properties of the steel compared to microstructure analysis. During service life of the steel YTS ranging 10.01% (24.9 MPa), UTS ranging 9.42% (39.5 MPa), while elongation ranging 43.16% and Reduction of area ranging 39.58%. These values can be explained by strain ageing of the S235J2 steel during service life. It has been stated that fractal size of pearlite and strength have good correspondence (YTS has fractal model correlation coefficient R² = 0.6194, while UTS has R² = 0.8068). The correspondence of ferrite fractal size to ductility has been calculated (Elongation has fractal model correlation coefficient R² = 0.6493 and Reduction of area has R² = 0.5258). Analysis of the S235J2 quality criteria fractal models let the range quality criteria depends on fractal size of the ferrite-pearlite structure of the steel. Experimental results prove the possibility of usage the fractal models for predicting remaining life of the steel constructions at NPP. Therefore, the fractal modelling may be assumed as a promising nondestructive test method for NPP.

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