ASPECTS OF EVOLUTION OF METAL NANOSCALE CHARACTERISTICS DURING THERMAL CYCLING

Relevance of the study is caused by the absence of experimental data, establishing a relation between submicroscopic (the third-order properties) and macroscopic properties of the first-order. Such interrelatedness, being a factor affecting the microstructure-strength ratio, can considerably facilitate a further success in solving the issues of life extension. The aim of the study is to establish the effect of cyclic thermal loads on the root-mean-square displacements of atoms as a sign of interatomic bond strength and microdamage of metal of superheater tubes. Object of study are specimens of heat-resistant pearlitic 12Cr1MoV steel from the weld-affected zone. Methods include physical simulation of operation conditions by thermal cycling of specimens in the MIMP-10UE furnace, X-ray dosimetry of specimens, evaluation of internal structural stresses using X-ray diffractometer DRON, and surface morphology by means of PENSCKOPE microscope. Results of the study established that thermal cyclic loads have effect on the root-mean-square displacements of atoms. It was shown that thermal effect results in increase of the strength of interatomic bonds. The paper demonstrates the correlation between submicro- and macroscopic characteristics of strength, as well as the fact that the amplitude of atomic thermal vibrations, being a characteristic of interatomic bonds strength, can be a diagnostic indicator of accumulation and development of metal damaging. Decrease of root-mean-square displacements amplitude during thermal cycling is explained by the hypothesis about occurrence of substitution solid solutions as a result of surface nitriding by molecular nitrogen of the atmospheric air. Under this hypothesis it was proved that one of oscillation mechanisms for internal stresses of the first-kind can be the process of surface nitriding during thermal cycling, which results in complex bonds between metal and nonmetal in the multicomponent alloyed steels, aimed at maintaining the stability of solid solution system.