Krechkovska H. Methodology of the critical state assessment of long-term operated structural steels based on the changes in their structure, micromechamisms, and fracture resistance

An important scientific and technical problem has been solved in this work devoted to the development, approbation and implementation of methodology for assessing the structural and mechanical state of structural steels after their long-term operation under various temperature and stress conditions in corrosive and hydrogenative environments. The methodology is based on the relationships between quantitative structural, fractographic and mechanical indicators sensitive to their degradation. Carbon steels produced in the late 19th and early 20th centuries (Shukhov towers, Wroclaw bridges) as well as low alloy steels of gas and oil pipelines and also heat-resistant low- and high-alloy steels (elements of steam pipelines of TPPs and rotor blades of steam turbines) have been analyzed. On the basis of a comprehensive study of the mechanical characteristics of steels, the choice of indicators sensitive to their degradation has been substantiated. The impact toughness KCV was used for steels operated under ambient conditions, and the effective threshold of fatigue crack growth resistance ΔKth eff for ones operated at elevated temperatures. As the structural indicator of the state of heat-resistant steels, the sizes of carbides and grains were used, and as the fractographic one the fraction of the fracture surface with fractographic features of steel degradation. For the steels long term operated under ambient conditions, such signs were the delaminations and transgranular cleavages on the fracture surfaces of impact specimens, and in the case of high-temperature conditions the fragments of intergranular fracture against the background of transgranular fatigue relief on the fracture surfaces of specimens after fatigue crack growth testing. The methodology for assessing the structural and mechanical state of a number of structural steels after their operation on various objects has been scientifically substantiated basing on the plotted unified correlations betwїeen the drops of their mechanical characteristics and the change in informative indicators of the structural and fractographic changes in steels due to their degradation after long-term operation in both ambient and high-temperature conditions