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ABSTRACT

Krechkovska H.V. Methodology of the critical state assessment of long-term
operated structural steels based on the changes in their structure, micromechamisms,
and fracture resistance. – The manuscript.
Thesis for the Doctorʼs degree of Engineering Sciences by speciality 05.02.01 – Ma-
terials Science. – Karpenko Physico-Mechanical Institute of the National Academy of Sci-
ences of Ukraine, Lviv, 2020.
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, fracto-
graphic and mechanical indicators sensitive to their degradation.
th
th
Carbon steels produced in the late 19 and early 20 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 me-
chanical characteristics of steels, the choice of indicators sensitive to their degradation has
been substantiated. The impact toughness KCV was used for steels operated under ambi-
ent conditions, and the effective threshold of fatigue crack growth resistance ΔK th 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 back-
ground of transgranular fatigue relief on the fracture surfaces of specimens after fatigue
crack growth testing.
It was proved that the sizes of microstructure elements (carbides and grains), meas-
ured across the wall thickness of steam pipelines (in pipe radial cross-section), correlate
with circular tensile stresses acting permanently in the cross section of pipe wall under
steady-state operating conditions or occurred for a short time during planned or forced
shutdowns. It was shown that the pipe outer surface is subjected to tensile stresses at all
stages of their operation, and this stimulates the grain growth, intensifies the coagulation
of carbides along grain boundaries, and accelerates the pipe fracture due to creep.
It was established that a common fractographic sign of cohesion weakening between
adjacent grains in operated low- and high-alloy heat-resistant steels is intergranular frac-
ture fragments found on the operational fracture surfaces of power equipment elements
(straight sections and bends of steam lines of TPPs, the rotor blades of steam turbines) as
well as on the fracture surfaces of exploited steel specimens after fatigue testing. For both
types of heat-resistant steels, dependencies were plotted between the change in their ΔK th
values and fraction of the fracture surface with fractographic features of steel degrada-
eff
tion in the form of intergranular fragments against the background of transgranular fatigue
relief on the fracture surfaces of specimens. The critical states of both low- and high-alloy