Page 8 - Дисертація на здобуття наукового ступеня доктора філософії

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are exposed to elevated temperatures and steam pressure, leading to creep and

corresponding changes in their structure as well as physical and mechanical properties.

The dissertation analyses the features of the changes in the technical state of

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heat-resistant 12Kh1MF steel after long-term (~2,86∙10 hours) high-temperature
operation (545°C at a steam pressure of 14 MPa) in the bend of the main steam pipeline

of a thermal power plant. It was revealed that the strength characteristics of the steel in

all bending zones (stretched, neutral and compressed) and at all levels along the pipe

wall thickness (near its outer and inner surfaces and in the center of the pipe cross-

section) were below the regulated values. The highest degree of metal degradation was

observed in the stretched zone of the bend. Despite the retention of higher yield strength

values here, both plasticity characteristics – relative elongation and reduction of area –

no longer met the required standards. Additionally, it was found that the metal

degraded most intensively near the outer surface of the pipe in the stretched bend zone,

which is associated with more favorable stress conditions for creep here. This

degradation was manifested by a change in the shape of the grains, which stretched

through the pipe wall thickness, as well as the precipitation and coagulation of carbides

along grain boundaries. The steel near the outer surface of the pipe bend had a
particularly low plasticity reserve, since both of its plasticity characteristics were

significantly lower than the regulated values. The low impact toughness of the steel
near the outer surface is consistent with low values of reduction of area and elongation.

Such low values of brittle fracture resistance and plasticity characteristics of the
operated steel were considered direct evidence that the metal in the stretched bend zone

had reached a critical state.

Fractographic analysis of the fractures obtained after tensile and impact testing

of specimens confirmed the more intense degradation of steel near the outer surface of

the pipe from the stretched bend zone. This was manifested by the decohesion of

carbides from the matrix during specimen fracture. Taking into account the

experimentally established maximum degradation of steel in the stretched bend zone,

as well as its lowest mechanical characteristics near the outer surface of the pipe, this

zone was chosen for further substantiation of the possibility of restoring the exploited