Page 11 - Дисертація на здобуття наукового ступеня доктора філософії

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restorative heat treatment was also characterized by a relief of ductile dimples, but

small (up to 0,7 μm) carbides were most often found at their bottom. These carbides

retained a significant degree of cohesion with the matrix even after tensile testing of

the specimen. This was interpreted as direct evidence of the steel restoration. The

identified fractographic signs of operational degradation and subsequent restoration of

steel, after a proposed heat treatment regime, were also confirmed on fractures of

specimens tested for impact toughness. This once again confirms the universality of

the conclusions made, even when the restored steel is subjected to dynamic loads.

The described fracture mechanisms of the operated and restored 12Kh1MF steel

during tensile and impact tests demonstrated the restoration of the cohesion of carbides

to the matrix and a decrease in the percentage of large defects along grain boundaries.

The results of these observations are in good agreement with the recorded changes in

the steel's mechanical properties. Certification of the restored steel for hardness,

strength, ductility, and resistance to brittle fracture confirmed a significant

improvement in these mechanical properties compared to the corresponding

characteristics of the operated steel. Furthermore, the restored steel met the regulatory

requirements regarding these indicators, which justifies the advisability of using the
proposed mode of restorative heat treatment to extend the service life of critically

degraded, long-operated elements of steam pipelines of thermal power plants.
Considering the negative impact of hydrogenation on the properties of steels

(due to hydrogen embrittlement) and the inevitability of its manifestation during long-
term operation in steam pipelines, substantiation of the stability of the positive effect

of steel restoration on its characteristics under the conditions of the detrimental impact

of hydrogenation has become another urgent task.

Tensile tests of specimens in air from both operated and restored steels, after

their preliminary hydrogenation in both cases, showed a decrease in their mechanical

properties. The positive effect of steel restoration decreased somewhat after

hydrogenation, but nevertheless, remained positive in relation to the properties of the

operated steel. In particular, the strength characteristics of the hydrogenated specimens

of the operated steel decreased by a maximum of 3% compared to the non-