Page 10 - Дисертація на здобуття наукового ступеня доктора філософії

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To determine the trends in the change in the structural parameters of heat-

resistant 12Kh1MF steel in the stretched bending zone, an analysis was carried out of

the content of small (up to 10 μm), medium (from 10 to 30 μm), and large (over 30 μm)

grains in its structure after operation and restorative heat treatment. It was shown that

after the restorative heat treatment, the percentage of large grains in the steel structure

decreased by 10%, while the percentage of small grains increased by 60%. The increase

in both the fine-grain amount and the hardness of steel after the recovery heat treatment

is consistent with each other. As a result, the hardness after the treatment increased to

170 HB compared to 120 HB for the operated steel.

The number of carbides (or pores) inside and at the grain boundaries was also

analyzed. It was found that after restorative heat treatment, the percentage of carbides

at the grain boundaries decreased, and their percentage inside the grains increased

slightly relative to the exploited steel. Moreover, a similar trend was observed

throughout the entire thickness of the pipe wall. The proposed mode of restorative heat

treatment proved to be effective in increasing both the strength and plasticity of the

steel. The restoration effect was most pronounced near the outer surface of the pipe,

where the improvement in plasticity reached 60%. Additionally, after restorative heat
treatment, the impact toughness of the steel increased more than twofold across the

entire thickness of the pipe wall.
Fractographic signs of degradation in the operated steel and features associated

with its restoration were identified. At the bottom of large dimples of ductile fracture
obtained during tensile testing of a smooth tensile specimen of operated steel,

nanosized particles (up to 300 nm) were detected using the fractographic method.

These particles were considered to be fragments of large carbides that had precipitated

and coagulated at grain boundaries during long-term operation of steel in a steam

pipeline. The cohesion between these particles and the surrounding matrix was finally

broken when the specimen failed during tensile testing. Nano-sized particles found at

the bottom of the dimple relief were interpreted as evidence of the preservation of

cohesion between the matrix and large inclusions (at least partially) even at such a high

level of steel degradation. The fracture of a specimen of the operated steel after its