Page 9 - Дисертація на здобуття наукового ступеня доктора філософії

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metal. It was assumed that if the properties of critically degraded steel could be

successfully restored, then the restoration of metal from less degraded sections of the

steam pipeline would become even more possible.

To improve the microstructure and, consequently, increase the mechanical

properties of the operated steel, a mode of restorative heat treatment was proposed,

namely, double normalisation followed by tempering. The grain size and distribution

of carbides within the grains and along their boundaries were used as structural

parameters to assess the degree of restoration of the steel structure. At the first stage of

normalization of the steel in use at a temperature of 1100°C, its austenitization time

varied from 30 to 630 minutes. Holding samples at temperatures above Ac₃ intensifies

diffusion processes necessary for the redistribution of elements and homogenization of

austenite. Since these processes occur through diffusion, the duration of austenitization

determines the uniformity of element distribution in the matrix and the size of structural

components in the steel. In the second stage of the restorative heat treatment, the

samples were normalized at a lower temperature (960°C for 30 minutes) and then

tempered for 180 minutes at 740°C. This processing mode is recommended for

12Kh1MF steel in its initial state by regulatory documents.
The duration of holding samples of operated steel at the first stage of

austenitization with subsequent cooling in air was substantiated. To justify the holding
time, the grain sizes and distribution of grains of different sizes after different holding

times were analyzed. Based on the graphical dependencies of the percentage content
of grains of different sizes on the holding time of samples at a temperature of 1100°C,

the duration (approximately 150 minutes) was established, sufficient for homogenizing

of steel and refinement of its grain structure. It was believed that such a transformation

of the steel structure was promising for enhancing its creep resistance during operation

in steam pipelines. It was established that the distribution of grain sizes in the cross-

section of the pipes, formed at the first stage of normalization, remained practically

unchanged after the second stage of normalization (at 960°C), followed by tempering

at 740°C. This was considered as a factor of positive influence on the steel's creep

resistance.