The article presents the results of a study on the impact of multiple normalization of the thermal conductivity of carbon steel 20 used in the power system for the manufacture of pipes of heating surfaces.The effect of heat treatment on the emerging nature of corrosive deposits.It has been found that in addition to the phase state of steel 20 on the thermal conductivity value is influenced by structural changes in the steel during heat treatment.With increasing interlamellar distance and cementite platelets thickness thermal conductivity increases.Studies have shown that the thermal conductivity of sample tubes in the delivery state at a temperature close to the operating temperature (300 °C), has a considerable scatter in the data (10 to 20%).Analysis of the samples destroyed showed corrosion rate of thermal fatigue 1.4 times vary depending on the level and type of the thermophysical properties of the scale.

Relevance of the research is caused by the need to minimize the corrosion losses of metal of tubular heating surfaces at their operation. This largely affects the urgency of the problem of ensuring the reliable operation of boiler units at thermal power plants and industrial power facilities. The increase in reliability and safety of operation of heat-mechanical power equipment, while improving the safety of life and reducing the risk of potential harm to operating personnel and the environment, is directly related to energy-efficient use of resources. The important aspect of the relevance of the work lies in the fact that the object of the study is carbon steel pipe (steel grade 20), which belongs to the most widely used in the Russian power engineering for manufacturing boilers heating surfaces. The main aim of the study is to analyze the microrelief of the outer and inner surface of new pipe size 60x6,0 mm, made of steel 20; to study structural heterogeneity in wall thickness and size of the microstructure parameters for predictive conclusions on the reliability and the prevailing mechanisms of damage that will be developed at operation; to investigate possible additional measures improving the operational properties and service life of pipes of heating surfaces. The methods used in the study: metallographic methods of investigating the structural components of steel, micro-hardness measurement methods. The results. Different types of defects (scab unrolled, intergranular cracks, defects in the form of grooves with rounded bottoms), which are evenly distributed over the surface of the pipe, were found on the surfaces in delivery state. The defects depth reaches a half of the acceptance level for the exploited pipes. Structural inhomogeneity and difference in dimensional parameters of microstructure are detected over the thickness of the pipe wall. The presence of these defects and inhomogeneity of the pipe material lead to significant changes in failure development rate and reduce the life of boiler tubes. The authors have proposed the proprietary methods to improve surface properties and reduce structural heterogeneity of the pipes. The methods (steam-pipe processing mode, double normalization mode at 920 degrees C) are available for implementing at thermal generation companies and reduce effectively the corrosion rate.

The necessity to enhance the operating characteristics of boiler steels is related to a continuous increase in corrosion damages of pipes of heating surfaces. Therefore, the actual task remains the development of ways to enhance the corrosion resistance of pipes made of grade 20, which are used as heat-absorbing elements in heat power engineering. The effect of cyclic modes of normalization (repeated structural recrystallization) on microstructural characteristics and the mechanical and corrosion properties of grade 20 steel in accordance with the regulatory requirements for products of this kind is studied. It is established that twofold normalization for grade 20 carbon steel is the optimum heat treatment mode for equalizing the ferrite grain sizes and decreasing the corrosion rate. It is revealed that this heat treatment mode increases the inequigranularity factor by three times in comparison with the original magnitude. Subsequent normalization cycles result in the formation of rejected microstructures and a decrease in mechanical properties of metal. The increased homogeneity of the microstructure at the double normalization decreases the corrosion rate by 38–51% of the original magnitude. The obtained results can be used for prolongation of the operation life by a decrease in the corrosion rate in pipes normalized twice as well as for the calculation of the remaining life of heating surfaces of boilers of heat power plants.