Effect of repeated structural recrystallization of grade 20 steel on corrosion resistance of pipes of heating surfaces

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.