Predicting the state of heat-resistant steel of a steam pipeline at a TPP taking into account changes in its strength due to operational damage

Abstract

The microstructure of heat-resistant 15Kh1M1F steel is analysed in the initial state and after long-term operation on the main steam pipelines of TPPs. According to the criteria adopted in fracture mechanics, the low volumetric content of fine carbides, which did not exceed 0.5% in the structure of the analysed steel in the initial state, does not significantly affect its strength. However, as a result of long-term high-temperature operation, a redistribution of carbon and alloying elements occurred in its structure with the precipitation and coagulation of carbides along the grain boundaries. Large carbides concentrated along the grain boundaries have a low cohesion with the matrix, which promotes the formation of pores along their interfaces. As a result, the cohesion of adjacent grains decreases, which contributes to the formation of a system of intergranular microcracks. Merging, they form a macrocrack, which has a significant effect on the strength of the material.A mathematical model is proposed for predicting changes in the ultimate strength of steel, based on the well-known solution of the problem with tension of a body with a periodic system of collinear cracks. The relationship between the intensity of tensile forces, the distance between the cracks, their size and the size of the pre-fracture zones in the vicinity of operational defects is found. The beginning of the merging of the pre-fracture zones at the tips of the nearest defects was taken as the condition for material destruction.A dependence has been obtained that makes it possible to make predictive estimates of a decrease in the ultimate strength of long-term operation of steel on steam pipelines of different units, due to its additional damage under the influence of shutdown numbers of TPP units corresponding to this steel. To do this, it is only necessary to calculate the fraction of the steel resource loss according to normalized technological indicators, such as the operation lifetime and the number of shutdowns of the units.