PSI - Issue 42

Halyna Krechkovska et al. / Procedia Structural Integrity 42 (2022) 1398–1405 Halyna Krechkovska / Structural Integrity Procedia 00 (2022) 000 – 000

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adjacent grains as a result of steel operation under severe conditions is possible to detect only at the stage of uncontrolled fracture. It should be pointed out that not all intergranular facets observed on the fracture surfaces of impact specimens are associated with completely damaged grain boundaries as a result of creep during long-term operation of the steel; however, they clearly indicate a decrease in cohesion between adjacent grains caused by steel degradation. Thus the area of intergranular facets in the unit area of the fracture surfaces of impact specimens has been used as a quantitative parameter for estimating the current structural and mechanical state of the steel with a different degradation degree, but, first of all, their critical state since we analysed the areas of uncontrolled (spontaneous) fracture. The changes of the proposed fractographic indicator across the pipe wall of the investigated steam pipelines have been analised. It is the largest on the fracture surfaces of the specimens cut out near the outer surface of the pipe, the minimum on the fracture surfaces of the specimens from the central part of the section, and, in the vicinity of the inner surface, this indicator has an intermediate value. Krechkovs ’ ka et al. (2019) explained a different intensity of steel degradation across the pipe wall by a gradient of thermal stresses in it, caused by shutdowns of power units. Taking this into account, in further studies on fracture toughness and fatigue crack growth resistance, we analyzed the part of intergranular facets on the fracture surfaces of specimens cut exactly near the outer surface of the pipes.

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Fig. 2. Fractographic signs of cohesion weakening between adjacent grains in the form of intergranular fracture fragments on the fracture surfaces of the specimens of the operated steel subjected to 501 (a ,с ) and 576 (b,d) shutdowns of TPP power units after impact (a, b) and fracture toughness ( c, d) tests. Fractographic signs of operational degradation on the fracture surfaces of steel specimens after fracture toughness tests. Analysis of the fracture surfaces of specimens used for fracture toughness evaluation revealed a narrower band of ductile static crack growth from the pre-crack for the steel with more shutdowns of the technological process, similar to the case of impact specimens. This is a typical sign of in-service embrittlement of steel. In addition, bond weakening between adjacent grains in the form of intergranular fracture fragments against the background of cleavage with typical river patterns is confirmed, detected only for the operated steel specimens. In this case, the part of intergranular fragments in the unit area of the fracture surface is also bigger for the steel subjected to more shutdowns (Fig. 2 c, d). Therefore, fractographic analysis of the steel specimens after fracture toughness tests confirmed the conclusion derived from the impact tests. Fractographic signs of operational degradation on the fracture surfaces of steel specimens after fatigue crack growth tests. In this case, the local parameters of fracture mechanics, namely, the threshold values of fatigue crack