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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V-notch impact specimens. Thus, the pre-fracture zone in the impact specimen encompasses a larger number of grains with damaged boundaries or weakened cohesion between them. As a result, the fracture in the degraded steel occurs along the path of least resistance involving the grains located at a much greater distance from the main crack path. Obviously, in this case, the probability of coalescence of a larger number of damaged grain boundaries increases, and, therefore, an almost intergranular fracture surface is formed. In contrast, the pre-fracture zone in the SENB specimen is considerably smaller under fracture toughness testing and contains a fewer grain boundaries damaged due to long-term operation. The pre-fracture zone schemes (Fig. 4) illustrate their sizes and the difference in stress distribution in the fatigue crack tip and V-shaped notch under the testing of corresponding specimens. It is obvious that intergranular fragments on the fracture surfaces were visualized at the stage of uncontrolled fracture of specimens due to the presence of damaged or weakened grain boundaries in the pre-fracture zone encountered on the path of the fracture front. When this zone is large enough as in the impact specimens, the crack front can considerably deviate from the main crack path, following the direction of the least resistance. Thus, it is the difference in the size of the pre fracture zone and the corresponding amount of operationally damaged grain boundaries that determine the sensitivity of the proposed fractographic indicator to in-service damages in the steel. It is admitted that the fractographic indicator of steel degradation determined on the fracture surfaces of the specimens after their fatigue testing quantitatively characterizes a part of the grain boundaries, damaged (with complete separation) during high temperature creep of the steel, whereas the one determined on the fracture surfaces of the impact specimens encompasses both types of boundaries, i.e., completely separated due to the intergranular mechanism of creep and only weakened by partial damage in the form of pores which reduce cohesion between adjacent grains.

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Fig. 4. Schemes of the grain structure and stress distribution in the pre-fracture zone of the specimens for fatigue crack growth testing (I), fracture toughness (ІІ) and Charpy (ІІІ) testing, substantiating a different sensitivity of the proposed fractographic indicator to the steel degradation. Thus, the Charpy impact test can be recommended for wide use for the evaluation of the degradation degree of the steel since the fracture surfaces of impact specimens ensured the highest sensitivity in the fractographic assessment of the number of grains with damaged boundaries as a result of creep and structural transformations during the steel operation. In addition, it is the fastest type of the mechanical testing to implement. The obtained values of the proposed fractographic indicator of the state of the steel 15H1M1F after its long-term high-temperature operation on steam pipelines of PPP are summarized in Fig. 5. In particular, after fatigue crack growth tests, the part of intergranular fragments S near the outer surface of the pipe (where the effect of creep is maximum) reached 34% for the operated steel subjected to 576 shutdowns during operation, whereas for the steel after 501 shutdowns this parameter is only 22% (Fig. 5). After fracture toughness tests, the part of intergranular fragments was 25% and 41%, respectively, for the steel with less and more shutdowns. Finally, the highest values were observed after the impact tests when the proposed fractographic indicator reached 60% and 92%, respectively, for 501 and 576 shutdowns of the technological process. Thus, fractographic studies of the long-term operated heat-resistant steel 15H1M1F made it possible to visualize and quantify protruding fragments of intergranular fracture against the background of generally shallow fracture surfaces of the specimens tested for impact toughness, fracture toughness and fatigue crack growth resistance.