PSI - Issue 59

Ivan Tsybailo et al. / Procedia Structural Integrity 59 (2024) 307–313 Ivan Tsybailo et al. / Structural Integrity Procedia 00 (2019) 000 – 000

309

3

for hydrogen dissolved in the metal. Getting into these traps hydrogen is molyzed and creates pressure inside them. This promotes the separation of carbides from the matrix and facilitates the coalescence of microvoids along grain boundaries. In other words, hydrogen can promote microcracking along grain boundaries by coalescing adjacent microvoids (Martin et al., 2020). In addition, such structural transformations during the long-term operation of heat resistant steels at high temperatures in a hydrogenation environment worsen their mechanical characteristics, which ensures their serviceability at the beginning of operation (Student et al. (2018), Wang et al. (2017)). Under such conditions, hydrogen can form unevenly distributed segregations along grain boundaries or chemically interact with carbon to form microvoids filled with methane under high pressure, decreasing steel's mechanical properties (Saba (2003), Louthan (2008)). All these damage accumulation processes are intensified by creep and, at the same time, contribute to its occurrence. The appearance of voids and the accumulation of damage in the metal usually occurs during operation in several stages. During the first three stages of steel operation, a gradual diffusion redistribution of carbide-forming elements to the grain boundaries occurs with the precipitation and coagulation of carbides along them (Fig. 1). At stage 4, single microvoids appear at the triple junctions of ferrite grains. At the next stage, their number increases, and chains of the voids are formed along the grain boundaries, predominantly oriented normally to the direction of action of the applied stresses. In the fifth stage, microvoids combine to form microcracks, the length of which can exceed the grain size. At the last stage of damage, the formation of a main crack occurs due to the merging of individual microcracks and its propagation along grain boundaries. The destruction of the element at this stage of steel degradation occurs in a relatively short time, not exceeding 5% of the total durability. Of course, the duration of each stage of damage accumulation depends on the service conditions and the metal's structural state at the beginning of its operation.

Fig. 1. Conditional curves of changes in creep strain ε (I) and damage accumulation along grain boundaries ω (II) during operation time τ op . Stages of accumulation of microdamage in metal as a result of creep: 1 – enrichment of the steel solid solution with carbide-forming elements; 2 – precipitation of carbides at the junctions of three grains; 3 – coagulation of carbides and the formation of their chains along grain boundaries, normally oriented relative to the applied stresses; 4 – decohesion of carbides from the matrix with the formation of voids; 5 – a coalescence of voids with the formation of intergranular cracks. Similar stages of damage accumulation are also characteristic of the bend metal of a steam pipeline at a TPP. However, due to the different thicknesses of the pipe wall in the area of its bending, creep processes and, accordingly, steel degradation in the SZ occur more intensely than in the neutral and compressed zones. As a result of the uneven distribution of stresses in the cross-section of the pipe in different zones, prerequisites may arise for the simultaneous implementation of all five stages of the development of micro damages. With increasing stress levels, the number of newly formed voids increases and with increasing temperature, their growth accelerates. It is clear that the rate of void formation also depends on the metal structure. In particular, in steels with a ferrite-carbide