PSI - Issue 36

Halyna Krechkovska et al. / Procedia Structural Integrity 36 (2022) 43–50 Halyna Krechkovska, Viktor Sylovanyuk, Oleksandra Student et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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indices τ N and N N ), the value of the proposed indicator approaches unity. The fraction of the exhaustion of the regulated values of the operational indicators (using (τ op ٠ N op ) / (τ N ٠ N N ) ratio) was 0.91 for steel after 501 shutdowns, and achieved 1.04 for steel after 576 shutdowns. Hence, it was concluded that the steel from block No. 2 has already exhausted its allocated resource, and the steel from block No. 1 still has 10% of its resource in reserve. If merger of the pre-fracture zones (marked as II in Fig. 3 d ) in the tip of one of the defects (marked as l in Fig.3 d ) with another defect closest to it is taken as the condition for material destruction, from relation (1) it is possible to obtain dependence for predicting the ultimate strength of the operated metal: = 0 (1 − ) , (2) where ω = ω τ = a av / d av is the damage index. This indicator was experimentally determined for steel in service during the time τ op . For this, the results of metallographic analysis of the microstructure of steel near the outer surface of the pipe were used, where maximum circumferential tensile stresses appeared at all stages of operation of steam pipelines. Measurements of both parameters of the microstructure ( a i and d i ) were carried out at a distance of ~ 1 mm from the outer surface of the pipes and only along those grain boundaries that were oriented perpendicular to the direction of action of circumferential tensile stresses in the analysed steam lines. The base of measurements of both parameters of the microstructure was ~ 1 mm. The measured values were used to determine the average values of the sizes of carbides d av and the distance between their centres a av . In particular, the values of the damage index ω τ for two variants of the steel after practically the same operating time (  2  10 5 h), but with a different number of shutdowns (501 and 576) of the blocks were 0.11 and 0.27, respectively. Based on the calculated damageability indexes ω τ for both steel variants and the corresponding proportions of the exhaustion for them of standard indicators (guaranteed operating time τ N and the normalized number of shutdowns of the unit N N ) a correlation was obtained between the proposed technological indicator (τ op ٠ N op ) / (τ N ٠ N N ) and by calculated damageability index ω τ . It was conventionally assumed that for steel in the initial state, these both indicators are close to zero, while for critically degraded steel the indicator (τ op ٠ N op ) / (τ N ٠ N N ) should tend to 1 (Fig. 4). It can be seen from the figure that the steel of unit No. 2 has already exhausted the allocated resource, and the steel of unit No. 1 has near ~ 10% of the resource in reserve.

1,25

N op = 576

1,00

 op N op ) / (  N N N )

N op = 501

0,75

0,50

0,25

(

0,00

0,05 0,10 0,15 0,20 0,25 0,30

w 

Fig. 4. The relationship between the experimentally determined values of damageability indices ω τ for all steel variants and the corresponding values of the proposed technological indicator (τ op ٠ N op ) / (τ N ٠ N N ), which characterizes the degree of exhaustion of the normative characteristics, such as the operating time τ N and the number of shutdowns N N of TPP units for their entire operation time τ op . Based on relation (2) and taking into account the dependence shown in Fig. 4, a graphical relationship was built between the loss of the ultimate strength  / 0 of the operated steel  relative to the corresponding value in the initial state 0 and the fraction of resource exhaustion according to the proposed indicator (τ op ٠ N op ) / (τ N ٠ N N ) (Fig. 5). Recall that the service life of both considered variants of steel on the different steam pipelines was the same. Consequently, the obtained difference in the effects of reducing their ultimate strength can only be associated with increased degradation of the steel microstructure under the influence of a temperature gradient and, as a consequence, thermal stresses in the cross sections of pipes due to shut downing blocks. This dependence makes it