PSI - Issue 16

Valentyn Uchanin et al. / Procedia Structural Integrity 16 (2019) 192–197 Valentyn Uchanin, Orest Ostash / Structural Integrity Procedia 00 (2019) 000 – 000

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4. Е valuation of the steam pipelines steels during long-term operation

This chapter is related to the steam pipelines (SP) of power plants operated under high pressure (up to 16 MPa) and elevated temperatures (up to 570º C). As a result of long -term operation the initial structural-phase state of materials are transformed. These factors lead to changes in the mechanical characteristics of materials. It is especially important for the areas of SP with high level of stress concentration. In this study the regularities of CF changes for 12Kh1MF and 15Kh1M1F steels of the SP bends after long-term operation (175 –280·10 3 h) depending on the phase composition and microdefects are established. The purpose of this investigation is to determine the ultimate values of the CF, possible to be applied for prediction of the critical state of SP. A complex of properties of SP made of 12Kh1MF and 15Kh1M1F steels operated at the Ukrainian thermal power plants was investigated. The mechanical properties of steels under short-term tension were determined. The fatigue tests of the metal after in service degradation in different zones of the bends of steam pipelines were carried out under the conditions of pulsating ( R = 0.1) loading cycles with constant values of the range (Δơ nom ) at a frequency of 10 – 12 Hz. Analyzing the influence of structural factors on the delay of motion of the domain boundaries and retardation of dislocation glide, it was emphasized that this influence is similar for the magnetic and mechanical characteristics of steel, which means that these characteristics are correlated. Thus, in particular, the CF H C , as a rule, increases with the yield strength σ 0,2 , ultimate strength σ u , and hardness (НВ and HRC) of the material and, hence, as the relative elongation δ and relative narrowing ψ of the material decrease. Similar regularities were also established for heat resistant steel in the as-received state (prior to operation). After different modes of normalizing of cold-deformed 12Kh1MF steel, its strength ( σ 0,2 and σ u ) became 1.5 – 3 times lower, its plasticity ( δ) became 2.5 – 3.5 times higher, and the CF became twice lower (Ostash et al. 2014b)). The heat-treated steels in the as-received state (including the heat-resistant steels of SP) are characterized by the correlation between their strength and plasticity characteristics and the CF (Fig. 4 а): the increase in CF is attended by the increase in strength and the decrease in the plasticity of steel. For 12Kh1MF and 15Kh1M1F steels after long-term operation, absolutely different relationships between the CF and the mechanical characteristics were recorded (Figs. 4b, c), as compared with the relationships obtained for steels in the as-received state (after heat treatment), when the CF decreases as the strength of the steel decreases (Fig. 4 а). Obtained results (Figs. 4b, c) indicate that the mechanical characteristics of the steels specified by the current normative document change slightly depending on the degree of degradation, demonstrating some decrease, which is accompanied by an increase in CF H C . Here, the parameters of fatigue fracture mechanics, in particular, FCGR characteristics N i and  K th in the stages of initiation and initial growth of a fatigue macrocrack, are more sensitive (Figs. 4b, c). Hence, the possibility to evaluate the structural-mechanical state of SP steels after long-term operation by the CF measurements was justified.

Fig. 4. Relationships of mechanical characteristics versus CF for (a) 12Kh1 М F steel after different heat treatment regimes and (b) 12Kh1 М F and (c) 15Kh1 М 1F steels after exploitation degradation:   ultimate strength σ В ;   yield strength σ 0,2 ;  – elongation δ;   fatigue threshold Δ K th ;   period N i to fatigue macrocrack initiation at ∆σ nom = 60…66 M Pa. 5. Е stimation of stresses along ship load-bearing elements Investigations were concerned with the estimation of stresses by the determination of the CF distribution along ship load-bearing elements material during operational loading. The measurements were done on the hatch coamings of the “river - sea” cargo ship. To determine the distribution of CF along the ship structure the upper horizontal surfaces of coatings of the ship (as important load-bearing elements) are selected (Zavalnuk et al. (2013)).