PSI - Issue 68

Halyna Krechkovska et al. / Procedia Structural Integrity 68 (2025) 762–768 Halyna Krechkovska et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction Long-term operation of low-alloy Cr-Mo-V heat-resistant steels at high temperatures and under stress results in creep and structural changes. Creep damage is primarily caused by microstructural degradation, associated with the formation and coalescence of voids on second-phase particles such as carbides and non-metallic inclusions. These particles align along the direction of the applied load during creep tests (Song (2022)). At the same time, the grain size increases, and the carbides released at their boundaries coagulate, which creates the preconditions for the formation of pores and intergranular cracking (Student (1998), Romaniv O.M. et al. (1998), Dzioba et al. ( 1 2010), Dzioba ( 2 2010), Hutsaylyuk et al. (2024), Krechkovs’ka (2008)). Ultimately, the degradation of heat-resistant steels due to changes at the substructural level (Babii et al. (2011)) enhances the creep process (Babii et al. (2007)). The growth of subgrain sizes (Sawada et al. (2011), Armaki et al. (2011)), depletion of the ferrite matrix in alloying elements (Chen et al. (2011), Maruyama et al. (2023), Kvapilová (2022)) and the redistribution of carbides at grain boundaries, as well as the formation of pores at their boundary with the matrix, contribute to intergranular failure of steels due to creep (Krechkovska et al. (2023), Ostash et al. (2009), Krechkovska (2008), Krechkovska et al. (2022)). Such structural changes in steels occur over decades of operation, reducing their mechanical properties. In particular, structural changes (increase in grain size) and phase transformations (precipitation and coagulation of carbides along grain boundaries) cause deterioration of the properties of heat-resistant steels operated at elevated temperatures (Dobrzanski et al. (2007), Student et al. (2021), Student et al. (2018)). Restorative heat treatment (RHT) of used steels is aimed at improving their microstructure and, as a consequence, mechanical properties. In particular, it is known (Student et al. (2021)) that after additional heat treatment of 25Kh2M1F steel after 21∙10 4 hours of operation as fasteners (bolts and studs) in the high-pressure cylinder body at a thermal power plant (TPP), it was possible to achieve a certain improvement in the properties of the steel in terms of hardness, strength, and plasticity, and most importantly, to significantly increase its impact toughness. At the same time, repeated quenching followed by high-temperature tempering of 14MoV6-3 steel used in steam pipelines (according to the heat treatment regime provided for this class of heat-resistant steels in the original state) did not lead to a complete restoration of its properties despite the noted improvement in its microstructure (Hodžić et al. (2014, 2016)). In particular, the hardness and strength of the steel have increased, but the fatigue limit decreased significantly. Its decrease was explained by inclusions in the pearlite and ferrite matrix that didn’t dissolve during heat treatment. Négyesi et al. (2023) also used RHT to restore the creep resistance of in-service 14MoV6-3 steel and achieved encouraging results. The analysis of available research papers describing the results of using RHT to improve the properties of heat resistant steels after their long-term high-temperature operation confirms the relevance of such studies as promising for extending the service life of components in expensive large-sized structures. The purpose of this work is to study the structure, strength and plastic characteristics, as well as resistance to brittle fracture of 12Kh1MF steel both after long-term operation in the stretched bending zone (SBZ) of the TPP steam pipeline and after its subsequent restorative heat treatment, which is a necessary prerequisite for extending the resource of its high-temperature operation. 2. Materials and methods Heat-resistant 12Kh1MF steel (wt.%: 0.1 C, 0.22 Si, 0.5 Mn, 0.84 Cr, 0.01 S, 0.0005 P, 0.2 Cu, 0.19 Ni, 0.23 V, 0.28 Mo, Fe – balance) was studied after 2.86×10 5 hours of operation in the vertical bends of the main steam pipelines of the TPP at a temperature of 540 °C and a coolant pressure of 14 MPa. The outer diameter of the straight section of the pipe was 325 mm, and the wall thickness was 38 mm. The bend radius was 1370 mm, and the bending angle was 90°. To improve the microstructure and increase the mechanical properties of the studied 12Kh1MF steel, a RHT mode (two-stage normalization followed by tempering) is proposed. To optimize the RHT mode of the operated steel during the first stage of processing, the holding time at an austenitization temperature of 1100 °C was varied from 30 to 630 min (Tsybailo (2024)). In this way, the degree of austenite homogenization was regulated before normalizing the steel by cooling it in the air. After all, the duration of holding samples from pre-exploited 12Kh1MF steel above the critical point Aс 3 , where diffusion processes are intensified, predetermines the completeness of the redistribution of elements