PSI - Issue 72

Halyna Krechkovska et al. / Procedia Structural Integrity 72 (2025) 149–156

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2. Materials and methods Heat-resistant steel 12Kh1MF (analogue 14MoV6-3 steel, according to DIN 17175-1979 and DSTU EN 10216 2:2015 after ~28.6  104h of operation on the main steam pipelines bend of the TPP was studied. The strength characteristics (  UTS and  YS) of the metal cut from the outer surface of the pipe bend, obtained by tensile testing of samples in air, were 415 and 283 MPa, respectively, and the plasticity characteristics (elongation and RA) – 13.8 and 39.3% respectively. The radius and bending angle of the pipe were 1370 mm and 90  , respectively. The steam temperature in the pipe during operation reached 545 ºС, and the pressure was 14 MPa. Templates with a thickness of 12 mm were used for the RHT. They were cut out from the stretched zone of the operated bend of the pipe Ø325×38 mm. The previously substantiated optimal RHT mode was used, which included two-stage normalization (N1: T=1100°С, τ=150 min, N2: T=960°С, τ=30 min) and tempering T=740°С, τ=180 min ( Tsybailo ( 2 2023)). The mechanical properties of steel from the stretched bend zone (SBZ) were assessed at three levels by the pipe wall thickness (near the outer and inner surfaces and in the centre of its cross-section). The degree of steel degradation was evaluated by its strength (ultimate strength  UTS and yield strength  YS) and plasticity (elongation and reduction of area RA) characteristics. Cylindrical specimens  5 mm and with a working part length of 25 mm were tested on a UME-10T tensile testing machine with a strain rate of 3  10 – 3s – 1. The mechanical properties were determined as the average value of three tests carried out on samples of each of the analysed variants. The EVO 40XVP scanning electron microscope was used for fractographic studies. To assess the effect of hydrogenation on the stability of steel properties after RHT, tensile tests in the air were carried out on smooth samples of both steel variants (operated and restored) after their preliminary electrolytic hydrogenation. Hydrogenation of the samples was carried out for 15 min at a cathodic polarization current density of 50 mA/cm2 in an aqueous solution of sulfuric acid (pH0) with the addition of 2 g/l of thiourea. After hydrogenation, the thoroughly washed and dried samples were tested by tension in the air. 3. Results and discussion Analysis of the RHT effect on the mechanical properties of heat-resistant steel. It was found that the microstructure of the long-term used 12Kh1MF steel changes after RHT. In particular, the number of large grains decreases, while the number of small grains increases. At the same time, the number of voids along the grain boundaries decreases (Krechkovska (2024). After the RHT, the proportion of fine grains, as an important structural indicator of the state of steel, increased, and its hardness also increased (up to 170 HB). Therefore, it is logical to expect an improvement in the mechanical properties of the restored steel. As can be seen from Table 1, its mechanical properties increased after RHT and meet the requirements established by the regulatory document TU14-3460:2009. The results obtained by Krechkovska (2025), indicate the restoration of the steel's serviceability in all these characteristics.

Table 1. Mechanical properties of the operated 12Kh1MF steel after restorative heat treatment.

Specimen location in the pipe Near the outer surface (I) The center of the pipe section Near the inner surface (III) TU14-3460:2009 [Ukraine]

 UTS , MPa

 YS, MPa

Elongation, % RA, %

492 505 501

315 326 335

20.7 21.1 21.5 >19

62.7 68.2 66.2 >50

441…637

> 274

Changes in the mechanical properties of restored steel corresponding to the operation were analysed using the indicator: = – ∙ 100% (1) Tensile tests of restored and operated steel samples showed that after RHT, both strength characteristics (σUTS and σYS) increased slightly (Fig. 1). In particular, after RHT, the σUTS value increased by 18,5%, and σYS – by 11% for steel located near the outer surface of the pipe, where the most favorable conditions for creep are usually created