PSI - Issue 43

Marie Ohanková et al. / Procedia Structural Integrity 43 (2023) 300–305 Author name / StructuralIntegrity Procedia 00 (2022) 000 – 000

301

2

tube with a nominal outer diameter 457 mm and a thickness 40 mm was made of low-alloyed Cr steel and was operated at designed parameters: temperature 528 °C, internal pressure of overheated steam 12.6 MPa. A part of cut off segment was subjected to routine basic material tests in a laboratory of pipeline operator. These tests comprised an analysis of chemical composition, metallographic analyses of cross-sections using light microscopy, hardness measurement, tensile test at room temperature, and Charpy impact test, which resulted in very low impact energy of the pipe material (below 20 J). All other results met the requirements of materials standard. Because root cause of this embrittlement needed to be assessed, another part of the cut-off segment along with a comparative non-exposed sample, that originated in a similar non-welded steam pipeline tube with an outer diameter 243 mm and a thickness 25 mm made of same low-alloyed Cr steel, were then subjected to a detailed examination in the laboratories of UJP PRAHA a.s. and the Institute of Physics of Materials, Czech Academy of Sciences. This examination was based on two assumptions. First, the material of steam pipeline could be susceptible to a rapid thermal ageing or creep embrittlement, that can be verified through embrittlement factors J, J*, X, K and CEF, introduced by Watanabe (1974), Bruscato (1987) and Gooch et al. (1977). And second, the material of steam pipeline could be already aged in advanced, so the real exposure time could be much more then only 7 years. That could be manifested with a presence of nitrides at the grain boundaries, according to Briant and Barenji (1978), and/or a bainite spheroidization, as reported by e.g. Zhao et al. (2016). If the first assumption were to be confirmed, it would lead to the immediate shutdown of the steam pipeline followed by the associated economic and production loss. In the latter case, a non-destructive testing of the steam pipeline would take place and its inconvenient parts would be easily replaced. 2. Experimental 2.1. Material A cut-off segment from the steam pipeline was consisted of three parts: weld metal (WM) and both base metals (BM1, BM2). Their chemical composition including of a comparative non-exposed sample (NE) is compared with the Czech material standard for Cr-Mo-V steel 15128 in Table 1.

Table 1. A chemical composition of material discussed ( wt. % ). Material C Mn Si Cr

Mo

V

Al total

P

S

Standard

0.10-0.18 0.45-0.70 0.15-0.40 0.50-0.75 0.40-0.60 0.22-0.35 max. 0.025

max. 0.040

max. 0.040

BM1 WM BM2

0.15 0.09 0.17 0.16

0.54 0.89 0.51 0.56

0.20 0.43 0.20 0.21

0.53 0.59 0.52 0.59

0.49 0.57 0.55 0.43

0.25 0.32 0.24 0.33

0.004 0.004 0.005 0.013

0.008 0.014 0.008 0.011

0.013 0.010 0.004 0.004

NE

2.2. Methods All parts of studied materials were further cut into specimens for a following mechanical testing, which has included detailed Vickers hardness measurement (HV10), extensive tensile testing at room and operating temperature 528 °C as well, evaluation of transition temperature by Charpy notch impact testing, and middle-term creep testing at operating temperature and with an applied tensile stress in a range of 120 - 300 MPa. The specimens had a transverse or a longitudinal orientation and were cut either near both surfaces (outer, inner) or from the middle part of tube thickness. All mechanical testing was appropriately performed in accordance with the Czech standards. Metallographic investigations included SEM, TEM, and EDX, and chemical analysis of impurities were simultaneously carried out on cross-sections of all four material samples. The embrittlement factors were determined by following formulae (1) - all elements are in wt. %.