PSI - Issue 59

Hryhoriy Nykyforchyn et al. / Procedia Structural Integrity 59 (2024) 82–89 H. Nykyforchyn et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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A series of experimental tests were carried out in order to assess changes in the mechanical behaviour of the steel in the different states, as well as resistance to stress corrosion cracking (SCC). All specimens for tests were machined from the pipe steel section longitudinally oriented with respect to the pipe axis. Strength and plasticity characteristics (yield strength σ YS , ultimate strength σ UTS , and reduction in area RA), as well as SCC resistance, were determined by tension tests of cylindrical specimens with a diameter and a gauge length of 5 mm and 25 mm, respectively. Reduction in area was preferred over elongation in assessing plasticity because it consistently decreases under the influence of embrittling factors. In contrast, elongation may not fully reflect embrittlement effects or even atypically increase due to the opening of operational microdefects in steels, as demonstrated by Nykyforchyn et al. (2004) in the case of a heat-resistant power plant steel and in gas pipeline steels by Gredil (2008), Hredil (2011) and Zvirko et al. (2021). A similar situation can be expected in SCC tests, which is why RA was preferred in this case, as well. The strain rate for the mechanical properties determination of the steel in air was 3×10 -3 s -1 , while the slow strain rate tension method was used for SCC testing with a strain rate of 3×10 -7 s -1 . As a corrosive environment for SCC tests, the NS4 test solution was used, which is commonly used in order to simulate the embrittling effect of groundwater on gas pipeline steels (Voloshyn et al. (2015), Nykyforchyn et al. (2019), Shtoyko et al. (2019), Zvirko et al. (2023) and others). The chemical composition of the NS4 test solution is presented in Table 1. The resistance to brittle fracture was determined by measurements of characteristics of Charpy impact toughness (KCV) and fracture toughness by the J-integral method with the J 0.2 parameter, which corresponds to the J -integral level at a 0.2 mm crack growth increment, as specified by the standard ASTM E 813.

Table 1. Chemical composition of the NS4 test solution. Components KCl

NaHCO 3 CaCl 2  2H 2 O MgSO 4  7H 2 O

Concentration in g/L 0.122 0.483

0.181

0.131

In order to perform fracture surface examinations and to define the fracture mechanisms of the steel in different conditions, a SEM EVO 40-XVP scanning electron microscope was employed. 3. Results and analysis Table 2 summarizes the mechanical properties of the 17H1S steel in different treated states, obtained through testing in air and using slow strain rate tension tests in the NS4 solution.

Table 2. Mechanical properties and SCC resistance of the studied pipeline steel 17H1S in different states. Steel state Environment

Ultimate strength σ UTS [MPa]

Yield strength σ YS [MPa]

Reduction in area RA [%]

Impact toughness KCV [J/сm 2 ]

Fracture toughness J 0.2 [N/mm]

531 535 533 529 538 537

428 433 435 429 431 434

71 72 74 69 68 53

129 125 131

322 330 286

As-delivered state

Air Air Air

LTT250

PEH + LTT250 As-delivered state

NS4 solution NS4 solution NS4 solution

LTT250

PEH + LTT250

Based on the provided data in Table 2, the following conclusions have been drawn. The application of the low temperature tempering for 1 hour at 250 ºС (LTT250) had no significant impact on the mechanical properties of the investigated steel and its resistance to SCC. The lack of influence from this treatment regime suggests the stability of the normalized steel state due to the relatively low-temperature tempering, which aligns with the principles of classical materials science. Combining PEH with low-temperature tempering for 1 hour at 250 ºС according to the (PEH + LTT250) regime, while not affecting the strength, plasticity, and impact toughness of the steel, slightly