PSI - Issue 68

Andriy Syrotyuk et al. / Procedia Structural Integrity 68 (2025) 880–886 Andriy Syrotyuk et al. / Structural Integrity Procedia 00 (2025) 000–000

885

6

After testing the notched specimens, their fracture surfaces were examined using a scanning electron microscope. It was focused on the study of zones I, II, and III (see Fig. 4). The obtained images of the fracture surfaces (Fig. 6a) confirm the above-presented results. For zone I, where the diffusible hydrogen only exists, the typical ductile fracture relief is observed: there are voids, dimples, and their coalescence. At the beginning of zone II, there is some intensification of the ductile fracture that leads to a decrease in the parameter W f (see Fig. 4). It can be explained by the increase in the diffusible hydrogen content C H(dif) . In our recent work (Dmytrakh et al. (2022)) we have found a formal reduction of the yield stress of low-alloyed pipeline steel under the presence of the diffusible hydrogen of low content. There it was shown that the low content of diffusible hydrogen leads to the change in the material’s microstructure increasing its defectiveness, and consequently, some change in its macroscopic mechanical behaviour can be expected (Dmytrakh et al. (2015)). At the hydrogen content C H = 0.456 ppm, where the residual hydrogen appears, we observed some signs of quasi-cleavage relief which intensifies at hydrogen content C H = 0.699 ppm. This indicates the beginning of the realization of the brittle fracture mechanism caused by the hydrogen embrittlement of material (Djukic et al. (2015); Lynch (2003)). So, zone II can be classified as the transition zone where the mechanism of fracture is gradually mixed. Zone III can be characterized as the zone of brittle fracture where the quasi-cleavage relief and microcracking are dominated (Djukic et al. (2015)). Hence, it may be concluded that in the case of the presence of a notch (model of damage like corrosion ulcer in the pipe wall), the hydrogen content C H ≥ 0.7 ppm should be considered potentially dangerous because it leads to significant loss of resistance to fracture of given pipeline steel (three times or more). Here, it should be also noted that in majority of the absorbed hydrogen is a residual which is trapped in the imperfection of the material microstructure (so-called ‘deep’ traps) (Lynch (2003). The features of the fracture surface of specimens with crack-like defects depending on the hydrogen content C H in the metal (Fig. 6b) correlate with the results obtained for the notched specimens (see Fig. 6a). There is also a ductile mechanism of fracture of the steel under a low content of diffusible hydrogen, which with increasing hydrogen content turns into mixed (voids, dimples, and quasi-cleavage). At the hydrogen content C H ≥ 0.6 ppm, the mechanism of brittle fracture becomes dominant (quasi-cleavage and microcracking), i.e. there is the hydrogen embrittlement of the material. Since the hydrogen content C H causes various effects on the fracture resistance of the steel, its value should be taken into account. For this purpose, we propose the parameter that is the index showing the loss of the resistance to fracture of the steel depending on the hydrogen concentration C H in the metal: ! " #

C C # =

(

)

'

! !

!"#$%"!

(3)

,

(

=

'

! " !

C

'

#

#

( ) ! "#$%&#" !

! " ! !

where is the fracture energy of the specimen, which was hydrogen charged to some defined hydrogen concentration С H(defined) . " # " # is the fracture energy of the specimen without the hydrogen charging ( C H » 0) and

b d e 3

;S2-0<#D #C4-%HH5#C#DH 7;19

?@a

!"#$%C#DE)*%H,)D0H$,

?@C

Z0D#)03)$-%H%$.5) $0D$#DH-.H%0D > ;7$-%H%.59

!"#$%C#DE)*%H,)$-.$L

?@B

?@A

1D,.D$#2)2#30-C.4%5%HS 7189

18W;1

?

> b? ; c)""C

?@?b

?@b

b

Fig. 7. Diagram for evaluation of hydrogen content effect on fracture behaviour of pipeline steel.