12 Industrial Pipeline Welding
411
The presence of protective atmosphere can increase cost. Thus, the following
gases are preferably used for the shielding atmosphere:
• In the case of non-active gas, either nitrogen, helium, and argon, or mixed gas of
at least two of them can be used.
• In the case of deoxidation gas, hydrogen and carbon monoxide, or mixed gas of
the two is recommended.
• In the case of utilization of mix of non-active and deoxidation gases, mixed gas
of the above [29].
12.4.4 Hydrogen-Induced-Cracking-Related Issues
As mentioned previously, the hydrogen atoms can be trapped in interstitial sites in
the lattice and accumulated in a way that it can form H 2 increasing the local pressure
to such an extend that cracks might form and propagate. According to Park et al.
and Carneiro et al. [49, 50], the trapping affinity increases in the order as follows:
second phase particles, coarse pearlite (P c ), fine pearlite (P f ), bainite (B), and martensite (M). Consequently, the martensite/austenite (M/A) constituents, the cementite
Fe 3 C (θ), interfaces, and the pearlite banding greatly affect the tendency for HIC
compared even with Al–Ca–Si oxides, sulfides, and (Ti, Nb, V, Mo)(C, N) precipitates which are irreversible traps for H
+ . Larger iron carbide particles are formed
during tempering by agglomeration of smaller ones, reducing the total interfacial area
of ferrite (F)/Fe 3 C. These act as reversible traps for H atoms [4, 50–52]. AF is a HIC
resistant due to its fine size and the absence of Fe 3 C [50, 51, 52, 23]. Titanium (Ti)
promotes the AF nucleation and weld metal ductility [51]. The various types of acicular ferrite reported in literature are: irregular grain boundary with small sub-grain
boundaries [50], randomly oriented, and needle-shaped after welding. HIC susceptibility is increased by bainite (B) morphologies and M/A constituents. Cracks may
form when H atoms accumulate and reach a critical value at the bainitic lath boundaries [53]. Increased cooling rate during the heat cycle will lead to finer BF and M/A
islands [4]. Experiments revealed that the diffusible H in steel with F/AF was higher
than with F/B steel [54] that means reversible H trapping of AF is higher, preventing
the H migration to the irreversible traps, thus increasing the HIC resistance. The structure of degenerated pearlite (DP) resembles pearlite without the banding. At intense
cooling, carbon diffusion is insufficient to form cementite lamellas. Steels with DP
exhibit good HIC resistance [4, 55]. Other researchers claim the most HIC resistant
structures consist mainly of polygonal or quasi-polygonal ferrite (PF) [4, 56].
The effect of the most important alloying elements is as follows:
i. Carbon (C) has a moderate tendency to segregate during solidification and
create various steel phases. The lower the carbon content (0.04–0.05 wt %),
the less the segregation and thus the higher the HIC susceptibility [57]. Finally,
methane (CH 4 ) can be formed by the reaction of hydrogen with carbon at high
411
The presence of protective atmosphere can increase cost. Thus, the following
gases are preferably used for the shielding atmosphere:
• In the case of non-active gas, either nitrogen, helium, and argon, or mixed gas of
at least two of them can be used.
• In the case of deoxidation gas, hydrogen and carbon monoxide, or mixed gas of
the two is recommended.
• In the case of utilization of mix of non-active and deoxidation gases, mixed gas
of the above [29].
12.4.4 Hydrogen-Induced-Cracking-Related Issues
As mentioned previously, the hydrogen atoms can be trapped in interstitial sites in
the lattice and accumulated in a way that it can form H 2 increasing the local pressure
to such an extend that cracks might form and propagate. According to Park et al.
and Carneiro et al. [49, 50], the trapping affinity increases in the order as follows:
second phase particles, coarse pearlite (P c ), fine pearlite (P f ), bainite (B), and martensite (M). Consequently, the martensite/austenite (M/A) constituents, the cementite
Fe 3 C (θ), interfaces, and the pearlite banding greatly affect the tendency for HIC
compared even with Al–Ca–Si oxides, sulfides, and (Ti, Nb, V, Mo)(C, N) precipitates which are irreversible traps for H
+ . Larger iron carbide particles are formed
during tempering by agglomeration of smaller ones, reducing the total interfacial area
of ferrite (F)/Fe 3 C. These act as reversible traps for H atoms [4, 50–52]. AF is a HIC
resistant due to its fine size and the absence of Fe 3 C [50, 51, 52, 23]. Titanium (Ti)
promotes the AF nucleation and weld metal ductility [51]. The various types of acicular ferrite reported in literature are: irregular grain boundary with small sub-grain
boundaries [50], randomly oriented, and needle-shaped after welding. HIC susceptibility is increased by bainite (B) morphologies and M/A constituents. Cracks may
form when H atoms accumulate and reach a critical value at the bainitic lath boundaries [53]. Increased cooling rate during the heat cycle will lead to finer BF and M/A
islands [4]. Experiments revealed that the diffusible H in steel with F/AF was higher
than with F/B steel [54] that means reversible H trapping of AF is higher, preventing
the H migration to the irreversible traps, thus increasing the HIC resistance. The structure of degenerated pearlite (DP) resembles pearlite without the banding. At intense
cooling, carbon diffusion is insufficient to form cementite lamellas. Steels with DP
exhibit good HIC resistance [4, 55]. Other researchers claim the most HIC resistant
structures consist mainly of polygonal or quasi-polygonal ferrite (PF) [4, 56].
The effect of the most important alloying elements is as follows:
i. Carbon (C) has a moderate tendency to segregate during solidification and
create various steel phases. The lower the carbon content (0.04–0.05 wt %),
the less the segregation and thus the higher the HIC susceptibility [57]. Finally,
methane (CH 4 ) can be formed by the reaction of hydrogen with carbon at high