412
S. Papaefthymiou
temperatures when hydrogen is accumulated at traps in the microstructure,
causing cracks [58].
ii. Manganese (Mn) is used for solid solution strengthening. Generally, it is kept
low in steek in order to avoid centerline segregation [56]. The tolerable Mn
amount depends on the carbon content and should not exceed 1.2 wt% for HIC
resistant steels. HIC is preventable when the Mn content is 0.30–0.50 wt% [56],
57.
iii. Sulfur and Calcium: Mn contents, in conjunction with sulfur contents (restricted
between 0.001–0.002 wt%) are critical for avoiding MnS formation, as
manganese sulfides (MnS) are the main inclusion, affecting the HIC from
the viewpoint of crack initiation and propagation. Elongated MnS inclusions,
even a small number of them, can lower the fracture toughness and HIC
resistance. They can be eliminated by low S-content and controlling sulfide
morphology by the adding calcium (Ca) [4]. This is attainable through the
calcium–silicon (CaSi) treatment, which takes place at the end of secondary
metallurgy processing. This Ca addition not only modifies the sulfides, but also
promotes the oxides to become more globular. A typical Ca/S ratio is around 2
[57].
iv. Phosphorus (P) increases strength and hardness of steel, given the right content
and segregation conditions. The usual P content of pipeline steel ranges from
0.005 to 0.025 wt.% [3].
v. Other alloying elements of steel including copper (Cu), nickel (Ni), and
chromium (Cr) can effectively improve the steel’s HIC resistance, because
they have a corrosion retarding effect (reduce the corrosion rate) for mild sour
conditions and thus the H entrance in the steel [57, 58].
vi. Titanium (Ti) has a strong effect on nucleation of AF, which improves HIC
resistance. The Mn amount in the inclusions is reduced with the addition of Ti,
while the Ti levels are increased in the same time. This happens since TiN, TiC,
and TiO are formed preferably to MnS. The precipitated TiC and TiN can act
as beneficial hydrogen traps [4, 51].
vii. Molybdenum (Mo) is reported to improve HIC susceptibility, owning to the
formation of fine carbides during tempering. It is also observed that Mo limits
segregation of P at the grain boundaries and, thereby, affects the HIC resistance
[58]. Mo promotes the formation of bainite or bainitic ferrite (lowering the
amount of acicular ferrite formed) [59]. In combination with Niobium (Nb), as
micro-alloying element, positively affects strength and toughness through the
precipitation of Mo, Nb carbonitrides [56]. Ti, Nb, and Mo carbonitrides (CN),
which are nano-sized and precipitate within the grains and on dislocations,
act as harmless, numerous hydrogen sites not large enough to form cracks in
contrast to oxides [4, 52].
A high strength pipeline with low HIC susceptibility cannot be achieved without a
fine-grained microstructure having significant volume fraction of low transformation
temperature constituents. HIC resistance is improved after a tempering heat treatment to toughen the formed at low–temperature phase(s), e.g., bainite (B), bainitic
S. Papaefthymiou
temperatures when hydrogen is accumulated at traps in the microstructure,
causing cracks [58].
ii. Manganese (Mn) is used for solid solution strengthening. Generally, it is kept
low in steek in order to avoid centerline segregation [56]. The tolerable Mn
amount depends on the carbon content and should not exceed 1.2 wt% for HIC
resistant steels. HIC is preventable when the Mn content is 0.30–0.50 wt% [56],
57.
iii. Sulfur and Calcium: Mn contents, in conjunction with sulfur contents (restricted
between 0.001–0.002 wt%) are critical for avoiding MnS formation, as
manganese sulfides (MnS) are the main inclusion, affecting the HIC from
the viewpoint of crack initiation and propagation. Elongated MnS inclusions,
even a small number of them, can lower the fracture toughness and HIC
resistance. They can be eliminated by low S-content and controlling sulfide
morphology by the adding calcium (Ca) [4]. This is attainable through the
calcium–silicon (CaSi) treatment, which takes place at the end of secondary
metallurgy processing. This Ca addition not only modifies the sulfides, but also
promotes the oxides to become more globular. A typical Ca/S ratio is around 2
[57].
iv. Phosphorus (P) increases strength and hardness of steel, given the right content
and segregation conditions. The usual P content of pipeline steel ranges from
0.005 to 0.025 wt.% [3].
v. Other alloying elements of steel including copper (Cu), nickel (Ni), and
chromium (Cr) can effectively improve the steel’s HIC resistance, because
they have a corrosion retarding effect (reduce the corrosion rate) for mild sour
conditions and thus the H entrance in the steel [57, 58].
vi. Titanium (Ti) has a strong effect on nucleation of AF, which improves HIC
resistance. The Mn amount in the inclusions is reduced with the addition of Ti,
while the Ti levels are increased in the same time. This happens since TiN, TiC,
and TiO are formed preferably to MnS. The precipitated TiC and TiN can act
as beneficial hydrogen traps [4, 51].
vii. Molybdenum (Mo) is reported to improve HIC susceptibility, owning to the
formation of fine carbides during tempering. It is also observed that Mo limits
segregation of P at the grain boundaries and, thereby, affects the HIC resistance
[58]. Mo promotes the formation of bainite or bainitic ferrite (lowering the
amount of acicular ferrite formed) [59]. In combination with Niobium (Nb), as
micro-alloying element, positively affects strength and toughness through the
precipitation of Mo, Nb carbonitrides [56]. Ti, Nb, and Mo carbonitrides (CN),
which are nano-sized and precipitate within the grains and on dislocations,
act as harmless, numerous hydrogen sites not large enough to form cracks in
contrast to oxides [4, 52].
A high strength pipeline with low HIC susceptibility cannot be achieved without a
fine-grained microstructure having significant volume fraction of low transformation
temperature constituents. HIC resistance is improved after a tempering heat treatment to toughen the formed at low–temperature phase(s), e.g., bainite (B), bainitic