414
S. Papaefthymiou
Researchers have studied fractures of both H charged and uncharged steels. They
showed that the fracture of the uncharged steel is characterized by micro-voids and
shear dimples, whereas with higher H concentration (2 and 4 ppm), the fracture
mode changes into a more brittle, quasi-cleavage fracture, [4, 51, 52]. These same
researchers agree that nano-sized carbonitrides could delay crack propagation due
to HIC and act as beneficial traps. For this reason, stringers of Ca-Al oxides (d =
2-3 μm) up to 30 μm length are detrimental as they induce interfacial de-cohesion,
support local void growth and coalescence, or directly lead to cleavage fracture.
With regard the proper pipeline welding for sour service, we can conclude the
following:
1. Pipelines with excellent resistance under sour environment should obtain a
microstructure based mainly on acicular ferrite and less on degenerated pearlite
avoiding hard phases.
2. Low-C, medium-Mn micro-alloyed steel grades are the most suitable materials
for such applications. Micro-alloying allows for more H+ entrapment.
3. The hydrogen entrapment as related to HIC susceptibility can be empirically
defined as follows:
(Fe 3 C) > (Al, Mn, Mg, Ca)(O, S) > MnS > (Nb, V, Ti)(C, N) > (Nb, V, Ti)C >
MoC. This sequence can be altered based on C, Mn levels.
4. Post-weld heat treatments must be carefully selected based on steel chemistry, customer’s required mechanical properties, and the pipe-manufacturers
capabilities.
5. After austenitization quenching, a relatively short annealing (or tempering) positively influences the second-phase morphology and distribution that contributes
to stress relieving, mobilizes atomic hydrogen avoiding its irreversible entrapment, and lowers HIC susceptibility of the weld and heat-affected zones.
12.4.5 Low Temperature Behavior and Weld Microstructures
Reduction of the grain size improves yield strength and toughness simultaneously.
A fine-grained microstructure provides numerous grain boundaries hindering crack
propagation. Therefore, a higher energy threshold is required for fracture to propagate. The formation of fine ferrite grains can be promoted by introducing dislocations in austenite at the deformation stage during controlled rolling. Moreover, the
restriction of austenite growth with the addition of micro-alloying elements further
promotes fine ferrite grain formation [7].
The microstructural constituent which contributes to high strength and good
toughness is acicular ferrite. The needle-like microstructure of acicular ferrite developing to different directions impedes the fracture propagation compared to more
oriented microstructural constituents [61]. Fine packets of bainitic ferrite are also
known to promote high yield strength and toughness at low temperatures due to the
high dislocation density and multiple grain boundaries. However, the presence of
cementite laths and/or M/A constituents in bainite may act as trap of H
+ , leading to
S. Papaefthymiou
Researchers have studied fractures of both H charged and uncharged steels. They
showed that the fracture of the uncharged steel is characterized by micro-voids and
shear dimples, whereas with higher H concentration (2 and 4 ppm), the fracture
mode changes into a more brittle, quasi-cleavage fracture, [4, 51, 52]. These same
researchers agree that nano-sized carbonitrides could delay crack propagation due
to HIC and act as beneficial traps. For this reason, stringers of Ca-Al oxides (d =
2-3 μm) up to 30 μm length are detrimental as they induce interfacial de-cohesion,
support local void growth and coalescence, or directly lead to cleavage fracture.
With regard the proper pipeline welding for sour service, we can conclude the
following:
1. Pipelines with excellent resistance under sour environment should obtain a
microstructure based mainly on acicular ferrite and less on degenerated pearlite
avoiding hard phases.
2. Low-C, medium-Mn micro-alloyed steel grades are the most suitable materials
for such applications. Micro-alloying allows for more H+ entrapment.
3. The hydrogen entrapment as related to HIC susceptibility can be empirically
defined as follows:
(Fe 3 C) > (Al, Mn, Mg, Ca)(O, S) > MnS > (Nb, V, Ti)(C, N) > (Nb, V, Ti)C >
MoC. This sequence can be altered based on C, Mn levels.
4. Post-weld heat treatments must be carefully selected based on steel chemistry, customer’s required mechanical properties, and the pipe-manufacturers
capabilities.
5. After austenitization quenching, a relatively short annealing (or tempering) positively influences the second-phase morphology and distribution that contributes
to stress relieving, mobilizes atomic hydrogen avoiding its irreversible entrapment, and lowers HIC susceptibility of the weld and heat-affected zones.
12.4.5 Low Temperature Behavior and Weld Microstructures
Reduction of the grain size improves yield strength and toughness simultaneously.
A fine-grained microstructure provides numerous grain boundaries hindering crack
propagation. Therefore, a higher energy threshold is required for fracture to propagate. The formation of fine ferrite grains can be promoted by introducing dislocations in austenite at the deformation stage during controlled rolling. Moreover, the
restriction of austenite growth with the addition of micro-alloying elements further
promotes fine ferrite grain formation [7].
The microstructural constituent which contributes to high strength and good
toughness is acicular ferrite. The needle-like microstructure of acicular ferrite developing to different directions impedes the fracture propagation compared to more
oriented microstructural constituents [61]. Fine packets of bainitic ferrite are also
known to promote high yield strength and toughness at low temperatures due to the
high dislocation density and multiple grain boundaries. However, the presence of
cementite laths and/or M/A constituents in bainite may act as trap of H
+ , leading to