12 Industrial Pipeline Welding
413
ferrite (BF), or even martensite in cases of higher C contents or extra high undercooling. Tempering lowers dislocation density and residual micro-stresses [50, 60].
HIC resistance can be increased by a post-weld heat treatment (PWHT) in the weld
and heat-affected zones. Post-weld heat treatment (PWHT) mainly aims to reduce
trapping sites for diffusible H atoms, dissolve local hard phases (carbides, B, M, M/A
constituents), and relieve residual stresses, especially at interphase grain boundaries
of BF laths, P, B, and/or M laths [52].
Controlling the microstructure after welding is really challenging, due to the
different microstructure regions and constituents that are generated. It is practically impossible to conduct PWHT in the welded area without affecting somewhat
the surrounding metal [3]. Many studies are focused on determining the optimum
chemical composition for these applications. Low-carbon micro-alloyed ultra-clean
pipeline steels, i.e., X70, X80, and X100, are increasingly used for sour service applications. The Mn level is crucial as combined with the Ca/S ratio, affects the formation
of MnS inclusions, and directly deteriorates HIC. Various researchers studied independently the Mn effect and the micro-alloying additions on HIC service condition.
In [4], X70 is reported to be much cleaner in terms of inclusions/precipitates density
than its medium-Mn version. In [56], low-C, low-Mn, and Nb micro-alloyed steels
with a PF microstructure show excellent mechanical properties (toughness, YS, TEL)
and improved HIC service behavior versus high Mn steels. It is claimed by Nayak
et al. [56] that the S-content (in low-Mn, micro-alloyed steels) is not needed to be
further reduced below 10–20 ppm in contradiction to the efforts made by other steel
producers that aim to lowering S-content in the range of 6–8 ppm.
HIC service is deteriorated by the presence of Ca-Al oxides with average size
above 3 μm. A less significant role to the HIC behavior is played by enlarged clustered
precipitates, mainly (Ti, Nb) (C, N) [56].
During pipe welding, some precipitates in the HAZ dissolve and then precipitate
again after solidification. A rapid cooling after welding is mandatory to homogeneously distribute fine precipitates in the WZ. Interphase precipitation including
the NbC prevents grain coarsening in the HAZ during welding. This is due to
the low surface energy of the stable precipitates which do not coarsen in temperature cycling; during welding. Their existence at the grain boundaries blocks grain
growth. Additionally, interphase precipitates contribute to the reduction of the hardness loss created by melting/resolidification process [56]. Thus, fine precipitates at
grain boundaries contribute to improved HIC behavior.
The X70 steel grades typical base metal hardness is within the range of 250HV–
280HV. After welding, a 50HV–80HV reduction in the HAZ can occur [56]. Hardness
relies on the microstructure constituents set after PWHT which are based on chemical
composition, micro-alloying additions, and subsequent cooling rate. AF and BF are
accompanied by increased dislocation content which increases strength. The higher
the WZ solidification cooling rate, the finer the microstructure components. Coarse
austenite grains lead to coarser final microstructure after PWHT and enable the
creation of hard constituents (based on C-Mn-content). Finer grained microstructure
has high grain boundary density and enables H movement and thus can affect HIC
behavior.
413
ferrite (BF), or even martensite in cases of higher C contents or extra high undercooling. Tempering lowers dislocation density and residual micro-stresses [50, 60].
HIC resistance can be increased by a post-weld heat treatment (PWHT) in the weld
and heat-affected zones. Post-weld heat treatment (PWHT) mainly aims to reduce
trapping sites for diffusible H atoms, dissolve local hard phases (carbides, B, M, M/A
constituents), and relieve residual stresses, especially at interphase grain boundaries
of BF laths, P, B, and/or M laths [52].
Controlling the microstructure after welding is really challenging, due to the
different microstructure regions and constituents that are generated. It is practically impossible to conduct PWHT in the welded area without affecting somewhat
the surrounding metal [3]. Many studies are focused on determining the optimum
chemical composition for these applications. Low-carbon micro-alloyed ultra-clean
pipeline steels, i.e., X70, X80, and X100, are increasingly used for sour service applications. The Mn level is crucial as combined with the Ca/S ratio, affects the formation
of MnS inclusions, and directly deteriorates HIC. Various researchers studied independently the Mn effect and the micro-alloying additions on HIC service condition.
In [4], X70 is reported to be much cleaner in terms of inclusions/precipitates density
than its medium-Mn version. In [56], low-C, low-Mn, and Nb micro-alloyed steels
with a PF microstructure show excellent mechanical properties (toughness, YS, TEL)
and improved HIC service behavior versus high Mn steels. It is claimed by Nayak
et al. [56] that the S-content (in low-Mn, micro-alloyed steels) is not needed to be
further reduced below 10–20 ppm in contradiction to the efforts made by other steel
producers that aim to lowering S-content in the range of 6–8 ppm.
HIC service is deteriorated by the presence of Ca-Al oxides with average size
above 3 μm. A less significant role to the HIC behavior is played by enlarged clustered
precipitates, mainly (Ti, Nb) (C, N) [56].
During pipe welding, some precipitates in the HAZ dissolve and then precipitate
again after solidification. A rapid cooling after welding is mandatory to homogeneously distribute fine precipitates in the WZ. Interphase precipitation including
the NbC prevents grain coarsening in the HAZ during welding. This is due to
the low surface energy of the stable precipitates which do not coarsen in temperature cycling; during welding. Their existence at the grain boundaries blocks grain
growth. Additionally, interphase precipitates contribute to the reduction of the hardness loss created by melting/resolidification process [56]. Thus, fine precipitates at
grain boundaries contribute to improved HIC behavior.
The X70 steel grades typical base metal hardness is within the range of 250HV–
280HV. After welding, a 50HV–80HV reduction in the HAZ can occur [56]. Hardness
relies on the microstructure constituents set after PWHT which are based on chemical
composition, micro-alloying additions, and subsequent cooling rate. AF and BF are
accompanied by increased dislocation content which increases strength. The higher
the WZ solidification cooling rate, the finer the microstructure components. Coarse
austenite grains lead to coarser final microstructure after PWHT and enable the
creation of hard constituents (based on C-Mn-content). Finer grained microstructure
has high grain boundary density and enables H movement and thus can affect HIC
behavior.