Hydrogen-induced Cracking And Hydrogen Trapping Behavior Of Welded X100 Pipeline Steel In H₂S Environments

X100 pipeline steel has been proposed for the third period of China's energy west-east pipeline project construction.To meet the requirement of long distance transportation,the actual pipes are jointed with each other by welding technique.The formed microstructure of welded joint is complex and consists of base metal,heat affected zone and weld metal,which makes it vulnerable to be corroded.The pipes that transporting crude oil and natural gas containing some H₂S corrosive medium are much easy to cause hydrogen induced cracking?HIC?,and the essence to HIC is that hydrogen atoms?H?are trapped by sites in the steel.Therefore,the investigation of HIC and H trapping behavior of X100 pipeline steel and its welded joints in wet H₂S environment can provide theoretical basis and data supporting for the later application and protection measures.In this work,X100 pipeline steel base metal and its welded joint were acted as research subjects and the HIC susceptibility of specimens was investigated according to the NACE TM0284-2011 standard.Moreover,the HIC cracks were observed and analyzed to clarify the reason of difference HIC susceptibility and the rule of HIC cracks initiation and propagation.And,the specimens'H permeation,diffusion and trapping behaviors in H₂S environment were observed and analyzed by hydrogen microprint test,hydrogen permeation and electrochemical cyclic H charging and discharging methods.The reversible and irreversible H concentration were quantitatively calculated to essentially discuss the influence of H on HIC behavior.Finally,X100 pipeline steel base metal was used to simulate the heat affected zone and weld metal by heat treatments,and then specimens'HIC behavior was carried out to define the weakest micro zone of X100 pipeline steel welded joint.The experimental results are shown as follows:X100 pipeline steel welded joint with complex and inhomogeneous microstructure is more susceptible to HIC than base metal,and the HIC cracks all mainly initiate from Ca-Al-Si-O composite inclusions,showing a typical stepwise cracking,and propagate mainly in transgranular form mode accompanied with little intergranular mode;The irreversible H concentration of X100 pipeline steel welded joint is almost 100 times that of base metal,which is the essential reason to cause a higher HIC susceptibility;The weakest micro-area of X100 pipeline steel welded joint is heat affected zone;The HIC cracks of X100 pipeline steel base metal and simulated heat affected zone specimens mainly initiate from hard brittle precipitates,extend and propagate along center segregation bands and the interfaces between Ca-Al-Si-O composite inclusions and steel matrix,and the cracks in the simulated weld zone initiate and expand along the center segregation zone;For the welded joints of X100 pipeline tested steel,the corresponding hydrogen penetration parameters with smaller apparent H diffusivity(D_app),H permeability?J_?L?and larger apparent H solubility(C_app)mean that more H are trapped and the HIC sensitivity is higher.