Study On The Hydrogen Embrittlement Susceptibility Of X100 Pipeline Steel

The high strength pipeline steels gain more popularity in the engineering construction of petroleum and gas pipeline in deep-sea areas because of their excellent performance.As a relatively new grade pipeline steel,X100 steel possesses a satisfactory combination of strength,low temperature toughness,weldability and formability.However,increasing strength levels also tend to decrease the resistance of steels to hydrogen embrittlement?HE?.Besides,during the process of manufacturing,laying and servicing of the X100 pipeline steel,it is inevitable that the steel will undergo artificial or sudden large plastic strain,which is called pre-strain.The pre-strain will also influence the HE susceptibility of materials.Therefore,in this article,the influences of hydrogen on the mechanical properties as well as the pre-strain on the HE susceptibility of X100 pipeline steel were investigated by combining electrochemical hydrogen charging and tensile experiments.And the study also take advantage of acoustic emission testsing,SEM,hydrogen permeation and TEM for analyzing the mechanism of hydrogen embrittlement susceptibility of X100pipeline steel.The main contents are as follows:?1?Electrochemical method is applied to produce hydrogen atoms,which then are adsorpted and diffuse into the specimen.The charging time is changed and then the hydrogen concentrartion is measured.The conclusions are as follows:hydrogen concentrartion increase with the the charging time,but the tendency of increment is gradually slower.When the charging time is 12h,the hydrogen concentration is 18.77×10^-6mol/cm³ and the absorption efficiency is only 0.000335,which is only 1/4 of the efficiency of 1h.?2?The influences of hydrogen on the mechanical properties of X100 pipeline steel are investigated by combining electrochemical hydrogen charging and tensile experiments as well as the fracture toughness expereiments.The conclusions are as follows:Both the yield strength and tensile strength decrease with the increase of hydrogen charging time.When the charging time is 12h,the yield strength and tensile strength was reduced by 3%and 9%,respectively.And the fracture decreases with the charging time too.The metal shows good mechanical properties in the air,and the CTOD value is 0.72mm.When the charging time is 12h,the CTOD value reduces to the 0.06mm.The acoustic emission signal shows that the hydrogen can promote the local movement of the dislocation and the crack initiation.?3?The HE susceptibility of X100 pipeline steel increases with the hydrogen charging time no matter the values of the pre-strain,indicating that the longer the charing time,the stronger of the HE damage for the steel.Besides,when the hydrogen charging time is same,HE sensitivity increases with the pre-strain values.For example,when the charging time is 6h,the HE sensitivity coefficient is 40.24%for the X100 pipeline without pre-strain,and it is increased to 49.26%with the pre-strain value increase to the 2%,indicating that the pre-strain would promote the HE of the steel.?4?Double electrolytic cell is used to perform the hydrogen permeation test of X100 pipeline steel.The results show that the effective diffusion coefficient of the hydrogen reduces gradually with the increase of pre-strain values.The coefficeint is5.45×10^-7cm²/s for the steel without pre-strain and it reduces to 1.21×10^-7cm²/s with the pre-strain value increase to the 3%.The TEM analysis shows that the density of the dislocation increase with the plastic strain.During the process of hydrogen charging,dislocations behave as hydrogen traps,which can capture the hydrogen.As a result,hydrogen diffusion coefficient reduces and the total absorption of hydrogen increases with the increase of the pre-strain values.Therefore,when the hydrogen charging time is same,HE sensitivity increases with the pre-strain values.