Study On Hydrogen Embrittlement Of X80 Pipeline Steel Welded Joints In High Pressure Coal Gas Environment

Hydrogen gas is usually involved in coal gas,so hydrogen induced permeation and decrease of mechanical properties in steel pipeline,especially for welded joint region would be a potential problem.In this dissertation,the hydrogen embrittlement(HE)susceptibility of X80 coal gas pipeline was systematically studied.A high pressure gaseous hydrogen permeation device was used to study the hydrogen permeation behavior of different sub-regions of welded joint,and the effect of diffused hydrogen on the HE susceptibility of pipeline was then studied by slow strain rate tension(SSRT)tests.Results show that the effect of hydrogen on embrittlement susceptibility of material depends on its microstructure,hydrogen diffusivity and sub-surface concentration.Compared with other regions in welded joint,the coarsed grained granular bainite in CGHAZ is more susceptible to HE,and the HE index is 34.56%,while the highest HE index for other regions is no more than 25%.During SSRT test,the diffused hydrogen atoms trapped at dislocations would decrease the deformation ability,and the side effect increase with the accumulation of hydrogen.Morever,the hydrogen diffusivity can affect the diffusion speed of hydrogen towards dislocations.Compared with other regions,the apparent hydrogen diffusivity of CGHAZ is the highest,which means the quickest accumulation rate of hydrogen towards dislocations,leading to the highest HE susceptibility in this area.Taking the residual stress and microstructure inhomogeneity affected hydrogen permeation parameters difference into consideration,the hydrogen enrichment in welded joint was simulated by numerical simulation,and the SSRT tests were then performed at the equivalent hydrogen pressure considering the hydrogen enrichment.Results show that there is an obvious hydrogen enrichment phenomenon in welded joints,especially for weld metal,the enriched hydrogen concentration at central area is 2.7 times higher than that without considering residual stress induced hydrogen enrichment.Combining the effect of microstructure inhomogeneity and residual stress in pipeline welded joint,CGHAZ also has the most significant HE susceptibility with a HE index of 39.58%.With the increase in hydrogen partial pressure in coal gas,there is also an uptrend for hydrogen concentration and HE susceptibility in different sub-regions of welded joint,among which CGHAZ is more susceptible to hydrogen concentration increase.According to the calculated results,once the hydrogen concentration in CGHAZ increases 1 ppm,the HE index would be 5 times higher.Under the action of tensile stress,the increased surface energy can lead to the increase in sub-surface hydrogen concentration,and the surface adsorbed hydrogen increases significantly after the local plastic deformation,which induces stress concentration and promotes the increase in hydrostatic stress and hydrogen trap density such as dislocations,and then caused local hydrogen enrichment.Once the axial stress is greater than a threshold value,the cracks initiated and propagated with a mode of brittle.With the increase in crack propagation rate,the diffused cannot keep up with the moving speed of dislocations.Then the effect of hydrogen on HE susceptibility decreases,leading to the diminishment of brittle fracture characteristics.The covered oxide layer on steel surface may effectively decrease the absorbed hydrogen concentration on steel substrate.However,if there is residual water left in pipeline,it may cause corrosion of pipe steel combined with carbon dioxide in coal gas.Finally,a layer of ferrous carbonate film may replace the original oxide film.Based on the pre-built theoretical model for hydrogen diffusion in duplex medium,calculated results show that the ratio of defect in ferrous carbonate film induced hydrogen flux is about 80%.Besides,film observation in scanning electron microscope(SEM)presents a lot of defects,such as film voids,and the bonding property between film and steel substrate is poor.All these defects provide paths for hydrogen atoms adsorption,absorbtion and diffusion in steel substrate,and the sub-surface hydrogen concentration is 2.7-3 times higher than that covered with oxide film,leading to a higher HE susceptibility.