Numerical Simulation Of Hydrogen Diffusion And Crack Propagation In Welded Joint Of Incoloy825/X65 Composite Pipe

Corrosion has a great negative impact on the service life of the pipeline.High welding residual stress will accelerate corrosion and reduce the strength of the pipeline.Because traditional carbon pipe is easy to be corroded and stainless steel pipe is expensive,bimetal composite pipe is a good choice for high-pressure corrosion-resistant pipe.Based on ABAQUS finite element software,this paper carried out numerical simulation of Incoloy825/X65 bimetallic composite pipe and established a three-dimensional finite element model to analyze the welding temperature field and residual stress field of the welded joint of the composite pipe.Based on the results of residual stress field,hydrogen diffusion analysis of welded joints was carried out,and the influence law of stress and hydrogen distribution was discussed.Finally,the XFEM was used to analyze the crack on the inner surface of the welded joint.Firstly,based on ABAQUS,the 3D finite element model of multi-pass butt welding of composite pipe welded joint was established,and the heat source subroutine was programmed.And temperature field and welding residual stress were analyzed by sequential thermal-stress coupling method.The results showed that: The distribution of axial residual stress on the inner surface of the Incoloy825/X65 composite pipe is obviously different from that of single metal pipe.The axial stress distribution of the inner surface changes obviously with the weld pass.After the final cooling,the weld and the near weld zone are subjected to tensile stress,and the tensile stress peak appears in the center of the weld.The distribution of axial residual stress on the outer surface is basically the same with the change of the weld pass,and the stress increases with the increase of the number of weld pass layers.After cooling,the weld area was subjected to compressive stress.With the increase of the distance from the weld center plane,the stress gradually manifested as tensile stress,and reached the peak of tensile stress in the heat-affected zone.Metal damage is quite frequent due to the influence of hydrogen and stress corrosion cracking.Based on the above stress field results,the hydrogen diffusion model of welded joint of composite pipe was established to discuss the influence of residual stress on hydrogen enrichment and the corrosion resistance of composite pipe.Before hydrogen diffusion simulation,the relevant hydrogen diffusion parameters were obtained by electrochemical hydrogen permeation experiment.The results showed that: The Incoloy825/X65 bimetallic composite pipe used in this study has good resistance to hydrogen corrosion.When the residual stress is considered,the hydrogen distribution is basically consistent with Mises stress distribution,and obvious hydrogen enrichment appears in the welded joint.In the horizontal direction,hydrogen is mainly concentrated in the weld seam and HAZ.In the thickness direction,the hydrogen concentration first increases and then decreases with the increase of the distance from the inner surface.When the residual stress is not considered,the hydrogen diffusion in the welded joint is only affected by the relevant hydrogen diffusion coefficient and the hydrogen concentration gradient,and the hydrogen concentration is basically the same in the horizontal direction.Aiming at the hot crack problem of the nickel base alloy during welding,an elastic-plastic mechanical model was established.The crack propagation behavior on the inner surface of the joint was analyzed by XFEM,and the influence of mechanical property parameters on the stress field distribution at the crack tip was studied.The results showed that: Under the effect of stress induction,there are both stress concentration and hydrogen accumulation at the crack tip.There is an obvious plastic deformation stage before crack initiation.The range of high stress zone and the maximum stress at crack tip increase with the increase of yield strength and tensile strength.When the crack is located in different positions,the propagation direction deflects to the softer Incoloy825,and the propagation speed is faster in Incoloy825.