Investigation Of Laser Shock Peening Process And Stress Corrosion Resistance Of X70 Pipeline Steel

With the rapid development of the China’s industry,the demand for oil and natural gas has increased.X70 pipeline steel is a high-grade pipeline steel,and it is widely used in project of ―Natural Gas Transmission from West to East in China‖.Due to the service environment of X70 pipeline steel contains many acid gases and substances,X70 pipeline steel is vulnerable to stress corrosion cracking(SCC),resulting in serious accidents.Therefore,it is of great significance to study how to improve the stress corrosion resistance of X70 pipeline steel.Laser shock peening(LSP)is a new method of surface treatment,which can produce a certain depth of compressive residual stress on the surface of the material,and optimize the microstructure to improve the stress corrosion resistance of the material.In this paper,X70 pipeline steel specimens were treated by LSP with different coverage layers and slow strain rate test was conducted.In addition,fracture morphology,mechanical properties and microstructure were observed.The influence of LSP with different coverage layers on stress corrosion resistance was analyzed,and the intrinsic mechanism was clarified that LSP could improve the stress corrosion resistance of X70 pipeline steel.The main research work is as follows:(1)The influence of different coverage layers and different corrosive solutions on the stress corrosion resistance of X70 pipeline steel was investigated by means of the slow strain rate tensile test(SSRT).The results showed that the X70 pipeline steel had SCC susceptibility in the corrosive solution,and the higher the concentration,the stronger the SCC susceptibility of the material.LSP increased the ultimate tensile strength,prolonged the failure time,improved the SCC resistance of X70 pipeline steel.Increase the coverage layers of LSP could further improve the SCC resistance at the same pulse energy.(2)The fracture morphology of X70 pipeline steel after SSRT was investigated.In the air,the area of the shear-lip region of the LSPed specimen increased and the dimple distribution was more uniform,compared with the as-machined specimen.In the corrosive solution,the as-machined specimen changed from ductile fracture to brittle fracture,while the LSPed specimens changed from ductile fracture to mixed fracture.This is due to the fact that the initiation of crack was restrained and the direction of crack propagation increased,resulting in a decrease of the propagation rate.It showed that the SCC resistance of X70 pipeline steel was improved after LSP.(3)The influence of the microhardness,residual stress and microstructure of X70 pipeline steel specimens after LSP with different coverage layers on SCC resistance was investigated.LSP significantly increased the surface microhardness of the X70 pipeline steel specimen.The microhardness of the LSPed specimen with one coverage layer increased by 16%,and the microhardness of the LSPed specimen with two coverage layers increased by 25%,compared with the as-machined specimen.LSP induced compressive residual stress on the surface of the X70 pipeline steel specimens.The initial surface stress value of the as-machined specimen was 30 MPa.The surface stress value of LSPed specimen with one coverage layer was-590 MPa,and the impact depth of LSP was 310 μm.The surface stress value of LSPed specimen with two coverage layers was-701 MPa,and the impact depth of LSP was 360 μm.Compressive residual stress restrained the initiation of the SCC on the surface of the material,increased the direction of crack propagation,and decreased the crack propagation rate.The original grain size of X70 pipeline steel was large.After LSP with one coverage layer,the surface grain size was reduced to nanometer.The refined grains had more grain boundaries,which made the diffusion path of hydrogen atoms complicated in the grains.At the same time,the more grain boundaries restrained the crack propagation and decrease the crack propagation rate.After LSP with two coverage layers,some pearlite underwent a phase change,which produced bainitic ferrite with higher dislocation density,more uniform structure and better SCC resistance.(4)The influence of different pulse energy and different coverage layers on the residual stress distribution of X70 pipeline steel was investigated by ABAQUS software simulation.When pulse energy was 3 J,compressive residual stress was generated on the surface of the model,but the impact depth was shallow and the distribution was not uniform.When the pulse energy was 6 J,the value of surface compressive residual stress increased significantly,the impact depth was deeper,and the stress distribution was more uniform.Under the same pulse energy,the LSP with two coverage layers induced a larger compressive residual stress and a deeper impact depth,compared with LSP with one coverage layer.The actual experimental pulse energy is 6 J,which is consistent with the simulation results.In summary,this article provides a theoretical reference and experiment basis for investigating the improvement of SCC resistance of X70 pipeline steel by LSP.