Microstructural Formation Mechanism And Sulfide Stress Corrosion Behaviors Of Submarine High Strength Low Alloy Pipeline Steel

With the depletion of onshore oil and gas resources,more attention has been paid to offshore oil and gas exploitation.The service environment of submarine pipeline is more complex and changeable than that of land pipeline.Hence,the requirements for submarine high strength low alloy pipeline steels are more demanding,requiring the excellent comprehensive mechanical properties,corrosion resistance and welding performance.In this paper,a systematic study has been carried out on formation of multi-phase microstructure during continuous cooling or after hot rolling of the high strength low alloy pipeline steel,the controlling mechanism of multi-phase microstructure and the optimization of mechanical properties,and the sulfide stress corrosion fracture process and the impact of environmental factors.The main research results are as follows:(1)Formation of multi-phase microstructure during continuous cooling in the high strength low alloy pipeline steelThe increase of cooling rate during the continuous cooling process inhibits the diffusion controlled polygonal ferrite and pearlite phase transformation,and promotes the shear controlled bainite and martensitic phase transformation.The ferrite/bainite overlap phase transformation kinetics behaviors during continuous cooling have been clarified.Accordingly,a universal modular phase transformation kinetics model with the two-dimensional visual analog output was developed,which can describe the overlap phase transformation during continuous cooling transformation kinetics.It was found that increase of cooling rate enhances the saturated nucleation density for ferrite and the pre-exponential factor for bainitic nucleation,resulting in grain refinement.(2)Microstructural formation after rolling in the austenitic recrystallization zone of the high strength low alloy pipeline steelRolling temperature,deformation amount,cooling rate after rolling,and deformation rate have the remarkable effects on the microstructural formation of high strength low alloy pipeline steel.The low rolling temperature results in the incomplete austenite dynamic recrystallization,and thus formation of band-structure resulted from alloy element segregation.The increase of rolling deformation can refine the austenite recrystallized grains,and thus promote the formation of polygonal ferrite and inhibit the formation of bainite during continuous cooling.The increase of cooling rate after rolling changes the morphology of bainite,promoting the formation of acicular ferrite.The increase of rolling deformation rate leads to the decrease of time interval between the starting point for ferrite and bainite transformation,and the increase of the beginning temperatures for ferrite and bainite transformation,which indicates that ferrite and bainite transformation is promoted at the same time.(3)Multi-phase controlling and the thus-obtained mechanical properties of the high strength low alloy pipeline steelThe effects of the different heat treatment routings,as direct cooling treatment(DCT),intercritical cooling treatment(ICT),step cooling treatment(SCT),on microstructural formation and mechanical properties were investigated.ICT and SCT routings can improve the strength and decrease yield ratio at the same time.The constitutive model for yield ratio,based on the microstructural parameters,was developed,which can clarify the impact mechanism of the microstructural ratio and morphology upon the different treatment routings on the mechanical properties.It was found that the yield ratio is decreased by increase of the contents of acicular ferrite or M/A constituent or fine martensite,the coordinate deformation of the multi-phase microstructure,and increase of the mobile dislocation density.(4)Sulfide stress corrosion fracture process of the high strength low alloy pipeline steelThe microstructural mechanism of the electrochemical noise signals change during sulfide stress corrosion process was revealed by the interrupted corrosion experiments.The change process from uniform corrosion/passivation to local corrosion/pitting corrosion was clarified.The methods for defining the different stages of sulfide stress corrosion,based on the electrochemical signals real-time monitoring,was developed.It was found that the sulfide stress corrosion process of the high strength low alloy pipeline steel can be divided into three stages:uniform corrosion/passivation,local corrosion/pitting,and crack initiation and propagation.The electrochemical analysis methods can accurately distinguish the stages of uniform corrosion and local corrosion,and the current kurtosis analysis method based on in-situ monitoring can obtain the information of crack origin early.(5)Environmental factors analysis for sulfide stress corrosion in the high strength low alloy pipeline steelThe influence of external environmental factors(including temperature and pH value of the corrosive medium,and the loading stress value)on the sulfide stress corrosion was studied.The environmental temperature could affect the stress corrosion process by changing the concentration of in the corrosive medium and the type of corrosion product film on the sample surface.The pH value of the corrosion medium can affect the concentration of Fe²⁺and in the solution near the anode metal,and thus change the types of corrosion products on the sample surface.The change of pH value can also affect the corrosion potential of the anode,and thus influence the local corrosion sensitivity of the anode metal.The increase of the applied stress value loosens the corrosion product film on the sample surface,and thus worsens the protection of the corrosion product film,accelerating the stress corrosion cracking process.