Study On Stress Corrosion Behavior Of Welded Joints Of Marine Steel In Simulated Marine Environment

With the continuous consumption of global energy,the resources available on land are constantly decreasing,while the ocean occupies over 70% of the Earth’s area and holds infinite resources.The development of marine resources cannot be separated from the shipbuilding industry.Low alloy high-strength steel,as a type of marine engineering structural steel,has advantages such as high tensile strength and low-temperature toughness,good corrosion resistance,low price,and easy processing and manufacturing.It is widely used in ship manufacturing,deep submersibles,large oil drilling platforms,and so on.However,the deep-sea environment in which it operates has the characteristics of high hydrostatic pressure,low temperature,and low dissolved oxygen,making low alloy steel prone to corrosion in harsh corrosive environments.In addition,the manufacturing of ships and marine engineering equipment cannot be separated from welding as a processing method.The residual stress generated during the welding process can affect the corrosion resistance of materials.Therefore,it is of great significance to explore and study the influence of stress corrosion behavior of welded joints of ship steel in deep-sea environments.In this thesis,10Ni5 CrMoV and 10 MnNiCr steels used for offshore engineering,10Ni5 CrMoV steel used for electrode arc welding,10 MnNiCr steel used for submerged arc welding and carbon dioxide gas shielded welding,respectively,were selected to study the microstructure characteristics of three kinds of welded joints and their differences in corrosion behavior in seawater by using static weight-loss method,electrochemical test,scanning electron microscope and other test methods.The elongation after fracture is19.64% and 19.32%,and the reduction of area is 45.48% and 42.36%,respectively;and the yield strength of 10 MnNiCr steel is 456 MPa and 452 MPa,respectively.The elongation after fracture is 28.14% and 22.4%,and the reduction of area is 51.14% and 48.53%,respectively.The three types of welded joints all have ductile fracture in air and sea water.For the arc welding welding specimen,the fracture location is the base metal zone.The fracture of the submerged arc welding and gas shielded welding specimen occurs in the Heat-affected zone.This is because the carbon element in 10 MnNiCr steel accumulates in the Heat-affected zone due to the high temperature effect during the processing of the submerged arc welding and gas shielded welding specimen,and the brittleness increases,making it easier to fracture.Under the simulated marine fully immersed environment,the corrosion rates of the three types of welded joints showed consistent changes,gradually decreasing and stabilizing;The main components of the yellow outer rust layer on the test steel are γ-Fe OOH,the main component of the black inner rust layer is α-Fe OOH,Fe3O4.The electrochemical test results show that the corrosion resistance of the three types of welded joints in seawater is different.When the corrosion cycle is 28 days,the self corrosion potential of 10 MnNiCr steel submerged arc welding is the lowest at-0.891 V,indicating the highest corrosion tendency;The self corrosion potential of 10Ni5 CrMoV steel arc welding is-0.7435 V,the corrosion current density is small,and the corrosion resistance is the best.The electrochemical test of corrosion in different areas of the same welded joint shows that the corrosion resistance of the base metal of arc welding welding and gas shielded welding is poor,and the corrosion resistance of the weld is good;Submerged arc welding,on the contrary,has poor corrosion resistance in the weld zone.Because there is a small amount of granular bainite in the weld zone of submerged arc welding,bainite is a hard and brittle high carbon particle,which is more likely to produce stress concentration in the matrix,which will lead to matrix slippage,so the corrosion resistance in the weld zone of submerged arc welding is worse.In deep-sea environments,under the coupling effect of environmental factors such as low temperature,low dissolved oxygen,and high hydrostatic pressure,the corrosion rate of materials decreases.Using a deep-sea pressure tank to simulate the deep-sea environment in which low alloy steel is in service,the four point bending method is used to apply external loads to the specimens to study the stress corrosion behavior and mechanism of 10Ni5 CrMoV and10 MnNiCr steel and their welded joints under simulated shallow and deep-sea environments.The results show that when the immersion period in the simulated shallow sea environment is 7 days,the external stress of 10Ni5 CrMoV steel is 75% σ The self corrosion potential of s is-0.3727 V,and the applied stress is 100% σ The self corrosion potential of s is negatively shifted by 400 m V,and the self corrosion current density is1.874 m A/cm2,which is lower than 75% σ S increases.As the external load increases,the material exhibits a stronger tendency towards stress corrosion,and the external load will accelerate the corrosion rate of the material.This is because as the external load increases,the corrosion product film on the metal surface becomes more loose,with more obvious pores and crack defects on the film.The bonding force between film atoms decreases,the atomic gap increases,the diffusion of external media is easier,and chloride ions and oxygen are more likely to come into contact with the substrate metal,resulting in poorer corrosion resistance of the material.