Research On Microstructure And Mechanical Property Of Flexible Pipe Armor Layer Steel In High Pressure And High Concentrations Of Acid Environment

With the increasing development of the ocean oil-gas exploration and production activities during the last few years,oil-gas development has been extended to deeper ocean.Submarine pipelines are the lifeline in the exploitation of offshore oil-gas resources,and they play an important role in the development and production of offshore oil and gas fields as well as transportation of products.Due to the harsh marine gathering and transferring environment where most of these pipelines are located in,with the extension of service time the disadvantages of rigid pipes will gradually been exposed.The marine flexible pipe is made of polymer and flat steel,and the marine flexible pipe has became a great alternative compared with rigid pipelines because of its excellent performance,and it has broad prospects for development.At present,only a few manufactures can design and manufacture flexible pipe,because that it must specifically be designed for users and its manufacturing process is complicated.The relative research in our country is still in its infancy,so it is urgent need to develop high-strength and corrosion resistant steel used in flexible pipe armor layer.The research object is developing the strength of 700MPa corrosion resistant flexible pipe armor layer steel.This paper studies its microstructure,mechanical properties,resistance to seawater and carbon dioxide.The main research work and results are as follows:(1)The undeformed and deformed austenite cooling process and microstructure evolution regular of armor layer steel are studied,and the continuous cooling transformation(CCT)diagrams were conducted through the methods of thermal expansion and microstructure observation.The results show that:with the cooling rate increasing,the microstructure develops gradually from polygonal ferrite and pearlite to amount of acicular ferrite and granular bainite,finally changes to martensite.(2)Though industrial trial-produced and quenching and tempering,the influences of holding times and tempering temperature on microstructure and properties of armor layer steel at quenching heating 900? was researched,making appropriate quenching and tempering process is achieve the expected aim.The results show that:the strength decreases and the plasticity enhances with the extension of holding time of quenching,the increase of tempering temperature and holding time of tempering.Optimal quenching and tempering processing parameters were proposed to meet the mechanical performance requirements Quenching process should be conducted when the heating temperature is 900?,and holding time 30 min,while the tempering time should be carried out at 580?and holding time 45min for armor layer steel,The microstructure of armor layer steel is tempered martensite,the tensile strength of armor layer steel is 802MPa,and the yield strength is 752MPa with elongation 10%.(3)Though the experiment of full immersion corrosion in seawater,the average annual corrosion rate of armor layer steel is calculated using weight-loss method.The surface morphology and phase composition of the corrosion product are observed and analyzed to explore the corrosion mechanism and analyze the applicability of armor layer steel in seawater.The results show that the corrosion of armor layer steel with this designed alloy composition and rolling and heat treatment processes is uniform and the steel surface does not appear obvious corrosion pits.The average annual corrosion rate of the steel is 0.0689mm/a.The corrosion process can be divided into three stages:fast corrosion,plain corrosion and slow corrosion.The surface morphology presents different sizes of flocculent mass,honeycomb cellular and dense material.The corrosion products include lepidocrocite,magnetite,iron oxide and goethite.(4)Though the full immersion corrosion experiment of high temperature and high pressure of oil-gas transportation under gas and liquid CO2 corrosion are simulated,Weight-loss method were used to calculate average annual corrosion rate,The surface morphology and phase composition of the corrosion product are observed and analyzed,and the effect of alloy elements and corrosion mechanism were explored.Results show that the corrosion of armor layer steel is uniform when the corrosion environment is 75 ? and 1.2MPa and the average annual corrosion rate of armor layer steel is 0.05mm/a in gas phase and is 1.75 mm/a in liquid phase.The corrosion resistance remains to be further improved in liquid phase.With the extension of time,corrosion rate flattens out.Corrosion product film has two layers,the out layer is density iron carbonate and the inner one is loose compounds comprising chromium.