Exploit In High Strength And Toughness OCTG Of V140 And Corrosion Resistance Research

With the depletion of energy and the continuing rise in oil prices,making oil and gas field exploration environment has become more severe,more harsh mining conditions.Oil industry growing demand for OCTG(Oil Country Tubular Goods),technology increasingly demanding high.Ultra high strength OCTG which used for deep well or ultra deep well has become increasingly strong,Ultra high strength OCTG material requires high toughness,simultaneously OCTG in harsh corrosive environment requires OCTG has certain corrosion resistance.Currently,OCTG of high strength and toughness which has certain corrosion resistance production technology and market is almost monopolized by foreign.Domestic OCTG products still need to import for lacking of autonomous production capacity.Therefore,it is important to develop appropriate steel and technology to produce OCTG of high strength and toughness which has certain corrosion resistance.The research object is OCTG of V140,and this paper studies its component,microstructure,mechanical properties,corrosion resistance and corrosion mechanism.The main research work and results are as follows:(1)To meet the requirement of properties,the component method of experiment steel is using low C,Mn,P and S contents,and add in Cr,Mo,V and Nb elements.The CCT curve of tested steels under undeformation condition was obtained by using Formastor-F II machine and metallographic studying.The effect law of cooling rate to microstructures was given.The results show that,on the condition of static continuous cooling transformation,When the cooling speed less than 5?/s,microstructure are main lath bainite and a few ferritic;when the cooling rate was between 5?10?/s,microstructure are Bainite and martensite mixed organization;when the cooling rate is more than 10?/s,microstructure are main lath martensite.(2)Rolling and heat treatment experiment research in Laboratory show that:The hot rolling process and heat treatment parameters of on-line quenchd and tempered steel should be:The heating temperature of hot rolling is 1200?,two regions rolling,the finish rolling temperature of on-line quenched steel in 950-980?,tempering temperature is 680? and holding time is 50min;the finish rolling temperature of off-line quenched and tempered steel in 850?880?,the heating temperature of quenching is 870? and holding time is 40min.Mechanical properties of two kinds of heat treatment processes have reached OCTG performance requirements of V140 and the microstructure are tempered sorbite.(3)After carbon dioxide corrosion experiment,the average annual corrosion rate of steel is calculated using weight-loss method.To determine the applicability of the designed steel and explore its corrosion mechanism,the surface profile and phase composition of the corrosion product are observed and analyzed.The results showed that:the OCTG of V140 at a temperature of 75 ?,total pressure of 1.2Mpa environment average annual corrosion rate is 0.834mm/a,the CO2 corrosion resistance is fine.The partial corrosion of the experimental steel is obvious,but the pitting corrosion is little,the pitting resistance is generally.Fe3C,FeOOH,Fe2O3 produced in short time of corrosion,where Fe3C is the main reason for the steel surface pitting phenomenon,the finally corrosion products are FeCO3,fine and compact FeCO3 improves the metal CO2 corrosion resistant ability.(4)The formation mechanism and influencing factors of HIC was researched,influencing factors of each element in mechanical properties and anti-acid corrosion of the OCTG steel was discussed.According to the requirement of NACE Standard TM0284-2003,resistance HIC experiment are made for OCTG of V140 steel.Resistance HIC experimental results:the samples in saturated H2S solution(A solution)for corrosion in 96 hours,there are different degrees of hydrogen blister in the experimental steel,and HIC cracks appear in the microstructure.The tempering at 700? of on-line quenched and tempered steel and the tempering at 680? of off-line quenched and tempered steel specimens crack sensitive rate,crack length,crack thickness ratio are lower than NACE TM0284 resistance to hydrogen induced cracking standards,with good resistance to hydrogen induced cracking.