Research On Sulfide Stress Corrosion Behavior Of High Strength SSCC Resistant Tubing And Casing Steel

With the development of oil and gas exploration,the geological conditions of oil and gas well become more and more severe,and H2S gas produced in the well could cause sulfide stress corrosion cracking(SSCC)on the tubing and casing,leading to serious corrosion problems of tubing and casing,even destroying tubing and casing,which may cause enormous economic losses and casualties.Considering the mechanisms of SSCC are complicated and the influence factors on SSCC behaviors are numerous,it is necessary to carry on further research on the SSCC mechanisms of high strength 110-ksi grade SSCC resistant tubing and casing.In this dissertation,110-ksi grade high strength steel used in SSCC resistant tubing and casing is selected to study the surface electrochemical behaviors,hydrogen diffusion coefficient and type and distribution of precipitates,investigating the effect of alloy elements on sulfide stress corrosion resistant of the steel.By studying the stress intensity factor of the steel when immersing in H2S saturated NACE solution and when hydrogen charging with electrochemical technique,the SSCC mechanisms of the steel are discussed.The effects of grain size and microstructure on the SSCC resistant properties of the steel are studied through rolling and heat treatment processes.Based on the above investigations,the main conclusions obtained are as follows:1.A layer of corrosion product film is formed on the surface of 110-ksi grade test steel when the steel is soaked in H2S saturated NACE solution,and the corrosion product film is single layer structure with Mackinawite FeS1-x when soaking time is shorter,while the corrosion product film is double-layer structure when soaking time is longer.The outer layer corrosion product film is composed of the loose of the Mackinawite FeS1-x phase and MoS2 phase,while the inner layer is made up of relatively dense Cu2S and Cr2O3 phases for the corrosion product film with double-layer structure,which could reduce the corrosion rate of steel in the H2S saturated NACE solution.2.The alloy elements have significant influence on the hydrogen permeation behaviors of 110-ksi grade test steel.A small amount of B atoms decreases the apparent hydrogen diffusion coefficient,which is beneficial to improvement of the SSCC resistant property of the steel.There are no the effect of Cr and Mo atoms on the apparent hydrogen diffusion coefficient,while composition segregation caused by Mn atom could significantly increase the apparent hydrogen diffusion coefficient in the test steel.3.The main phases(Cr,Fe)23C6,Mo2C and(V,Nb,Ti)(C,N)precipitated in 110-ksi test steel are irreversible hydrogen traps,which is beneficial to improving the ability of SSCC resistant of the steel.And the(Cr,Fe)23C6 phase precipitated along the grain boundary and MnS inclusions will reduce the SSCC resistant property of the test steel.4.The critical stress intensity factor(KISSC)of 110-ksi grade test steel in H2S saturated NACE solution is smaller than that(KIH)with electrochemical hydrogen charging technique.The initiation and extension of crack when the steel soaked in H2S saturated NACE solution are the results of the combined effect of anodic dissolution and hydrogen-induced cracking.5.By simulating the hot rolling process of 110-ksi grade the test steel,a recrystallization kinetic model including nucleation,growth and collision during the dynamic recrystallization is constructed.The dynamic recrystallization process of 110-ksi grade test steel could be reasonably described by this recrystallization kinetic model.6.Grain refinement could increase the critical stress intensity factor KISSC of the steel which increases the ability of SSCC resistant of the tese steel.7.The mixture microstructure of the ferrite and pearlite is beneficial of the formation of thick and dense corrosion film,improving the SSCC resistant property of 110-ksi steel.