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Effect Of Quenching Temperature On Sulfide-stress-corrosion Resistance Of Grade C110 OCTG Steel

Posted on:2017-10-03Degree:MasterType:Thesis
Country:ChinaCandidate:Z N HeFull Text:PDF
GTID:2311330536454015Subject:Materials science
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Sulfide stress corrosion(SSC)of the oil country tubular goods(OCTG)steel is a serious security problem during the exploitation of oil and gas.Literature review showed that optimal SSC resistance could be obtained through quenching followed by a high temperature tempering.In this work,the effect of quenching temperature on the SSC resistance of grade C110 OCTG steel has been investigated by quenching at different temperatures from 900 ? to 1 200 ? followed by tempering at 710 ?.First,the mechanical properties of all the QT-treated samples were tested,and the microstructures of which were observed by using optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM)and X-ray diffraction(XRD).Secondly,double cantilever beam(DCB)test was conducted to determine the SSC critical stress intensity factor(the value of KISSC)of the samples,and the SSC fractures of each sample were observed by using SEM,respectively.Then,Devanathan-Stachurski hydrogen permeation technique was adopted to study the penetration behavior of hydrogen in the steel.Finally,the effects of quenching temperature on SSC resistance and hydrogen permeation behavior were analyzed,the relationship between them was studied,and the SSC resistance mechanism was revealed.Results show that the strength and hardness of the tested steel are improved as the quenching temperature increased,while the impact energy decreased.The microstructure of the tested steel after QT-treated is tempered martensite.As the quenching temperature increased,the size of the prior austenite grain increases gradually as well as the martensitic packet and block,the width of the martensitic lath has no obvious change,the proportion of high angle grain boundary gradually decreases,the volume fraction of precipitated particles is almost constant,the equivalent diameter of precipitated particles decreased,and the dislocation density increases gradually.The DCB test results show that with the increase of quenching temperature,the KISSC value of the tested steel decreases gradually,the number of hydrogen blistering on the surface of DCB samples increases gradually,and the fracture mode of the last part of SSCfracture gradually changes from transgranular fracture to intergranular fracture.The results of hydrogen permeation experiments show that the apparent diffusion coefficient(D)of hydrogen decreases with increasing quenching temperature,and the diffusible hydrogen content(C0)gradually increases.Based on the above research,it can be drawn that the increased quenching temperature enlarges the size of the tempered martensitic microstructures and increases the number of the reversible hydrogen traps consisting of the low angle grain boundaries and the dislocations,which leads to the decrease of the apparent diffusion coefficient(D)of hydrogen,the increase of the diffusible hydrogen content(C0),and the decrease of SSC resistance.The formation of large-size precipitation particles at the grain boundaries or the lath boundaries resulting from excessive quenching temperature should be avoided to prevent intergranular fracture.
Keywords/Search Tags:grade C110 OCTG steel, sulfide stress corrosion, quenching temperature, tempered martensite, double cantilever beam test, hydrogen permeation
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