Microstructure And Mechanism Of Sulfide Stress Cracking Of C110 Casing Steel

Casing steel is one kind of oil country tubular goods?OCTG?supporting the walls of oil and gas wells to ensure the drilling works successfully and operating orderly after wells completion.High temperature and high pressure always existed in the oil and gas fields,and various corrosion mediums?such as H₂S?were also contained.Higher requirements for the strength and corrosion resistance of casing steels were put forward by petroleum industry especially along with the exploration and development of the deep and ultra-deep oil and gas wells.When the casing steel applied in oil and gas fields with wet H₂S and other corrosive environments,electrochemical corrosions occurred,resulting in wall thinning,pitting corrosion and various corrosion failures.More serious is that multiple hydrogen damage accidents were likely to be caused.Hydrogen damage is the hydrogen embrittlement fracture resulted from the atomic hydrogen,which generated by electrochemical reactions,diffusing into steel,including:hydrogen blistering?HB?,sulfide stress cracking?SSC?,hydrogen induced cracking?HIC?,and stress oriented hydrogen induced cracking?SOHIC?.Among them,SSC is a failure occurred in the material below its yield strength under the coexistence of applied stress and H₂S-containing corrosion medium.SSC is the most common fracture phenomenon when casing steel was serviced,which perhaps led to serious consequences such as oil well failure,property damage,ecological damage,and even casualties.Therefore,the safety and reliability of the oil casing are very critical to the exploitation of oil and gas fields.The production of casing steels with both high strength and superior SSC resistance is an important part of safeguarding the sustainable development of national oil and gas resources.In this paper,multiple types of C110 casing steels produced by Baoshan Iron&Steel Co.,Ltd.were selected.The effects of heat treatments on microstructure and mechanical properties of the rolled casing steel were investigated.On this basis,the SSC resistance performances of casing steels were evaluated according to methods A and D of NACE TM 0177-2005 standard.Firstly,this work investigated the growth characteristics of SSC crack in a crack-centric approach.Then,the effects of quenching-and-tempering processes and interrupted accelerated cooling processing on SSC resistance of casing steels were studied.Finally,X-ray diffraction?XRD?,scanning electron microscope?SEM?,transmission electron microscope?TEM?,and electron backscatter diffraction?EBSD?combined with hydrogen permeation tests were performed to systematically investigate the effects of microstructure,precipitates,grain boundaries character?GBC?and dislocation density on SSC behavior.The mechanism of SSC was revealed in the sight of hydrogen interaction with steel.The results indicated:1.The microstructures of the casing steels after normalized,quenched,and quenched-and-tempered?Q&T?were ferrite-bainite,martensite,and tempered martensite,respectively.XRD analysis showed that the major extracted precipitates from the normarlized and Q&T specimens were cementites?M₃C?,and MC-type carbide was the dominating phase in the quenched specimen.The semi-quantitative analysis for the compositions of the relevant phases by energy dispersive X-ray spectroscopy confirmed that the bainite transformation is a paraequilibrium mechanism.Meanwhile,a twinned M₃C and M₇C₃-type carbides were observed by TEM in the Q&T specimen.A good combination of strength and toughness was obtained in Q&T specimen and the quenching-and-tempering process is a suitable heat treatment for the production of high-performance casing steel.2.EBSD analysis of the GBCs for the regions near a crack,more distant from a crack,and from specimen with no cracks of the casing steel found that,a higher proportion of low-angle grain boundaries appeared in the regions near the crack,while the fractions of high-angle grain boundaries in the regions distant from cracks or no cracks were higher.Kernel average misorientation?KAM?and Taylor factor maps were respectively used to evaluate the dislocation densities and deformation capacities of grains for the detected areas.The results found that the areas beside cracks contained a higher dislocation density and more grains with higher Taylor factor.At the beginning of SSC cracks occurred in a casing steel,corrosive solutions were squeezed into cracks and promoted cracks continue to growth.3.Casing steels with different quenching-and-tempering processes obtained tempered martensites,M₇C₃ carbides and similar mechanical properties,but distinct SSC susceptibilities.The results of EBSD examination and NACE-method D tests indicated that the specimen with higher dislocation density was more susceptible to SSC.Since the abilities of hydrogen trapping for dislocations are stronger than grain boundaries,the dislocations affected the SSC behavior more significantly than did the grain boundaries.Thus,compared to the casing steel Q&T for once,the dislocation density was further decreased by the second quenching-and-tempering process performed,the.SSC resistance was then improved.4.Grain boundaries,dislocation and precipitates are hydrogen traps.They could make the casing steels exhibit distinct SSC behaviors by influencing hydrogen diffusion,transportation and trapping.In the presence of external stress,the dislocations trapped hydrogen was transferred into the potential crack sites by dislocations moving,leading to the initiation of SSC cracks.Thus,the tendency of SSC was mainly determined by the dislocation density.Steel with a higher density of dislocations provided more“transporters”for hydrogen transportation to the crack sensitive sites and showed worse SSC resistance.5.Interrupted accelerated cooling?IAC?after rolling was introduced into the production of the C110 casing steel.It was found that the SSC resistance of casing steel is improved by IAC processing without reducing its strength,when compared with the specimen with normal air cooling?NAC?after rolling.The low dislocation density obtained in the IAC specimen with quenching-and-tempering performed once was comparable with that of the NAC specimen with quenching-and-tempering performed twice,which reduced the hydrogen“transporters”and provided the precondition for the steel with superior SSC behavior.The hydrogen permeation tests revealed that the Q&T IAC and NAC specimens have near-equal effective hydrogen diffusivities,but the amount of irreversible traps of the former specimen is larger than that of later.The irreversible hydrogen traps were mainly precipitates,which redistributed the diffused hydrogen and inhibited the dislocation moving,making the SSC resistance of casing steel was then further enhanced.