Study On Microstructure And Mechanical Properties And SSC Resistance Of Grade C110 Octg Steel

The seamless Oil Country Tubular Goods?OCTGs?of C110 grade with superior sulfide stress cracking?SSC?resistance have been essentially and increasingly applicated for the oil/gas resource development in seriously corrosive circumstance for decades.However,considerable quantity of the C110 product still remains low-level and unreliable in performance,probably due to poorly understanding of the correlations among the heat treatment process,martensitic microstruture and overall properties.Hence,in this attempt,a 28CrMo48VTiB martensitic steel was designed and prepared for the improved SSC resistance OCTGs of C110 grade,with the processes of quenching?Q?at 890 ^oC and tempering?T?at 600⁷20 ^oC×30⁹0 min condisered.For each Q&T-treated sample,the martensitic microstructure was characterized through optical microscope?OM?,scanning electric microscope?SEM?,electron back-scattered diffraction?EBSD?,transmission electric microscope?TEM?and X-ray diffraction?XRD?,and the tension,impact and hardness properties determined.The double cantilever beam?DCB?test was subqsequently carried out for the measurement of the SSC propagation length??a?variation against the immersion time?t?in the standard NACE A solution,the SSC propagating rate?da/dt?variation against the instantaneous applied stress intensity factor(K_Iapplied),from which the threshold stress intensity factor for SSC,K_ISSC,was finally determinded.The hytrogen?H?mobility was also quantitatively estimated via the electrochemical hydrogen permeation?EHP?test at 298 K using the classical Devanathan-Stachursky technique.The strengthing contributions from various mictrostructural features to the yield strength?YS?were determined quantitatively,and accordingly the underlying mechanism controlling the YS of high-temperature tempering martensitic steel was clarified.In addition,the relationship among the martensitic miscrostructure,the H mobility and the SSC initiation,as well as the relationship between the various grain boundaries and the SSC propagation,were estimated quantitatively and thoroughly.The mechanism dominating the SSC resistance of 28CrMo48VtiB martensitic steel was therefore revealed more obviously.Consequently,the industry trial of a high-performance SSC-resistance OCTGs has been successfully accomplished on the basis of aforemensioned experimental and theoratical investigations.The major rusults are summarized as below.The typical martersitic miscrostrucuture developed in experimental 28CrMo48VTiB steel subjected to quenching at 890 ^oC and tempering at 600⁷20 ^oC×30⁹0 min,which consisted of martensitic packet?block and lath substructures in descending order of size.The increases in tempering termperature?T_t?and time?t_t?led to a significant decrease in martensitc packet/block/lath size and an increase in volume fration of high angle grain boundaries?HAGBs?.The increased T_t and t_t also brough about increases in the equivelent diamter and the volume fraction of precipites,and a derease in the dislocation density.The yield/tensile strength and hardness dereased with the increased T_t and t_t,whereas the impact toughness was enhanced.The solution and grain bounariry strengthing contributed dominantly to the YS,whereas the enhanced impact toughness could be attributed to the increased volume fraction of HAGBs.It is also confirmed that the relationship between the YS and the martensitic packet/block size satisfied the classic Hall-Petch formula.Additonally,the martensitic lath width was the key microstructural parameter controlling the YS of this heavily-tempered martensitic steel,whereas the corresponding relationship was in accordance with the Langford-Cohen model.With 28CrMo48VTiB steel subuected to quenching at 890 ^oC and tempering at600⁷20 ^oC×30⁹0 min,The K_ISSC increased obviously due to the increased T_t and t_t,and accordingly the overall resistance to SSC was enhanced.The mechanism for this enhanced resistance to SSC could be attributed to the following tow aspects.Firstly,the increasing T_t and t_t led mainly to the martensitic lath polygonization,widening and coalescence,providing a decreased amount of the HAGBs and accordingly the reversible H traps,inducing a lowered overall and local diffusion H concentration?C₀ and C??,causing a depressed stress concentration around the inclusions and simultaneously a prolonged incubation time for SSC initiation,and finally resulted in a decreased susceptibility to SSC initiation.Secondly,the increased T_t and t_t also brought about a higher fraction of HAGBs,especially the increased amounts of lath boundaries with higher misorientation angle,and resulted in enhanced resistance to SSC propagation by more frequently diverting the propagating direction and lowering the propagation rate.As an optimized heat treatment process of 28CrMo48VTiB steel,the quenching at890 ^oC and tempering at 700 ^oC for 60 min could offer a superior overall performance,including yield strength of 776 MPa,tensile strength of 827 MPa,Rockwell hardness of25 HRC,longitudinal Charpy-V-notch impact energy at 0 ^oC of 133 J,and K_ISSC value of29.02 MPa·m^0.5,etc.This indicates that the 28CrMo48VTiB steel material can fully comply with the tensile and Charpy requirements for the stipulated SSC resistance OCTGs of C110 grade by API 5CT specification with comfortable marginsThe seamless OCTGs of C110 grade with superior SSC resistance have been successfully trial-produced using 28CrMo48VTiB steel and the optimized heat treatment process,based on the aforemensioned results.The trial products were featured with high metallugical qualities,satisfied mechanical properties and superior SSC resistance,and fully satisfied the requirements...