Strength And Sulfide Stress Cracking Behavior Of A Medium Carbon Cr-Mo Steel Controlled By Nb And Heat Treatment

The worldwide demand for oil country tubular goods?OCTGs?with higher yield strength and sulfide stress corrosion?SSC?resistance has become more evident with the growth of sour oil and gas fields in deeper positions and the higher pressure of H₂S.The increased strength,however,might lead to a lowered SSC resistance for tempered martensite OCTGs of C110 grade or higher,due to the complexity of related microstructure factors.The niobium?Nb?microalloying has been attempted lately for the SSC resistance of high-strength steels.The mechanism of Nb in controlling the SSC behavior,however,has rarely been paid until recently to our knowledge.So Cr-Mo steels with varied Nb content were designed and prepared base on the Thermo-Calc?thermodynamic calculation.Then the effect of Nb and the heat treatment on the microstructure?PAG,packet,block,dislocation and precipitate?,yield strength,hydrogen permeation behavior,SSC behavior and SSC resistance property were investigated by heat treatment test,mechanical performance test,double cantilever beam?DCB?test,Devanathan-Stachurski hydrogen permeation technique and characterization technique,incluing optical microscope?OM?,scanning electron microscope?SEM?,transmission electron microscopy?TEM?and X-ray diffraction?XRD?.The main results are as follows:Equilibrium phase component diagram of Cr-Mo steels with varied Nb,V and Ti content were calculated and the effect of Nb,V and Ti on the precipitation behavior of the equilibrium phases were also investigated.The results showed that carbides precipitated at temperature range of 1600⁴00 ^oC were Ti rich-?Ti,Nb,V??C,N?,Nb rich-?Ti,Nb,V??C,N?,V rich-?Ti,Nb,V??C,N?,M₂₃C₆ and cementite.With the addition of Nb,V and Ti and its increasing content,both the precipitate temperature and the mass fraction of Nb rich-?Ti,Nb,V??C,N?,Ti rich-?Ti,Nb,V??C,N?and V rich-?Ti,Nb,V??C,N?increased respectively.Steels with four different Nb content were designed and prepared based on the above thermal-cal results.The yield strength of the quenching?Q?&tempering?T?treated Cr-Mo martensitic steel was improved significantly with 0.030⁰.060 wt.%Nb addition.The yield strength of martensitic steels increased 77 MPa under the same Q&T heat treatment with Nb content increasing from 0.003⁰.060 wt.%.Moreover,the martensitic steels with increasing Nb content from 0.003⁰.060 wt.%tempered accordingly at675⁷10 ^oC,acquired an almost identical yield strength of⁷80 MPa,indicating an enhanced resistance to tempering.The related mechanisms were as follows.The mass of the Nb-containing precipitates increased with the increasing Nb content,which enhanced the precipitation strengthening.With the enhanced precipitation of Nb-containing particles,the austenite grain and correspondingly martensitic packet and block were refined evidently,which enhanced the grain boundary strengthening.Besides,the dislocation strengthening enhanced with the increased dislocation density by Nb.The mechanisms responsible for enhanced yield strength could be attributed to improved strengthening of grain boundaries,precipitates and dislocations in sequence.With the increased Nb content from 0.003⁰.060 wt.%and accordingly the elevated tempering temperature from 675⁷10 ^oC,the high angle grain boundaries?HAGBs?increased,which raised the diffusible hydrogen concentration and the local hydrogen concentration around the inclusions,and finally promoted the SSC initiation.The increased HAGBs decreased significantly the SSC propagation rate by diverting more frequently the propagating direction,and accordingly restricted the SSC propagation.The increased Nb content contributed to the resistance to SSC propagation more than to the assistance to SSC initiation,leading to an enhanced overall SSC resistance.The overall resistance to SSC(K_ISSC)of this martensitic steel enhanced significantly with the T_q decreasing from 1200 to 900 ^oC.As the T_q decreased,the overall and local hydrogen concentration around the inclusions decreased owing to the decreased solute atoms and dislocation density,resulting in a longer incubation time,and finally an increased resistance to the SSC initiation.The decreased T_q refined the martensitic microstructure and accordingly increased the fraction of the HAGBs which could divert more frequently the propagating direction,thus further preventing the SSC propagation.The decreased T_q contributed to resistance to SSC initiation and propagation,leading to an enhanced overall SSC resistance.As an optimized heat treatment process of Cr-Mo steel,the quenching at 890 ^oC and tempering at 700 ^oC could offer a superior overall performance,including good yield strength conbining with perfect SSC resistance(K_ISSC),which can fully comply with the using requirements for the stipulated SSC resistance OCTGs of C110 grade by API 5CT specification with comfortable margins.