Effect Of Reversed Austenite On Mechanical And Corrosion Properties Of 13Cr Super Martensitic Stainless Steel

13Cr super martensitic stainless steel(13Cr SMSS)is widely used in the oil and gas field because of its high strength,good welding performance and good corrosion resistance.The excellent comprehensive properties of 13Cr SMSS are closely related to the content of reversed austenite and the transformation mechanisms.With the development of oil and gas exploration in deep and ultradeep wells,13Cr SMSS,as a casing material,is facing the influence of high temperature and high pressure,which puts forward higher requirement for its corrosion resistance,ductility and fatigue properties.At present,there is a lack of systematic research on the effect of the reversed austenite on the mechanical and corrosion properties of 13Cr SMSS.In this thesis,13Cr SMSS is used as the research material,and the reversed austenite is taken as the research core.Based on in-situ tensile electron backscatter diffraction(EBSD)and three-dimensional atomic probe tomography(3D-APT)techniques,the microstructure evolution and formation mechanism of reversed austenite and the effects of reversed austenite on mechanical properties and corrosion resistance under three types of heat treatment processes,namely,intercritical quenching+two-step tempering,quenching+twostep tempering and intercritical quenching+cyclic transformation+two-step tempering,were investigated.The main research contents and conclusions are as follows:(1)Reversed austenite with different contents was obtained by intercritical quenching and two-step tempering.The microstructure and formation mechanism of reversed austenite were studied by transmission electron microscope(TEM)and 3D-APT.The results showed that high content of reversed austenite(13.3 vol.%)was obtained by intercritical quenching and two-step tempering,which was 9.7 vol.%higher than one-step tempering.When the primary tempering temperature is 675℃,the rod-like reversed austenite is mainly distributed at the lath interface of the martensite,and meets the Kurdjumov-Sachs(K-S)orientation relationship with adj acent martensite,(111)γ//(011)α’and[101]γ//[111]α’.When the primary tempering temperature rises to 700℃,the reversed austenite distributed at the lath boundaries and in the lath evolves from rod-like to circular block.By analysis of 3D-APT results,Ni was found to be enriched in the reversed austenite,and the degree of C enrichment was not high and C mainly formed MoC.The concentration of Ni in MoC was low,which promoted the enrichment of Ni around MoC and caused composition fluctuation.The nucleation core of reversed austenite is MoC,and the enrichment of Ni provides energy for the nucleation and growth of reversed austenite,which promotes the formation of reversed austenite.When the content of Ni≥7.6 wt.%,the reversed austenite can stably exist at room temperature,and the main reason is the enrichment of Ni.(2)The effect of reversed austenite content on tensile and fatigue properties was studied.The results showed that the more the content of reversed austenite,the higher the uniform elongation,total elongation and strength-plastic.The transformation-induced plasticity(TRIP)effect occurred during the tensile process.The secondary martensite formed by the reversed austenite transformation and the ability of high strain hardening in a wide strain range were the reasons for the increase of strength.The stress-induced transformation of reversed austenite into martensite at the crack tip consumed the plastic property at the tip of the microcrack,slowed down the stress concentration,and the phase transformation promoted the crack closure,which hindered the further propagation of the crack,and improved the total elongation and the toughness.When the content of reversed austenite is less and the content of martensite is more,the martensite will hinder the movement of dislocations.Therefore,the cyclic hardening will occur at the initial fatigue stage.During fatigue process,with the increase of the reversed austenite,the austenite is more prone to TRIP effect,which leads to lower density of dislocations and cyclic softening.(3)Different contents of reversed austenite were obtained by quenching and two-step tempering.The effect of reversed austenite content on corrosion properties was studied.The results showed that with the increase of reversed austenite content,the self-corrosion potential increased from-426 mVSCE to-311 mVSCE,the pitting potential increased from 78.71 mVSCE to 155.87 mVscE,the self-corrosion current density decreased from 3.29 μA·cm-2 to 0.0955 μA·cm-2,the thickness of passivation film increased from 0.2 nm to 1.6 nm,and the pitting resistance increased from 2424 Ω·cm2 to 502400 Ω·cm2.The results showed that the reverse austenite had a beneficial effect on the corrosion behavior of 13Cr SMSS.(4)Based on the traditional intercritical quenching+two-step tempering process,a 4-step cyclic phase transformation was added,and this improved process was applied to 13Cr SMSS.The results showed that the martensite packets,blocks and martensite lath were obviously refined and the content of reversed austenite was increased by 5.4 vol.%,up to 40.6%growth rate.Compared with the traditional heat treatment processes,the strength,plasticity and toughness of 13Cr SMSS were improved after the improved heat treatment process.The evolution of straininduced martensite transformation of 13Cr SMSS during the tensile process clarified by in-situ EBSD followed the classical model of γ→α’.The nucleation ofα’ martensite was first observed at the phase boundary of the reversed austenite when the strain increased to 8%during the in-situ tensile process.When the strain increases to 18%,the α’ martensite nucleus grows along the direction of the phase boundary of the reversed austenite and gradually transformed into the reversed austenite.In the process of tensile process,the large size of reversed austenite was unstable and the phase transformation occurred preferentially,while the small size of reversed austenite has better stability and diffcult occur phase transformation.During deformation,martensite mainly formed textures of(011)[311],(114)[041]and(110)[113],and the austenite mainly formed textures of(015)[451],(041)[314]and(112)[131]and with the increased of strain,the texture strength of(011)[311]and(015)[451]gradually decreased.