Research On Microstructure Evolution And Phase Transformation Behavior Of 00Cr13Ni6Mo2 Supermartensitic Stainless Steel

Supermartensitic stainless steel has outstanding mechanical properties,including a high strength,good toughness,weldability and excellent corrosion resistance in the CO₂ and H₂S corrosion environment.It has such excellent mechanical properties that has already become an alternative product to replaceaustenitic stainless steel and duplex phase stainless steelmainlyusedin the offshore and deep sea oil and gas extraction and transportation industry.The excellent mechanical properties are strongly dependent on the microstructure evolution during hot working and heat treatment.The research object wasbased on the chemical composition of the 00Cr13Ni6Mo2 supermartensitic stainless steel in the paper.The hot deformation behavior was studied by the Gleeble-3500 thermal simulator machine.The austenite grain growth behavior and phase transformation was investigated bythe high-temperature Laser Scanning Confocal Microscope.The effect of heat treatment process on microstructure and mechanical properties was studied of the experimental steel.The microstructurecharacteristics,element distribution in the reverted austeniteand the crystal orientation relationship between the reverted austenite and termpered martensite were studiedby optical microscope(OP),scanning electron microscope(SEM),lectron backscatter diffraction(EBSD)and transmission electron microscope(TEM)equiped with energy spectrum.The kinetic dynamic model was established by the dataof the equilibrium reverted austeniteunder the isothermal tempering processes.The morphological characteristics,elements distribution and enrichment in the reverted austenite,and crystallographic relationship between the reverted austeniteand tempered martensite was analyzedto reveal the phase transformation mechanism of the reverted austenite.The research paper enriched the knowledge of the phase transformation of reverted austenite,and provided theoretical guidance for the formulation during hot working process of super martensitic stainless steel.The mainconclusion of the thesis work as follows:(1)Based on the hyperbolic-sinemathematics model,the constitutive relationship equation of the experimental steel was established between the thermo mechanical parameters with strain and stress.The deformation activation energy was 412k J/mol during hot deformation of the super martensitic stainless steel.The flow stress of the super martensitic stainless steel increases with the decrease of the deformation temperature and increases with the increase of the strain rate.Under low strain rate and high deformation temperature conditions,the supermartensitic stainless steel prones to generatedynamic recrystallization,the recrystallized grain becomes more smaller and uniform,and the grain size increases with increasing deformation temperature.The deformationcondition have a great influence on the microstructure.The result shows that a higher deformation temperature(1050?)and a lower deformation rate(0.01 s^-1)help to improve the uniformity of the microstructure after thermal deformation,and the more reverted austenitewas generated at low strain rate and high deformation temperature after hot working process.The high temperature deformation behavior was investigated in supermartensitic stainless steel,which can provide a reference for the controlling hot rolled microstructure of the experimental steel.(2)In situ observation of the austenite grain growth behavior was investigated by high-temperature laser scanning confocal microscope(LSCM).The growth of austenite grain size was in accordance with the Arrhenius relationship in the range of 950?1150?.The average apparent activation energy of grain boundary migration was 160.6 k J/mol.The austenite grain size increased parabolically with holding timeand the grain growth index was about 0.3whenthe experimental steelheated at 1050?.The martensite transformation startingtemperature(Ms)of the experimental steelgradually increases with the austenitizing temperature under the same cooling rate.It was found that the martensite growth from prior austenite grain boundary into the crystal in shear mechanism by the in situ observation of the experimental steel.The higher heating temperature is,the larger grain size of the martensite laths are.It was found that the reverted austenite decomposedinthe experimental steel during the tempering cooling process by the in situobservation.The in situobservation of dynamic microstructure provides analysis tools for controlling austenite grain growth behavior and reverted austenite stability research in the temperingprocess.(3)The microstructure is mainly composed of tempered martensite and reverted austenite when the experimental steel quenched at 1050?and tempered at 580?700?.With the tempering temperature increasing,the amount ofreverted austenite increases first and then decreases,which is stronglydepended on the tempering temperature.The content of reverted austenite reached the maximum value during tempering at 620?.When the tempering temperature continues to increase,the amount of inverted austenite decreases.The reason is that the stability of the reverted austenite decreasedduring the cooling process,the reverted austenite decomposed and retransformed into martensite with the increase of tempering temperature.The microhardness showed an opposite trends with the amount of reverted austenite.The amount of reverted austenite gradually increase with the tempering holding timewhen the experimental steel tempered at 620?for 1?32 hours.The morphology of reverted austenite changed from granular to massive,and finally showed a lath shape.The lath-shaped of the reverted austenite refined the martensite matrix.However,the microhardness gradually decreased with the increasing of tempering holding time.(4)The EBSD characterization of the microstructure showed that that the reverted austenite was mainly nucleated and precipitated on the tempered martensite packets,blocks and laths,and a small amount distributed at the prior austenite grain boundaries.The martensite matrix has high density dislocation and the misorientation angles range of0°?60°in the stucture according to the EBSD results.The crystallographic orientation relationship between the reverted austenite and martensite conforms to the Kurdjumov-Sachs(K-S)relationshipaccording to(100)pole figure analysis.The crystal plane is(111)_??(011)_?,and the crystal orientation is[11^-0]_??[11^-1]_?.The deviation angle from the K-S orientation relationship was mainly concentrated at about 2 degrees.The reverted austenite and martensite interface with K-S orientation relationship has a low interface energy,which is beneficial to the nucleation of reverted austenite.(5)It was found that the formation of reverted austenite is mainly related to the distribution and enrichment of elements by TEM observationequiped with energy spectrum analysis and XRD refinement and fitting,especially the Ni content in reverted austenite is significantly higher than that in the surrounding martensite matrix.The formation of reverted austenite is a diffusion mechanism related to element partitioning and enrichment.According to the volume fraction of the equilibrium reverted austenite at room temperature,the Johnson-Mehl-Avrami(JAM)kinetics equation was established during the tempering process under isothermal temperature conditions.The activation energy of reverted austenite formation is369k J/mol and the Avrami exponent value n is about 0.5.The kinetic equation and Avrami exponent valueindicates that the mechanism of reverted austenite transformation is diffusion-controlled mechanism and the growth of reverted austenite closely relies on the elementaldiffusion.