Creep,Oxidation And Creep-Oxidation Interaction Behavior And Mechanism Of P92/G115 Steel In Near-Service Environment

P92/G115 martensite heat-resistant steels are used for superheaters,reheaters,headers and main steam pipes of ultra-supercritical critical thermal power units.Under the harsh service conditions of high temperature and supercritical water（SCW）inside,creep,oxidation and the creep-oxidation interaction led to the failure of the material.In order to ensure the safe operation of thermal power units and provide guidance for the alloy design and microstructure design of materials,it is necessary to study the service behavior of 9%Cr martensitic heat-resistant steel in a near-service environment.The creep test of P92/G115 steel in high temperature air and superheated water vapor,and the oxidation test in SCW were carried out in this study.Combined with scanning electron microscope,energy dispersive spectrometer,electron backscatter diffraction technique,Raman spectrometer,focused ion beam,nanoindentation and transmission electron microscope and other characterization methods,the microstructure（dislocation,precipitate and hierarchical martensite microstructure）before and after creep,oxide film structure,element distribution and phase after oxidation were systematically characterized.The effects of the evolution of hierarchical martensite microstructure and δ-ferrite on the creep behavior and mechanism of P92 steel under different stresses were studied.The oxidation behavior and mechanism of P92/G115 steel in ultra-supercritical water were studied;The creep tests in air and water vapor environment were carried out to study the effects and mechanisms of superheated steam on the creep behavior of P92/G115 steel.The main results obtained in this paper are as follows:（1）In the air environment,under different stresses,different evolution mechanisms of hierarchical martensite microstructure led to different accelerating creep and cavity initiation mechanisms of P92 steel.As stress decreases,the preferential coarsening of precipitates occurred on packet/block boundaries.The migration of packet/block boundaries occurred due to the lack of pinning of precipitates,resulting in an increase in lath width.Under high stress,the hierarchical martensite micro structure disappeared completely due to recrystallization,while under low stress,lath coarsening occurred due to the migration of fine packet/block boundaries.The accelerating creep under high and low stresses was caused by the increasing density of random high-angle grain boundaries and the coarsening of laths,respectively.On the other hand,in the stress concentration zone around the inclusions,the sliding of fine recrystallized grain boundaries under high stress led to the initiation of a large number of fine wedgeshaped cracks,while under low stress,cavities were initiated at the coarsened precipitate/matrix interface and propagated along high-angle grain boundaries.（2）In the air environment,inconsistent strain of δ-ferrite/martensite under high stress and large strain promoted the cavity initiation at the δ-ferrite/martensite interface.The elemental segregation in the central region of δ-ferrite accelerated the nucleation of precipitates and strengthening the central region,while solute depletion caused by initiation and coarsening of precipitates at ferrite grain boundaries led to the formation of a softened precipitation-free zone（PFZ）withinδ-ferrite close to the martensite/δ-ferrite interface.Under different stresses,different coordinated deformation behaviors between δ-ferrite/martensite led to different cavity initiation behaviors in the accelerated creep stage.Under high stress and severe plastic deformation,the activation of multiple sliding systems was still unable to coordinate the strain of martensite and δ-ferrite,and creep cavities initiated at the martensite/δ-ferrite interface.Under low stress,the softened PFZ hindered the initiation of creep cavities at the martensite/δ-ferrite interface,causing coordinated deformation of martensite and δ-ferrite in the segregation zone.（3）Compared with that in the air environment,in the water vapor environment,the dense oxide film and the crack tip blunting under high stress increase the creep life,while the loose oxide film and the damage caused by crack growth reduced the creep life under low stress.For P92/G115 steel,under high stress,the oxide film formed in the air environment was loose and easy to peel off.In contrast,the dense inner layer of oxide film with dispersed FeCr₂O₄ particles formed in the water environment enhanced the strength of the creep sample,reduced the steady-state creep rate,and prolonged the duration of the steady-state creep stage.Compared with that in the air environment,although a large number of cracks were initiated on the surface of the sample in the water vapor environment,the crack tip blunting occurred due to recrystallization of martensite at crack tip,resulting in a longer accelerating creep stage.For G115 steel,under lower stress,the loose and easily cracked oxide film could not bear the load,and the thinning of the material due to oxidation increased the effective stress,leading to an increase in the steady-state creep rate.On the other hand,oxidation of the water vapor environment promoted the initiation and growth of surface cracks,accelerated the connection of isolated creep cavities,aggravated creep damage,and significantly reduced creep life.For P92 steel,in the water vapor environment,oxidation promoted crack propagation along high-angle grain boundaries under low stress,accelerated creep crack growth,and reduced creep life.（4）In ultra-supercritical water environment,the surface morphology,crosssectional structure,phase and oxidation rate of the oxide film on P92/G115 steels were affected by the martensite boundary types and dissolved oxygen.For P92/G115 steel,high-angle grain boundaries,such as prior austenite boundaries and packet/block boundaries,contributed to the formation of the Cr-rich oxide layer.Unlike the discontinuous Cr-rich oxide layer formed in P92 steel,the Co-rich residual matrix accelerated the formation of the continuous and dense Cr-rich oxide layer in G115 steel at the oxide film/matrix interface.The continuous Cr-rich oxide layer could effectively reduce the oxidation rate and improve the oxidation resistance of G115 steel.A higher dissolved oxygen accelerated the transformation process from relatively dense Fe₃O₄ to porous Fe₂O₃ on the surface of P92/G115 steel,thereby increasing the material’s oxidation rate.（5）In ultra-supercritical water environment,the diffusion behavior of Co and Cu elements during the oxidation process affected the morphology of the internal oxide layer.When the material was oxidized,the difficult-to-oxidize Co element diffused and enriched into the residual matrix,hindering the complete oxidation of the internal oxide layer,resulting in the formation of a thicker internal oxide layer in G115 steel than that in P92 steel.On the other hand,the residual matrix rich in Co accelerated the formation of Cr-rich layer.In the internal oxide layer,Cu precipitated at the oxide/residual matrix interface to form Cu-rich elemental particles.When the internal oxide layer was completely oxidized,Cu diffused below the Cr-rich layer to form a Cu-rich layer.