Mechanism Of Abnormal Rapid Sigma Precipitation And IGCS Desensitization Optimization Of Super304H Austenitic Stainless Steel With Critical Deformation Degree

The novel high-carbon Super304H austenitic stainless steel(SS)has been widely used in ultra-supercritical boilers due to its excellent high-temperature performance.However,its high carbon content may bring about massive M₂₃C₆ precipitation and chromium-depleted zones along grain boundaries in high-temperature service,resulting in high intergranular corrosion susceptibility(IGCS).Although surface nanocrystallization via shot peening(SP)has been found to accelerate the healing of chromium-depleted zones to reach desensitization state by long range chromium diffusion enhancement in the nano-grained austenite,abnormal precipitation of chromium-rich sigma phases was triggered in the high-intensity SPed nanostructures,leading to the deterioration of properties.This thesis attempted to control the plastic deformation degree by optimizing the SP parameters to obtain a fast desensitization rate under the premise of no sigma phase precipitation.The mechanisms of early nucleation and abnormal growth of sigma phase in the deformed austenite were clarified.On the basis of the optimized desensitization techniques,the evolutions of microstructure and corrosion resistance of the SPed specimen at high temperature were studied.The main contents and conclusions are as follows:The saturation value of plastic deformation in Super304H SS was defined and the complete curve of critical deformation parameters under different SP pressures was determined.At unsaturated deformation state,uniform plastic deformation occurred and austenitic matrix developed deformation layer rapidly with the increase of SP time.When reaching the oversaturated deformation state,uneven plastic deformation accumulated locally at some boundaries to form high-energy stress/strain concentrations.Abnormal sigma phase precipitation was triggered in the oversaturated SPed austenite during aging.Massive sigma phase increased the IGCS and brought a positive move of the reactivation potential of chromium-depleted zones in the Double Loop Electrochemical Potentiokinetic Reactivation(DL-EPR)curve.In order to avoid the sigma phase precipitation in the pursuing of rapid desensitization process,it is necessary to choose SP parameters with unsaturated deformation degree which is close to the critical values.The extremely early nucleation of sigma phase in oversaturated deformed Super304H SS during aging at 650°C was observed by transmission electron microscope.Nano-scaled sigma phase nuclei were found before recrystallization and its preferential nucleation sites were grain boundaries,twin boundaries,phase boundaries and especially the intersections of multi-interfaces.Unlike grain boundaries and phase boundaries with stored high deformation energy to become stress/strain concentration sites,the deformation energy at twin boundaries was relatively low and sigma phase nucleation at the low-energy twin boundaries was due to the partial microstructure inheritance from the original austenite twins,which lowered the energy barrier of transformation.Moreover,the high-distortion areas favored the segregation of chromium which worked as a transient phase to facilitate the structure transformation from face-centered cubic austenite to topologically close-packed sigma phase.Sigma phase particles were found to grow slowly in the deformed austenite matrix due to the residual compressive stress field,but began to grow quickly to 1–2?m in size at the recrystallizing boundaries when recrystallization commences,due to both the release of residual compressive stress at the recrystallized region and the fast chromium diffusion at the deformed nanostructure region.Desensitization efficiency was greatly improved by nanocrystallization with optimized SP deformation degree to solve the problem of high IGCS in Super304H SS.For the optimally deformed specimen by SP-0.5MPa-5min,a short ageing time of only 10 h at 650°C was acquired to reach the desensitization state.At the same time,the surface nanometer grains retained and the passivation behavior was not affected obviously.Although the desensitization time was shortened(8 h)of the specimen with higher SP pressure(0.6 MPa),its uniform corrosion resistance was obviously lower than others,due to the high density of residual deformation defects in its desensitized state.The recommended desensitization process is SP0.5 MPa-5min/Ageing 650°C-10 h.The evolution of deformed microstructure and uniform corrosion behaviour during ageing of the SPed Super304H SS(including unsaturated and oversaturated deformation degrees)were studied.In the SPed Super304H steel,the grain refinement increased the compactness of the passive film,while the existence of deformation defects reduced the stability of the passive film.The competition of the two influencing factors led to the minor change of corrosion resistance after SP treatment.After long-term aging,partial recrystallization happened in the SPed specimen and brought the co-existence of recrystallized and residual deformed regions,which formed the galvanic couple.When the passive film was thin at early passivation and sensitive to galvanic corrosion,the recrystallized region with coarse grains was selectively dissolved due to its inferior corrosion resistance,bringing an abnormal dissolution current peak in the potentiodynamic polarization curve at early passivation.As the passivation potential proceeded to complete compact passive film,the selective dissolution terminated.Increased recrystallization region in the over-deformed state intensified the selective dissolution process.This research elucidates comprehensively and deeply the internal relationship among SP deformation degree,high temperature precipitation behavior,deformed microstructure evolution and self-healing of chromium-depleted zones in the rapid desensitization process of nanocrystalline Super304H SS,which provides a scientific basis for the optimization of SP treatment and desensitization process,and shows the important values of scientific research and engineering application.