| The high intergranular corrosion of the high-carbon and high strength heat-resistant austenitic stainless steel has drawn increasing attention in recent years.In this thesis,Super304 H steel of high carbon content,which has been widely used in ultra supercritical fossil-fired boilers recently,was selected as the investigation material and the fine grained surface microstructure via shot peening,which has been used for improving the high temperature oxidation resistance of Super304 H steel,was utilized to solve the problem of its high intergranular corrosion susceptibility(IGCS),which may realize the co-existence of high temperature strength,oxidation resistance and intergranular corrosion resistance in the Super304 H SS.Firstly,severe plastic deformation layer with ultrafine/nano grains on Super304 H steel was obtained by surface shot peening.X-ray diffraction(XRD)analysis,optical micrographs analysis,scanning electron microscope(SEM)and transmission electron microscope(TEM)observation,and hardness tests were carried out to investigate the microstructure and mechanical properties superelasticity of the plastic deformation layer.Secondly,the effect of grain refinement on the IGCS of austenitic stainless steel after being sensitizated\aged was analyzed by Double Loop Electrochemical Potentiokinetic Reactivation(DL-EPR)tests.At the same time,scanning electron microscopy(SEM)and transmission electron microscopy(TEM)were used to observe the precipitation and distribution of M23C6 in the coarse and fine grained Super304 H steel so as to understand the effect of grain refinement on the precipitation kinetics of M23C6 in Super304 H steel.Based on the DL-EPR results and the corrosion morphologies after DL-EPR tests,the time-temperature-sensitization(TTS)diagrams were plotted to clarify the general rules and mechanism of grain refinement influence on the sensitization and desensitization characteristics of austenitic stainless steels.Finally,the rechromization possibility of eliminating the chromium depletion zones in Super304 H steel was probed into by the non-traditional diffusion heat-treatment in austenitic stainless steels under the condition that the surface nanograin microstructure was stable.It is found that surface nanocrystallization accompanied by strain-induced α?-martensite was obtained in the top surface of the Super304 H steel by optimizing the surface shot peening parameters and deformation twinning intersection was the dominating mechanism for the grain refining.With the increase of shot peening time,the grain size decreased,the volume fraction of strain induced martensite increased.and as a result,surface hardness was improved significantly.Under 0.5MPa/3~20min shot peening,surface hardness was more than twice that of the untreated specimen.The depth of the refined layer increased with shot peening time and was almost constant under 0.5MPa/3~8min shot peening,which were recommended as shot peening parameters for the Super304 H steel in this paper from the engineering view of point.As compared to the solution-treated Super304 H steel,deformation twins and more grain boundaries in shot-peened specimens(0.5MPa/3~20min)provided more nucleation sites for carbide precipitates therefore more carbide precipitates at deformation twins and grain boundaries were observed in shot-peened specimens after being sensitized at 600-700℃ for 2h,which increased with shot peening time.The degree of sensitization(DOS)from DL-EPR tests of shot-peened Super304 H steel sensitized at 600℃ to 650℃ for 2 h was found to increase with shot peening time and was much higher than that of the solution-treated one.The strain-induced martensite was blamed mainly for this DOS deterioration.The high IGCS of shot-peened Super304 H stainless steels could be eliminated by annealing at 700℃ for 2 h due to the reverse transformation of strain-induced martensite and faster healing of chromium depletion zones by long range chromium diffusion enhancement in ultrafine grained austenite at this temperature.Further more,two recommended shot-peened specimens(0.5MPa/3min,0.5MPa/8min)were choosed to be aged at 600-700℃ for varied time to see the DOS change with aging time.The nanostructure and strain-induced α?-martensite in shot-peened Super304 H steel had a thermal stability at 600 ℃ and the martensite-induced sensitization dominated over the aging process and increased the degree of sensitization of shot-peened specimens at this temperature.At the same time,more carbide nucleations and small carbide size helped Cr diffusion into the Cr depleted zones from the matrix thus shot-peened specimens(0.5MPa/3min,0.5MPa/8min)came into desensitization phase in a relatively short time(24h,96h).Self-healing of sensitization of shot-peened specimens happened when being aged at 650 ℃ for 24 h because of the reverse transformation of ɑ?-martensite and faster healing rate of chromium depletion zones by long range Cr diffusion enhancement in nano-grained austenite at this temperature.At 700 ℃,the self-healing time of sensitization was decreased to 2 h,which was attributed to the ultrafine-grained microstructure as well as the higher temperature which further accelerated the desensitization process via faster long range Cr diffusion.But the negative consequence was the growth of nano-sized grains into ultrafine grains at this temperature,which may weaken the oxidation resistance enhancement by grain refinement.Finally,attempt was made to eliminate the IGCS of Super304 H steel via desensitization in a shorter time at 650℃ by further increasing the shot peening time(0.5MPa/20min).However,the poor stability of strain-induced martensite deteriorated the thermal stability of surface nanostructure in the specimen,and promoted the change of precipitates from M23C6-type carbide to σ-phase,which nucleated and grew at recrystallized interfaces from 24 h to 168 h at 650℃.At the early stage of aging at 650℃ up to 10 h,the intergranular corrosion behavior of the high-intensity shot-peened specimen was primarily governed by the kinetics of the formation of M23C6-type carbides,and the specimen underwent a sensitization and desensitization process.When the aging time increased to 24 h and 168 h,however,an increase in the DOS value appearred again in the specimen because the high content martensite with poor thermal stability accelerated the formation of σ phase at the recrystallized austenite grain boundaries and induced additional Cr-depleted zones,which resulted in a decrease in IGC resistance of the shot-peened Super304 H steel. |
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