| In this thesis,the research object is the nuclear power duplex stainless steel S32101.Duplex stainless steel(DSS)consist of ferrite and austenite,the phase composition is usually 50 pct of each phase.The advantage of mixing the two phases is that the resulting material has good retained properties from both phases.Some examples are the high mechanical strength and resistance to stress corrosion cracking in the ferrite and the high toughness in the austenite.Compared with the traditional 304 austenitic stainless steel,the DSS S32101 has higher yield strength,better corrosion resistance and lower cost.However,there are still some problems in the production process and application in sub-zero temperatures of the DDS S32101.The studies of these issues are limited,in this thesis,hot workability,development of constitutive model and processing map have been investigated.In addition,impact toughness testing was conducted on plate of the DDS S32101.The main research results and original achievements are drawn as follows.(1)Hot compression tests were conducted using Gleeble 3800 simulator to obtain flow stress curves as a function of temperature and strain rate.The deformation behavior and microstructure evolution of DSS S32101 under different hot-working parameters were studied.At lower strain rate and lower deformation temperature,the ferrite undergoes DRX while the austenite only takes DRV.At lower deformation temperature or higher strain rate,the micro structure exhibits a typical flow localization band caused by the uneven deformation.Generally,such deformation bands can easily cause the cracks and cavities of the material.(2)Based on the compression results,the constitutive model with the compensation of strain was developed on the basis of hyperbolic sine equation to predict the flow stress of DSS S32101.The material constant in the model such as ??Q?n and lnA was functioned with the strain by seventh order polynomial.The results show that the developed constitutive model has excellent predictability of flow stress.The standard statistical parameter,correlation coefficient R is 0.986.Moreover,the average absolute relative error for the entire compression tests is 6.46%.The constitutive model can be used to predict flow stress precisely.(3)Instability of DSS S32101 are flow localization band in the specimen and cracks at subsurface on the equatorial plane of bulge surface.The processing maps based on DMM gave a poor prediction of the occurrence of flow instability.While,the processing map based on the critical specific plastic work matched well with the instability of the specimens.(4)The factors affecting the impact toughness of the duplex stainless steel S32101 were explored.The conclusion from the fractographic investigation was that the delaminations that occur in DSS S32101.The delaminations can be explained by the cleavage fracture and the appearance of the austenite phase microstructure.Fracture process mainly propagated in the ferritic phase.Fracture was largely dependent on the distribution and austenite phase microstructure geometrical properties of the austenite lamellae which influence the impact toughness of the duplex stainless steel S32101.(5)The effective methods to improve the impact toughness of DSS S32101 were investigated.The maximum impact energy of the half impact specimen(55×10×5mm)is 67J,which was hot rolled by heavy reduction rate,then adopted ultra fast cooling technology,in the last annealed at 1000? for 15 minutes.And the yield strength and tensile strength of the specimen are higher than the standard values.Heavy reduction rate hot rolling increases the density of the austenite lamellae.The adoption of ultra fast cooling avoids the production of the precipitates.The behavior of the impact specimen at cleavage fracture was largely dependent on the distribution and geometrical properties of the austenite lamellae which gave rise by annealed at 1000??... |
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