| Duplex stainless steels(DSS),which consist of approximately similar amounts of austenite(y)and ferrite(a),are designed to combine the best features of austenitic and ferritic stainless steels,and have an attractive combination of mechanical and corrosion properties and are at present used in the petrochemical,marine,pulp and paper,power industries.In the petroleum chemical industry,papermaking,food processing and other industries duplex stainless steel need to work in high temperature environment.But subjected to aging treatment at tempetures of 500 to 1000?,duplex stainless steel will precipitate sigma phase,nitrides,? phase and other intermetallic phases,which decrease mechanical properties and corrosion resistance of these steels.The heavily use of nickel in duplex stainless steel increase the cost,this paper attempt to use the low cost manganese instead of nickel,research the effect of manganese on the precipitation behavior of duplex stainless steel,provide a reference for the development and application of low nickel duplex stainless steel.In this dissertation,the effects of alloy compositions and aging tempeture on the microstructure,mechanical properties of these super DSS have been investigated systemically.Optical microscope(OM),X-ray diffraction(XRD),scanning electron microscope(SEM),transimission electron microscope(TEM),the precipitation kinetics and the Charpy impact test and other analytical instruments and methods have been employed.Special attention has been paid to the effects of Mn on the precipitate behavior of the super DSS aging at 500?,600?,700? and 800?.The results show that:The solution treatment results show that austenite proportion decrease with the solution temperature rise from 1040?,1120? to 1220?,manganese can expand the austenite proportion in the duplex stainless steel,increase the impact toughness.The proportion of ?/? phase is balanced and the impact toughness is better after solution treatment at 1040?.The aging treatment results show that precipitation behavior of duplex stainless steel mainly occurs at the 8/y grain boundary and the ? phase,increase aging temperature can promote the precipitation behavior,different aging temperatures can change precipitate type.Under the aging temperature of 500?,the precipitate phases are needle-like phase and ?phase,the needle-like phase change smaller and transform into ? phase with aging time extending.Under the aging temperature of 600?,the precipitate phases are a phase,?2 phase and ? phase,and the precipitation mechanism is 8??2+?(?),the thin ? phase precipitate first at grain boundary,a phase and ?2 phase precipitate following.Under the aging temperature of 700? and 800?,the precipitate phases are a phase,72 phase and M23C6 carbide,which precipitation mechanism is ???2+M23C6,???2+?.Manganese can accelerate the precipitation of ? phase and ?phase.Under the aging temperature of 500?,increasing of manganese content in DSS can increase the content of manganese and decress the content of molybdenum content in x phase.Manganese atoms replace the molybdenum atoms in x phase which change the lattice constant of ? phase,? phase and ? phase turn into coherent relationship,therefore ? phase can precipitate and exist largely.Under the aging temperature of 600?,compared with x phase,manganese is more enriched in a phase,accelerate the precipitation of a phase.Under the aging temperature of 700? and 800?,manganese can accelerate the precipitation of ? phase and M23C6 carbide,increase the maximum precipitation of sigma phase.The results of precipitation kinetics and the impact test show that both the manganese content and the aging temperature affect the kinetic parameters of the 8 phase decomposition.At the same aging temperature,increasing of manganese enlarge k value and n value,the decomposition time of ? phase was earlier,and the decomposition rate was accelerated,the k value increased significantly with the aging temperature increasing,the 8 phase decomposition time was advanced with the aging temperature increased from 600? to 800?,little amount of decomposition of 8 phase had little effect on the impact toughness of the samples,and the impact toughness of the samples decreased rapidly when the volume fraction of decomposition was more than 3%.At the early stage of aging treatment,?phase decomposition quantity is less,the impact toughness is mainly affected by the organization of ? phase and y phase,samples with higher Mn content have larger amount of austenitic proportion and higher impact toughness,toughness is mainly affected by the influence of sigma phase,samples with higher Mn content have larger amount of precipitation leads to poor impact toughness at aging period. |
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