Effect Of B?Al And Ti On Microstructure And Properties Of K325 Alloy

K325 alloy is a solid solution strengthened nickel-based casting superalloy.Because of its high strength,excellent processing performance,good oxidation resistance and casting performance,it is considered as an appropriate candidate for the valve body of 700? ultra-supercritical coal-fired power plants.However,during the service process of the above parts,the grain boundary coarsening usually leads to the lack of plasticity,which resulting in the phenomenon of cracking.Therefore,in this study,trace element B is added to improve the strength of grain boundary and reduce the tendency of grain boundary crack initiation.In addition,large castings can not be melted by vacuum induction,so they need to be melted by vacuum induction melting,with the refining of large castings by means of 30-ton electric furnace and ladle refining furnace.A deoxidizer added to ensure the quality of the alloy during this process,contains a small amount of Al and Ti.Therefore,this study focuses on the effect of Al and Ti elements on the stability of alloy structure in the long-term aging process.The effects of trace elements B,Al and Ti on the as-cast microstructure,solid solution microstructure,long-term aging structure and mechanical properties of K325 alloy at room temperature and 700? were studied systematically.The as-cast structure of the K325 alloy exhibited a typical dendrite morphology,and the precipitates in the alloy was Nb-rich MC carbides,which were distributed in the interdendritic region and the grain boundary randomly.The effect of solution treatment on the microstructure and mechanical properties of K325 alloy was studied.When the holding time was 1 h and the solution temperature is 1150?1250?,with the increase of solution temperature,the degree of carbide dissolution increases gradually,the dendrite segregation phenomenon is no longer obvious,and the homogeneity of alloy is improved.When the solution temperature was 1200?,the amount of carbides in the alloy decreased gradually with holding time prolonging from 0.5 h to 2 h.After solution treatment at 1200? for 1h,the high-temperature yield strength of the alloy reached the maximum value of 196MPa,and the elongation reached 52%.The degradation reaction of MC carbides occured with the increase of temperature and time during the long-term aging process of K325 alloy at 650-750?,and the degradation reaction equation is MC+??M23C6.When the alloy was aged at 700?for 300 h,film-like M23C6 carbide precipitated on the grain boundary.With the increase of temperature and aging time,the grain boundary coarsened,and some M23C6 carbide transformed into M6C carbide.The y"phase precipitated in the temperature range of 650-750?,its coarsening process was in accordance with LSW theory.When the alloy was aged at 650?,the nucleation and growth of ?" phase were slower,while ?" phase would transform to ? phase with prolonging aging time at 750?.The y" phase phase had the best growth kinetics when the alloy aged at 700?.The ? phase was also precipitated within the aging temperature of 650?750 ?,and the roughening rate was fastest when the aging temperature is 750?.The effect of B on the microstructure and properties of K325 alloy was studied.Boride precipitation was not found in the alloy with different B contents.The addition of B intensified the segregation of Nb in the interdendritic regions,but had no obvious effect on the segregation of other elements.Besides,the addition of B element reduced the solid-liquid solidification temperature range of the alloy and the secondary dendrite arm spacing.The effect of trace element B had little effect on the solid solution microstructure,strength of room temperature and high temperature of the alloy,but the grain boundary strength was improved.The fracture mode of K325 alloy at high temperature changed from intergranular fracture to a mixed fracture mode,and the transgranular fracture phenomenon becomes more obvious with the increase of B content.When the B element content was 0.0025%,the room temperature elongation of the alloy increased from 40%to 68.5%,and the high temperature plasticity increased from 45%to 60%,After long-term aging,the addition of trace element B in the alloy was segregated at grain boundary and ?/M23C6 interface,which hindered the diffusion of M23C6 carbide forming elements to the grain boundary,making the transformation of the grain boundary carbide from continuous film shape to discrete chain shape,which improved the performance of grain boundary.The additionofB element had little effect on tensile strength at 700? after long-term aging,but greatly improved the plasticity of the alloy,and made the fracture mode of the alloy change from intergranular fracture to a mixed fracture.Morever,the addition of B element changed the morphology of the grain boundary carbides from a continuous film-like to a discrete chain,which reduced the stress concentration at the grain boundary,thereby reducing the occurrence of cracks at the grain boundary.In addition,the number of dimples on the section increasedsignificantly,so the plasticity of the alloy was improved.The effect of Al and Ti on the microstructure and properties of K325 alloy was studied.Addition of Al and Ti elements improves the precipitation of MC carbides in as-cast microstructure and solid solution microstructure,because Ti was the forming elements of MC carbides.Al and Ti had little effect on the instantaneous tensile properties at room temperature and high temperature,but significantly improve the stress rupture life of the alloy.After long-term aging,the addition of a small amount of Al and Ti promotes the precipitation of ?" phase.Since Al and Ti atoms can replace the Nb atoms in the ?" phase,the lattice constant of the y" phase was changed,resulting in the reduction of the coherent strain of ?/?" and reducing the nucleation energy of the ?" phase.Besides,Al and Ti increased the activity of the Nb element,so that the diffusion rate in the matrix was increased,which lead to increasing the content of the y"phase and increasing the size of the y" phase.Therefore,the addition of Al and Ti improves the content and thermal stability of ?" phase in the microstructure,thus impro ving the high temperature tensile strength of the alloy after long-term aging.After long-term aged at 700? for 1000 h,the yield strength of the alloy increased from 421 MPa to 563MPa,and the tensile strength increased from626MPa to 719MPa.B,Al and Ti elements had no effects on the deformation mechanism of aged K325 alloy in tensile deformation at 700?,and the interaction between dislocation and ?"phase was Orowan bypass mechanism.In addition,the dense distribution of needle like? phase and carbides on the grain boundary played a role of an obstacle to the moving dislocation,which enhances the precipitation strengthening effect.