Study On Electrode Cracking Behavior Of Nickel-base Superalloy By Vacuum Induction Furnace Melting

Vacuum induction electrode is the fundamental link of the nickel-base superalloy production process.The crack of electrode could seriously influence the subsequent refining process.These influences including the wide fluctuation of parameters such as voltage,current,melting speed and droplet rate,which results in disturbance of molten pool.And then it will cause macroscopic defects such as black spot,white spot and dot segregation.As a result,the prouducts can not be used.In this paper,the factors that cause the cracking of the vacuum induction electrode are investigated based on the alloy of GH4105.The effects of primary dendrite arm spacing,secondary dendrite arm spacing,dendrite segregation and the distribution of carbides on the cracking tendency were disccused.Additionally,the influence of ?'precipitation on the internal stress of the GH4742 alloy was studied as well to provide theoretical guidance for reducing the cracking tendency of the electrode.It is found that the primary dendrite arm spacing and the secondary dendrite arm spacing increases obviously with the decrease of cooling rate from the chilling layer to the center of electrode ingot.The average values of the secondary dendrite arm spacing at no-crack,crack and fracture are 61 ?m,81 ?m and 120 ?m,respectively,at the same cooling rate.The tensile test at 800? shows that the strength and ductility of the alloy increase with the decrease of the secondary dendrite arm spacing.The coarsening behavior of the secondary dendrite arm reduces the strength and ductility of the alloy,and leads to higher cracking tendency.The segregation of alloying elements usually induces inhomogeneity.The result of thermodynamic calculation shows that the inhomogeneity can lead to obvious difference in melting point in different regions.This results in higher cracking tendency of the post solidification zone,i.e.,interdendrites and grain boundaries which suffer tensile stress.Additionally,MC carbides distribute in the interdendrites and grain boundaries.They have sharp edges and corners.During solidification,it is easier to generate stress concentration in carbides and matrix interface,and finally result in fragmentation.Through the study of GH4742 alloy,it is found that the volume fraction and the size of ?' phases and the lattice misfits between ? phase and ?' phase increase with increasing aging time when aged at both 950? and 1050?.The shape of ?' phase is spherical at all the testing time when aged at the 950?,while the shape of ?' phase change from spherical to cubic with increasing aging time when aged at 1050?.These phenomena induce the increasing of the internal stress with increasing aging time.As a result,the hot cracking tendency of the alloy increases as well.