Formation And Evolution Of Dendritic Carbides And Its Effect On Microstructure In Alloy 690

The quality stability,microstructure uniformity and corrosion resistance of Alloy 690 steam generator tubes are crucial for the safe operation of PWRs,and carbide evolution control is the a decisive factor for the microstructure and properties of Alloy 690 tubes.Based on the deformation microstructure control of a 3-ton industrial-scale Alloy 690 ingot,the precipitation evolution behavior of dendritic carbides during continuous cooling are systematically studied.The formation mechanism of dendritic carbides and its influences on grain boundary,microstructure uniformity and corrosion resistance of the material are discussed in detail.The controlling method of carbide morphology for industrial-scale Alloy 690 ingots is proposed.The main results are as follows:The precipitation evolution behavior of intergranular carbides in Alloy 690 during continuous cooling was studied by the method of high temperature solution-annealing,controlled immediate cooling,and water quenching.During cooling,firstly carbides nucleate at a relatively high temperature and grow with a rod-like morphology.When carbides increase to a certain size,interfacial destabilization occurs with the growth of secondary branches.With the decrease of temperature,carbides and their branches extend and interfacial destabilization occurs further,resulting in a developed dendritic morphology with multiple branches.The interfacial stability of carbides mainly depends on the solute concentration gradient on the grain boundary near carbides.The solute concentration gradient and degree of undercooling increase,the critical size of interfacial destabilization decreases,and dendritic carbides are easier to form.Higher solution temperatures(>1120℃)and appropriate cooling rates(2-10℃/min)can enhance the solute concentration gradient at grain boundary and promote the precipitation of dendritic carbides.By studying the relationship between the morphologies of carbides and grain boundaries,the mechanism of grain boundary serration(GBS)during continuous cooling was clarified.M23C6 carbides have different orientation relationship to the matrix on two sides in Alloy 690.Carbides grow at a higher speed on the incoherent side and consume a large amount of solute elements on this side,which results in a significant solute concentration difference and a vacancy gradient between the two sides of grain boundary,which drives the migration of grain boundary and induces the grain boundary protrude towards the incoherent side.The branches on dendritic carbides can lead to a much more complex grain boundary morphology change.The effect of dendritic carbides on the corrosion resistance of Alloy 690 was studied by the slow strain rate tension(SSRT)stress corrosion test.Results show that dendritic carbides are easy to break under stress due to their large sizes,which can induce grain boundary cracks and increase the stress corrosion cracking(SCC)tendency of Alloy 690.By studying the hot deformation process of Alloy 690,it is found that dendritic carbides are difficult to dissolve during forging,and the residual undissolved carbides can form carbide strips in deformation,hinder grain growth and evolve into fine-grain bands,which significantly affects the microstructure uniformity and corrosion resistance of the material.In order to eliminate the adverse effect of dendritic carbides on the microstructure and properties of Alloy 690,high temperature forging is used to eliminate the dendritic carbides in large-size alloy ingot and avoid the formation of fine-grain band.