| It is of great significance for energy saving and emission reduction to improve thermal efficiency of thermal power generating units.Now many developed countries have already started to develop coal-fired power generation technology in 700?Ultra-supercritical(A-USC)units.Inconel 740H alloy is an important candidate material for superheater/reheater tubes of plants in 700? A-USC,because of its good thermal endurance and corrosion resistance.Electroslag remelting(ESR)is currently an important method of smelting Inconel 740H nickel-based superalloy,but there are still following problems:Al,Ti and other easily oxidized elements burn and distribute unevenly along the ingot.Nickel-based alloys with low melting point and high viscosity in liquid state always have surface quality problems of ingot such as slag runners,waist defects and shunt holes.High alloying degree of Inconel 740H results in its severe dendritic segregation and complicated precipitation,which brings about inhomogeneity of structure and effects the mechanical properties and corrosion resistance of alloy.In order to solve these problems,in this paper the electroslag remelting slag and ingot homogenization have been studied.The research work of the ESR slagconsists of the following contents:Firstly,according to the technological characteristics of the ESR Inconel 740H alloy and its requirements for the slag,five slag systems based on 51.5%CaF2-322.7%Al2O-20.6%CaO-5.2%MgO slag,were designed by adding 0?3%?5%?7%and 9%TiO2,regarded as T0#,T3#,T5#,T7#and T9#.The melting characteristics,viscosity,density,optical basicity and conductivity of the five slag systems were measured or calculated,and XRD analysis of the slag was carried out.Finally,the equilibrium experiments between slag and Inconel 740H alloy smelted by vacuum induction furnace were performed for five slags.The alloy and slag samples during reaction process and at the end were taken and analyzed.The research on the ingot homogenization consists of following works:Microstructure,precipitation and element segregation of as-cast Inconel740H from vacuum induction melting were studied,by means of OM,SEM and EDS.Their kinetic homogenization curves were calculated by DICTRA software through the residual segregation parameter model.Based on the theoretical results,six homogenizing treatments were tested.Microstructure analysis was used to verify the effect of homogenizing treatment to get a reasonable homogenizing treatment.The main conclusions are obtained as follows:(1)Within 0?9%range content of TiO2,based on 51.5%CaF2-22.7%-Al2O3-20.6%CaO-5.2%MgO slag,melting point was in the range between 1264? to 1296?.With the increase of TiO2 content,the low melting point phase was gradually formed in the slag phase and the melting point of the slag system gradually decreased.The viscosity of the slag system did not change much in the high temperature range(1350-1550?),which was in favor of slag pool.In the low temperature range,with the increase of TiO2 content,the viscosity of slag in each type showed a decreasing trend.The calculated conductivity,optical basicity and density decreased with the increase of TiO2 content.(2)Adjusting the Al2O3 and TiO2 content ratio in the slag system can change the activity of the slag and thus affect the control of the Al,Ti alloy content ratio lg([Al%]4/[Ti%]3).Within 0?9%range content of TiO2 and with the increase of TiO2 content,the phenomenon of "the increase of aluminum and decrease of titanium"happens in T0#slag system.the phenomenon of "the increase of titanium and decrease of aluminum" happens in T5#?T7#and T9#slag system obviously.T3#(composed of 50%CaF2-22%Al2O3-20%CaO-5%MgO-3%TiO2)can make Al,Ti content more stable.T3#slag is more suitable for use of ESR of Inconel 740H alloy.(3)As-cast Inconel 740H exhibits severe dendritic segregation,and Nb and Ti elements are intensively segregated to the interdendritic regions,leading to the main precipitation of Laves phase.The segregation degree in ingot center and 1/2 radius are greater than in the edge.During homogenizing,Ni and Ti are gradually distributed evenly by diffusion with the increase of homogenizing temperature and holding time.It is found that the segregation of elements is basically eliminated and the harmful precipitation phases are redissolved for the Inconel740H alloy after homogenizing treatment at 1180? for 20 h. |
PDF Full Text Request