Fundamental Research On Thermal Physical Behavior In Electroslag Remelting Process Of Steam Turbine Rotor In Combined Cycle Power Generation

A transient three-dimensional comprehensive model of the electroslag remelting(ESR)process,based on the dynamic mesh technique,has been established using the finite volume method.The ESR process was employed to produce a multiple steam turbine rotor used in the combined cycle power generation.According to the features of the ESR process,Maxwell's equations were simplified,and the magnetic diffusion coefficient of the two-phase was created.A transportation equation of the magnetic field therefore was obtained.Alternating current phasor was invoked to solve the Lorentz force and the Joule heating distributions,and they were coupled with the momentum and energy equations as a source term.The volume of fluid approach was used to describe the momentum and heat transport between the slag and the metal,and the turbulent viscosity was determined by RNG k-? turbulence model.Besides,the damping of the melt momentum during the solidification was modeled by an enthalpy-based technique,where the mushy zone was treated as a porous medium.In order to consider the directional solidification of the ESR ingot,an anisotropic permeability module was developed.Thus,the damping force was related to the metal pool shape,the primary and the secondary dendrite arm spacings.The redistribution of solutes at the freezing front was described by Lever algorithm,and the diffusion of solutes in the solid state was corrected.In order to distinguish the effects of the Lorentz force,the thermal buoyancy force,and the solutal buoyancy force on the flow in the metal pool,two non-dimensioanl numbers,ST and LT numbers,were proposed.A new remelting method was put forward throught analyzing the evolution of the ST number.An ESR furnace with an open atmosphere was employed to accomplish the remelting.A single-phase power serviced for the furnace.The consumable electrode,which was constituted of two pieces,one was AISI 202 stainless steel and another was AISI 316 stainless steel.They were connected through welding.During this process,slag temperature was measured by W3Re/W25Re thermocouple.Descent speed of the electrode was recorded for the calculation of melt rate.The completely solidified ingot was splited along the longitudinal direction using wire-electrode cutting.Distributions of nickel mass fraction along the longitudinal and radial directions were examined by an optical emission spectrometer.Macrostructure of the ingot was then observed with the help of aqua regia.Moreover,the influences of the applied current,the slag weight and the mold fill ratio on the solute content distribution as well as the the transition zone length were studied in the experiments.The results indicates that negative segregation o nickel happens at the ingot bottom,and the segregation index along the vertical axis decreases with the increasing of the ingot height.The nickel concentration becomes higher and reaches the nominal concentration of the AISI 316 stainless steel after the alloy transition.Positive segregation of nickel appears at the upper part of the ingot,and the maximum positive segregation index is found at the center of at the ingot top.Besides,positive segregation of nickel is observed at the middle of the ingot,while negative segregation is found at the outer side.The nickel content gradually decreases along the radial diection.The concentration difference of the nickel at the middle and the outer side of the ingot reduces from the ingot bottom to the ingot top.The relationship of the transition zone length in the dual alloy ingot with the current and the slag thickness can be expressed:Lc=368.013-0.098×I+1.214×10-5×I2-3.755×hs+0.032×hs2,Lm=261.204+0.007×I-1.214×10-5×I2-2.207×hs+0.019×hs2,where Lc represents the transition zone length along the vertical centerline of the ingot,Lm indicates the length along the vertical midradius line.The units are mm.I is the current which ranges from 1000 A to 2000 A,and hs is the slag thickness which changes from 40 mm to 80 mm.Additionally,the maximum positive and negative segregation indexes of nickel along the vertical axis can be determined by the current and slag thickness as:Mp=-0.123-1.268×10-4×I+8.182×10-8×I2+0.009×hs-7.559×10-4×hs2,Mn=0.012 + 8.814×10-6×I-1.482×10-8×I2-7.175×10-4×hs+5.943×10-6×hs2,where Mp is the maximal positive segregation index,and Mn is the maximal negative segregation index.The magnitude as well as the growth rate of the ST number must be increased for the sake of shortening the transition zone.The ST number is expected to exceed 1 as soon as possible during the ESR process.The AISI 316 stainless steel therefore is placed at the lower part of the assembled electrode,and the AISI 202 stainless steel is placed at the upper part.The dual alloy ingot would be produced by the new assembled electrode.As a result,the unavoidable macrosegregation could be utilized to decrease the transition zone lenrth.The length of the transition zone along the vertical centerline and the vertical midradius line were 108 mm and 150 mm,respectivetly,when the slag thickness is 60 mm,and the current is 1500 A.The maximal positive and negative segregation indexes of nickel along the vertical centerline are +0.1138 and-0.0124,respectivitly.The two transition zone lengthes decrease 18%and 16%,and the maximal positive and negative segregation indexes reduce 23%and 56%,when compared with the dual alloy ingot produced using the traditional method.