Observations of melt rate as a function of arc power,CO pressure,and electrode gap during vacuum consumable arc remelting of Inconel 718

Statistically designed experiments were conducted at two different production melt shops to evaluate the influence of arc power, CO pressure, and electrode gap on melt rate. Approximately 11,000 kg of Inconel 718 alloy 0.4 m diameter electrodes were vacuum consumable arc remelted into 0.5 m diameter ingots. Analysis of the experimental results revealed that melting efficiency (melting rate/kW) was maximized when CO pressure and electrode gap were held at low levels. Under these conditions, the heat distribution (created by the vacuum arc) on the electrode tip and the molten pool exhibited macro uniformity. Increased CO pressure and/or electrode gap depressed the melt rate, and at 13.3 Pa (100 microns) and a 0.050 m electrode gap, this depression exceeds 46 pct. Increasing these parameters also changed the arc behavior to that of a constricted arc with a highly localized heat input. It is hypothesized that the change from the usual diffuse arc to this constricted arc results in intense Lorentz pumping in a localized region of the molten pool atop the ingot causing fluid flow transients. These transients could, in turn, create solidification defects.     

Model-based melt rate control during vacuum arc remelting of alloy 718

Vacuum arc remelting (VAR) is used widely throughout the specialty metals industry to produce superalloy and titanium alloy cast ingots. Optimum VAR casting requires that the electrode melting rate be controlled at all times during the process. This is especially difficult when process conditions are such that the temperature distribution in the electrode has not achieved, or has been driven away from, steady state. This condition is encountered during the beginning and closing stages of the VAR process, and also during some process disturbances such as when the melt zone passes through a transverse crack. To address these transient melting situations, a new method of VAR melt rate control has been developed that incorporates an accurate, low-order melting model to continually estimate the temperature distribution in the electrode. This method of model-based control was tested at Carpenter Technology Corporation. In the first test, two 0.43-m-diameter alloy 718 electrodes were melted into 0.51-m-diameter ingots. Aggressive start-up and hot-top procedures were used to test the dynamic capabilities of the control technique. Additionally, a transverse cut was placed in each electrode with an abrasive saw to mimic an electrode crack. Accurate melt rate control was demonstrated throughout each melt. The second test used an electrode size and grade proprietary to the host company. Because it was not stress relieved after the primary casting process, the electrode was known to possess multiple cracks that make accurate melt rate control impossible using standard VAR controller technology. This electrode was also successfully melted with good melt rate control using the model-based controller.     

Analytical determination of the vacuum arc remelting parameters

An analytical method is proposed for determining the optimal parameters of vacuum arc remelting at which the refinement of a melt is most efficient. The processes of removing nonmetallic inclusions (NIs), gases, and impurities are considered separately.     

Vacuum consumable arc remelting electrode gap control strategies based on drop short properties

An investigation was conducted to determine the influence of electrode gap on the properties of drop shorts during vacuum consumable arc remelting. The goal of this work was to yield an electrode gap control strategy based on easily measured electrical parameters. For a particular alloy, at one current level and low pressure (<4 X 10^-3 Torr), this goal was achieved, in that drop short period was found to be highly correlated with electrode gap. However, this model was found to be constrained by statistics which require counting 100 drop shorts in order to obtain a valid measurement. Other control strategies involving anode spikes, drop short resistance, and energy/resistance ratios were also evaluated and these properties showed poor correlation with electrode gap.     

Stabilization of vacuum arc remelting of steels and alloys

The main cause of the electrode mass melting rate oscillations during vacuum arc remelting (VAR) of steels and alloys is shown to be the displacement of an arc into zones with different metallic vapor pressures. For the remelting process to be stabilized, the arc space length should be controlled as a function of the electrode melting rate and the shrinkage defects in cast electrodes should be removed by high-temperature gasostatic treatment.     

镍基高温合金真空自耗数值模拟

摘要：基于某特钢厂生产过程,对镍基高温合金的真空自耗过程进行数值模拟,研究了不同电流强度、熔速和通入氦气对铸锭Nb元素宏观偏析、黑斑形成的影响规律。铸锭在顶部和1/4处产生较为严重的宏观偏析。随着熔炼电流强度增加,铸锭顶部的磁场增加,顶部宏观偏析和1/4处黑斑逐渐加重。随着熔速的增加,熔池深度增加,铸锭顶部的Nb元素偏析加重,铸锭1/4处黑斑增加。熔炼时通入氦气,铸锭冷却速率大幅度提高,铸锭的元素偏析程度和黑斑明显减少,且最大偏析部位向铸锭顶部移动。     

真空电弧重熔炉控制策略的分析

通过对真空电弧重熔炉工艺过程的分析,根据冶炼实际情况,提出了一些有助于控制冶炼目标的局部策略,如弧长控制、熔速控制、解耦控制等。实际应用表明,当熔炼过程出现各种特殊情况时,采用这些局部策略能得到更好的控制效果。     

真空自耗熔炼过程宏观偏析的数值模拟

基于国内某特钢厂真空自耗生产过程,利用ProCAST软件建立三维熔炼模型,研究了不同电流强度和熔速对铸锭熔池形状和宏观偏析的影响规律.铸锭侧面表层基本不发生宏观偏析,铸锭1/4处偏析度约为1.03的正偏析,铸锭中心处由于枝晶沉降形成偏析度为0.96的负偏析.电流强度从4kA增加至8kA,熔滴滴落温度增加,熔池深度加深,...     

Numerical simulation of vacuum arc remelting nickel based superalloy

Based on the process in a special steel plant, the vacuum arc remelting process of nickel based superalloys was simulated to investigate the effect of current intensity, melting rate, and helium gas cooling on macrosegregation of Nb and formation of freckles. The ingot has severe macrosegregation in the upper and 1/4 part of the ingot. With the current intensity increasing, the magnetic field in the upper part of the ingot increases, and macrosegregation and freckles increase gradually both in the upper and 1/4 part of the ingot. With the melting rate increasing, the depth of the molten pool increases, the macrosegregation of Nb increases in the upper part of the ingot, and the freckles increase significantly in the 1/4 part of the ingot. When helium is introduced into the ingot during smelting, the cooling rate increases greatly, the macrosegregation and freckles reduce significantly in the ingot, and the maximum segregation position moves to the top of the ingot.     

Numerical simulation of dendrite white spot formation during vacuum arc remelting of INCONEL718

White spot is the term for a particulate dispersion lean in niobium found in vacuum arc remelted (VAR) ingots of niobium containing nickel-based superalloys, such as INCONEL718, that can be detrimental to the mechanical properties. While spot can result from exogenous fragments that fall into the VAR melt pool and remain incompletely melted. In this study, white spot formed when dendrite clusters fall-in from the shrinkage pipe of vacuum induction melted (VIM) electrodes is considered by simulations. The motion and dissolution of the dendrite cluster particles were simulated in the framework of a macroscopic heat and fluid flow model of the VAR process. Two scales of heat and mass transfer are considered within the cluster: interdendritic solute diffusion within particles and the thermal interaction between the particle and the bulk convective melt. The dissolution behavior of dendrite cluster fall-in was investigated for a range of initial particle conditions including solid fraction, Nb content, drop height, and initial temperature. The operational window where the exogenous particles completely dissolve was determined as a function of cluster size, density, and location. It was found that particles smaller than 3 mm are completely dissolved under all conditions simulated in this study. All factors studied demonstrated significant influence on particle dissolution. Particles with a solid fraction less than 0.5, a Nb content greater than 4 pct, or an initial temperature greater than 1400 K are likely to be dissolved immediately after entering the melt pool. Drop height and initial density had the greatest effect on particle dissolution.     

真空自耗冶炼过程数值仿真研究进展

借助计算机仿真技术研究真空自耗冶炼过程已成为一种新的发展趋势,此方法不仅有助于探明各工艺参数对真空自耗冶炼过程的影响与作用机理,还可对其冶炼过程的稳定性与铸锭的冶金质量进行有效预测。对此,简要介绍了国内外近30年来真空自耗冶炼过程数值模拟的发展历程与取得的科研成果。同时,以真空自耗冶炼过程模拟、宏观尺度模拟、微观尺度模拟为切入点,详细综述了国内外关于真空自耗冶炼模型、金属熔池模型及微观组织模型的研究进展与应用现状。最后,对国内进一步推动真空自耗冶炼过程数值模拟的开发与应用提出了新的发展方向。     

Tree-ring formation during vacuum arc remelting of INCONEL 718: Part I. Experimental investigation

In the production of turbine discs, the final wrought structure is critically dependent on aspects of the ingot grain structure produced by vacuum arc remelting (VAR) prior to cogging and forging. Variations in the as-cast grain structure in the nickel-based superalloy INCONEL 718 were investigated, focusing upon regions where strings of equiaxed grains interrupt a predominately columnar-dendritic structure. These features, termed “tree rings,” form concentric circles that can be observed visually on etched transverse sections of the VAR ingot. These structures are of interest because they correlate with perturbations in control of the process and have also been associated with the occurrence of defects. This article describes an experimental study of these tree rings, which both characterizes them and investigates possible mechanisms for their formation. Fluctuations in the macroscopic heat, mass, and momentum transfer, which can increase the grain nucleation at the mushy zone front (similar to a columnar-to-equiaxed transition), were considered to be the most likely of the mechanisms hypothesized. In a second article, a multiscale mathematical model is developed to quantify these concepts and to determine whether they account for the features observed.     

Tree-ring formation during vacuum arc remelting of INCONEL 718: Part II. Mathematical modeling

Tree-ring grain formations, a common microstructural feature found in vacuum arc remelted (VAR) ingots of nickel-based superalloys, were characterized experimentally in Part I. The experimental observations led to the conclusion that tree rings are chains of fine-equiaxed grains interrupting a predominately columnar-dendritic structure. Several possible mechanisms for their formation were considered, and their implications correlated with experimental observations. The most likely mechanism was determined to be that process perturbations cause changes in the thermal (or solutal) fields ahead of the columnar-dendrite tips, temporarily altering the conditions to increase grain nucleation and, hence, forming fine-equiaxed grains. In this article, Part II, a multiscale mathematical model of the VAR process is presented that simulates the macroscopic heat and momentum transport and combines it with a mesoscopic model of the nucleation and growth of grains. Using this multiscale model, the transient development of the VAR grain structure was simulated with varying levels and durations of fluctuations in the principal process parameters: power supply, arc focus, melt rate, and the ingot-crucible heat-transfer coefficient. The simulations were shown to agree with optical and electron back-scattered diffraction (EBSD) measurements of grain morphology and crystallographic orientation. The model results predict that tree-ring structures (consistent with those observed experimentally) can be formed by process perturbations that alter the thermal field conditions at the solidification front. A sensitivity study of the effect of the different process fluctuations on the microstructure formation was performed, providing process maps predicting the range of conditions where tree rings will not form.     

Achieving the maximum melting rate for various vacuum arc remelting schemes

For traditional vacuum arc remelting, the electrode diameter is found at which the remelting rate is maximal under given electrical conditions. For double-electrode vacuum arc remelting, the dependence of the mutual radial shift of electrodes on the electrode diameter that ensures the maximum output without degrading the ingot quality is determined.     

Quasi-steady-state characteristics of solidification of alloys made from VT3-1 alloy during vacuum arc remelting

Numerical calculations are presented of the depth of a liquid pool, time of local solidification, and temperature gradient in the axial region of an ingot during vacuum arc remelting (VAR) of VT3-1 titanium alloy (Ti-6.5Al-2.5Mo-1.5Cr-0.5Fe-0.3Si) corresponding to quasi-equilibrium conditions. Calculations were carried out for ingots of diameters D = 400, 800, and 1200 mm in the ranges of mass melting rates \(\dot M\) = 0.5–12.0, 0.5–35.0, and 1.5–30.0 kg/min, respectively. It is found that the depth of the liquid pool (H, mm) linearly increases with increasing \(\dot M\) and is virtually independent of D in the conditions under consideration and, therefore, can be presented as a unique dependence H = 66.63 \(\dot M\) + 71.91. A finite depth of the liquid pool at a zero mass melting rate is associated with that the state \(\dot M\) = 0 corresponds to a finite current of the arc, which holds a part of metal in the liquid state. It is shown that the time of local solidification depending on \(\dot M\) has a minimum associated with various physical processes, which determine the kinetics of the solidification front at small and large solidification rates. In relative units, which correspond to a minimum, these dependences are identical for all considered diameters of the ingot. In addition, based on autoradiographic investigations of solidification of VT3-1 alloy under the VAR conditions, critical values of the temperature gradient (G) and velocity of motion of the crystallization front (v) along the ingot axis, which determine the passage from the column structure to the equiaxial one, are determined. Starting from these results, the plots v(G) are constructed, which turn out to be very useful in the development of remelting modes, excluding the appearance of some type of liquation process. It is revealed that at the specified ingot diameter, the dependence v(G) is decreasing and, at the specified temperature gradient, the velocity of motion of the solidification front decreases as D increases, which indicates an increase in D for the smelting of the ingots of highly-doped alloys.     

USS122G钢锭真空电弧重熔工艺的数值模拟

 超高强度不锈钢以其超高强度和良好的韧性以及优异的耐腐蚀性能而广泛地应用在航空、航天等领域。真空自耗重熔(VAR)作为生产超高强度不锈钢铸锭的主要生产技术,具有去除钢中有害杂质、改善钢中元素偏析的功能。为了研究新型Cr-Co-Ni-Mo合金体系超高强度不锈钢USS122G的真空电弧重熔过程,通过工艺仿真优化软件(Melf-Flow-VAR),对VAR过程的宏观传热、传质和流动现象进行模拟,建立USS122G合金VAR过程的二维轴对称数学模型,预测了不同冷却速度下的温度场和熔池形貌,并着重分析了特定熔速下的温度场、流场的演变,有无氦气冷却的元素宏观偏析情况,最后以模拟工艺制备了USS122G合金660 mm铸锭进行验证。结果表明,熔速增加,熔池深度加深,熔池形貌由低熔速扁平状圆弧状高熔速深“U”变化,熔炼速率为4.5 kg/min的熔池形貌具有较窄的糊状区,在此熔速下,熔池形貌呈现圆弧状,且真空自耗炉的输入功率较低,流场模拟结果显示流体的流动方向沿边部向下,中部向上,在铸锭右侧呈现顺时针运动规律;模拟熔池在达到稳态后深度为132 mm,此时模拟熔池深度与实测结果基本一致;在熔炼过程中Cr和C元素均发生正向偏析,采用氦气冷却的铸锭中元素偏析程度较小,在距钢锭1/2R处到边部Cr和C元素分布规律与模拟结果吻合较好。本项研究成果为钢的工业化稳定生产提供了可靠的数据支撑。