Prediction of Melt Rate in Electroslag Remelting Process Based on LSTM-PINNs

A new method for manufacturing Mn18Cr18N steel applied to retaining ring has been developed based on hollow ESR ingot. The ESR hollow ingot not only keep away from upsetting and punching, provide hollow billet for forging directly, but also meet the need of high quality of the raw material. ESR hollow ingot of Mn18Cr18N steel has been produced successfully in our lab. Melt rate is the most important technological parameter of electroslag remelting (ESR) process, which influences ingot quality and production efficiency. To further investigate the influence of surface quality on melt rate, the experimental results comparing to the simulations results have been implemented in this paper. A mathematical model to describe the temperature distribution and the solidification structure of ESR hollow ingot of Mn18Cr18N steel has been established. The change of metal pool profile, pool cylinder part height, and grain growth angles during hollow ESR process are calculated based on the coupled technology of CAFE method and the moving boundary method, which are consistent with the experimental results. The experiment of hollow ESR Mn18Cr18N steel at the most suitable melt rate obtained by numerical simulation results has been carried out to reveal the law of cylinder part height of metal pool changing with the withdrawal speed. The optimal technological parameters applied in the experiment of hollow ESR process can reasonably control the surface quality of Mn18Cr18N hollow ingot.

The electroslag remelting (ESR) process is a widely used secondary remelting process for the production of high-value-added alloys and steels. The grain structure of ESR ingot has a great effect on the final properties of products. A multiscale mathematical model combining the macroscopic heat transport with the mesoscopic nucleation and grain growth was developed to predict the grain structure evolution of solidification ingot during the ESR process. A moving cell frame, which dynamically defines the calculation domain for grain structure simulation, was proposed to save the computation resources and time. The thermophysical properties of steel related to the solidification of rotor steel 30Cr1Mo1V were adopted in present model and the nucleation parameters, which were suitable for the ESR process, were determined using the trial and error method in numerical simulation. The multiscale mathematical model was validated by the comparison between predicted and experimentally observed grain structure, and the results showed that the model was capable of simulating the grain structure evolution during the ESR process. Finally, the preliminary investigation on the effect of industrial process parameters on the grain structure was carried out and the results showed that increasing melting rate caused finer columnar grain structure and changed the growth direction of columnar grain structure from the axial–radial growth into the radial growth at very high melting rate. Meanwhile, increasing the molten slag temperature made the columnar grain structure finer and reduced the thickness of the refined equiaxed grain layer both at the surface and bottom of the ESR ingot.

Experimental and theoretical studies have been carried out to investigate the effects of the slag on desulfurization during the electroslag remelting (ESR) process with a focus of developing a mass transfer model to understand the mechanism of desulfurization. Stainless steel 1Cr21Ni5Ti was used as the electrode and remelted with two different kinds of slags using a 50-kg ESR furnace. The contents of sulfur along the axial direction of product ingots were analyzed. It was found that the sulfur content of 350 ppm in the electrode is reduced to 71 to 95 ppm in the ingot by remelting with the slag containing 5 wt pct of CaO, and lowered more to 47 to 59 ppm with another slag having 20 wt pct CaO. On the basis of the penetration and film theories, the theoretical model developed in this work well elucidates the kinetics of desulfurization revealing the mechanism of sulfur transfer during the ESR process. The calculation results obtained from the model agree well with the experimental results. The model indicates that when sulfur content in electrode is given, there is a corresponding minimum value of sulfur content in the ingot due to the kinetics limit. This lowest sulfur content cannot be further reduced even with increasing L S (sulfur distribution coefficient between metal and slag phases) or decreasing sulfur content in the slag. Constant addition of extra amount of CaO to the molten slag with the increase of sulfur content in the slag during the remelting process can improve the macrosegregation of sulfur distributed along the axial direction of ESR ingots. Since the rate-determining steps of the sulfur mass transfer lie in the metal phase, adding calcium as deoxidizer can change mass transfer of sulfur and thus promote desulfurization further during the ESR process.

The equilibrium reaction between Ni alloys and CaO-Al2O3-CaF2-TiO2 system electroslag remelting (ESR) slags was investigated in the temperature range of 1773 K to 1873 K (1500 °C to 1600 °C) at p(O2) = 10−16 atm in order to obtain the optimized composition of the slags for producing Ni alloys with various Al and Ti ratios. In addition, the temperature dependence of the reaction equilibria between the ESR slags and Ni alloys was also evaluated. The stable ionic species of titanium in the ESR slag under the present experimental conditions was experimentally confirmed to be mainly Ti4+ (i.e., TiO2) by X-ray photoelectron spectroscopy analysis of the quenched samples. The activity-composition relationship of TiO2 and Al2O3 in the ESR slag was determined as a function of the Al/Ti ratio of the alloys and the CaF2 content of the slags in conjunction with the activity ratio of Al to Ti in the alloys calculated from the FactSageTM 7.0 software. The temperature dependence of the activity-composition relationship of TiO2 and Al2O3 in the slag showed good linear correlations, and the equilibrium content ratio of TiO2 to Al2O3 at a fixed activity ratio increased with increasing temperature, which was expected based on the standard enthalpy change of the reaction. Thus, higher amounts of TiO2 should be added at higher operation temperatures in the ESR process. A 120 kg scale pilot ESR test (2000 A and 16 V) was performed to produce a commercial grade Ni-based superalloy based on the activity-composition relationship of the slag components obtained in the present study. Consequently, the contents of Al and Ti in the solidified ESR ingot were nearly the same as that of the original electrode throughout the entire length (280 mm) after the ESR process.

The electroslag remelting (ESR) process has been used widely to produce large ingots of high quality based on the controlled solidification and chemical refining that can be achieved. The vibrating electrode method was used in the ESR process in this paper, which can improve the quality of solidification and reduce the energy consumption. A transient three-dimensional (3D) coupled mathematical model was developed to simulate the electromagnetic phenomenon, fluid flow as well as pool shape in the ESR process with vibrating electrode. The finite element volume method is developed to solve the electromagnetic field using mechanical APDL software. Moreover, the electromagnetic force and Joule heating are interpolated as the source term of the momentum and energy equations. The flow field, temperature profiles and pool shapes are demonstrated by the finite volume model of FLUENT software. The volume of fluid (VOF) approach is implemented for the two-phase flow. The solidification of metal is simulated by an enthalpy-porosity formulation. The multi-physical fields have been investigated and compared between the traditional electrode and the vibrating electrode in the ESR process. The results show that the behavior of metal droplets with traditional electrode is scattered randomly. However, the behavior of metal droplets with vibrating electrode is periodic. The maximum temperature of slag layer with vibrating electrode is higher than that with traditional electrode, which can increase the melting rate as to the enhanced heat transfer in the vicinity of the electrode tip. The parameters study show that when the amplitude and frequency of vibrating electrode increases, the cycle of behavior of metal droplets decreases significantly.

Numerical Simulation of the Influence of Electrode Shrinkage Cavity on ESR Process of IN718 Alloy

The aim of this study was to investigate the effect of under electro slag re-melting (ESR) process on the fatigue behavior of GTD-450 steel used for manufacturing gas turbin blades. These parts are always at risk of failure due to fatigue. Refining of steel by ESR process is one of the most important modern methods of steelmaking in order to achieve a homogeneous structure, minimal elemental and structural segeregation, as well as reducing non-metallic impurities in terms of their number and size and most importantly their uniform distribution in the steel structure. Due to dependency of fatigue properties of steels on structural factors of steel, the role of ESR process using different slag compositions on this important property of GTD-450 steel was investigated. Microstructural studies by optical and electron microscopes showed that grain size of ESR samples were averagely 10% finer than non-ESR ones. As well as performing various mechanical tests including tensile and fatigue on samples prepared in different conditions, showed that although ESR had a minor effect on tensile properties of steel but impact energy of steel in ESR condition were averagely 16% more than non-ESR condition. Finally, the core finding of this paper is that, fatigue life of GTD-steel turbine blade produced via ESR method can be improved by 30%.

This study analyses the mechanism of oxygen increase during the electroslag remelting (ESR) process and proposes a countermeasure to control oxygen content of electroslag ingots. The results show that when the oxygen in the electrode is high, the oxygen in the ingots decreases after the ESR process. However, when the oxygen in the electrode is low, the oxygen in the ingots increases after the ESR process. Oxygen increase is closely related to the slag system that was used during the process. According to the experiment and the theoretical analysis, the [Al]-[O] reaction is the control reaction, and the oxygen content of the ingots is determined by the Al2O3 activity in the slag pool when Al in the electrode is kept constant. Based on this finding, the new slag system that decreases the oxygen content of the electroslag ingot is designed and it contains the following components: 63–67%CaF2, 4–6%Al2O3, 18–22%CaO, 8–12%RExOy or 43–47%CaF2, 4–6%Al2O3, 18–22%CaO, 8–12%MgO and 18–22%RExOy.

COMPUTER CONTROLLING FOR THE ESR PROCESS – AN APPLICATION OF MODERN CONTROL THEORY

The electroslag remelting (ESR) process has been effectively applied to produce high grade special steels and super alloys based on the controllable solidification and chemical refining process. Due to the difficulties of precise measurements in a high temperature environment and the excessive expenses, mathematical models have been more and more attractive in terms of investigating the transport phenomena in ESR process. In this paper, the numerical models for different ESR processes made by our lab in last decade have been introduced. The first topic deals with traditional ESR process predicting the relationship between operating parameters and metallurgical parameters of interest. The second topic is concerning the new ESR technology process including ESR with current-conductive mould (CCM), ESR hollow ingot technology, electroslag casting with liquid metal(ESC LM), and so on. Finally, the numerical simulation of solidification microstructure with multi-scale model is presented, which reveals the formation mechanism of microstructure.

Based on laboratory experiments and thermodynamic analysis, this article reveals the reaction mechanism behind the increase in aluminium content during the electroslag remelting (ESR) process in a protective atmosphere. It investigates the effects of slag composition, steel composition, reaction temperature, and reaction atmosphere on the aluminium content in ingots to inhibit the increase of aluminium during the remelting process. The results indicate that the increase in aluminium content is accompanied by significant silicon loss in the steel and an increase in silicon dioxide in the slag. The aluminium content in the ingots is controlled by the thermodynamic reaction equilibrium between [Si] in the steel and (Al2O3) in the slag. The high-temperature conditions during the ESR process promote the occurrence of the aluminium addition reaction. In the ESR process without a protective atmosphere, the oxidation of the consumable electrode in air forms a ferrous oxide, leading to a decrease in silicon activity. Consequently, the trend of aluminium increase in ingots remelted in a nonprotective atmosphere is lower than that in ingots remelted in a protective atmosphere. Increasing the silicon dioxide content in the slag to 1.96%–7.62% is an effective approach to inhibit aluminium addition in the ingot. However, excessive addition of silicon dioxide can cause the burning loss of alloy elements and increase the content of silicate inclusions. Additionally, reducing the initial silicon content in the electrode from 0.55% to 0.40% further reduces the aluminium content in the ingot. Industrial experiments were conducted using the optimised S2 slag system (68.63% CaF2–19.61% Al2O3–4.90% CaO–4.90% MgO–1.96% SiO2). The results were consistent with laboratory experiments, and the aluminium content in the ingots met the standard requirements.

Since 1970s of last century, the electroslag remelting (ESR) process began to be used in the production of cold roll steel, which led to an obvious improvement in chemical homogeneity, cleanliness and compact microstructure. The properties of cold roll steel have been significantly improved. ESR process is invisible, most previous researches focused on temperature field, concentration field and velocity field of ESR by establishing mathematical model and using Fluent software. The influence of process parameters on ESR, based on commercial software Melt-Flow and Cr5 cold roll, was studied in this paper. Results showed that increasing electrode filling ratio could reduce molten pool depth and local solidification time, and this could also play a role in energy conservation; With the increase of slag pool depth, molten pool became shallower, but local solidification time increased; A greater melting rate led to the increase of molten pool depth and local solidification time.

Steel solidification process control, especially in the solidification process of high alloy steel, and improvement of the solidification structure have been increasingly gaining interest among metallurgists, particularly the electroslag workers. To further develop the electroslag remelting (ESR) process and to improve the advantage of the ingot solidification structure, the effects of relative motion between the consumable electrodes and the mould (namely, mould rotation) on chemical element distribution were observed in this study, as well as the compact density changes in electroslag ingots. Experiment results show that applying relative motion between the mould and the consumable electrodes in ESR results in a more uniform chemical element distribution in the electroslag ingots. Compared with the electroslag ingot of conventional ESR, maximum segregation of carbon could decrease from 3·19 to 1·146, and statistical segregation decreased from 0·2636 to 0·0608. Maximum segregation of chromium could decrease from 1·316 to 1·253, and statistical segregation decreased from 0·2753 to 0·1201. The compact density for the stationary mould increased from 0·7693 to a compact density of 0·9501 for the rotating mould. The improvement in the solidification structure of the electroslag ingot can be attributed to mould motion, which led to the generation of a shallow pool and the improvement of the solidification structure. But the excessive rotation rate is harmful to solidification structure instead due to the molten metal pool motion caused by violent slag pool motion.

Computer Controlling for the ESR Process - An Application of Modern Control Theory

Insight into droplet formation in electroslag remelting process by numerical simulation