Electroslag Remelting Ingot Research Articles

Ferrite formation and decomposition in 316H austenitic stainless steel electro slag remelting ingot for nuclear power applications

316H austenitic stainless steel used in nuclear power has higher requirements for material magnetism, and the main magnetic phase is ferrite. Thus, the ferrite content in 316H steel castings needs to be strictly controlled. This study investigated the ferrite phases in 316H austenitic stainless steel electro-slag remelting (ESR) ingot used in nuclear power applications. The morphology, content and decomposition of ferrite as well as microsegregation in 316H ESR ingot are studied by optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA). The solidification process is calculated by thermodynamic calculation using Thermo-Calc. Besides, the empirical chromium and nickel equivalent formulas and phase stability diagram are used for prediction of the solidification mode and ferrite content. The experiment results show that the ferrite morphology changes greatly in the direction of the thickness of ESR ingot. From surface to center of the ESR ingot, the ferrite morphology varies as granular → short rod-shaped → blocky → skeletal → parallel short rod-like + network-like. The ferrite content in the thickness direction of the electroslag ingot varied from 1.92 to 3.57 %, showing an “A” shape distribution. There are three kinds of decomposition behavior of ferrite: decomposition into Sigma phase and austenite phase by the eutectoid reaction, transformation into Chi phase in regions enriched with high Mo element along grain boundaries, and direct transformation into secondary austenite phase. In addition, the thermodynamic calculation and most of empirical formulas predicts the solidification mode is FA mode. From the microstructure analysis, the solidification mode of ESR ingots change from AF mode to FA mode from surface to center. Furthermore, the formulas proposed by Hull provides most accurate predictions results in ferrite content.

Study on Evolution Behavior of Carbides in Industrial‐Grade American Iron and Steel Institute M35 High‐Speed Steel Produced by Electroslag Remelting

In order to optimize the heating schedule before forging and improve the breaking and deformation effects of carbides in high‐speed steel, it is of great significance to study the transformation of M2C carbides at high temperatures. The evolution of carbides in the industrial‐grade American Iron and Steel Institute M35 steel produced by electroslag remelting (ESR) is analyzed and observed using thermodynamic calculations and experimental methods. The results indicate that the carbides in the ESR ingot are mainly MC and M2C, and the microstructures of M2C carbides with the highest volume fraction are lamellar and brain like. As the heating temperature increases and holding time prolongs, the lamellar M2C carbides gradually transform into MC and M6C carbides, accompanied by protrusion, dissolution, separation, and spheroidization of the microstructure, until significant coarsening occurs at 1180 °C for 90 min. The newly transformed carbides are embedded and stacked with each other, occupying the original position of M2C carbides. Based on the theories of Gibbs free energy and atomic diffusion, the evolution mechanism of M2C carbides is discussed. Ultimately, the appropriate heating schedule is proposed, and it is validated by combining the characteristics of carbides after forging.

Multi‐Stage Enhanced Removal of Inclusions During Electroslag Remelting Process by a Static Magnetic Field

An axial static magnetic field (ASMF) is superimposed during the electroslag remelting (ESR) process to regulate the inclusion removal (GCr15 bearing steel) in the current study. After applying a 50 mT ASMF, the metal pool of the ESR ingot becomes shallower and the solidified dendritic structure tends to grow axially, together with a higher inclusion removal efficiency. The number of inclusions in ESR ingot decreases, and the inclusion with larger size (>10 μm) is almost eliminated. By tailoring the morphology of the liquid melt film (LMF), droplet, and metal pool, the ASMF applied in this study strengthens the kinetic conditions (inclusion migrated to slag/metal interface) for the inclusion removal, thus realizing the multi‐stage enhanced removal of inclusion during ESR process.

Morphology tailoring of metal pool and eutectic carbides in magnetic-controlled electroslag remelted M2 high-speed steel

A transverse static magnetic field (TSMF) was superimposed during electroslag remelting (ESR) process to regulate the shape of metal pool, the eutectic carbides morphology as well as the mechanical properties of electroslag remelted M2 high-speed steel (HSS) in current study. The metal pool became shallower and flatter after applying a 65 mT TSMF. Moreover, refined eutectic carbides and solidification structure (tended to grow axial) of the ESR ingot were obtained, together with uniform/improved mechanical properties (hardness, wear). Modifications of the morphology of metal pool and grain growth angle under the TSMF were attributed to a periodic electromagnetic excitation effect on droplets transition characteristics, which changed the drop path of the droplets (single to multiple drop) and homogenized the temperature distribution in metal pool. In addition, local solidification time (LST) became shorter and the number of inclusions ((Ti, V)N–Al2O3 and (Ti, V)N) which acted as heterogeneous nuclei for eutectic carbides formation increased (the inclusions also became finer) due to TSMF, resulting in the refinement of the eutectic carbides in ESR ingots.

Growth and agglomeration behaviors of eutectic M7C3 carbide in electroslag remelted martensitic stainless steel

The solidification microstructure of 8Cr13MoV martensitic stainless steel electroslag remelting (ESR) ingot is observed by scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM). The results show that eutectic carbides in ESR ingot are composed of three types of morphology, which are clustered blocky, fibrous and cerebriform. Transmission electron microscopy (TEM) results indicate that the eutectic carbides are M7C3 carbides. Electron backscattered diffraction (EBSD) results show that although the morphology of eutectic M7C3 carbides is diverse, they are composed of three carbide substructures, which are bulky carbides, fibrous carbides and spherulitic carbides, respectively. Eutectic M7C3 carbides in the center of the ingot are general carbide clusters composed of bulky carbide grains, fibrous carbide grains and a small amount of spherulitic carbide grains. However, eutectic M7C3 carbides at the edge of the ingot edge are almost cerebriform carbides composed of fibrous carbide grains and spherulitic carbide grains. Microsegregation promotes the formation of heterogeneous nuclei Al2O3-(Ti,V)N at the solidification end, which promotes the formation of bulky carbides. In the meanwhile, microsegregation also substantially increases the supersaturation of the residual liquid and drives the morphology evolution of eutectic carbides from bulky to fibrous and spherulitic. The cooling rate increase restrains carbide growth, intensifies the solute enrichment in the residual liquid, obviously increases the supersaturation and finally promotes the morphology change from bulky to fibrous and spherulitic. Furthermore, microsegregation and the cooling rate change have less effect on the agglomeration and coalescence behaviors of carbide substructures.

The Design of Slag and Electroslag Remelting Production Technology of Steel Containing Zirconium

Slag–metal reaction experiments in MoSi2 resistance furnace combined with electroslag remelting (ESR) experiments in ESR furnace are used to study the effect of slag on zirconium distribution in ESR ingot by establishing a new mass transfer model of slag–metal reaction. The mass transfer model consists of Al + Al2O3, Si + SiO2, Zr + ZrO2, and Fe + FeO systems based on the penetration and film theories. Both experimental and simulated results show that the returned slag (CaF2:CaO:Al2O3:MgO:ZrO2 = 57:20:16:3:3) combined with extra 4% Al2O3 added into molten slag in the first slag-temperature-rising period can control the zirconium in ESR ingot ranging from 0.35 to 0.40% and improve the homogeneous distribution of zirconium in ESR ingot. The returned slag of Exp.A containing low silica being used in Exp.C can not only contribute to the recycling of returned slag, but also improve the homogeneous distribution of Zr along the height of ESR ingot.

Characteristics and Formation Tendency of Freckle Segregation in Electroslag Remelted Bearing Steel

Undesirable macro segregation defects, freckles, restrict the commercial production of large-sized electroslag remelting (ESR) bearing steel ingots through degradation of the mechanical properties and service lifetime. In order to clarify the freckle characteristics and formation tendency as well as the formation mechanism, freckles from an industrial large-sized GCr15SiMn ESR ingot were investigated through structural and compositional analysis, along with simulation calculation. The results show that freckles consist of (Si, Mn, Cr)-enriched equiaxed grains and occur in about the 1/2 radius region at the middle-upper part of the ESR ingot, where the secondary dendritic arm spacing (SDAS) and solidification front angle are large but cooling rate is small. The absolute value of relative Rayleigh number, Ra, also reaches its maximum in the 1/2 radius region, with a liquid fraction of 0.3–0.5, corresponding to the region where freckles form. Based on the experimental and simulation results, to evaluate the freckle formation in industrial-scale GCr15SiMn ESR ingots, the threshold value of relative Ra, a freckle criterion considering the compositional and thermal effects, was determined to be about −0.023.

Effect of CeO2 addition in the slag on inclusions of FGH96 superalloy during electroslag remelting

The FGH96 superalloy was electroslag remelted by utilizing CeO2 containing slag. CeO2/Al2O3 ratio and atmosphere are found to be the main factors that control the oxide inclusion contents during electroslag remelting (ESR). The reaction between CeO2 in the molten slag and Al in the molten alloy leads to the increased dissolved Ce in the molten alloy, which is effective to remove oxide inclusions. Dissolved Ce reacts with MgO in the inclusions, leading to the transformation of Al2O3–MgO inclusion in the electrode to Al2O3–MgO–Ce2O3 and Al2O3–Ce2O3 inclusions in the ESR ingot. In case of low CeO2/Al2O3 ratio of 0.263, oxygen content decreases to as low as 8 ppm after ESR under vacuum, and the number density of oxide inclusion decreased by an order of magnitude in comparison to the electrode. In case of high CeO2/Al2O3 ratio of 5.0, oxygen content decreases further to 6 ppm no matter what the condition is.

Cleanliness improvement and microstructure refinement of ingot processed by vacuum electroslag remelting

The composition analysis, inclusion and microstructure characterizations for two ingots processed by Ar gas protective electroslag remelting (ESR) and vacuum ESR as well as thermodynamic calculation were carried out to explore mechanisms of cleanliness improvement and microstructure refinement of ingot processed by vacuum ESR. The results show that compared with the ingot processed by Ar gas protective ESR, the Al pickup in ingot processed by vacuum ESR is less due to the lower Al2O3 activity in molten slag resulted by fluoride vaporization from molten slag. During Ar gas protective ESR, only Al deoxidation occurs in molten steel. But, the new equilibriums among liquid steel, inclusions and gas phase are established, and C deoxidation could occur upon 1635°C during vacuum ESR. The inclusions size in vacuum ESR ingot is finer because the time for Al2O3 inclusion growth is shorter. The C deoxidation and less steel reoxidation resulted by slag-steel interaction during vacuum ESR contributes to the lower oxygen content in vacuum ESR ingot. Furthermore, because of more uniform temperature distribution in molten slag pool, the smaller SDAS is acquired in vacuum ESR ingot. The higher cleanliness, microstructure refinement and smaller size of carbides contribute to the performance improvement in vacuum ESR ingot.

Investigation of Primary Carbides in a Commercial-Sized Electroslag Remelting Ingot of H13 Steel

The characteristics of primary carbides in a commercial-sized (one ton) electroslag remelting (ESR) ingot of AISI H13 steel were investigated. The interaction between the primary carbides and inclusions was also clarified. The results indicate that there are two types of primary carbides, V-rich and Mo-rich primary carbides, in the H13 ESR ingot. The quantity, the area fraction, and the size of the two primary carbides tend to decrease from the center of the H13 ESR ingot to the outer surface. Additionally, the V-rich primary carbide is obviously larger than the Mo-rich primary carbide. The Al2O3 inclusion can promote the precipitation of the V-rich primary carbide, while the MnS inclusion encourages the precipitation of Mo-rich primary carbide. The CaO∙Al2O3 inclusion cannot act as the nucleation site for the precipitation of the two primary carbides. The solid fraction that the V-rich primary carbide begins to precipitate ranges from 0.965 to 0.983, and that for the Mo-rich primary carbide and the MnS inclusion change from 0.9990 to 0.9998 and from 0.989 to 0.990, respectively.

Evolution of Tantalum Content During Vacuum Induction Melting and Electroslag Remelting of a Novel Martensitic Steel

The need for materials with superior thermal and mechanical properties while mitigating cost increases interest in new complex alloy compositions which brings challenges to manufacturing processes. In this investigation, vacuum induction melting (VIM) and electroslag remelting (ESR) of a novel tantalum (Ta)-containing martensitic steel was performed using standard industry practices at a laboratory scale. A 25 pct loss of Ta was measured from the VIM electrode to the ESR ingot using X-ray fluorescence. Several tools were used for broad characterization of the ingots, including LECO for chemistry analysis, scanning electron microscopy, and electron probe microanalysis for observation of the precipitate and inclusion phases post-VIM and post-ESR as well as computational modeling of the ESR process for the calculation of macrosegregation and inclusion travel. It was found that a significant amount of Ta2O5 inclusions formed during VIM and were transferred to the slag during ESR. While ESR was particularly successful at decreasing the number density of inclusions by 95 pct, additional efforts are needed with regard to vacuum leak rate and purity of stock material when melting novel advanced steels.

Investigation on Band Segregate Formation during the Electroslag Remelting of H13 Die Steel

Band segregation has been found in the H13 die steel produced by the electroslag remelting (ESR) technology. Chemical and metallographic studies have been carried out on a one ton ESR ingot of H13 die steel, so as to understand the formation mechanism of the band segregation. The results indicate that the T.O content and S content decreased because of cleanliness improvement of ESR process. Transverse macrosegregation of S content decreased after ESR. The overall removal ratio of the inclusion is around 65.8%. The original complex inclusions would be modified to the CaO·Al2O3 inclusions. Al2O3 and MnS inclusions can be found after ESR. Both of Al2O3 and MnS inclusions were found to be the core of primary carbides. The net like structure in ESR ingot and banded structure in the forged steel were observed. V, Mo, Cr and S are rich in the segregation areas of ESR ingot. Besides, black and white segregation bands can be observed on the forged steel samples after etching. Uneven distribution of carbides rich in V, Mo and Cr was observed in banded structure.

Investigation of desulfurization of Inconel 718 superalloys by ESR type slags with different TiO2 content

In the present study, effect of CaF2–CaO–Al2O3–MgO–TiO2 slag with various TiO2 contents on the desulfurization of Inconel 718 alloy was investigated at 1773K to provide the fundamental information for the establishment of desulfurization procedure during electroslag remelting (ESR) process. The melt-quenching method did not accurately determine the change in content of sulfur and oxygen in the nickel-based alloy with time for the present study, which uses a novel experimental apparatus. The results indicated that the final sulfur content in the nickel-based alloy increased from 7 to 9ppm when the TiO2 content in slag of CaF2–CaO–Al2O3–MgO–TiO2 system increased from 2.17 to 10% without addition of deoxidizer. In order to further expound the desulfurization mechanism, meanwhile, the relationship between the oxygen activity at slag–metal interface and reaction time under conditions of various TiO2 in the slag was studied by coupling with the developed mass transfer model. The calculated oxygen content in bulk metal has a good corresponding relationship with the measured oxygen content in the nickel-based alloy. The oxygen activity at slag–metal interface increased up to 9.6ppm when TiO2 content was 10% in the slag, giving rise to the decrease in desulfurization ratio in high TiO2 content. The optimal condition for simultaneously ensuring low sulfur content and maintaining oxidative elements, such as Al and Ti, uniformly from bottom to top of the ESR ingot is also discussed.

Effect of electroslag remelting and homogenization on hydrogen flaking in AMS-4340 ultra-high-strength steels

Hydrogen flakes and elemental segregation are the main causes of steel rejection. To eliminate hydrogen flaking, the present study focuses on the manufacture of AMS-4340 ultra-high-strength steel through an alternate route. AMS-4340 was prepared using three different processing routes. The primary processing route consisted of melting in an electric arc furnace, refining in a ladle refining furnace, and vacuum degassing. After primary processing, the heat processes (D1, D2, and D3) were cast into cylindrical electrodes. For secondary processing, electroslag remelting (ESR) was carried out on the primary heats to obtain four secondary heats: E1, E2, E3, and E4. Homogenization of ingots E1, E2, E3, and E4 was carried out at 1220°C for 14, 12, 12, and 30 h, respectively, followed by an antiflaking treatment at 680°C and air cooling. In addition, the semi-finished ESR ingot E4 was again homogenized at 1220°C for 6–8 h and a second antiflaking treatment was performed at 680°C for 130 h followed by air cooling. The chemical segregation of each heat was monitored through a spectroscopy technique. The least segregation was observed for heat E4. Macrostructure examination revealed the presence of hydrogen flakes in heats E1, E2, and E3, whereas no hydrogen flakes were observed in heat E4. Ultrasonic testing revealed no internal defects in heat E4, whereas internal defects were observed in the other heats. A grain size investigation revealed a finer grain size for E4 compared with those for the other heats. Steel produced in heat E4 also exhibited superior mechanical properties. Therefore, the processing route used for heat E4 can be used to manufacture an AMS-4340 ultra-high-strength steel with superior properties compared with those of AMS-4340 prepared by the other investigated routes.

Crystallization Characteristics and In-Mold Performance of Electroslag Remelting-Type TiO2-Bearing Slag

The non-isothermal crystallization characteristics of the electroslag remelting (ESR)-type TiO2-bearing slag were studied. The effect of crystallization characteristics of the slag on the in-mold performance was verified by ESR pilot trials operating in a static mode. The results showed that increasing TiO2 content decreased the crystallization temperature and liquidus temperature of the slag and suppressed the crystallization tendency of the slag. There is no change in the types of the crystalline phases in the slag with different TiO2 contents during cooling, i.e., 11CaO·7Al2O3·CaF2, CaTiO3, and CaF2. The sequence of crystal precipitation during cooling was 11CaO·7Al2O3·CaF2 to CaTiO3, and then CaF2 with increasing TiO2 content from 4.2 to 12.6 mass pct. With adding 16.8 mass pct TiO2 in the slag, the precipitation of 11CaO·7Al2O3·CaF2 and CaTiO3 took place simultaneously, followed by CaF2. With the increase in the TiO2 content from 4.2 to 16.8 mass pct in the slag, the dominant crystalline phase changed from 11CaO·7Al2O3·CaF2 to CaTiO3. The morphology of 11CaO·7Al2O3·CaF2 and CaTiO3 changed from faceted and bonelike to elliptically faceted and blocky, respectively. The morphology of CaF2 were spherical in all cases, irrespective of TiO2 contents in the slag. The decrease in the crystallization tendency of the slag with increasing TiO2 contents was attributed to the decrease in the activity of CaO in the slag. The increasing of TiO2 content in the slag are favorable for providing thin slag skin and stable heat flux across the slag skin, which improved the surface quality of the as-cast ESR ingot as demonstrated by ESR pilot trials operating in a static mode.

Effect of initial large-sized inclusion content on inclusion removal during electroslag remelting of H13 die steel

ABSTRACTThe current paper focuses on the influence of initial large-sized inclusion content in the consumable electrode on inclusion removal during electroslag remelting (ESR) of H13 die steel. Considering the relationship between the inclusion size and the interfacial energy change of the slag/inclusion/steel system during the inclusion transfer across the steel/slag interface, the thermodynamic conditions for inclusion removal from steel to the slag were put forward. The results showed that the content of large-sized inclusions in final ESR ingot was decreased by approximately 13.66% with the increase in large-sized inclusion content in the consumable electrode from 11.36 mg/10 kg to 16.50 mg/10 kg. The interfacial energy change of the slag/inclusion/steel system decreases with the increase in inclusion radius during the absorption process of inclusions by slag, which is beneficial for inclusion removal.

Review on Modeling and Simulation of Electroslag Remelting

The Electroslag Remelting (ESR) is an advanced technology for the production of high quality materials, for example, hot work tool steels or nickel base alloys. In the past years, several models are developed aiming to predict the way in which the operational parameters affect the structure and chemical composition of the final ESR ingot. Proper modeling of this process depends on the ability of the model to predict the Multiphysics resulting from the complex coupling between many physical phenomena. This review includes the main findings starting from the 1970's, with a special focus on the results obtained in the period of 1999–2017. The difficulties related to the poorly known physical properties of ESR slags are discussed. Then, the main achievements in the field of electromagnetism, fluid flow, heat transfer, and solidification are also summarized. The review finishes by presenting the special topics representing the actual scientific frontiers, such as the physics of mold current, the importance of multiphase phenomena, and the difficulties in predicting the electrode melting rate.

Distribution Behavior of Aluminum and Titanium Between Nickel-Based Alloys and Molten Slags in the Electro Slag Remelting (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 Structural Evolution and Segregation in a Dual Alloy Ingot Processed by Electroslag Remelting

The structural evolution and segregation in a dual alloy made by electroslag remelting (ESR) was investigated by various analytical techniques. The results show that the macrostructure of the ingot consists of two crystallization structures: one is a quite narrow, fine, equiaxed grain region at the edge and the other is a columnar grain region, which plays a leading role. The typical columnar structure shows no discontinuity between the CrMoV, NiCrMoV, and transition zones. The average secondary arm-spacing is coarsened from 35.3 to 49.2 μm and 61.5 μm from the bottom to the top of the ingot. The distinctive features of the structure are attributed to the different cooling conditions during the ESR process. The Ni, Cr, and C contents markedly increase in the transition zone (TZ) and show a slight increase from the bottom to the top and from the surface to the center of the ESR ingot due to the partition ratios, gravity segregation, the thermal buoyancy flow, the solutal buoyancy flow, and the inward Lorentz force. Less dendrite segregation exists in the CrMoV zone and the transition zone due to a stronger cooling rate (11.1 and 4.5 °C/s) and lower Cr and C contents. The precipitation of carbides was observed in the ingot due to a lower solid solubility of the carbon element in the α phase.

Effect of Melt Rate on Surface Quality and Solidification Structure of Mn18Cr18N Hollow Ingot during Electroslag Remelting Process

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.