Simulation on Structure of Electroslag Remelting Ingot by Cellular Automata Method

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.

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.

The cooperative control of both electrode remelting rate and position was applied in order to obtain optimal electro-slag remelting metallurgy effect and improve melting production efficiency. The nonlinear dynamic model of electro-slag remelting process was linearized. And the lineariztion model was analyzed including its stability, coupling property and variable matching strategy. The multi-loop PID controllers with cascade PID controllers were applied to the linearization model based on matching strategy. The dynamic performance was analyzed to electro-slag remelting control system. Study results show that linearizition model of electro-slag remelting process have characteristic of stability and weakly-coupled property. Multi-loop PID control system can realize stable control both electrode melting rate and electrode position. The control system has faster dynamic response and better anti-interference capability.

The purity of a steel is an important factor influencing the quality of the final products. Therefore, it is important to optimize the existing and develop new steelmaking technologies that affect the resulting purity. Electro slag remelting is a technology of tertiary metallurgy, which can advantageously be used to fabricate high quality steels. The study presents selected theoretical aspects of oxide systems and their specific influences on effectiveness of the electro slag remelting technology. The aim of this work was to experimentally analyze the purity of a tool steel fabricated by electro slag remelting using two different oxide systems (fused slags). The core of the study is the determination of the overall presence of elements in the steels, a thorough investigation of the presence of (not only) oxide-based inclusions within the investigated tool steel, and a detailed analysis of their chemical composition, including the size of these non-metallic inclusions, using energy dispersive X-ray (EDX) on the scanning electron microscope (SEM). Last but not least, the determination of the modification of the occurring non-metallic inclusions and verification of the experimentally acquired results as well as the calculation of the liquid and solid temperature and the calculation of the viscosity of the slags using the FactSage calculation software was performed. The results showed that the used slag influenced especially the occurrence of Mg and Al-based oxide inclusions. The CaS-type inclusions were present within all of the examined samples. The slag type influenced not only the typical morphology and size of the inclusions (especially of the CaS type), but also the tendency of the steel to exhibit localized corrosion when exposed to the ambient environment. This research can contribute to a better understanding of the effect of oxidation systems on the resulting purity and properties of ESR steels, thereby advancing the production of tool steels with higher quality and performance requirements.

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.

Macrosegregation is a very common problem for the quality control of all cast ingots. The effect of current on the structure and macrosegregation in dual alloy ingot processed by electroslag remelting (ESR) was investigated experimentally with various analytical methods. In this study, the electrode consisted of NiCrMoV alloy bar (upper part) and CrMoV alloy (lower part) with a diameter of 55 mm, was remelted in a laboratory-scale ESR furnace with the slag containing 30 mass pct alumina and 70 mass pct calcium fluoride under an open air atmosphere. The results show that the macrostructures of three ingots processed by electroslagremelting with different currents are nearly similar. The thin equiaxed grains region and the columnar grains region are formed under the ingot surface, the latter region is the dominant part of the ingot. The typical columnar structure shows no discontinuity among the NiCrMoV alloy zone, the CrMoV alloy zone, and the transition zone in three ingots. With the increase of the current, the grain growth angle increases due to the deeper molten metal pool. The secondary dendrite arm spacing (SDAS) firstly decreases, then increases. The SDAS is dominated by the combined effect of the local solidification rate and the width of mushy region. With the current increasing from 1500 A to 1800 A and 2100 A, the width of the transition zone decreases from 147 mm to 115 mm and 102 mm. The macrosegregation becomes more severe due to the fiercer flows forced by the Lorentz force and the thermal buoyancy force. The cooling rate firstly increases, then decreases, due to the effect of the flows between the mushy region and metal pool and the temperature gradient at the mushy zone of the solidification front. With a current of 1800 A, the SDAS is the smallest and cooling rate is the fastest, indicating that less dendrite segregation and finer precipitates exist in the ingot. Under the comprehensive consideration, the dual alloy ingot processed by the ESR with a current of 1800 A is the best because it has the smallest SDAS, the appropriate grain growth angle, moderate macrosegregation and thickness of the transition zone.

X-750 is a nickel-chromium based super alloy with high strength, high toughness, excellent creep resistance and good stress relaxation resistance at high temperature. It can be used in the manufacture of holddown springs for reactor vessel internals (RVI). Because of high content of alloying elements and the large size of ingot for the RVI holddown spring, observations by optical microscopy (OM) and scanning electron microscopy (SEM) showed that there was obvious dendrite segregation and serious carbonitride net in the as-cast electroslag remelting (ESR) ingot. Before forging, homogenization treatment was carried out for the ESR ingot to reduce element segregation, and promote the secondary carbonitrides dissolve into the alloy, which could significantly improve the malleability of the billet and reduce forging cracks. The homogenization process adopted in this study was annealing at 1220°C for 70 hours. In addition, the final forging temperature also played a key role in the control of formability of the billet and grain size of the forging. The carbonitrides or grain size of both the as-cast and forged samples annealed at 900°C∼1200°C for 10h were observed, and also charpy impact tests were carried out on the forged and annealed samples. The results showed that the final forging temperature for upsetting and drawing process should be no less than 1100°C, and the final forging temperature for forming process could be reduced to 1000°C. After solution and aging treatment, the comprehensive mechanical properties of the forging were excellent, with KV impact value at room temperature as high as 92J, yield strength of tensile test at 350°C up to 696MPa, and tensile strength up to 1019MPa.

The possibility of controlling the carbide heterogeneity in tool and high-speed steels by electroslag remelting with the rotation of a consumable electrode is considered. The elimination of a possible significant overheating of a metal during forced removal of the electrode metal, its supply to the zone of maximum cooling, and the equalization of a temperature gradient lead to an increase in the solidification rate, which favors the formation of a more homogeneous and fine carbide component.

Dynamic solidification process during laser cladding of IN718: Multi-physics model, solute suppressed nucleation and microstructure evolution

A two-dimensional stochastic model for the prediction of the columnar-to-equiaxed transition (CET) during solidification processes of Al–Cu alloy ingots is developed, based on a finite differential method (FDM) for macroscopic modelling of heat transfer and a cellular automaton method (CA) for microscopic modelling of nucleation and growth. The formation of a shrinkage cavity at the top of the ingot is taken into account. The effects of the casting variables on the CET are presented and discussed. A quantitative relation between CET position and casting variables is obtained. The columnar zone is found to expand with decreasing alloy composition and mould preheated temperature, and with increasing superheat and initial heat transfer coefficient. However, increasing the mould preheated temperature in a particular range has a negligible influence on the CET. With a low rate of heat extraction, the CET is found to be independent of melt superheat.

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

This paper discusses the development of columnar grains in as-deposited C-Mn-Ni(-Mo) low-carbon low-alloy weld metals and the influence of the alloying elements, particularly nickel. It was found that the austenite columnar grain size prior to the γ-α transformation of the weld metals was mainly controlled by the alloying contents rather than HAZ grains adjacent to the fusion boundaries, although the latter determined the size of the initial columnar grain size. The addition of nickel initially depressed the prior austenite grain size and subsequently dramatically coarsened it. This was related to the nickel equivalent (Nieq) of the weld metals and the peritectic reaction during the solidification process. Small columnar grains were associated with a Nieq between 3.4 and 6.2% which resulted in a peritectic reaction when the weld melt solidified, whilst a Nieq higher than 6.2% produced very large columnar grains because the weld pool would directly solidify into austenite and have a subsequent continuous growth.

A two-dimensional, coupled, cellular automaton-continuum model was developed to model the evolution of solidification grain structures and the deflection behavior of the columnar dendritic grains solidified in a flowing melt. Al-Cu alloys were solidified on an inclined Cu chill in order to investigate the deflection behavior of columnar grains. The growth velocity of a dendrite was determined by the KGT (Kurz-Giovanola-Trivedi) model based on the calculated temperature and solute profiles around a dendrite tip during solidification. The deflection behavior of the growing grains in a flowing melt was studied by the macroscopic observation of the solidified shell on the inclined Cu chill plate, and the interpretation of this phenomenon was made with the aid of numerical calculation, including Cu solute redistribution and velocity vector profile around dendrite tips during solidification, with various solidification parameters. Effects of flow velocity and solute content on the deflection angle of the columnar grains were investigated through experiments, and the results were compared with the numerical simulations.

This investigation was conducted to determine whether there are correlations between CVN, HRc and KIc test results at room temperature for AISI H11/H13 hot‐work tool steels that could be developed to a tool for materials engineers and the structural designers. The results showed that such correlations do exist for electro‐slag‐remelted (ESR) and conventional AISI H11 hot‐work tool steel. Static KIc values were correlated with the results of standard CVN impact tests and the obtained hardness values at room temperature. Furthermore, the results show that the correlation can also be used to calculate KIc values from standard CVN impact tests and the respective Rockwell‐C hardness for AISI H13 tool steel. In general, the results of this investigation show that the correlation between the values for KIc, CVN and HRc can be used to estimate the KIc values from the CVN test results and the Rockwell‐C hardness.

An innovative method for determining the structural zones in the large static steel ingots has been described. It is based on the mathematical interpretation of some functions obtained due to simulation of temperature field and thermal gradient field for solidifying massive ingot. The method is associated with the extrema of an analyzed function and with its points of inflection. Particularly, the CET transformation is predicted as a time-consuming transition from the columnar- into equiaxed structure. The equations dealing with heat transfer balance for the continuous casting are presented and used for the simulation of temperature field in the solidifying virtual static brass ingot. The developed method for the prediction of structural zones formation is applied to determine these zones in the solidifying brass static ingot. Some differences / similarities between structure formation during solidification of the steel static ingot and virtual brass static ingot are studied. The developed method allows to predict the following structural zones: fine columnar grains zone, (FC), columnar grains zone, (C), equiaxed grains zone, (E). The FCCT-transformation and CET-transformation are forecast as sharp transitions of the analyzed structures. Similarities between steel static ingot morphology and that predicted for the virtual brass static ingot are described.