The Evolution of Particles during Solidification of Nb-Ti micro-alloyed Steels and Mn/Si deoxidized High S bearing Steels : Nb-Ti 첨가강과 Mn/Si 탈산 쾌삭강의 응고시 개재물 생성 거동

우대희 포항공과대학교 철강대학원 2010 국내박사

The evolution of particles during solidification of steels was investigated by experiments and microsegregation calculations employing simple microsegregation models with a help of computing software such as ThermoClac and FactSage for the better control and utilization of inclusions and precipitates. In order to elucidate the formation of large Nb-Ti carbonitride particles which cause defects in final steel products such as linepipes or shipbuilding plates, the formation of Nb-Ti carbonitride particles during solidification was investigated employing unidirectional solidification technique with a constant cooling rate (8?aC/min) and the effects of microsegregation was estimated using simple microsegregation models. During unidirectional solidification, most (Nb,Ti)(C,N) particles were formed in the interdendritic region due to microsegregation and their size and morphologies were affected by the steel composition. With increasing alloying elements such as Ti, Nb, C, and N, the size of carbonitride particles increased. Type II Cubic Ti-rich (Nb,Ti)(C,N) particles were heterogeneously nucleated on the surface of oxide inclusions at earlier stage of solidification than Nb-rich (Nb,Ti)(C,N) particles. Type III Plate- and type IV fishbone-like Nb-rich (Nb,Ti)(C,N) particles were formed at the final stage of solidification. Unintended formation of type I dendritic Nb-rich (Nb,Ti)(C,N) particles occurred due to solidification of the remaining liquid steel during quenching stage. Under the solidification condition of the present study, modified Scheil model considering complete diffusion of interstitial elements such as C and N was well applicable to simulate microsegregation and formation of (Nb,Ti)(C,N) in the interdendritic liquid. Considering macrosegregation in the slab center, with increasing macrosegregation degree, size of (Nb,Ti)(C,N) increased. Especially, size of Nb-rich (Nb,Ti)(C,N) particle sharply increased when the macrosegregation degree exceeded a certain value. In order to utilize oxysulfide inclusions more efficiently in the free-cutting steels, phase equilibria of the MnO-SiO2-Al2O3-MnS quaternary system in the temperature range of 1185 to 1500?XC was measured and behavior of oxysulfide inclusions which consist of the quaternary system was investigated in the unidirectionally solidified steel. In the MnO-MnS system, the eutectic temperature and composition of the MnO-MnS system were also determined to be 1256∂3?XC and 64∂1mass% MnS. In the MnO-SiO2-MnS system, ternary compound ¨A〃(67.3mass% MnO- 23.4mass% SiO2- 9.3mass% MnS) which melted incongruently at a temperature between 1280 and 1300?XC and ternary compound ¨B〃(74.4mass% MnO- 22.0mass% SiO2-3.6mass% MnS) which melted incongruently at a temperature between 1300?XC and 1315?XC and decomposed into MnO, Mn2SiO4 and the ternary compound ¨A〃 were observed. Based on the experimental results, polythermal projection of the MnO-SiO2-MnS system below 1500?XC was estimated. In the MnO-SiO2-Al2O3-MnS system, the MnS solubility is strongly affected by the Al2O3 content and also the MnO/SiO2 ratio. It increased as increasing MnO/SiO2 ratio at a constant Al2O3 content and decreased as increasing the Al2O3 content at a constant MnO/SiO2 ratio. In addition, depending on the Al2O3 content and the MnO/SiO2 ration, the liquid phase is possibly saturated with not only MnS but also other solid oxide phases. In the aspect of inclusions utilization for free-cutting steel, it might have an advantage to decrease Al2O3 content and increase MnO/SiO2 ratio. Thermodynamic calculation of the MnO-SiO2-Al2O3-MnS system using FactSage with newly developed database by Kang and Pelton shows a good agreement with the results of present study. Therefore, it was confidently used for the calculation of phase equilibria in the oxysulfide inclusion during solidification steels. The behavior of oxysulfide inclusions was investigated employing unidirectional solidification technique and the effect of microsegregation was estimated using Scheil model with a help of thermodynamic calculation software (FactSage). The oxysulfide inclusions which were formed in the liquid before solidification had high content of Al and they were dispersed uniformly in the intradendritic region after solidification. With increasing solid fraction, Al2O3/(SiO2+Al2O3) ratio in the inclusions gradually decreased. According to the Scheil calculation, soluble Al in molten steel was almost consumed when the solid fraction exceeded about 0.5 thereby newly precipitated inclusions during solidification or after solidification had low Al content. It shows a good agreement with the experimental results. Al2O3/(SiO2+Al2O3) ratio in the inclusions which were located on the interdendritic region was much lower than the primary inclusions. In addition, oxysulfide inclusions which were precipitated after solidification had also low value of Al2O3/(SiO2+Al2O3) ratio.

Development of a Primary Solidification Mode Diagram for Austenitic Stainless Steel Weld Metals using CALPHAD-based Modeling

Sutton, Benjamin J ProQuest Dissertations & Theses The Ohio State Uni 2021 해외박사(DDOD)

The primary solidification mode of austenitic stainless steel weld metals strongly dictates solidification cracking susceptibility. Provided that primary solidification mode selection is highly dependent on chemical composition, predictive tools such as the WRC-1992 diagram are often used to assess risk and/or design around potential solidification cracking concerns. In recent years, solidification simulations are becoming more commonplace with the advent and ever-growing adoption of CALPHAD methodologies, providing an additional avenue to predict primary solidification mode in austenitic stainless steels.In this work, high-throughput computational thermodynamic calculations have been used to develop a diagram to predict primary solidification mode for austenitic stainless steel weld metals. By simulating the stable and metastable liquidus temperatures for randomly generated austenitic stainless steel chemistries, a new set of nickel and chromium equivalency relationships have been developed that provide a sharp delineation between primary austenite and primary ferrite solidification modes under equilibrium conditions. Comparisons between legacy experimental data and computational thermodynamic calculations suggest that undercooling at the solid-liquid interface promotes metastable primary austenite solidification in stainless steel chemistries that fall near the equilibrium austenite-ferrite transition during conventional arc welding solidification conditions. Multicomponent dendrite growth theory has also been applied to help rationalize the occurrence of metastable primary austenite solidification. Using this information, a correction scheme has been established to modify the new primary solidification mode diagram to account for dendrite growth kinetics. A series of controlled gas tungsten arc spot welds have been performed on various arc-cast alloy chemistries that fall near the austenite-ferrite transition to assist with validation of the new primary solidification mode diagram. Validation efforts highlight that the location of the metastable austenite-ferrite transition is sensitive to both alloy chemistry and solidification conditions. An overview of the computational thermodynamic simulation framework, diagram construction, and experimental validation will be provided.While the new primary solidification mode diagram was constructed using a chemical composition range that covers many common austenitic stainless steel grades, the methodology developed here can be used to generate similar diagrams in the future and greatly reduce experimental burden. Examples where this methodology can be applied include cases where the chemical compositions and/or solidification conditions of interest differ from those explored here, or when increased resolution is needed within a narrow chemical composition range.

IN 625 초내열 합금의 응고거동 및 상변태 온도 분석

함성효 창원대학교 2023 국내석사

Solidification and phase transformation behaviors of IN 625 with low C and Fe were investigated by directional solidification and quenching experiments. The primary and secondary dendrite arm spacings decreased exponentially with increasing solidification rate, showing good agreement with theoretical equations. The MC carbide formed at the constant temperature regardless of solidification rate. The laves phase solidified at the final stage of solidification. Thus, the laves phase was found at the bottom of mushy zone. Morphologies and sizes of the MC carbide and the laves phase were closely related to solidification rate. Increasing solidification rate at a constant thermal gradient developed the MC carbide from blocky to script and eventually spotty shape along with decreasing size. The size of Laves phase also decreased together with bulky and coarse lamellar to fine lamellar structure with increasing solidification rate. The weldability of the alloy could be improved with low C and Fe contents and relative high rate solidification which generate finely distributed the MC and laves phase.

Solidification Microstructure and Mechanical Properties of Directionally Solidified Ni3Al Alloys

노운 창원대학교 2006 국내박사

Ni₃Al has been considerable research area due to its high temperature behavior increasing strength with increasing temperature. However, it is not well established whether the eutectic is composed of γ /γ´, γ´/β, or γ/ β. In order to understand solidification behavior of the eutectic structure, directional solidification experiments have been carried out with solidification rates near the Ni₃Al composition in this study. The effects of the solidification rate and composition on formation of the equilibrium and metastable eutectics have been discussed by using Jackson-Hunt eutectic model. The (γ '+ γ) band and cellular coupled structure were formed in hypoperitectic Al23 alloy at low solidification rates. The effect of convection, sample length and solidification rates on the structure development have been calculated qualitative. To increase both tensile ductility and strength of Ni₃Al alloys, the ductile phase of γ (Ni-rich) dendrite fibers or strong phase of β (NiAl) dendrite fibers are often arrayed in the γ´-(Ni₃Al) matrix by directional solidification. Dendrite spacing could be controlled by solidification rate, and the volume fraction of γ or β phase could be altered by alloy composition, for example, from 23 at.% Al to 27 at.% Al. With increasing Al content, the γ dendritic microstructure transformed to the β dendrite in γ´ matrix, reducing the tensile ductility by increasing the volume fraction of brittle β dendrites in γ´ matrix. With increasing solidification rate, the dendrite spacing decreased and the tensile properties of Ni₃Al varied in a complex manner. The microstructural evolution affecting the tensile behavior of directionally solidified Ni₃Al alloy specimens with different solidification rates and Al contents have been discussed.

일방향응고 247LC 초내열합금의 용접 응고균열 거동에 대한 연구

김경민 부경대학교 2023 국내석사

Directionally solidificated 247LC superalloy applied to gas turbine blade has been reported to have solidification cracking phenomena during welding. Therefore, in order to secure a sound tip welds of 247LC superalloy gas turbine blade, it is essential to suppress solidification cracking susceptibility. In this study, BTR was evaluated through a varestraint test incorporating heterogeneous welding (solid/liquid coexistence temperature range reduction) and oscillation welding(solidification mode control) using dissimilar filler materials to suppress solidification cracking. As a result, BTR was reduced through dissimilar(400 K → 217 K) and oscillation welding(400 K → 275 K). However, in both welding processes, solidification cracks occurred during welding under without external load. Therefore, the correlation between epitaxial growth and solidification crack inhibition was investigated by performing high-speed single-mode fiber laser welding accompanied by rapid solidification. As a result, solidification cracks were completely suppressed with an epitaxial growth rate of 99% when laser welding at a welding speed of 1000 mm/s within one crystal grain of 247LC-DS superalloy. Finally, based on the results, actual gas turbine blade tip welding guidelines were presented.

일방향 응고속도에 따른 고크롬 백주철의 미세조직 및 기계적특성

장득원 창원대학교 2017 국내석사

High chromium cast iron is well known for its excellent resistance to abrasion and oxidation, and has been widely used for wear resistant components in mining and metallurgical machinery. In this work, the changes behavior of the solidification interface and microstructure with the solidification rate through directional solidification in High chrome white cast iron alloy with the main alloying elements 26wt%Cr and 2.8wt%C has been studied. When rate is slow, solidification mode is eutectic solidification. When rate is fast, solidification mode is hypo-eutectic solidification. Solidification rates are 1, 5, 10, 25, 50μm/s both of samples. To observe solid/liquid interface microstructure use OM(Optical Microscope), SEM(Scanning Electron Microscope). In order to compare the mechanical properties, a tensile test and an abrasion test were conducted.

Studies on heat transfer and fluid flow during solidification processes using finite element method

유재석 University of California at Berkeley 1984 해외박사

A numerical method employing finite elements, the front tracking finite eleme nt method, has been developed for the solution of multidimensional problems of h eat transfer with phase change. Specific to the method is that both the energy e quation in the media and the energy equation on the change of phase interface ar e regarded as independent governing equation which, when solved through the use of a finite element formulation, yield the temperature distribution in the media as well as the displacement of the interface. Numerical examples illustrate the ability of the method to handle problems with arbitrary boundary conditions whi ch result in irregular two-dimensional shapes of the change of the phase interfa ce. The front tracking finite element method was then used to investigate two di fferent problems of practical importance in manufacturing processes involving so lidification. The first was a numerical investigation of the solidification of u ndercooled liquid. The result of this study indicates that the solid-liquid inte rface is morphologically unstable during recalescence, in contrast to the solidi fication process in saturated liquids where the interface is always stable. It w as found that the stability strongly depends on the amount of undercooling. This study indicates that the small disturbance can initiate cellular growth in under cooled liquid and consequently effect the structure of materials produced by rap id solidification. It was noticed that, after recalescence, the interface become s stable and the interface will return to planar shape. This transition occurs w hen the ratio between the dimensionless liquid temperature gradient at the inter face and the dimensionless growth velocity is about 0. 5. The second problem inve stigated in this work dealt with the effects of natural convection induced by te mperature difference in the liquid on the solidification process in an enclosure . This study required the development of a new numerical technique also based on the front tracking finite element method for the study of solidification proces ses in the presence of natural convection. The new method using penalty formulat ion is the first to be able to solve problems of this kind with transient and ir regular boundaries of the liquid domain in which natural convection occurs. The results indicate the importance of natural convection and show that the change o fphase morphology was decisively different from that in a conventional correspon ding pure-conduction model. The effects of the fluid flow due to natural convect ion on the solidification process were studied to a Rayleigh number of 1,000,000 . The study indicates that the morphology of the interface was affected by the n atural convection. The findings indicate that the natural convection effects are of importance and have to be considered in studying solidification. The new nu merical method developed developed here is an important tool for future studies of these problems.

Numerical Modeling of Equiaxed Solidification in Direct Chill Casting

Coleman, John S Purdue University ProQuest Dissertations & Theses 2020 해외박사(DDOD)

Direct chill (DC) casting is the main production method for wrought aluminum alloys. In this semi-continuous process, significant heat is extracted through a narrow, solidified shell by impinging water jets. A combination of rapid cooling and inoculation of the liquid metal with heterogenous nucleation sites (grain refiner) produces the proper conditions for equiaxed solidification. As equiaxed grains nucleate and grow in the slurry, they are transported by natural convection until their eventual coalescence into a rigid mush. The preferential accumulation of these solute-depleted grains in localized regions of the casting can lead to long range composition differences known as macrosegregation. Because macrosegregation cannot be mitigated by subsequent processing, it is critical to understand and prevent its development during casting.Numerical models are often used to gain insight into the interplay of the different transport phenomena that cause macrosegregation. The formation of mobile equiaxed grains creates a multiphase system with many moving interfaces, causing several modeling challenges. In principle, a model could be formulated in terms of local instantaneous variables describing the evolution of these interfaces, however the associated computational cost prohibits its extension to the length scale of industrial castings. For this reason, macroscopic transport equations are mathematically formulated using volume averaging methods. Two different volume-averaged model formulations can be distinguished in the solidification literature. The first approach is the multiphase formulation, which solves separate sets of governing equations for each phase that are coupled using microscale interfacial balances. While this approach retains closure models to describe the behavior of the sub-grid interfaces, these interfacial models introduce significant uncertainty that is propagated through the model. The second approach is the mixture formulation, which solves a single set of governing equations for the mixture and utilizes more pragmatic closure relationships. While this approach significantly reduces the complexity and computational cost of the model, previous formulations have oversimplified the microscale transport. Recognizing the advantages and disadvantages of both formulations, a mixture model is rigorously derived, retaining appropriate relationships for the grain structure and microsegregation behavior in equiaxed solidification.Implementation of this model into a 3-D finite volume method (FVM) code using a co-located grid is discussed along with appropriate treatment of the discontinuous body forces and phase mass fluxes across the interface between the slurry and rigid mush. More specifically, body forces in the momentum equation are treated at the face-centers of a control volume to prevent erroneous velocity oscillations near this interface, and a diffuse phase flux method is proposed to reduce the sensitivity of composition predictions to the numerical grid. The proposed methods are verified across a wide range of conditions present in equiaxed solidification.This model is then used to investigate the role of grain motion on macrosegregation development in equiaxed solidification, specifically in horizontal and vertical DC casting. In horizontal DC casting, the casting axis is perpendicular to gravity and there is a tendency for grains to accumulate along the bottom of the ingot. Feeding liquid metal through a constrained inlet near the bottom suspends grains in the slurry, both reducing the overall macrosegregation and improving the macrosegregation symmetry in the ingot.

Ni기 초합금에서 합금 원소가 응고거동 및 주조성에 미치는 영향

박상현 창원대학교 일반대학원 2024 국내석사

IN792, Ni-based superalloy, shows excellent mechanical properties at medium and high temperatures (600~900℃). However, it is known as very sensitive to casting defects such as hot cracking and hot tearing during casting. In this study, effect of alloy elements such as Hf, Zr, and B on solidification behavior and castability in IN792. To analyze, 4 types of ingots manufactured by adjusting the contents of Hf(0~1.0wt%), Zr and B were prepared, and directional solidification and castability tests were performed. As a results of directional solidification, as the Hf content increased, there was little change in the dendritic spacing, which is a measure of segregation behavior, but the mushy zone increased. In addition, the volume fraction of residual liquid at 2, 4, 6, and 8mm below the dendritic tip was measured to confirm the change rate during solidification. As a result of the measurement, 0.8wt%Hf showed the largest change rate. It is predicted that rapid changes in the residual liquid during solidification will lead to a higher strain rate. Castability test was quantified by the length and frequency of cracks after casting to determine the effects and causes of grain boundary strengthening elements and process variables on casting cracks. As a results, specimen containing 0.8wt% Hf showed the worst castability. Misorientation was confirmed between 30˚and 40˚ around cracks and no differences were found for each specimen. Mapping results showed that Zr content around crack was highest in case of 0.8wt% Hf. The castability is determined by the complex effects of not only Hf but also Zr and B.

Study on growth of optoelectronic materials (LiNbO3 and Si) by directional solidification method based on numerical simulation

서중원 성균관대학교 일반대학원 2010 국내박사

Recently, certain materials have attracted attention for a new generation of high speed, efficient, multi-functional optical devices. Among these materials, small-diameter long-length bulk crystals are of considerable interest for miniaturization and high efficiency. In particular, fiber single crystals have already received attention as attractive materials for a variety of electro-optical application because of their extended interaction length and high optical intensity. There is increasing interest in the growth of rod or fiber-like micro-single crystals, suitable for application in non-linear optical and electro-optical devices, such as second harmonic generation (SHG), micro-laser sources or optical memory arrangements. An improved efficiency can be expected because of the high surface-volume ratio, the long interaction length and the high crystal quality. Fiber single crystals were grown by variously modified melt growth like the laser heated pedestal growth (LHPG) method, the drawing down method and the micro-Czochralski (µ-CZ) method. For micro-single crystals the habit is of special importance because the dimension of the cross section approaches the diameter of the stimulating laser mode. Therefore, the correlation between surface morphology and growth parameters, like starting composition of the melt and temperature field, are of considerable interest for optimization of the SHG properties. LiNbO3 is a widely studied optoelectronic material because of its technological applications. LiNbO3 is a material that combines excellent electro-optic, acousto-optic and non-linear properties with the possibility of rare earth or transition metal doping. Doping with foreign ions modifies the optical properties of the matrix and makes the system useful for a great variety of application such as photorefractive devices, solid-state lasers or optical waveguides. In this study, rare-earth elements-doped LiNbO3 single crystals from congruent and stoichiometric melts composition were simulated and grown by the micro-pulling down (µ-PD) method. The characteristics of this method has a high pulling rate, a low thermal strain compared with other growth methods and it is possible to grow a crystal from incongruent melt composition. The grown fiber single crystals were investigated for the change of the photoluminescence (PL) properties in various wavelength according to influence on adding of rare-earth elements (Er, Nd and Tm). To guarantee the growth of the photovoltaic industry, the manufacturing process of solar silicon (Si) ingots must be further developed in order to lower the manufacturing costs of ingots, wafers, solar cells and solar modules. In the photovoltaic (PV) industry, more than 90% of the annual solar cell production is based on mono-crystalline, multi-crystalline wafer and thin film module Si. In recent years, the market share of multi-crystalline silicon has increased remarkably. Multi-crystalline Si wafers constitute more than 60% of the photovoltaic market because of the cost advantages compared to mono-crystalline Si wafers. Since the first paper that focused on the casting process in 1976, several solidification processes industry, including casting, heat exchanger method (HEM), and electromagnetic casting have been developed. However, the rapid market growth of mono and multi-crystalline Si wafers is being suppressed by the shortage of Si feedstock. Therefore, two of the main goals in the PV industry today are to increase the weight of ingots and to accelerate the growth rate of solar ingots. The directional solidification (DS) process is a cost-effective technique for producing multi-crystalline Si ingots. A detailed understanding of the DS process is important to producing high-quality multi-crystalline Si ingots and wafers. In this study, an open and closed heat insulation system with a heat exchanger was used to improve the DS process and to satisfy the above-mentioned goals. The improved DS process was beneficial because of its small heat loss, short cycle time and efficient directional solidification. Based on the FD model, a numerical simulation was performed on the thermal characteristics of the improved DS process. Using a commercial CFD code, Fluent, the heat transfer characteristics in the DS system were calculated, and the results were graphically depicted. From simulation results, multi-crystalline Si ingot was grown using the improved DS process, and the characteristic resistivity and life time of the ingot were investigated.