Xiuhong Kang

The Effect of Natural Convection on Equiaxed Dendritic Growth: Quantitative Phase-Field Simulation and Comparison with Synchrotron X-Ray Radiography Monitoring Data

A two-dimensional (2D) quantitative phase-field model solved by adaptive finite element method is employed to investigate the effect of natural convection on equiaxed dendritic growth of Al-4 wt.%Cu alloy under continuous cooling condition. The simulated results are compared with diffusion-limited simulations as well as the experimental data obtained by means of in situ and real-time X-ray imaging technique. The results demonstrate that natural convection induced by solute gradients around the dendritic crystal has an obvious influence on the dendrite morphology and growth dynamics. Since the rejected solute cooper from solid is heavier than aluminum, it sinks down along the interface from the top arm tip to the bottom arm which results in the formation of a circulatory flow vortex on both sides of the dendrite. Hence, the convection promotes the top arm advancing into the melt progressively whereas it suppresses the growth of bottom severely. As the dendrite grows into a large size, the convection becomes more intense and the morphology shows distinguished asymmetric shape. When compared with experimental data, the growth velocity is found to agree substantially better with the simulation incorporating natural convection than the purely diffusive phase-field predictions.

Numerical simulation of macrosegregation in large multiconcentration poured steel ingot

Abstract A continuum model for the transport phenomena in solidification systems is used to investigate the formation of macrosegregation in a 360 t multiconcentration poured steel ingot. A numerical scheme with explicit time stepping in solidification problems is employed to solve coupled temperature and concentration fields, and equations of momentum. The proposed scheme is tested against an experimental concentration field reported in the literature. The influences of the concentration in each ladle and the ladle pouring time upon macrosegregation are studied. The simulation results show that, with a shorter time for the last ladle, macrosegregation is not significantly reduced with MCP compared with conventional single concentration pouring. A longer ladle time produced a marked reduction in the extent of positive segregation at the top of the ingot. Any positive segregation in excess of the industrial requirement limit is shifted to the riser. If the last ladle is poured after a longer time, growth of channel segregates is initiated by local remelting.

Hydraulic Experiments of Mold Filling Process in Horizontal Centrifugal Casting

Mold filling process in horizontal centrifugal casting was analyzed in this paper. The effects of rotation speed on flow pattern were studied. Results show that the flow pattern changes greatly with increasing rotation speed and the first thread interval shows significant differences within each group. It is found that inappropriate rotation speed results in liquid drop during filling process, leading to casting defects.

Chemical Compositions, Microstructure and Mechanical Properties of Roll Core used Ductile Iron in Centrifugal Casting Composite Rolls

The industrial manufacture processes of three kinds of roll core used ductile irons have been investigated via systematical experiments. Effects of the ratio of C/Si, pig iron, nodularizer and alloying method on the microstructure and mechanical properties of the heavy section ductile iron have been analyzed. It has been found that when treated with RE-Mg plus Sb, high quality nodular castings can be produced even if much anti spheroidizing alloy elements are included in the pig iron. The alloy element Sb played an important role in the control of graphite morphology.

Study on macrosegregation in heavy steel ingots

Abstract Macrosegregation in heavy steel ingots was studied through numerical and experimental studies of a 500 kg ingot. The numerical model used heat conduction coupled with thermal convection. Simulation results confirm that a small 500 kg ingot poured in a sand mould has a solidification time that is equal to that of a 10 000 kg industrial ingot cast in an iron mould. Accordingly, the sand moulded ingot exhibits more severe macrosegregation compared to the iron moulded ingot, indicating the possibility that a relatively small ingot in sand can simulate conditions in a much heavier steel ingot in a conventional iron mould. Experiment demonstrated that a 500 kg ingot exhibited all the types of macrosegregation, including A- and V-segregates and negative and positive segregation commonly found in a 65 000 kg steel ingot.

Microstructure evolution of a cold-rolled 25Cr-7Ni-3Mo-0.2N duplex stainless steel during two-step aging treatments

A cold-rolled 25Cr-7Ni-3Mo-0.2N duplex stainless steel (DSS) has been aged in two steps. Firstly, the aging treatment at interval of 50 °C in a temperature range from 900 to 1050 °C was carried out in order to obtain fine grains. Secondly, another aging treatment at 850 °C was performed to reveal the σ-phase precipitation behavior. A detailed microstructure evolution during those two aging steps was observed by the optical microscope (OM), the scanning electron microscope (SEM), the electron backscatter diffraction (EBSD) and the transmission electron microscope (TEM). The results revealed that the micro-duplex structure with grain size of lower than 10 µm appeared after the first aging step. However, their grain size was rapidly increased with increasing aging temperature. Meanwhile, the δ → γ and/or δ → γ + σ transformations took place in association with the occurrence of the extensive recovery or a little recrystallization in δ-grains. During the second aging treatment, σ-phase mainly nucleated at δ/γ interfaces and further grew along those interfaces into various morphologies (e.g., butterfly and granule). A novel precipitation behavior was found in this study that the γ-grain boundaries bulged not only into the δ-grains as usual, but abnormally into the σ-phase precipitates without the prior precipitation of the isolated secondary austenite γ2 or another phases.

Modeling of coupled motion and growth interaction of equiaxed dendritic crystals in a binary alloy during solidification

Motion of growing dendrites is a common phenomenon during solidification but often neglected in numerical simulations because of the complicate underlying multiphysics. Here a phase-field model incorporating dendrite-melt two-phase flow is proposed for simulating the dynamically interacted process. The proposed model circumvents complexity to resolve dendritic growth, natural convection and solid motion simultaneously. Simulations are performed for single and multiple dendritic growth of an Al-based alloy in a gravity environment. Computing results of an isolated dendrite settling down in the convective supersaturated melt shows that solid motion is able to overwhelm solutal convection and causes a rather different growth morphology from the stationary dendrite that considers natural convection alone. The simulated tip growth dynamics are correlated with a modified boundary layer model in the presence of melt flow, which well accounts for the variation of tip velocity with flow direction. Polycrystalline simulations reveal that the motion of dendrites accelerates the occurrence of growth impingement which causes the behaviors of multiple dendrites are distinct from that of single dendrite, including growth dynamics, morphology evolution and movement path. These polycrystalline simulations provide a primary understanding of the sedimentation of crystals and resulting chemical homogeneity in industrial ingots.

Process modelings and simulations of heavy castings and forgings

The Materials Process Modeling Division, IMR, CAS has been promoting for more than 10 years research activities on modeling and experimental studies on heavy castings and forgings. In this report, we highlight some selected achievements and impacts in this area: To satisfy domestic strategic requirements, such as nuclear and hydraulic power, marine projects and high speed rail, we have developed a number of casting and forging technologies, which combine advanced computing simulations, X-ray real time observation techniques and industrial-scaled trial experiments. These technologies have been successfully applied in various industrial areas and yielded a series of scientific and technological breakthroughs and innovation. Important examples of this strategic research include the hot-processing technologies of the Three Gorge water turbine runner, marine crankshaft manufacturers, backup rolls for hot rolling mills and the production of hundreds-ton steel ingot.

Effect of The Sigma Phase on the Mechanical Properties of a Cast Duplex Stainless Steel during the Ageing Treatment at 850°C

With about equal amount of austenite and ferrite in volume fraction, duplex stainless steel (DSS) is in advantage of mechanical properties and corrosive behaviors. Hence it is widely applied to the heavy castings for nuclear power plants inshore, such as impellers, pumps and valves. However, lots of cracks usually occur in these castings during manufacturing processes, because it is susceptible to precipitate the brittle intermetallic compound of sigma phase when the castings are exposed from 600 to 1000oC. In this work, the precipitation of sigma phase was observed by optical microscope (OM) and scanning electron microscope (SEM) in a cast DSS named as MAS/6001, which aged at 850oC from 5 to 300 minutes. The effect of sigma phase on the mechanical properties was analyzed by the tensile at room temperature and impact tests at -10°C. The results show that sigma phase in the MAS/6001 steel precipitated simultaneously with the secondary austenite, which obeyed the eutectoid reaction. The interfaces between austenite or secondary austenite and sigma phase were the locations where cracks generated from the void aggregation. Cracks are susceptible to propagate along or cross these interfaces, and to promote the sigma phase breaking-off, which severely deteriorated the mechanical properties.

Numerical simulation of delayed pouring technique for a 360t heavy steel ingot

A continuum mathematical model for the transport phenomena in the solidification systems has been established to study the central axial macrosegregation in a 360t multi concentration pouring (MCP) steel ingot. A time explicit finite difference scheme is adopted to calculate the coupling of the temperature, concentration and velocity flow fields. The flow equations are solved by the solution algorithm for transient fluid flow (SOLA) technique. Simulations of Fe-C-Si-Mn quaternary alloy are performed. The established model has been validated by comparing with the experimental result of a 360t MCP steel ingot. The simulated carbon concentration profile along the centreline of the ingot shows a fair agreement with the measurement. The influence of the delay time for the last ladle on the macrosegregation along the centreline has been investigated. Simulation results show that the delay time for the last ladle has a significant effect on the macrosegregation, especially for the positive segregation below the hot top. A novel criterion of selecting the delay time for the last ladle has been proposed to reduce the macrosegregation. By selecting the proper delay time for the last ladle, the carbon concentration along the centreline in the ingot body can be controlled within the range of the industrial limit.