Numerical simulation of central shrinkage crack formation in a 234-t steel ingot

Central shrinkage crack is a common defect encountered in steel ingot casting. It is necessary to limit the degree of crack in case of further propagation in forging. A 234-t steel ingot was dissected to check the internal quality, and a central shrinkage crack band of 1,400 mm in height and 120 mm in width, was found at a distance of 450 mm under the riser bottom line. Then, thermo-mechanical simulation using an elasto-viscoplastic finite-element model was conducted to analyze the stress-strain evolution during ingot solidification. A new criterion considering mush mechanical property in the brittle temperature range as well as shrinkage porosity was used to identify the shrinkage crack potential, where the degree of shrinkage porosity is regarded as a probability factor using a modified sigmoid function. Different casting processes, such as pouring speed, mould preheating and riser insulation, were optimized with the simulation model. The results show that fast pouring, proper mould preheating and good riser insulation can alleviate shrinkage crack potential in the ingot center.     

Inhomogeneity of microstructure and mechanical properties of a 500 mm diameter heavy section semi-continuous cast AZ61 magnesium alloy ingot简

For the large magnesium alloy ingot, there is a considerable difference in cooling rate of different parts in the ingot, which leads to non-uniform distribution of the secondary phases, solute segregation and tensile properties. In the present research, an heavy AZ61 alloy ingot with a diameter of 500 mm was made by semi-continuous casting. The microstructure and mechanical properties at different positions along the radial direction of the large ingot were investigated by using an optical microscope(OM), a scanning electron microscope(SEM), an energy dispersive spectroscope(EDS), and a micro-hardness tester. The results indicate that the microstructure of the AZ61 ingot is non-uniform in different locations. It changes from equiaxed to columnar grains from the center to the edge; the average grain size gradually reduces from 1,005 μm to 763 μm, the secondary dendrite arm spacing reduces from 78 μm to 50 μm, and the Mg17(Al,Zn)12 phase is also refined. The micro-hardness value increases from 55.4 HV at the center to 72.5 HV at the edge of the ingot due to the microstructure differences, and the distribution of micro-hardness at the edge of the ingot is more uniform than that in the center. The tensile properties at room temperature show little difference from the center to the edge of the ingot except that the elongation at the edge is only 3.5%, much lower than that at other areas. The fracture mechanism is ductile fracture at the center and cleavage fracture at the edge of the ingot, and at the 1/2 radius of the ingot, a mixture of ductile and cleavage fracture is present.     

Effects of can parameters on canned—forging process of TiAl base alloy（I）——Microstructural analyses

By using thermal simulation technique,.the conventional canned-forging process of TiAl based alloy was studied.The effect of can parameters on the microstructures of TiAl alloy was analyzed in this process.The results show that,the deformation microstructure of TiAl based alloy without canning is inhomogeneous.In lateral area,crack and shearing lines can be found;while in central area,fine-grained shearing zone can be found.The effect of can is to reduce the secondary tensile stress.However,only when the deformation of the steel can is coincidental with that of TiAl alloy ingot,can this effect be effective.Moreover,a thick can would enhance the microstructural homogeneity in TiAl based alloy.With the H/D ratio of the ingot increasing.the deformation of TiAl alloy would be more unsteady,therefore,a thicker can should be needed.     

A Numerical Study of the Effect of Multiple Pouring on Macrosegregation in a 438-Ton Steel Ingot

In present paper, a ladle–tundish–mold CFD model and a macrosegregation model were utilized to investigate the effects of the multiple pouring(MP) process on the macrosegregation in a 438-ton steel ingot. Firstly, the model was partially proved as compared to the measured carbon distributions along the transverse sections in the riser of ingot. Then, the comparison between the single pouring(SP) and MP process has been carried out to study their influences on the macrosegregation in ingot. Besides, the predicted macrosegregation results in MP process which introduced the improved riser fixed with an insulating sleeve were compared with that in traditional MP process. The traditional MP process leads to certain favorable initial carbon distribution in the mold, which has some favorable influence on suppressing the positive segregation in ingot. The holding time of the low carbon in the riser is the main factor to suppress the positive segregation in ingot. Improved insulating sleeve can prolong the holding time of the low carbon in the riser and release the positive segregation in the riser of ingot. Improvement of the insulating effect of the riser is an efficient method to control macrosegregation in large steel ingot.     

Thermal stress analysis method considering geometric effect of risers in sand mold casting process

Solidification and fluid flow analysis using computer simulation is a current common practice. There is also a high demand for thermal stress analysis in the casting process because casting engineers want to control the defects related to thermal stresses, such as large deformation and crack generation during casting. The riser system is an essential part of preventing the shrinkage defects in the casting process, and it has a great influence on thermal phenomena. The analysis domain is dramatically expanded by attaching the riser system to a casting product due to its large volume, and it makes FEM mesh generation difficult. However, it is difficult to study and solve the above proposed problem caused by riser system using traditional analysis methods which use single numerical method such as FEM or FDM. In this paper, some research information is presented on the effects of the riser system on thermal stress analysis using a FDM/FEM hybrid method in the casting process simulation. The results show the optimal conditions for stress analysis of the riser model in order to save computation time and memory resources.     

Numerical simulation on multiple pouring process for a 292 t steel ingot简

A ladle-tundish-mould transportation model considering the entire multiple pouring （MP） process is proposed.Numerical simulation is carried out to study the carbon distribution and variation in both the tundish and the mould for making a 292 t steel ingot.Firstly,the fluid flow as well as the heat and mass transfer of the molten steel in the tundish is simulated based on the multiphase transient turbulence model.Then,the carbon mixing in the mould is calculated by using the species concentration at the tundish outlet as the inlet condition during the teeming process.The results show a high concentration of carbon at the bottom and a low concentration of carbon at the top of the mould after a MP process with carbon content high in the first ladle and low in the last ladle.Such carbon concentration distribution would help reduce the negative segregation at the bottom and the positive segregation at the top of the solidified ingot.     

Casting process design and practice for coolant pump impeller in AP1000 nuclear power station

The coolant pump impeller casting is the only rotating component in the nuclear island of an AP1000 nuclear power station, and is required to have a 60-year service time, which requires advanced materials and processing technologies to guarantee. In this paper, the casting process was studied, designed and modified by means of numerical simulation. The gating system was distributed symmetrically and the runner diameter was a little bigger for avoiding sand wash and turbulence;the feeding system focused on the solution of blades feeding, as some parts of which should reach Severity Level 1 radioactive testing standard. Therefore, upper and lower plates cooperating with chillers acted as feeding method besides additional 2-3 times thickness;in addition, lowering sand core strength, decreasing pouring temperature and increasing dimension allowance would be adopted to avoid crack defects. Finally, the pilot impeller was cast. The results show that the casting process design is reasonable, as the liquid rises very smoothly when pouring, and no volume defects are found by means of 100% radioactive testing. Based on this casting process, 16 coolant pump impellers have been successfully produced and delivered to customers.     

Numerical simulation and optimization of shell mould casting process for leaf spring bracket

Although the shell mould casting process has a wide range of application in many fields,the prediction of casting defects is still a problem.In the present work,a typical leaf spring bracket casting of ZG310-570 was fabricated by shell mold casting.The finite element model and ProCAST software were utilized for simulating the filling and solidification processes of the casting;and the formation mechanism of the gas pore,and shrinkage porosity defects were analyzed.The results indicate that the gas pore and shrinkage porosity defects are formed due to air entrapment,insufficient feeding and non-sequential solidification.Subsequently,through changing the position of risers,adding a connecting channel between the risers,and setting blind risers at the U-shaped brackets,an optimized gating and feeding system was established to improve the quality of the casting.After optimization,the gas pore and shrinkage porosity defects of the leaf spring bracket casting are effectively eliminated.The experiment results with the optimized casting process are in good agreement with the numerical simulation,which verifies the validity of the finite element model in the shell mould casting.     

Preparation of helicopter rotor counterbalance component by means of permanent-mold casting

Copper alloy was adopted to prepare helicopter rotor counterbalance component by means of permanent-mold casting. Process parameters were determined on the basis of theory calculation and computer numerical simulation. Through controlling mould temperature, pouring temperature and speed, the defects, such as gas cavity, shrinkage porosity, cold shut, can be effectively avoided. The results show that the best process parameters for smeking are as follows： pouting temperature is 1 100 ℃, pouring time is 14 s and opened mould time is 6 min. Mixture of 90% charcoal powder and 10% fluorite were selected as covering agent and 0.01% phosphorus copper acts as oxidizer. The density of rotor counterbalance component after casting in permanent-mold is 99.91% of its theory density. Mechanical properties are as follows： σb=315 MPa, σ0.2=143 MPa, δ=25%, HB=950. The mass deviation is between -5 g and ＋5 g, the curved surface distortion is less than 0.20 mm, and the largest tolerance of sectional thickness can be controlled between -0.10 mm and ＋0.10 mm.     

Simulation study on three casting processes for a marine propeller hub body简

The mold filling and solidification process of a marine propeller hub were simulated using ProCAST. Three casting processes - gravity casting, centrifugal casting and low pressure casting - were compared in order to get the best process. The heat transfer coefficient of the casting/mold interface was determined using a reverse method. The simulated results of velocity, temperature and shrinkage porosity distribution were discussed in detail for the three casting processes. A smooth filling was found in all three casting processes, especially the low pressure casting exhibiting a better filling performance than the other two, but the solidification processes were different. The casting did not experience the sequential solidification, and the feeding paths were blocked, leading to shrinkage porosity defects in the riser and the bottom of the casting in gravity casting and in the upper zone of the casting in low pressure casting. While, the sequential solidification was well controlled in the solidification process of centrifugal casting, and majority of the shrinkage porosity defects can only be observed in the riser. It could be concluded that the centrifugal casting process is the most suitable casting process for the production of propeller hub body. The casting experiments verified the simulation results, and a defect-free propeller hub was obtained by centrifugal casting with a rotational speed of 150 r.min-1.     

钢锭铸锻一体化开坯工艺数值模拟

针对13 t的12Cr2Mo1V钢锭开坯后探伤不合格问题,提出了铸锻一体化开坯数值模拟分析方法。首先在THERCAST软件中完成钢锭的浇铸和凝固过程的仿真分析,然后通过数据共享将铸造数据结果导入到锻造分析软件FORGE中,完成最终的开坯仿真分析。分析表明:通过铸锻一体化开坯数值模拟分析方法,有效地预测了锻件疏松出现的部位。钢锭最初的疏松出现在中心部位,其Niyama值较大,随着锻造过程的进行,中心部位的疏松区域逐渐消失,其Niyama值变小。通过调整开坯过程的重要工艺参数,如锻造比、相对送进量、温度控制等,制定合理的开坯工艺来减轻或消除疏松、缩孔影响,使显微空隙及疏松通过锻压焊合。     

Numerical Simulation of the formation of the Sedimentary Zone of Inclusions in Large Steel Ingots

Abstract

A mushy zone is formed in the lower part of an ingot by the showering of crystallites from the top and by free dendrite arms fused and broken from dendrites growing from the side mould wall. The zone obstructs the floating of inclusions. In addition, inclusions floating to the upper part of the ingot can be carried down to the bottom by natural convective flow and can be trapped by the mushy zone. An equiaxed crystal zone in the bottom of a steel ingot results from free crystals coming from the top and sides and from the local growth of crystals caused by heat transfer. The interval from the beginning of pouring to the time when the velocity of convective flow becomes too low to carry any inclusions larger than those of tolerable size down to the bottom is defined as the critical time. The size of the sedimentary zone of inclusions is then equal to that of the equiaxed crystal zone formed in the critical time. In this paper, a method for determing the critical time is proposed, and numerical simulation of the formation of the bottom equiaxed crystal zone has been carried out. A 27Cr2Mo1V steel ingot of 68 tonnes in weight was studied. From the results of the numerical simulation, the weight ratio of the offcut tail end to the ingot body was 8.11%. This result approximately agreed with the weight ratio of 8.8% obtained in production practice with the same ingot.     

70Cr3Mo钢支承辊超探不合格原因分析及改进措施

制造44800和19500 kg的两根70Cr3Mo钢支承辊大锻件。钢锭采用EAF+LF+VD+VC冶炼浇注,1锭锻制2件,锻后球化退火,出炉冷却后,毛坯进行超声波探伤检查。结果发现:小辊超探合格踩线,大辊在冒口端辊颈部分存在超标缺陷。为了探究锻造工艺合理性,切掉大辊两端辊颈部分,利用辊身合格的坯料改锻成小辊。改锻后的小辊经超声波探伤发现,除辊身外两端辊颈部分均存在超标密集缺陷。通过一系列试验,对孔洞缺陷和夹杂物进行了分析,确定了导致支承辊超探不合格的原因:铸锭缺陷中广泛分布的非金属夹杂物分割了钢基体的连续性,恶化了钢的性能,导致超探不合格。此外,分析了夹杂物的来源及其危害性,并提出了相应的工艺改进措施。     

锭模斜度及浇注工艺对钢锭质量影响的数值模拟研究

采用THERCAST模拟软件,模拟35 t钢锭的浇注和凝固过程,分析钢锭内部缺陷形成的原因。模拟计算锭模斜度、浇注温度和浇注速度对缺陷分布的影响,得出材料为30Cr Ni Mo8的35 t钢锭最佳浇注工艺方案。     

钢液导热系数对大型钢锭凝固过程的影响

数值模拟是提高大型钢锭质量、优化浇铸工艺、降低实验费用和帮助缺陷诊断分析的重要手段之一.模拟结果是否可靠很大程度上取决于数学模型假设是否合理,合金热物性参数和边界条件的设置是否准确等因素.利用ProCAST铸造模拟软件,分析了钢液导热系数对钢锭充型和凝固过程中的温度分布、凝固时间和缩孔、疏松的影响规律.结果表明,导热系数的提高显著缩短钢锭整体凝固时间,增加钢锭中心产生缺陷的可能性,因此有必要在模拟计算之前准确测量该物性参数.另外,对于在同一模具中浇铸具有不同物性参数的各类合金,该结果为了解其凝固规律提供了参考.     

42CrMo模铸钢锭凝固场数值仿真与超声检测分析

采用大型商业软件ProCAST建立了钢锭凝固传热数值仿真,钢锭模在凝固过程中的温度场是空间和时间的变化量,属于三维不稳定态传热.为了验证钢锭凝固过程温度场变化对钢锭冶金缺陷的影响,本文采用24寸42CrMo钢种钢锭模进行数值仿真.采取钢锭浇注过热度、断面温度梯度、液相线和固相线的分析,模拟了钢锭凝固过程中温度场、凝固分...     

42CrMo模铸钢锭凝固场数值仿真与超声检测分析

采用大型商业软件ProCAST建立了钢锭凝固传热数值仿真，钢锭模在凝固过程中的温度场是空间和时间的变化量，属于三维不稳定态传热。为了验证钢锭凝固过程温度场变化对钢锭冶金缺陷的影响，本文采用24寸42CrMo钢种钢锭模进行数值仿真。采取钢锭浇注过热度、断面温度梯度、液相线和固相线的分析，模拟了钢锭凝固过程中温度场、凝固分数、缩松率及Niyama的分布。通过仿真与实际操作工艺相结合，以及钢锭的超声检测和解剖验证的低倍检验，证明了仿真技术和超声检测可以提高钢锭的冶金质量。     

S304308大型不锈钢钢锭下注工艺的技术开发

介绍了30 t的S304308大型不锈钢钢锭的锭型设计、冶炼工艺、浇注工艺和试生产过程,并对钢锭成品的各项理化指标进行了检验分析。结果表明:采用EBT→LF→VD→IC氩气保护下注工艺,可以生产出质量优良的大型不锈钢钢锭。     

Numerical Simulation of Macrosegregation in Steel Ingot CastingEI北大核心CSCD

Many key forging components of heavy equipment are manufactured by large steel ingots. Macrosegregation in steel ingots is a key defect formed during the solidification process. Over the past few decades, numerical modeling has played a more and more important role in the study of macrosegregation. Various models have been developed and applied to different ingot casting processes. This paper focused on the application of macrosegregation models to the steel ingot. Firstly, the formation mechanism and influencing factors of macrosegregation were introduced. Then, the existing macrosegregation models and their recent development were summarized. Macrosegregation models accounting for such mechanisms as solidification shrinkage- induced flow and mushy zone deformation were analyzed, respectfully. To model macrosegregation due to solidification shrinkage, the key was to solve the free surface. A simple derivation showed that the multi-phase (including gas phase) models were equivalent to the VOF-based segregation models in dealing with the shrinkage-induced flow. Finally, our recent research work on numerical modeling of macrosegregation in steel ingots was illustrated, including application of the developed multi-component and multi-phase macrosegregation model to a 36 t steel ingot, and numerical simulation of multiple pouring process. The carbon and sulphur concentrations at about 1800 sampling points, covering the full section of a 36 t ingot, were measured. By detailed temperature recording, accurate heat transfer conditions between the ingot and mould were obtained. Typical macrosegregation patterns, including the bottom-located negative segregation and the pushpin-like positive segregation zone in the top riser, have been reproduced both in the measurements and the predictions, The carbon and sulphur concentrations predicted by the three dimensional multi-component and multi-phase macrosegregation models agreed well with the measurements, thus proving that the model can well predict the macrosegregation formation in steel ingots. As for the multi-pouring process simulation, the results show a high concentration of carbon at the bottom and a low concentration of carbon at the top of the mould after the multi-pouring process with carbon content high in the first ladle and low in the last ladle. Therefore, the multiple pouring process could get the initial solute distribution with the opposite form of segregation. Such carbon concentration distribution would reduce the negative segregation at the bottom and the positive segregation at the top of the solidified ingot, thus proving the ability of the multiple pouring process for the control of macrosegregation.