Structure Evolution and Solidification Behavior of Austenitic Stainless Steel in Pulsed Magnetic Field

 To understand the solidification behavior of austenitic stainless steel in pulsed magnetic field, the solidification process is investigated by means of the self made high voltage pulse power source and the solidification tester. The results show that the solidification structure of austenitic stainless steel can be remarkably refined in pulsed magnetic field, yet the grains become coarse again when the magnetic intensity is exceedingly large, indicating that an optimal intensity range existed for structure refinement. The solidification temperature can be enhanced with an increase in the magnetic intensity. The solidification time is shortened obviously, but the shortening degree is reduced with the increase of the magnetic intensity.     

脉冲磁场强度对钛处理低碳钢凝固过程的影响

采用自发研制的脉冲磁场发生装置将脉冲磁场非接触式地施加到钛处理低碳钢的凝固过程中,利用金相显微镜、扫描电镜和能谱仪研究了不同强度的脉冲磁场对试样的宏观形貌、枝晶和组织的影响。结果表明,随着脉冲磁场强度的增加,试样的宏观表面逐渐变得平整;枝晶和组织随着脉冲磁场强度的增加逐渐细化,但达到一定强度后,继续增加又出现了相反的结论。由此可以推断,脉冲磁场强度对枝晶和组织的影响存在一个最佳值。     

氧化物冶金与脉冲磁场参数影响钢组织的研究

 脉冲磁场处理与氧化物冶金技术是细化组织、提升材料性能的两种常用方法,将其有机结合可进一步优化钢铁材料的性能。利用自主研制的高频感应线圈加热炉与脉冲磁场发生装置将脉冲磁场非接触式地施加在钛处理低碳钢的凝固过程中,利用金相显微镜、多重分形软件与维氏硬度仪研究了不同脉冲磁场参数对凝固组织的影响。结果表明,脉冲磁场感应强度为135~190 mT、磁场作用时间为5~10 min时,试样的金相组织最细小均匀,原始奥氏体晶粒得到明显细化,原始奥氏体晶粒面积由15.79 mm2下降到1.25 mm2,试样的硬度值由118.1HV提升到165.4HV,此参数下的脉冲磁场对凝固组织的细化程度最佳。     

基于多重分形研究脉冲磁场周期对凝固组织的影响

为了研究脉冲磁场对凝固组织的影响,采用自发研制的脉冲磁场发生装置将脉冲磁场非接触式地施加到钛处理低碳钢的凝固过程中,利用金相显微镜和实验室设计的多重分形软件研究了不同周期的脉冲磁场对凝固组织的影响。结果表明,该钢种的凝固组织具有分形特征;随着脉冲磁场周期的延长,Δα和Δf(α)呈现为先减小后增大的规律,在脉冲磁场周期为0.5 s时,组织最为均匀。由此可以推断脉冲磁场周期对凝固组织的影响存在一个最佳值。     

Progress in Research on Solidification in a Strong Static Magnetic Field

Strong magnetic fields available from superconducting magnets are opening a way to new phenomena that could lead to new methods in materials processing including solidification. The principal research involving solidification in strong static magnetic fields is emphasizing four aspects: control of crystal orientation, convection damping, thermoelectric magnetohydrodynamics (TEMHD) and change in thermodynamics. Under high magnetic intensity, aligned structural textures are induced in both magnetic and non‐magnetic materials. Since in strong magnetic field the melt flow is suppressed by convection damping, the microstructure being formed during solidification is affected heavily; this phenomenon applies to eutectic, monotectic and peritectic alloys as well as to dendritic morphologies typical of directional solidification. If strength and orientation of a magnetic field are controlled appropriately, this strong damping effect will generate more homogeneous crystals as a result of achieving diffusion‐controlled solute transport conditions. TEMHD more easily occurs in strong magnetic fields, resulting in equiaxed crystals even under directional solidification. It is evidenced experimentally and theoretically that the thermodynamics of phase transformation and nucleation are changed by strong magnetic fields.     

Effect of Pressure on Graphite Morphology and Mechanical Properties of Squeeze Cast Hyper-Eutectic Grey Cast Iron

Squeeze casting is a process in which a high external pressure is applied and maintained on the molten metal during the whole solidification process resulting in the lower porosity, higher solidification rate and improved mechanical properties. In this research, the effects of applied pressure on the morphology of graphite flakes in a squeeze cast hyper-eutectic grey cast iron were investigated. The molten grey cast irons were solidified under atmospheric pressure as well as 25, 50 and 75 MPa externally applied pressures. The results indicated that the pressure increase during solidification decreased the amount of free graphite flakes, reduced the size of pro-eutectic Kish graphite flakes and increased the density of the castings. D-type graphite enclosing austenite dendrites, resembling that expected in hypo-eutectic grey cast iron, was observed near the surfaces of the castings solidified under externally applied pressures. Moreover, some compacted and spheroidal graphite particles were observed in the castings solidified under 50 and 75 MPa pressures. Furthermore, by increasing the squeeze casting pressure from atmospheric pressure to 75 MPa, the hardness, ultimate tensile strength and elongation increased by about 13, 68 and 128%, respectively. The results have been discussed in terms of increased cooling rate, expected change in the phase diagram of the alloy, different atomic structure of the interfaces of graphite and austenite as well as some proposed theories for formation of eutectic cells and spheroidal graphite.     

Tensile and Thermal Properties in Compacted Graphite Irons at Elevated Temperatures

Tensile and thermal properties of compacted graphite irons (CGIs), prepared with various molybdenum additions and solidification rates, have been investigated for temperatures between room temperature and 873 K (600 °C). A slower solidification rate resulted in larger and fewer graphite particles as well as in an increase of intercellular cementite, or carbides. Molybdenum is a carbide stabilizing element; i.e., increasing additions of molybdenum increased the amount of carbides. Young’s modulus decreased with increasing temperature, and a lower solidification rate increased this parameter slightly. Both increasing content of carbide and increasing nodularity increased the Young’s modulus. Strength parameters such as yield strength and ultimate tensile strength (R _m ) were affected in similar ways by temperature and solidification rate. The strength values were generally quite temperature independent for temperatures below 573 K (300 °C) but decreased rapidly for higher temperatures. Increasing nodularity increased the strength, while increasing content of carbide had little influence on the values. The thermal conductivity decreased with increasing content of carbide and increasing nodularity. The thermal conductivity generally showed a maximum value at 573 K (300 °C). A contradictory linear relationship was found between yield strength and thermal conductivity.     

磁场对金属材料凝固的影响

控制并得到合适的凝固组织一直是材料工作者研究的方向。近年来人们对金属在磁场中的凝固做了一系列研究,取得了一定成果。主要介绍了磁场对金属凝固过程的影响,包括磁场驱动流体流动、控制宏观偏析、改善凝固组织等,并对未来的研究工作进行了展望。     

Computer Simulation of Solidification Transport Behaviours in Blade‐Like Castings under Transverse Static Magnetic Fields up to 25T

Numerical simulation of solidification transport phenomena/processes in a TiAl alloy blade‐like casting, under transverse magnetic fields of different strengths, was carried out. The simulation was based on a continuum solidification model and the computer codes developed by the authors. The simulation results show that, although the liquid flow in the bulk liquid region can be suppressed efficiently, the feeding flow in the mushy zone caused by the volume contraction, due to solidification shrinkage and thermal/solutal expansion, cannot be suppressed even under an ultra‐strong magnetic field up to 25T. This indicates that the forces driven by volume contraction are much stronger than those caused by the gravity. The natural convection can delay the directional solidification process, while the applied static magnetic field accelerates it to some extent, by weakening the natural convection. The magnetic field changes the coupled heat and species mass transfer to a diffusion type mechanism. The natural convection may be the cause for horizontal segregation. An ultra‐strong magnetic field is not necessary to achieve sufficient suppression of natural convection.     

Fluidity and Temperature Profile of Ductile Iron in Thin Sections

 It has been shown that it is possible to produce thin wall ductile iron (TWDI) castings of considerable length using an Archimedes spirals with a wall of 1, 2 or 3 mm in thickness. The fluidities for different moulding materials ［(classical mould, chemically bonded silica sand and chemically bonded low-density alumina-silicate ceramic sand (LDASC)］, chemical composition, and pouring temperature were estimated. There is a significant temperature drop in thin sections (contrary to typical sections) during the mould filling. A profile of real temperature drop is presented along with theoretical predictions. The high temperature drop of liquid iron results in an increased cooling rate (before the eutectic equilibrium solidification temperature), which in turn affects the solidification and microstructure of TWDI castings. Microstructures were characterized quantitatively using an image analyser. Structure parameters for different wall thicknesses and moulding materials (graphite nodule count, ferrite and cementite fraction) are plotted, which is versus distance from the entrance to the mould cavity. It has been shown that the thin wall castings have a gradient structure. Moreover, a strong influence of LDASC sand (material with low ability to absorb the heat) on the structure parameters of TWDI castings is presented.     

Influence of Forced/Natural Convection on Segregation During the Directional Solidification of Al-Based Binary Alloys

We analyzed the columnar solidification of a binary alloy under the influence of an electromagnetic forced convection of various types and investigated the influence of a rotating magnetic field on segregation during directional solidification of Al-Si alloy as well as the influence of a travelling magnetic field on segregation during solidification of Al-Ni alloy through directional solidification experiments and numerical modeling of macrosegregation. The numerical model is capable of predicting fluid flow, heat transfer, solute concentration field, and columnar solidification and takes into account the existence of a mushy zone. Fluid flows are created by both natural convection as well as electromagnetic body forces. Both the experiments and the numerical modeling, which were achieved in axisymmetric geometry, show that the forced-flow configuration changes the segregation pattern. The change is a result of the coupling between the liquid flow and the top of the mushy zone via the pressure distribution along the solidification front. In a forced flow, the pressure difference along the front drives a mush flow that transports the solute within the mushy region. The channel forms at the junction of two meridional vortices in the liquid zone where the fluid leaves the front. The latter phenomenon is observed for both the rotating magnetic field (RMF) and traveling magnetic field (TMF) cases. The liquid enrichment in the segregated channel is strong enough that the local solute concentration may reach the eutectic composition.     

外磁场在定向凝固过程中的应用研究

以磁场对合金凝固组织的影响为基础,综述了近年来交变磁场和稳恒磁场在定向凝固过程中的应用和发展,总结了外磁场对定向凝固过程中合金微观组织的作用机制,并展望了今后的研究方向。     

Effect of Induction Heat on Initial Solidification During Electromagnetic Continuous Casting of Steel

 Aiming at the process of electromagnetic continuous casting (EMCC), a three dimensional finite element model on electromagnetic field and temperature field was developed through the commercial software ANSYS to investigate the effects of induction heat of high frequency electromagnetic field on the early solidification process of molten steel in soft contact mold under various conditions of exciting current parameters. The results show that the induction heat has significant effects on the early solidification process, which appear as increasing the billet surface temperature, thinning the initial solidified shell and lowering the starting point of the initial solidification. The increases of exciting current frequency and density make the effects of induction heat on solidification process increased remarkably. Especially, with the increase of exciting current frequency, the early solidification process and shell growth become non-uniform in billet circumferential direction. Morever, if the exciting current density exceeds a certain value, there occurs a high temperature region in the top of molten steel column, and then the initial solidification rate is decreased greatly.     

A study on inhomogeneous distribution of temper graphite particles in strip-cast Fe-C-Si alloys

This study examined the relationship between solidification structure and graphitization characteristics of white cast iron strips produced by strip casting. Experimental results showed that there was an unusual distribution of temper graphite particles along the through-thickness direction of the graphitized strips in comparison with gravity-cast chill plate. In particular, the graphite-free zones appeared in the vicinity of the strip surface after the completion of graphitization, especially in the strip with low carbon and silicon content. There were abnormally straight interfaces between matrix and eutectic cementite with a strong preferred [001]_c growth direction caused by the effect of directional solidification found in the near-surface regions of the strips. The interfaces did not form a site for the graphite to nucleate and gave rise to the graphite-free zones close to the strip surface. An increase in carbon and silicon content could significantly increase the number of temper graphite particles and shorten the time for the completion of graphitization, but an inhomogeneous distribution feature of graphite particles was still observed in strips with a higher carbon equivalent value (CE). Furthermore, variations in carbon and silicon content resulted in transitions in carbide morphology and composition, which had a tremendous effect on the graphitization characteristics of the cast iron strips.     

脉冲电磁场凝固组织细化和均质化技术研究与应用进展

由于金属凝固过程中选分结晶的作用,不可避免地会出现成分不均匀现象。连铸过程中由于强制冷却,这种成分不均匀现象更为严重。这不仅影响了铸坯和铸锭后续加工性能,并影响了最终产品质量和性能的均匀性和稳定性。研究和生产实践表明,细化凝固组织是解决成分不均匀性的有效手段。脉冲电磁场因能耗低、施加方便、细晶效果显著,近年来得到了广泛关注,有望成为冶金界广泛应用的凝固组织细化和均质化技术。分别介绍脉冲电流、脉冲磁场和脉冲磁致振荡3种脉冲电磁场凝固组织细化和均质化技术的研究现状和应用进展。     

The effect of solidification rate on structure and high-temperature strength of the eutectic NiAl-Cr

The eutectic NiAl-Cr, consisting of chromium rods in a NiAl matrix, was directionally solidified at rates varying from 1/4 to 30 in. per hr. The inter-rod spacing and elevated temperature tensile properties were measured and the structure determined for each solidification rate. The spacing, λ, obeyed the relation λ²V= Constant, whereV is the solidification rate. AtV greater than 1 in. per hr, a cell or colony structure formed and the cell size decreased with increasing solidification rate. At 600°C, the tensile strength increased substantially with decreasing inter-rod spacing, reaching 100,000 psi at the highestV. At 800° and 1000°C, the strength first increased then decreased at the highest solidification rates reaching maximum strengths of 63,000 psi and 47,000 psi, respectively. At 1200°C, the strength decreased slightly with increasing solidification rate. Fracture occurred by shear along grain boundaries and cell walls. The decreased strength at the highest solidification rates may be related to the increased cell boundary area.     

凝固过程中的脉冲磁场和脉冲磁力研究

用数值模拟方法研究了凝固样品中的脉冲磁感应强度和电磁力的分布特性。模拟结果表明，脉冲磁场凝固条件下，脉冲磁感就强度和电磁力分布具有轴向、径向分布的不对称性和不均匀性。这种分布特性有助于了解脉冲磁场对金属凝固组织的细化作用。     

烧结-热等静压炉石墨发热体的优化设计

通过对烧结-热等静压炉石墨发热体的优化设计,改善石墨筒内温度场的均匀性,以解决实际生产中存在的石墨筒内温度分布不均匀、底部空间和靠近两个炉门侧空间温度偏低的问题,并对石墨发热体改进前后石墨筒内温度场分布情况进行了数值仿真.结果显示,改进后石墨筒内及整炉烧结制品表面温度场均匀性得到明显的改善,减小了由于炉门处未布置发热体所带来的影响.