球墨铸铁缩松形成机理研究的现状

为全面、准确认识球墨铸铁缩松形成机理,从球墨铸铁的凝固特性、凝固过程以及生产工艺等方面综合评述了球墨铸铁缩松形成机理的研究现状.对球墨铸铁的缩松形成机理中存在的一些问题或矛盾之处进行剖析和总结,且对下一步研究工作的方向进行展望.     

采用钇基重稀土处理厚大断面球铁冷却壁的生产

厚大断面球铁冷却壁在制造过程中易出现球状石墨粗大及数量减少、石墨漂浮、缩孔及缩松等铸造缺陷.通过采取选择钇基重稀土球化剂,合理设计球铁化学成分,设置球化潜伏期控制球化反应,采取全程孕育处理工艺等系列工艺措施,生产出符合质量要求的厚大断面球铁冷却壁产品.     

Kinetics of graphite expansion during eutectic solidification of cast iron

The paper introduces a new linear displacement analysis (LDA)/thermal analysis (TA) experimental device for measuring linear displacement during the solidification of cast iron. The experimental device comprises a sand mould encased in a steel shell that prevents mould wall movements. Thus, only the linear displacement caused by the shrinkage or expansion of the metal is recorded by the transducers. Two quartz rods introduced directly at different heights into the liquid metal and connected to two transducers record the linear displacement during the liquid–solid transformation and subsequent cooling. Two thermocouples positioned at the same height with the quartz rods allow for the concomitant TA and LDA and thus for the direct correlation between expansion/contraction and the temperature change during solidification events such as graphite formation. The LDA device was used to study the differences in the solidification mechanisms of irons with different graphite morphologies (lamellar, compacted/vermicular and spheroidal) at carbon equivalent in the range of 3·7–4·4%. The analysis included the LDA and TA curves and full metallographic characterisation of the cast irons. In general, graphite expansion increased as the graphite shape changed from lamellar, to compacted and then to spheroidal. The most important process variables are the magnesium and carbon contents. Higher Mg residual and C in the iron produced more graphite expansion. Compacted graphite (CG) iron was particularly sensitive to the Mg residual. Indeed, the high Mg CG irons exhibited similar graphite expansion to that of spheroidal graphite (SG) iron, while the low Mg CG iron expansion was closer to that of the lamellar graphite (LG) iron. Graphite expansion increased for all data with the time interval over which graphite expansion occurred. It also increased with both carbon and carbon equivalent. The time for graphite expansion increased noticeably with the carbon content of the iron. It did not depend on the graphite shape. By combining TA and LDA, it was possible to plot the evolution of graphite expansion as a function of the fraction solid and thus to understand the kinetics of graphite expansion. The amount of expansion available at the end of solidification was quantified. Such data, when correlated with process variables, will be useful in decreasing microshrinkage and in producing riserless compacted and SG irons.     

Eutectic solidification mode of spheroidal graphite cast iron and graphitization

The shrinkage and chilling tendency of spheroidal graphite (abbreviated SG) cast iron is much greater than that of the flake graphite cast iron in spite of its higher amount of C and Si contents. Why? The main reason should be the difference in their graphitization during the eutectic solidification. In this paper, we discuss the difference in the solidification mechanism of both cast irons for solving these problems using unidirectional solidification and the cooling curves of the spheroidal graphite cast iron. The eutectic solidification rate of the SG cast iron is controlled by the diffusion of carbon through the austenite shell, and the final thickness is 1.4 times the radius of the SG, therefore, the reduction of the SG size, namely, the increase in the number, is the main solution of these problems.     

Precipitation and evolution of nodular graphite during solidification process of ductile iron

The quantity and morphology of spheroidal graphite have an important effect on the properties of ductile iron,and the characteristics of spheroidal graphite are determined by the solidification process.The aim of this work is to explore the precipitation and evolution of graphite nodules in hypoeutectic,eutectic,and hypereutectic ductile irons by thermal analysis,liquid quenching and metallographic technique.Results show that hypoeutectic ductile iron has the longest solidification time and the lowest eutectic temperature;eutectic ductile iron has the shortest solidification time;hypereutectic ductile iron has the highest eutectic temperature.After solidification is completed,hypoeutectic ductile iron has the lowest nodule count,nodularity and graphite fraction;eutectic ductile iron has the highest nodule count,nodularity and the smallest nodule diameter;hypereutectic has the highest nodule diameter and graphite fraction.The nucleation and growth of graphite nodules in hypereutectic ductile iron starts before bulk eutectic crystallization stage,however,the precipitation and evolution of graphite nodules of hypoeutectic and eutectic ductile irons mainly occur in the eutectic crystallization stage.The graphite precipitated in eutectic crystallization of hypoeutectic,eutectic,and hypereutectic ductile irons,are 61%,68% and 43% of total graphite volume fraction,respectively.Simultaneously,there are plenty of austenite dendrites in hypoeutectic and hypereutectic ductile irons,which are prone to shrinkage defects.Therefore,the eutectic ductile iron has the smallest shrinkage tendency.     

球墨铸铁轮毂的缩孔形成模拟和工艺改进

本文使用了收缩膨胀动态叠加（DECAM）法，对球铁轮毂的缩孔形成进行了模拟和工艺改进，DECAM方法是对凝固过程中液态收缩、凝固收缩和石墨膨胀进行动态叠加，并考虑石墨化膨胀力导致的铸型壁位移的影响，对轮毂件原工艺的模结果与实际生产中缩孔产生的情况吻合较好，改进工艺的模拟结果和实际试制结果基本吻合，改进工艺后铸件缩孔倾向明显减小。结果表明：收缩膨胀动态叠加法够较准确地模拟球铁缩孔的形成，能够进行了工艺方案的评价和改进，但应该进一步考虑球化剂加入量和残Mg量的影响。     

有关球铁铸件缩孔缩松形成及预防的文献综述

综述评述有关球墨铸铁凝固特性、凝固过程体积变化和缩孔、缩松形成机理的文献资料。笔者还结合本人的工作实践就缩孔、缩松的预防提出了看法。     

球墨铸铁缩松分析计算模型的建立

用系列试验铸件研究了主要工艺因素(碳当量、孕育量以及铸件模数)对球墨铸铁件缩松产生的影响规律。结果表明,在试验选择的范围内,随着碳当量增大、孕育量增加,缩松面积率减小;对亚共晶和共晶成分的球铁,模数的减小会导致缩松面积率的增大;对过共晶球铁,缩松面积率随模数增大而减小。通过对试验数据的回归分析,建立了球墨铸铁缩松面积率计算模型。为球墨铸铁缩松的预测模拟计算提供了依据。     

球墨铸铁微观组织形成过程数值模拟研究

基于球墨铸铁发生固态相变之前的热动力学和凝固原理建立了数值模拟的物理数学模型,提出局部单元替代放大算法,并编制程序对试样进行了数据模拟计算。得到的计算结果与试验值吻合得较好。同时,编制程序对微观组织的形成过程进行了二维动态显示。     

Numerical simulation of microstructure evolution on near eutectic spheroidal graphite cast iron

A multiphase cellular automaton model was developed to simulate microstructure evolution of near eutectic spheroidal graphite cast iron (SGI) during its solidification process, and both dendritic austenite and spheroidal graphite growth models were adopted. To deduce the mesh anisotropy of cellular automaton method, the composition averaging and geometrical parameter were introduced to simulate the spheroidal graphite growth. Solute balance method and decentered square algorithms were employed to simulate austenite dendrites growth with different crystallographic orientations. The simulated results indicate that the graphite nodule grows in a spherical morphology when the surrounding environment of a single graphite nodule is same. However, for two adjacent graphite nodules, the environment is different. The higher the carbon concentration, the faster the growth of graphite. By comparison with experimental results, it is found that the microstructure evolution of near eutectic spheroidal graphite cast iron during solidification process can be reproduced quantitatively by numerical simulation with this model.     

链轮球铁件缩孔、缩松的改进措施

对球铁链轮铸件缩孔、缩松的形成机理进行了分析研究,提出了提高砂型紧实度、控制碳当量在4.4%～4.6%之间、控制浇注温度在1350～1380℃之间、采用均衡凝固技术设置雨淋式浇注系统、压边冒口和冷铁等措施。经实际生产的验证,基本上消除了缩孔、缩松缺陷。     

Crystallography of graphite spheroids in cast iron

To further understand graphite growth mechanisms in cast irons, this study focuses on the crystal structure of a graphite spheroid in the vicinity of its nucleus. A sample of a graphite spheroid from a commercial cast iron was characterised using transmission electron microscopy. The chemical composition of the nucleating particle was studied at the local scale. Crystal orientation maps of the graphite spheroid revealed misorientations and twist boundaries. High-resolution lattice fringe images showed that the basal planes of graphite were wavy and distorted close to the nucleus and very straight further away from it. These techniques were complementary and provided new insights on spheroidal graphite nucleation and growth.     

Effect of pouring temperature,composition, mould strength and metal flow resistance on shrinkage cavities in spheroidal graphite cast iron

Abstract

Under the subject of this study, it was found that the growth of graphite in eutectic solidification is the most important factor. Also, shrinkage cavity increases in hypereutectic composition because the crystallisation volume of graphite in eutectic solidification becomes smaller. In addition, the Al amount contained in spheroidal agent or molten metal works negatively for shrinkage cavity, which could be said the same for P, Mo and Cr. As for the shrinkage cavity in non-alloyed ductile iron, the metal supply resistance P _MSR is the most significant element in regard to the occurrence of shrinkage cavity. For tight mould, there is no relationship between modulus and shrinkage cavity while the P _MSR of a product is the dominant factor. This theory of P _MSR can be applied to final solidification loops of products based on simulation. Based on this, it became possible to make shrinkage cavity free products without a riser.     

球墨铸铁中的奥氏体枝晶与缩松

采用着色腐蚀技术,金相显示了球墨铸铁缩松区中奥氏体枝晶组织形貌,分析了球铁缩松的形成机制。研究表明,奥氏体枝晶对缩松缺陷的类型及形成机制具有显著影响;宏观缩松常常出现在枝晶晶簇的间隙,产生于共晶凝固前期树枝晶骨架形成以后,是异地凝固收缩造成的对热节中心（厚壁处）铁水抽吸流动的结果;显微缩松是于凝固末期,晶簇间隙中的凝固收缩得不到补偿而产生的微小孔洞;枝晶数量增多,形态趋于发达,液态金属异地抽吸作用增强,易于形成宏观缩松;反之,枝晶数量减少,形态粗壮,倾向于形成显微缩松;共晶石墨化膨胀有利于消除缩松,而不是缩松形成的原因。     

球铁卡块铸件质量控制的工艺研究

针对球铁卡决铸件伸长率低的情况,在分析其结构和凝固过程特点的基础上,结合实际生产条件,采用均衡凝固理论设计浇冒口,通过强化补缩消除了缩松缺陷,使微观组织致密;采用瞬时孕育工艺等措施,使石墨明显细化,铁素体含量提高,铸件本体的力学性能明显得到改善,达到了QT450-10的要求,伸长率甚至达到15％。     

Understanding compacted graphite iron solidification through interrupted solidification experiments

While the manufacture of compacted graphite (CG) iron castings has seen significant expansion over the recent years, the growth of CG during iron solidification is still not fully understood. In this work, effort was expanded to experimentally reveal the evolution of graphite shape during early solidification and its relationship to the solid fraction. To this purpose, interrupted solidification experiments were carried out on hypereutectic irons with three magnesium levels. The graphite shape factors were measured and analysed as a function of chemical composition and solid fraction. Scanning electron microscopy was carried out to establish the fraction of solid at which the transition from spheroidal graphite (SG) to CG occurs. It was confirmed that solidification started with the development of SG for all CG irons. The SG-to-CG transition was considered to occur when the SG developed a tail (tadpole graphite). The findings were integrated in previous knowledge to attempt an understanding of the solidification of CG iron.     

Colour Metallography of Cast Iron

Chapter 3 Spheroidal Graphite Cast Iron（I） Spheroidal Graphite Cast Iron, SG iron in short, refers to the cast iron in which graphite precipitates as spheroidal shape during solidification of liquid iron. The graphite in common commercial cast iron can only be changed from flake to spheroidal shape by spheroidising treatment. Since spheroidal graphite reduces the cutting effect of stress concentration, the metal matrix strength of SG iron can be applied around 70%-90%, thus the mechanical property of SG iron is significantly superior to other cast irons;even the tensile strength of SG iron is higher than that carbon steel.     

表层石墨球化的灰铸铁件外表层片状石墨的防止

利用含有稀土的涂料实现了表层石墨球化的灰铸铁件,其组织形态由外到内依次为球化区、过渡区和灰铸铁本体。生产中发现,在某些条件下,已经实现了表层石墨球化的灰铸铁件的外表层又出现片状石墨。为防止这种现象发生,进行了研究。     

船用柴油机活塞铸造顺序凝固工艺设计及数值模拟

在生产壁厚变化较大的复杂球墨铸铁零件中,仅依靠球墨膨胀难以消除铸件缩孔、缩松等缺陷。通过数值模拟,分析了某船用柴油机活塞铸造缩孔、缩松产生的成因。采用阶梯流道、冷铁及冒口布置建立了顺序凝固方案,并通过数值模拟及试验方法进行验证。结果证明,通过顺序凝固方案能够消除活塞缩孔、缩松缺陷。     

Modeling and Experimental Validation of Ductile Iron Castings During Solidification

Ductile irons are still important engineering materials due to their high strength and toughness, and relatively low price. In the foundries, ductile irons suffer from shrinkage porosity formation during solidification, which is detrimental to the mechanical properties. In order to minimize porosity formation, large risers are normally used in the design, which reduces porosity level sometimes but leads to a low yield. In order to better understand the shrinkage behavior of ductile iron during solidification, a micro model was developed to simulate the microstructure formation. The density change during solidification and the room temperature mechanical properties can be calculated based on the microstructure. The simulation has been compared with the experimental results and found to be in good agreement.