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.     

Experimental and numerical analysis of effect of cooling rate on thermal–microstructural response of spheroidal graphite cast iron solidification

This work presents an experimental and numerical study of the solidification process of an eutectic spheroidal graphite cast iron (SGI). The effect of the cooling rate on the thermal–microstructural response is particularly analysed. To this end, experiments as well as numerical simulations were carried out. The experiments consisted in a solidification test in a wedge-like casting such that different cooling rates were measured at specific positions along the part. A metallographic analysis was also performed in five locations of the sample with the aim of obtaining the number and size of graphite nodules at the end of the process. The numerical simulations were made using multinodular based and uninodular based models. These two models predicted similar results in terms of cooling curves and nodule counts. Besides, good experimental–numerical agreements were obtained for both the cooling curves and the graphite nodule counts.     

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.     

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。     

基于CA模拟焊缝凝固过程枝晶生长的分析

将元胞自动机方法用于焊缝金属凝固组织演变的模拟中,不仅为焊缝微观组织演变的数值模拟开辟了一种新的途径,同时也有助于研究焊接接头组织并依此优化工艺来提高焊件质量.文中研究了焊缝凝固过程中的溶质扩散问题,构建了基于元胞自动机的焊缝枝晶生长速度模型,在此基础上进行了焊缝中心等轴晶和熔池边缘柱状晶生长的模拟.初步计算了焊缝中心等轴晶和熔池边缘柱状晶的生长形态,计算结果明显再现了二次、三次枝晶的生长及竞争生长等微观现象.结果表明,元胞自动机方法是研究和模拟焊缝微观组织的有效手段之一.     

球墨铸铁中白口共晶形成的数值模拟

建立了球铁稳定系和介稳定系共晶转变的数学模型。同时考虑了元素微观偏析和不同组织所产生潜热的影响。对球铁试样进行了定量金相实验分析，并与模拟结果进行了比较，结果表明，石墨球数量和尺寸以及碳化物体积分数的模拟结果与实验结果吻合，对白口共晶生长，碳化物形貌与凝固方式的关系进行了分析和讨论，表明凝固分析和模拟可以在一定程度上预测球铁中白口共晶的生长及最终碳化物的形貌。     

灰口铸铁中石墨分布的优化

控制石墨的形态和分布是优化灰口铸铁性能的关键;石墨的形态取决于凝固过程中经过共晶温度区间时的冷却条件;通过选择合适的化学成分和金属液处理方法可以控制共晶凝固过程,获得分散的、均匀分布的A型石墨,从而得到性能优良的灰铁铸件。     

Modelling of compacted graphite cast iron solidification-Discussion of microstructure parameters

A melt maintained for hours in a press pour unit allowed the following changes over time from spheroidal graphite to compacted graphite iron by casting thermal cups at regular time intervals.This provided extensive experimental information for checking the possibility of simulating solidification of compacted graphite irons by means of a microstructure modelling approach.During solidification,compacted graphite develops very much as lamellar graphite but with much less branching.On this basis,a simulation of the thermal analysis records was developed which considers solidification proceeding in a pseudo binary Fe-C system.The simulated curves were compared with the experimental ones obtained from three representative alloys that cover the whole microstructure change during the holding of the melt.The most relevant result is that the parameter describing branching capability of graphite is the most important for reproducing the minimum eutectic temperature and the recalescence which are so characteristic of the solidification of compacted graphite cast irons.     

Effect of nodule count and cooling rate on As-Cast matrix of a Cu-Mo spheroidal graphite

The transformation of austenite to ferrite and graphite or to pearlite in spheroidal graphite cast iron depends on a number of factors, among which are the nodule count and the cooling rate. In this study, the pearlite fraction decreased as the nodule count increased for a given cooling rate. Furthermore, as the cooling rate increased, the fraction of pearlite increased. Both effects were more sensitive at low nodule count. The effects of altering these parameters on the relative amount of pearlite and ferrite in the matrix of a copper-molybdenum (Cu-Mo) spheroidal graphite cast iron were addressed using heats carried out in an induction furnace, and the melts were treated with magnesium ferrosilicon in a ladle. To vary the nodule count, the melts were inoculated with two different amounts of ferrosilicon. Pouring was performed into sand molds of cylindrical cavities with different section size in order to achieve various cooling rates. Both the nodule count and the cooling rate affected the relative amount of ferrite and pearlite in the matrix.     

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。     

厚大断面球墨铸铁件生产中若干问题的探讨(1)

介绍了厚大断面球墨铸铁的凝固时间长、共晶结晶平台长、石墨球数少、易畸变、基体组织异常等凝固特性,通过理论与生产实例相结合的方法,阐述了碎块状石墨问题,石墨球数少与石墨畸变的关系,球化剂、w(Mg残)量、RE与石墨畸变的关系,CE及化学成分的选择问题,厚大断面球墨铸铁件若干问题的措施,球化剂中RE问题以及如何增加石墨球数问题。最后指出:要保证厚大断面球墨铸铁正常生产,除上述问题外,还有浇注系统优化和浇注时间控制的问题,生产过程质量控制以及生产组织等问题,这些都需要注意和严格控制。     

Study of shrinkage porosity in spheroidal graphite cast iron

This study aims at identifying the relationship between the shrinkage cavities and the solidification structure in spheroidal graphite cast iron. Cast samples specially designed to contain shrinkage cavities were used. The solidification macrostructure was revealed using the Direct Austempering After Solidification method, while the solidification microstructure was revealed by using colour etching. At the midsection of the pieces, the shrinkage cavities and the solidification structure were observed jointly. The study showed that the classification of shrinkage porosity found in literature does not correspond to the ductile iron solidification model recognized by most of the scientific community. Early solidification models, and therefore shrinkage formation mechanisms, were proposed in instances when there was not a thorough knowledge of the morphology of the solid phases during solidification. Nowadays, defects formation mechanisms can be described with higher accuracy. Therefore, an updated classification of shrinkage porosity for spheroidal graphite iron is proposed.     

同质焊缝球墨铸铁焊条的研制

研制了一种适合于球墨铸铁与球墨铸铁以及球墨铸铁与结构钢焊接的专用球墨铸铁焊条，通过试验确定了球墨铸铁焊条的合金成分并给出了比较合理的焊芯和焊条药皮化学成分。采用该焊条对球墨铸铁焊接时，可以达到同质焊缝要求。测试结果表明，焊缝金属的化学成分、金相组织均满足焊接球墨铸铁的要求，焊缝的硬度在190～230HB之间，焊缝中石墨球化稳定，焊条具有良好的焊接工艺性能。     

DISTRIBUTION OF SULFUR IN HIGH TEMPERATURE PHASES OF CAST IRON

The distribution of subversive element sulfur in high temperature phases during solidificationof cast iron was investigated by means of liquid quenching method and autoradiography.Theresults show that in grey cast iron,sulfur is concentrated in the liquid,flake graphite and in theperipherial area of D-type supercooled graphite eutectic cells,but less sulfur is in austenite.Besides,the content of sulfur in A-type graphite is higher than that in D-type graphite,andsulfur content in eutectic austenite is a little higher than that in primary austenite.While,innodular graphite cast iron less sulfur is in liquid and austenite,but more in nodular graphite.The blackness of the spots on the emulsion film corresponds to the atomic percentage ofradioactive substances located on the surface of the specimen.     

Simulation of Grain Growth during Solidification of Twin-Roll Continuously Cast Pure Aluminum with a Modified CA Model

基于枝晶生长的基本传输过程和元胞自动机(Cellular Automaton,简称CA)-有限元(Finite Element,简称FE)模型基本原理,建立了适应双辊连续铸轧纯铝薄带工艺特点的凝固过程形核和晶体生长的数学模型。模型耦合了宏观温度场和微观组织模拟计算,考虑了溶质扩散、曲率过冷和潜热释放等重要因素的影响,能够模拟凝固过程中枝晶生长的形态。应用本模型对双辊连续铸轧纯铝薄带凝固过程中等轴晶生长、等轴晶多晶粒生长及柱状晶生长、柱状晶向等轴晶演化进行模拟并与实验结果进行对比,模拟结果与实验结果吻合较好     

Effect of bismuth on refinement of graphite in thin wall spheroidal graphite cast iron

Abstract

Although it is known that the addition of bismuth refines the graphite nodule in spheroidal graphite cast iron, the refinement mechanism has not yet been clarified. In this research, the effect of bismuth on the refinement has been investigated by examining the microstructure of the spheroidal graphite cast iron containing a small amount of bismuth. Bismuth was added at 0˙01 mass% to the spheroidal graphite cast iron melt containing 3˙5–3˙7 mass% carbon and 2˙0– 2˙8 mass% silicon, then the melt was poured into the mould to obtain the stepped test bar with 2, 3, 5 and 10 mm thicknesses. The graphite nodule increased as the bismuth content increased. The diameter of the graphite nodule decreased as the thickness decreased, namely, as the cooling rate increased. The graphite nodule was further refined by the addition of bismuth. The increase in silicon content increased the graphite nodule count and the ferrite in the matrix. It postulated that bismuth exists as simple substance or a compound in the vicinity of the nucleus of the graphite.     

Influence of boron on ferrite formation in copper-added spheroidal graphite cast iron

This paper reviews the original work of the authors published recently,describing the influence of B on the matrix of the Cuadded spheroidal graphite cast iron.The effect of Cu has been corrected as a ferrite formation promoter in the matrix of the grey cast iron by the usage of high-purity material.Also,this paper focuses on the ferrite formation and the observation of the Cu distribution in the B-added and B-free Cu-containing spheroidal graphite cast iron.The Cu film on the spheroidal graphite can be successfully observed in the B-free sample using a special etching method.However,in the B-added sample,no Cu film could be found,while the secondary graphite was formed on the surface of the spheroidal graphite.The interaction between B and Cu is stressed as a peculiar phenomenon by the employment of a contrast experiment of B and Mn.The heat treatment could make Cu precipitate more significantly in the eutectic cells and in the matrix in the form of large Cu particles because of the limited solubility of Cu.     

Effect of microstructure on resonant vibration fatigue fracture of ferritic spheroidal graphite cast iron

The effect of graphite nodule size, counts and eutectic cell wall inclusions on the fatigue fracture behaviour under resonant vibration of spheroidal graphite (SG) cast iron was investigated in this study. Experimental results indicated that the variations of graphite nodule size and counts slightly influence resonant frequency. The D-N curves (deflection amplitude vs. number of vibration cycles) of all test materials show similar manner. The deflection amplitude maintains a constant value for a certain period of vibration cycles (region I) and then decreases significantly with increasing the vibration cycles (region II). The feature of fatigue fracture behaviour under resonant vibration of ferritic SG cast iron can be concisely divided into four steps, namely (1) crack initiation, (2) crack linking, (3) the major cracks formation, and (4) deepening of the major cracks in the through-thickness direction. The initial three steps correspond to the region I period of the D-N curves, and the fourth step occurs in the region II period. Increasing the graphite nodule counts and refining the microstructure increase the region I period, and improve the resonant vibration fatigue life. During fatigue fracture process under resonant vibration, cracks mainly initiate from graphite nodules and some from eutectic cell walls. The eutectic cell wall inclusions are more crowded in SF specimen than that in PF specimen. So, the crack path of SF specimen is more tortuosity. The feature of fracture surface is mostly the brittle fracture.     

定向凝固过程中NH_4Cl-H_2O枝晶生长的数值模拟

建立了一种改进的元胞自动机模型(MCA),通过考虑成分过冷、曲率过冷、择优取向系数、温度梯度和抽拉速度等因素,模拟了不同温度梯度方向、不同择优取向及不同抽拉速度对柱状晶形态的影响.模拟结果较好地刻画了温度梯度和生长方向夹角的变化对一次枝晶臂间距的影响,不同择优生长取向的柱状晶竞争生长过程以及柱状晶尖端分叉机制.为了验证模型的可靠性,使用NH_4Cl-H_2O透明合金进行了定向凝固实验,实验结果和模拟结果吻合较好.     

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.