Influence of furfuryl moulding sand on flake graphite formation in surface layer of ductile iron castings

The influence of the use of moulding sand with furan resin, prepared both with fresh sand and reclaimed matrix, on the formation of a flake graphite formation at the surface layer of ductile iron castings has been investigated. A series of experimental heats of ductile iron cast in moulds made of moulding sand characterised by different levels of surface active elements (sulphur, oxygen) were performed. The effect of the wall thickness and the initial temperature of the metal in the mould cavity on the formation of flake graphite in the surface layer of the casting is shown in the paper. Investigations carried out by means of scanning electron microscopy (energy dispersive X-ray spectroscopy and wavelength dispersive X-ray spectroscopy) showed concentration of gradient profiles of surface active elements in the castings surface layer, which are responsible for their quality. Finally, it has been shown that there exists a significant effect of the quality of the sand on the formation of the flake graphite layer and the surface characteristics of ductile iron castings.     

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.     

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.     

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.     

Occurrence and behaviour of Mo containing precipitates in nodular cast iron at high temperatures

In nodular cast iron, molybdenum is alloyed in the range of 0.5 to 1.5?wt.-% to increase the mechanical properties for high temperature application, especially for thick walled castings like housings of gas or steam turbines. The behaviour of Mo precipitates in the as cast state as well as under long exposure to heat was the object of this investigation. In the as cast state, Mo forms metastable carbidic grain boundary precipitates with iron, silicon and carbon. These transform into stable MoC precipitates during sustained temperature influence. Furthermore, there are finely dispersed Mo containing precipitates found in the matrix. These increase the high temperature strength of Mo alloyed cast iron due to the Orowan mechanism and the occurrence of additional grain boundaries. However, it has been shown that these Mo containing precipitates underlie Ostwald ripening under continuous heat exposure, and a precipitate free area occurs along the ferritic grain boundary.     

Occurrence and effect of casting skin in compacted graphite iron

Most iron castings retain their as cast surfaces because of their geometric complexity and to minimise the machining costs. However, the mechanical properties that are documented in standards (i.e. ASTM) are tested on fully machined test bars. Therefore, the effect of the as cast surface and subsurface features (commonly referred as ‘casting skin’) on mechanical properties should be evaluated. Preliminary works have shown the negative effect of the casting skin on mechanical properties of grey and ductile irons. This paper reviews the recent works on the casting skin effect on tensile and fatigue properties in compacted graphite and ductile irons. It was found that the tensile and fatigue strength were reduced by 9 and 40% respectively because of the presence of the casting skin. In addition, the correlations between processing parameters (e.g. nodularity and section thickness) and the casting skin features were presented. The experimental results suggest that the Mg depletion because of metal–mould and metal–air interactions was the main reason for the formation of the casting skin.     

物理模拟大断面球墨铸铁中异常石墨形态的观测

使用自行研制的大断面铸件冷却模拟装置,研究了不同共晶时间球墨铸铁样品的石墨形态及相关的力学性能。验证了这一装置对于厚大断面球铁铸件研究的有效性。随着共晶时间的延长,铸件的石墨形态逐渐偏离球形,向异常化趋势发展,石墨颗粒的圆整度随之降低。异常石墨的形态表现出相互联结的立体网络状。     

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.     

Unambiguous classification of complex microstructures by their three-dimensional parameters applied to graphite in cast iron

Three-dimensional (3D) quantitative analysis is indispensable for the unambiguous characterization and objective classification of complex microstructures. Focused ion beam (FIB) nanotomography provides complete information of the spatial arrangement, chemistry and orientation of different phases of real microstructures on scales especially important in materials science (10 nm–100 μm). Complex graphite particles were analyzed in three-dimensions. Whereas nodular, vermicular and temper graphite particles can be characterized individually, the whole network of flake graphite has to be considered due to the high spatial interconnection of particles. The characterization method was verified in comparison to established two-dimensional stereological methods. The influence of anisotropy and image resolution was discussed. Basic stereological characteristics (volume, surface area, integrals of mean and total curvature) as well as 3D connectivity (Euler number) and shape parameters objectively differentiate these graphite morphologies and contribute to the understanding of their growth mechanisms and the properties of the cast iron.     

Controlling graphite content in plasma sprayed cast iron coatings via in-flight particle diagnostic

Graphite formation and degradation in thermally sprayed cast iron coatings is a technological barrier for achieving superior wear resistant coatings. Therefore, there is a need to understand the in-flight particle behavior of cast iron powder and introduce new approaches to control the graphite content. In this study, it has been demonstrated that the graphite content can be controlled by means of in-flight particle diagnostic. For this purpose, cast iron coatings were plasma sprayed under a variety of spray conditions and characterized by using an optical microscope, X-ray diffractometer and electron probe micro-analyzer. As a result, a significant amount of graphite with respect to a wide range of in-flight particle temperature and velocity was preserved in cast iron coatings.     

锶硅铁在稀土镁球墨铸铁中的孕育效果

采用稀土镁合金、铜镁合金对立式电阻坩锅炉熔炼的铁水进行球化处理后,分别用75％Si-Fe和SrSiFe对其孕育考察了它们的孕育效果。     

Graphite degeneration in surface layer of ductile iron castings

Abstract

In the furan resin moulding technique sulphur in P-toluenesulphonic acid (PTSA), usually used as the hardener, has been identified as an important factor causing graphite degeneration at the metal/mould interface, especially at lower graphite nodularity levels. The greatest surface layer thickness and the lowest graphite nodularity, and shape factors, were obtained with irons solidified in moulds coated with an S bearing material. Uncoated moulds provided better results, but employing a MgO type coating effectively neutralised the sulphur migrating from the mould. In the present solidification conditions, the application of an active mould coating also influenced the graphite phase characteristics in the entire section of the casting, up to its centre. Negative effects were observed using an S bearing coating and positive effects from an MgO based coating.     

我国内燃机球墨铸铁零件的材料工艺概况

从铁水熔炼、球化处理、孕育处理、铸型成份、热处理及成品检查等方面,介绍了我国内燃机球铁零件的工艺和材质概况,并作了简要的评述。     

An experimental evaluation of graphite nanoplatelet based lubricant in micro-milling

Micro-milling is characterized by significant frictional interaction between the tool and workpiece, leading to relatively short tool life. This paper evaluates a graphite nanoplatelet based cutting fluid specifically developed to reduce friction and associated heat generation at the tool-workpiece interfaces in micro-milling. The results of micro-slot milling experiments on H13 tool steel (50 HRc) with and without graphite lubrication are presented. In particular, cutting forces, slot depth, and surface finish obtained under different lubrication conditions are compared and discussed. Possible explanations for the experimental observations are given.     

基于WORKBENCH平台对检查井盖结构优化设计的分析

基于WORKBENCH软件对常见的三种球墨铸铁井盖进行静力学分析,通过得到最大应力值、最大变形值等数据对比,分析不同加强筋形式对井盖抗压强度的影响关系,并找出相应的规律,为球墨铸铁检查井盖结构优化设计提供一种理论依据.     

An investigation of graphite nanoplatelets as lubricant in grinding

Cooling and lubrication are very critical to ensure workpiece quality in grinding due to the high friction and intense heat generation involved in the process. Liquid lubricants have traditionally been used in flood form or minimum quantity lubrication (MQL), raising however, major environmental and economic concerns. The focus of this study is to evaluate the performance of graphite nanoplatelets as a lubricant in surface grinding. The role of graphite's characteristics such as form, size and concentration; and the effect of the carrying medium and the graphite's application method are determined based on an experimental study. The results indicate that graphite nanoplatelets significantly reduce the grinding forces, specific energy, and improve surface finish during surface grinding of hardened D-2 tool steel. A comparison with results obtained in conventional MQL grinding is also provided. The proper selection of graphite, carrying medium and application method can lead to a low cost, nontoxic and simple alternative to solid lubrication or MQL grinding.     

铸铁基体及石墨形状超声波无损伤检验探讨

利用超声波（速度和强度）来测试铸铁中不同的石墨形状参数、基体结构及与此相关的机械性能。结果表明，石墨形状参数和超声波速度有着明显的对应关系，但是基体与超声波之间的影响则不明显。随着石墨形状参数的增加，超声波的幅值也随之变化，发现铁素体、珠光体、正火马氏体和贝氏体有相同的特性，但是基体中较大数量的正火马氏体或珠光体都会导致超声波幅值的增大。     

灰铸铁在酸性环境下的耐腐蚀性研究

采用失重法对含不同石墨相的灰口铸铁在酸性环境下的腐蚀速度与腐蚀深度进行了测试。分析了不同石墨相对灰口铸铁腐蚀性能的影响,同时还比较了灰口铸铁与白口铸铁的腐蚀行为。实验结果表明:含有A型石墨的灰口铸铁耐腐蚀性能最差;白口铸铁因为它的组织几乎都为碳化物,所以它的耐腐蚀性明显优于灰口铸铁。     

球墨铸铁热浸镀铝层的耐蚀性能

为了研究热浸镀铝后球墨铸铁的耐蚀性,通过对热浸镀铝前后球墨铸铁的电化学腐蚀性能进行测试,采用扫描电镜(SEM)和能谱仪(EDS)对腐蚀形貌和腐蚀产物的分析,得到了球墨铸铁和热浸镀铝后球墨铸铁的极化曲线和腐蚀形貌.结果表明:热浸镀铝后球墨铸铁的耐蚀性与腐蚀介质溶液有关.在3.5%NaCl溶液里的球墨铸铁热浸镀铝层耐腐蚀性能比球墨铸铁明显提高,但在5%NaOH溶液和5%HCl溶液里,球墨铸铁热浸镀铝层耐腐蚀性能不如基体球墨铸铁.