Effect of Cooling Rate on Microstructure and Mechanical Properties of Thin-Walled Ductile Iron Castings

This article addresses the effect of cooling rate on microstructure and mechanical properties as determined by changing molding media and section size. The research was conducted for thin-walled iron castings with 2-5-mm wall thickness and for the reference casting with 13-mm wall thickness, using different molding materials (silica sand and insulating sand “LDASC”) to achieve various cooling rates. Thermal analysis was performed to determine the real cooling rate at the beginning of the graphite eutectic solidification. In general, it was found that the predictions based on theoretical analysis of the solidification process of ductile iron are in good agreement with the experimental outcomes. Finally, the present study provides insights into the effect of cooling rate on the graphite nodule count, the ferrite fraction and mechanical properties of thin-walled ductile iron castings. The study shows that the cooling rate of thin-walled castings varies in a wide range (80-15 °C/s) when changing the wall thickness from 2 to 5 mm, accompanied by significantly changing the mechanical properties of ductile iron. The cooling rate can be effectively reduced by applying an insulating sand to obtain the desired properties of thin-walled castings practically in the whole range of ductile iron grades in accordance with the ASTM Standard.     

Effects of Titanium Addition on Microstructure and Mechanical Properties of Thin-Walled Compacted Graphite Iron Castings

This research studies the effects of titanium added in an amount up to 0.13 wt.% on the microstructure and mechanical properties. The research was conducted for thin-walled iron castings with 3-5 mm wall thickness and for the reference casting with 13 mm wall thickness to achieve various cooling rates. Microstructural changes were evaluated by analyzing quantitative data sets obtained by image analyzer and also using scanning electron microscope. Metallographic examination revealed in thin-walled castings a significant effect of the addition of Ti to compacted graphite, much stronger in comparison with castings with thicker sections. Moreover, thin-walled castings with high degrees of inoculation and which have been solidified under high cooling rates have a homogeneous structure, free of chills, and good mechanical properties, which may predispose them for potential use as substitutes for aluminum alloy castings in diverse applications.     

The influence of copper on microstructure and mechanical properties of compacted graphite iron

Abstract

The influence of copper content (0·26 to 1·31 wt-%) on microstructure formation and mechanical properties of compacted graphite iron (CGI) has been evaluated through standard metallographic analysis, colour etching techniques and tensile testing of machined test bars. The properties investigated are yield strength, tensile strength and elongation. The castings were made in an industrial environment from a combination of CGI returns, pig iron, cast iron- and steel scrap. A total of four heats were cast in specially designed sampling cups (3 different cooling rates), chill wedges as well as tensile test bars machined from sand moulded cylinders (20, 45 and 85 mm in diameter). The results clearly illustrate the combined effect of copper and cooling rate on nodularity, chilling tendency as well as pearlite content. A discussion concerning the effect of graphite morphology on the ferrite growth is also included.     

High-quality manufacturing method of complicated castings based on multi-material hybrid moulding process

A multi-material hybrid patternless moulding process for complicated castings has been proposed. Moulding sands used in the hybrid moulding process include silica sand, ceramic sand, chromite sand, zircon sand, and steel shot sand. Experimental method was used to study the effects of moulding sands on the temperature field, mechanical properties, and dimensional precision of the iron castings. Under the condition that the wall thickness on different sides of the casting is the same, when the wall thickness is greater than 10 mm, the heat storage capacity of the moulding sands from strong to weak is steel shot sand, zircon sand, chromite sand, ceramic foundry sand, and silica sand. Tensile strength of the obtained castings from high to low is zircon sand, chromite sand, steel shot sand, ceramic sand, and silica sand. Contraction rate of the obtained castings from high to low is steel shot sand, zircon sand, chromite sand, silica sand, and ceramic sand. Therefore, steel shot sand and zircon sand can be used as chilled sand, and even can be used instead of cold iron when the casting wall thickness is greater than 10 mm. Zircon sand and chromite sand can be used to obtain high mechanical properties, and silica sand and ceramic sand can be selected to obtain high dimensional precision of the castings. Finally, a typical iron casting piece was tested by experiment using the hybrid moulding process. Excellent performances of iron castings confirm the feasibility of the hybrid moulding process.     

The Influence of Section Size and Base Sulphur Content on the Structure of Compacted Vermicular Graphite Cast Iron Produced by the Inmold Process

Abstract

The paper discusses the effect of certain variables involved in the production of compacted graphite cast iron by the Inmold Process,? using a 5% Mg-FeSi alloy, such as sulphur content of the base iron, copper content and section size. Experimental results show that acceptable compacted graphite irons can be produced in castings with section sizes ranging from 12.5 to 50 mm. The required residual Mg was a function of the base sulphur as follows: 0.013–0.021 % Mg for a base sulphur of 0.0083–0.0092% and 0.019–0.024% Mg for a base sulphur of 0.017–0.019%. When the base sulphur was 0.025–0.030%, it was impossible to produce compacted graphite microstructures. As expected, copper additions increased the amount of pearlite in the microstructure. For example, a 0.42% Cu content resulted in microstructures containing more than 50% pearlite in sections of 12.5 and 25 mm.     

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.     

Effects of nodularity on thermal conductivity of cast iron

Abstract

The thermal transport properties of five predominately pearlitic grades of grey, compacted graphite and spheroidal graphite iron have been investigated by the laser flash technique. Samples have been taken from cylinders cast in controlled thermal environments designed to produce three dissimilar cooling rates. Digital image analysis has been utilised in order to characterise the different graphite morphologies. The results indicated linear relationships between the thermal transport properties and the roundness of the graphite and the nodularity for compacted graphite and spheroidal graphite iron. A pronounced decrease in the thermal conductivity occurred when the lamellar graphite structure was transformed into compacted graphite. The thermal conductivity of compacted and spheroidal graphite iron has been recalculated with good accuracy over a temperature range of 25–500°C by means of regression analysis.     

Numerical modeling of the gas evolution in furan binder-silica sand mold castings

Modelling of gas evolution during sand-mould castings is one of the most important technical and environmental issues facing the metal casting industry. The current effort focused on developing the capability of numerically predicting the gas evolution for the furan binder-silica sand system. Specifically, the decomposition of furan was experimentally analyzed and then predicted based upon the work developed in the current project. This methodology can be easily implemented into existing commercial casting codes. A parametric study was also performed for steel 4340 and aluminium A356 cylinders (D100 × H200 m) and bars (H50 mm × W50 mm × L250 mm) cast into silica sand moulds (furan binder) of 50-mm mould wall thickness to investigate the effects of superheat and heating/cooling conditions of the mould on the gas evolution. Such information would enable more technically and environmentally friendly decisions to be made concerning the process design used to make a given casting.     

Influence of cooling rate and antimony addition content on graphite morphology and mechanical properties of a ductile iron

Cooling rate and inoculation practice can greatly affect the graphite morphology of ductile irons.In the present research,the effects of the cooling rate and antimony addition on the graphite morpholog...     

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.     

Volume Expansion of Compacted Graphite Iron Induced by Pearlite Decomposition and the Effect of Oxidation at Elevated Temperature

Engine cylinder blocks and heads, made of compacted graphite iron, are subjected to prolonged periods of cyclic heating and cooling. These conditions may give rise to the decomposition of the pearlite matrix accompanied by the formation of lower-density graphite and oxides, which will lead to an increase of material volume. The microstructural instability deteriorates the physical and mechanical properties of CGI and accordingly the thermal fatigue properties. In the present work it was shown that the extent and mechanism of volume change are drastically affected by the presence of an oxide atmosphere. It was found that after annealing under atmospheric conditions internal oxidation largely inhibited the progress of pearlite decomposition and therefore much smaller growth rates were obtained as compared to those observed under vacuum conditions in the dilatometer. After 16 h of annealing time at 700 °C in vacuum, the CGI samples exhibited 6 times faster growth kinetics as compared to annealing in open atmosphere.     

Effects of transition from lamellar to compacted graphite on thermal conductivity of cast iron

Abstract

Different levels of magnesium were added to a standard grey iron alloy in order to obtain a range of graphite morphologies from lamellar to compacted graphite. The thermal conductivity/diffusivity of samples, solidified at different cooling rates, was investigated by means of the laser flash technique. There is a significant decrease in the thermal conductivity as the morphology transits from lamellar to compacted graphite. The thermal conductivity of grey iron decreases considerably at elevated temperatures, whereas the thermal conductivity of compacted graphite iron is less sensitive to changes in temperature. At increased nodularities, compacted graphite irons exhibit a maximum thermal conductivity at ～400°C. The influence from the cooling conditions on the thermal conductivity decreases as the morphology alters from lamellar graphite to compacted graphite. The effective thermal conductivity of cast iron is modelled by means of existing models for composites.     

Effect of antimony on the eutectic reaction of heavy section spheroidal graphite castings

Abstract

There is a strong demand for heavy section castings made of spheroidal graphite with a fully ferritic matrix, e.g. for manufacturing hubs for windmills. Such castings with slow solidification process are prone to graphite degeneration that leads to a dramatic decrease of the mechanical properties of the cast parts. Chunky graphite is certainly the most difficult case of graphite degeneracy, though it has long been known that the limited and controlled addition of antimony may help eliminate it. The drawback of this remedy is that too large Sb additions lead to other forms of degenerate graphite, and also that antimony is a pearlite promoter. As part of an investigation aimed at mastering low level additions to cast iron melts before casting, solidification of large blocks with or without Sb added was followed by thermal analysis. Comparison of the cooling curves and of the microstructures of these different castings gives suggestions to understand the controlling nucleation and growth mechanisms for chunky graphite cells.     

Improving chill control in iron powder treated slightly hypereutectic grey cast irons

Recent studies revealed that in eutectic to slightly hypereutectic grey irons (CE = 4.3%-4.5%) the presence of austenite dendrites provides an opportunity to improve the cast iron properties, as a high number of eutectic cells are "reinforced" by austenite dendrites. An iron powder addition proved to be important by promoting dendritic austenite in hypereutectic irons, but was accompanied by adverse effect on the characteristics of potential nuclei for graphite. The purpose of the present paper is to investigate the solidification pattern of these irons. Chill wedges with different cooling moduli (CM = 0.11 - 0.43 cm) were poured in resin bonded sand and metal moulds.Relative clear / mottled / total chill measurement criteria were applied. Iron powder additions led to a higher chill tendency, while single inoculation showed the strongest graphitizing effect. The various double treatments show an intermediate position, but the inoculant added after iron powder appears to be the most effective in reducing base iron chill tendency, for all cooling moduli and chill evaluation parameters. This performance reflects the improved properties of (Mn,X)S polygonal compounds as nucleation sites for graphite, especially in resin bonded sand mould castings. Both austenite and graphite nucleation benefit from a double addition of iron powder + inoculant,with positive effect on the final structure and chill tendency.     

Effects of Cooling Rate on High-temperature Mechanical Properties of A356 Alloys

High temperature mechanical properties of A356 alloy castings under different solidification cooling rates have been studied and the influence of cooling rates on secondary dendrite arm spacing (SDAS) and mechanical properties has been discussed. To get different cooling rates, three different types of mold—green sand, green sand with chill and permanent mold, were used to pour castings which would subsequently be machined into tensile test and metallographic specimens. The temperature curves of castings’ solidification in three different mold were recorded using thermal couples, which would be used to calculate their corresponding cooling rates. Tensile tests were carried out at 20, 200, 300, 400 and 500 ℃ and then mechanical properties, such as tensile strength, yield strength, elongation, of specimens from different mold types at different test temperatures were obtained. And SDAS of different specimens were measured using optical metallographic photos. From integrated analysis of all those results, following conclusions could be reached. The relationship between SDAS and cooling rates is negative, and the quantitative relationship has been obtained through data fitting analyzing. Generally speaking, tensile strength and yield strength decease as the temperature elevates while elongation behaves in the contrary trend. Through the regression analysis of SDAS, mechanical properties and temperature, the relationship among them is obtained, which makes quantitative prediction of A356 alloy’s mechanical properties at different temperatures with different solidification cooling rates be possible.     

冷却速度对铝合金铸件气孔形成的影响

为研究冷却速度对铝合金铸件气孔形成的影响,分别采用潮模砂型、树脂砂型和金属型浇注壁厚10mm、25mm、40mm的铸件,观察截取试样的气孔分布。试验结果表明,冷却速度较慢的铸件气孔数量较多,形状不规则、大小不一,且气孔分散程度大;冷却速度较快的铸件气孔数量少,形状为小圆形且集中分布。潮模砂型铸件因砂型中的水分增加了铝液中的含氢量,铸件中的气孔数量较多;树脂砂型铸件因其凝固时间长,形成的氢气在铝液中溢出一部分,减少了铸件中的气孔量;金属型厚壁铸件由于成分过冷严重,铸件中心部位的气孔数量比边缘部位多。     

造型与配砂车间改造项目设计

介绍了公司原造型和配砂车间的铸造设备和生产能力,为实现十二五增能改造项目,对造型和配砂车间进行大规模的改造:采用1条全自动湿型砂水平分型(有箱)静压造型线,集散型造型线电控系统;采用2台英国克莱德曼多功能机器人用于冷却后的铸件分拣;采用1台美国某公司气压保温浇包,主机采用转子式混砂机,间歇式混砂,并配置一套独立的型砂在线检测装置,为保证向新增静压造型线提供优质型砂,还配置砂冷却单元,同时对辅助设备也进行了新增与改造,并提出了项目实施过程中的注意事项。     

大型复杂薄壁铝合金铸件的复合型铸造

采用金属型、呋喃树脂砂、石墨砂等复合铸型,成功生产了大型、薄壁、复杂铝合金铸件。     

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.     

Effects of the Presence of Water Vapour on the Oxidation Behaviour of Low Carbon–Low Silicon Steel in 1 %O2–N2 at 1073 K

The effect of water vapour additions in the range of 2.5–24.8 % on the oxidation behaviour of a low carbon and low silicon steel in 1 %O₂–N₂ at 1073 K (800 °C) was examined. It was found that the characteristic of steel oxidation was completely changed by addition of water vapour in the atmosphere. First, the kinetics was changed from non-parabolic to parabolic. Second, the scale formed in 1 %O₂–N₂ for 30 min or longer was easy to spall upon cooling whereas the scales formed in the water-vapour containing atmospheres did not spall easily. Third, additions of 2.5–10 % of water vapour in 1 %O₂–N₂ resulted in the formation of numerous depressed locations in the scale, but the scale-steel adherence in the areas surrounding the depressed locations was very much strengthened. Finally, additions of 17.2 and 24.8 % of water vapour in the atmosphere prevented both scale spallation upon cooling and formation of depressed areas in the scale. The mechanisms of forming various scale structures and the roles of water vapour additions under different conditions were discussed.