Metallurgical parameters,mechanical properties and machinability of ductile cast iron

Results on the effect of solidification cooling rate on the microstructure, mechanical properties and machinability of spheroidal graphite (SG) iron have been presented. The effect of ferritic heat treatment on the same properties has been also investigated. The microstructural observation, tensile properties and hardness values of the present SG iron has been developed. The tool life criterion was used as a measure of machinability. It was found that during turning of SG iron by using a single point cutting tool, its life increased with decreasing the solidification cooling rate for both sand and metal moulds. The tool life was found to be significantly affected by the variation of nodule characteristics. A decrease in tool life due to an increase of nodule count was observed. The tool life was found to be directly proportional to the ductility of SG iron whether for the as cast or ferritic heat-treated ingots.     

Cooling slope casting of cast iron

The aim of the present work is to study the effect of semi‐solid processing using cooling slope method on the microstructure and mechanical properties of grey cast iron. A water cooled castable cement coated steel plate with a variable inclination angle is used to make a mechanical treatment on the semi‐solid flow of grey cast iron. The experiment is repeated for three different pouring temperatures and three inclination angles. The tensile test, hardness test and microstructure examination is used to compare the new properties of the new resulting grey cast irons. The results show that the sharp edged carbon flakes can be refined and the mechanical properties is enhanced. The carbon particles average circularity is increased followed with a decrease in carbon particle size due to the effect of the cooling plate.     

Influence of chunky graphite on mechanical and fatigue properties of heavy-section cast iron

This research work aimed to find out the influences of the different amounts of chunky graphite on mechanical and fatigue properties of GJS 800 ductile cast iron. Chunky graphite has been a problem of heavy section thick-walled ductile cast iron components. Chunky graphite is branched and interconnected as a network within eutectic cells and has been observed to form in thermal centres of heavy ductile cast iron sections during solidification. This research work proved that chunky graphite in the microstructure decreases the ultimate tensile strength, the elongation to fracture and fatigue life significantly, but does not influence on the yield stress of ductile cast iron GJS 800 substantially. Low nodular count and nodularity rate also decrease the fatigue life of ductile cast iron, and the difference of fatigue life of specimens containing chunky graphite or having low nodular count and nodularity rate is not large. Influence of the amount of chunky graphite on fatigue life increases until 20% chunky graphite content, and above that the fatigue life does not decrease substantially.     

球墨铸铁表面电弧增材制造Fe‒Cr合金结合区组织和性能

目的 在球墨铸铁基体上电弧增材制造Fe-Cr合金,研究结合区组织和性能,以期获得具有良好冶金结合、满足冲裁模具性能要求的双金属构件。方法 采用GMAW工艺增材制造,用金相显微镜和扫描电子显微镜表征结合区的显微组织,并分析其形成机制。结果 Fe-Cr合金与球墨铸铁结合区无明显裂纹和气孔,其凝固组织为柱状晶和等轴晶,冷却后转变为马氏体和残余奥氏体,但其分布不均匀,在界面处有一富奥氏体层。结合区内球墨铸铁受热影响发生奥氏体化和部分熔化,熔化发生在临近结合界面的石墨球周围,其冷却后形成一层马氏体和一层莱氏体的双层壳型组织结构,未熔化部位的组织为马氏体和铁素体,珠光体球墨铸铁比铁素体球墨铸铁形成的马氏体多。结合区内硬度分布不均匀,球墨铸铁的硬度从基材到结合界面逐渐升高,最高达630HV,Fe-Cr合金平均硬度为510HV。结论 电弧增材制造Fe-Cr合金与球墨铸铁基体冶金结合良好,Fe-Cr合金组织为马氏体和残余奥氏体,有较高的硬度,能满足冲裁模具的性能要求。     

Cooling rate inference in aluminum alloy squeeze casting

An indirect method for the evaluation of cooling rate of a squeeze cast aluminum alloy is proposed. Cooling rate is inferred by simple local mechanical tests such as microhardness or indentation. A predictive model is used to correlate cooling rate and mechanical properties. An aluminum alloy (EN-AB46000) was squeeze cast. Four cylindrical samples were fabricated at different values of the squeezing pressure (from 0 to 100 MPa). Samples were cut longitudinally at the middle height and microhardness was measured along the sample radius as well as dendrite size. Moreover yielding stress was evaluated by means of FIMEC test in the centre of the sample section. Mechanical properties can be related to dendrite size and used for cooling rate inference.     

Temperature dependence of tensile strength and hardness for nodular cast iron and their mutual correlation

Nodular cast iron has mechanical properties which make it superior to relatively brittle pig cast iron. As a matter of fact, by using appropriate heat treatment processes, the tensile strength of nodular cast iron can be improved to such a degree that its hardness corresponds to that of carbon steels. The main aims in this study are to find the most preferable heat treatment conditions which will yield high strength levels, and to clarify the temperature dependence of mechanical properties for nodular cast iron. The estimation of tensile strength from hardness is also discussed, since tensile tests at elevated temperatures are usually more expensive and time consuming than the hardness tests. Nodular cast irons, having four different microstructures were first prepared by performing the following heat treatments: (1) as-cast, (2) annealed, (3) normalized and (4) bainitized. Tensile property and hardness were then measured for the respective cast irons under elevated temperatures. The temperature dependence of the tensile strength as well as hardness was investigated. It was found that the dependence was well represented by an expression of =₀exp(–BT). Thus results were discussed from a view-point of the reaction rate process. The correlation between tensile strength and hardness was also examined and a significant linearity was found between them. Based on this strict correlation, an estimation procedure of the tensile strength was finally proposed.     

耐热球墨可锻铸铁

耐热球墨可锻铸铁是一种新型的耐热铸铁，具体工艺是在中硅耐热球铁铁水中加入适量合金元素及变质剂，使之在铸态下得到白口组织，再经过石墨化退火而得到球团状石墨。该铸铁胶中硅耐热球铁有更为良好的常温力学性能及高温抗氧化性。尤其重要是的，因其凝固与结晶方式发生了质的改变，铸铁的缩松倾向得以明显改善。     

改性纳米SiC粉体强化球墨铸铁的组织和力学性能研究

在生产条件下用改性纳米SiC粉体对球墨铸铁进行了强韧化处理,研究了不同的纳米SiC粉体加入量对球墨铸铁组织和力学性能的影响.结果表明,经改性纳米SiC粉体强韧化后,球墨铸铁中的石墨球尺寸减小,圆整度提高,铁素体含量增多,球墨铸铁的韧性提高.断口分析结果表明,经过强韧化处理后,球墨铸铁的断裂方式为脆性韧性混合断裂.     

Surface reactivity of thin wall ferritic ductile iron. The effect of nodule count and grinding variables

Thin wall ductile iron (TWDI) castings constitute an attractive alternative to several applications in which the strength to weight ratio becomes a key design variable. In TWDI, the nodule count for a given chemical composition is highly dependent on cooling rate during solidification, and hence on thickness.For mechanical parts, where accurate dimensional tolerance is mandatory, the most common machining process applied is grinding. This process induces significant temperature gradients and surface plastic deformations which could affect service performance, particularly in corrosion environments.In TWDI, surface properties become more relevant due to the high surface area to volume ratio. Therefore, the aim of this paper is to determine the effect of nodular count and grinding conditions on surface reactivity of ferritic TWDI. Electrochemical assays were carried out in a three-electrode cell in a 3.5 wt.% NaCl solution on both polished and ground samples. The results obtained indicate that surface reactivity increases with higher nodular counts and residual plastic strain.     

改善转8A贝铁斜楔组织与性能的探讨

对转8A贝氏体球墨铸铁斜楔生产中出现的球化率和渗碳体不合格问题进行了分析,并从铸造和热处理工艺方面探讨了影响材料性能因素及改善和提高材料性能的途径。试验结果表明,探讨碳当量和球化剂加入量,采用瞬时孕育等可提高球化率;采用预热砂型等可减少渗碳体。改进热处理工艺后材料的力学性能得到了改善。     

Fracture mechanics behaviour of austenitic compacted graphite cast iron

The fracture mechanics behaviour of high-nickel austenitic compacted graphite cast iron was studied and the effects of graphite morphology, alloying elements and specimen thickness on the mechanical properties, plane stress fracture toughness, and fatigue crack growth rate were evaluated. It was found that the graphite morphology, i.e. the percentage of compacted graphite present, was the major determinant of all properties of the materials investigated. The irons with a greater amount of compacted graphite (the balance was nodular graphite in austenitic matrix) resulted in lower tensile strength, yield strength, elongation and K _c fracture toughness but higher crack-growth index values (poorer crack-growth resistance). For 25 mm thick specimens, K _c values of the austenitic compacted graphite cast irons in this study were in the range of 58–64 MPa m^1/2. This is higher than ferritic/pearlitic ductile iron of 43–53 Mpa m^1/2, and is compatible to Ni-resist austenitic ductile iron of 64.1 Mpa m^1/2. The addition of cobalt not only contributed to slightly higher values of mechanical properties, but also higher plane stress fracture toughness and better crack growth resistance. Optical microscopy, scanning electron microscopy and X-ray diffraction techniques were applied to correlate the microstructural features to the properties attained.     

Modelling microstructural and mechanical properties of ferritic ductile cast iron

Abstract

It is well known that the mechanical properties of ductile cast iron (DCI) depend on its microstructure, and that the microstructure depends on the properties of the melt and the cooling conditions during casting. There have been many studies of the individual elements of the process of casting DCI, but as yet there have been very few examples of modelling the entire process to predict cooling rates, microstructure, and mechanical properties, particularly for large castings. The present paper describes a method of modelling the microstructural and mechanical properties of ferritic DCI, and applies the methods to the case of a large (13 t) thick walled (300 mm thickness) casting. The microstructure calculated includes nodule count, nodularity, ferrite grain size, and percentage ferrite. The mechanical properties calculated include yield stress, tensile strength, elongation, and static upper shelf fracture toughness (J _1C and K _JC). The calculated results compare well with those of a test casting.     

Influence of carbon diffusion on microstructure and wear behaviour of duplex stainless steel surface layers on lamellar grey cast iron

Surface welding with duplex stainless steel was performed to enhance the wear and corrosion properties of grey cast iron, which is used as material for applications as pump components in maritime and chemical environments. The method used for surface welding and the corresponding process parameters determine the chemical composition and microstructure, which both determine the corrosion and wear properties of the surface layer. High heat input leads to high chemical dilution and thus, reduced corrosion resistance. Slow cooling rates, which are recommended for welding of grey cast iron components, facilitate the formation of carbides in the fusion zone of the chromium‐rich duplex stainless steel surface layer. On the one hand, carbides lead to increased hardness and thus, improved wear resistance of the surface layers. On the other hand, carbides and high chemical dilution rates reduce the corrosion resistance and therefore should be avoided. Under high cooling rates, the risk of cracking in the heat affected zone of the grey cast iron increases due to martensitic phase transformations. The paper describes the correlation of process parameters, microstructure and chemical composition with a focus on carbon diffusion and carbide formation, ever considering the effect on the wear behaviour in an oscillation tribometer and under erosion‐corrosion conditions.     

厚断面球铁的声速

论述了影响厚断面球铁声速和声波衰减的各种因素。指出，必须在应力状态下同时考核声速和声波衰减，以作为判别厚断面球铁材质的一种手段。     

Solidification structures and mechanical properties of Zn–27Al alloy cast in metal moulds

Abstract

Rectangular and round cross-section test bars of Zn–Al alloy containing (wt-%) 27·67%Al; 2·30%Cu; 0·13%Mg; 0·04%Fe; <0·0025%Pb, Cd, and Sn; balance Zn were cast in metal moulds. The variables studied were: bar cross-section 3–25 mm thickness and 14·5–28 mm dia., pouring temperature 550–700°C, feeder location at bar shoulders or over full gauge length, and cast iron mould preheating temperature 100 or 200°C. The results show that the above variables can lead to variations of 0·14–1·72% bar porosity, 18–0·5% elongation, 428–305 MN m^?2 tensile strength, and 108–128 Brinell hardness. Metallographic examination leads to the conclusion that these properties are influenced by the volume, distribution, and morphology of shrinkage porosity and of eutectic constituent. The observed variations in the microstructure and their effects on the mechanical properties are related to the cooling and feeding conditions of the bars.

MST/549     

Influence of modification, solidification conditions and heat treatment on the microstructure and mechanical properties of A356 aluminum alloy

The effects of casting thickness, modification and heat treatment on the microstructure and mechanical properties of A356.2 alloy have been investigated. Experiments were conducted with unmodified, Sr-modified (0.02% Sr) and Sb-modified (0.2% Sb) on both sand cast test bars with various thicknesses (from 3 to 9 mm) and permanent mold cast test bars.The microstructural changes associated with these treatments have been studied by optical metallography, scanning electron microscopy (SEM) and image analysis.The tensile properties of all samples were determined and the relationship between cooling rate, modification and heat treatment has been investigated.The results show that modification has a beneficial effect on microstructure and improves the mechanical properties of the alloy. Modification has a major role in controlling the kinetics of the spheroidisation of silicon particles during heat treatment. Tensile properties improved more with heat treatment than with modification or cooling rate.Antimony is effective on mechanical properties at higher solidification rates, while Strontium is more effective at lower solidification rates.     

Cast Iron Casing Cracking Due to Chunky Graphite Formation

Quality inspection of heavy turbomachinery casings made of ductile cast iron has revealed anomalies in the form of characteristic relief pattern on roughly machined surfaces. Detailed microstructural observations determined that this anomaly is related to degeneration of cast iron microstructure where undesired chunky graphite was formed instead of evenly dispersed graphite nodules. Mechanical tests showed that this microstructural alteration led to deterioration of mechanical properties, especially plasticity. Regularity was found as this problem affected mainly thick sections of casts, especially where the cooling rate was limited. This article discusses the consequences of chunky graphite formation, detection methods, acceptance criteria, and preventive actions.     

Analysis of mechanical properties and its associated fracture surfaces in dual‐phase austempered ductile iron

This work aims at evaluating the fracture surfaces of tensile samples taken from a new kind of ductile iron referred to as ‘dual‐phase Austempered Ductile Iron (ADI)’, a material composed of ausferrite (regular ADI microstructure) and free (or allotriomorphic) ferrite. The tensile fracture surface characteristics and tensile properties of eight dual‐phase ADI microstructures, containing different relative quantities of ferrite and ausferrite, were studied in an alloyed ductile cast iron. Additionally, samples with fully ferritic and fully ausferritic (ADI) matrices were produced to be used as reference. Ferritic–pearlitic ductile irons (DI) were evaluated as well. For dual‐phase ADI microstructures, when the amount of ausferrite increases, tensile strength, yield stress and hardness do so too. Interesting combinations of strength and elongation until failure were found. The mechanisms of fracture that characterise DI under static uniaxial loading at room temperature are nucleation, growth and coalescence of microvoids. The fracture surface of fully ferritic DI exhibited an irregular topography with dimples and large deformation of the nodular cavities, characteristic of ductile fracture. Microstructures with small percentages of ausferrite (less than 20%) yielded better mechanical properties in relation to fully ferritic matrices. These microstructures presented regions of quasi‐cleavage fracture around last‐to‐freeze zones, related to the presence of ausferrite in those areas. As the amount of ausferrite increased, a decrease in nodular cavities deformation and a flatter fracture surface topography were noticed, which were ascribed to a higher amount of quasi‐cleavage zones. By means of a special thermal cycle, microstructures with pearlitic matrices containing a continuous and well‐defined net of allotriomorphic ferrite, located at the grain boundaries of recrystallised austenite, were obtained. The results of the mechanical tests leading to these microstructures revealed a significant enhancement of mechanical properties with respect to completely pearlitic matrices. The topographies of the fracture surfaces revealed a flat aspect and slightly or undeformed nodular cavities, as a result of high amount of pearlite. Still isolated dimple patterns associated to ferritic regions were observed.     

Optimized cast components in the drive train of wind turbines and inner ring creep in the main bearing seat

Using large components made of nodular cast iron (GJS) in wind turbines enables the application of lightweight construction through the high degree of design freedom. Besides the sand-casting process, casting into a permanent metal mould, i.e. chill casting, leads to a finer microstructure and higher quasi-static mechanical properties as well as higher fatigue strength. Unfortunately, in present design methodologies specific fatigue data is only available for sand cast and not for chilled cast GJS. Thus, lightweight design strategies for large, chilled cast components are not achievable, which led to the publicly funded project “Gusswelle”. Based on material investigations of EN-GJS-400-18-LT chill cast, an optimized hollow rotor shaft is developed. The design process and the resulting shaft design are presented. The optimized hollow rotor shaft prototype will be tested on a full-scale test bench to validate the design methodology. The intended validation plan as well as the test bench setup is shown in this paper. Furthermore, the decreasing wall thickness influences the interference fit between main bearing and hollow rotor shaft. Thus, through the applied bending moment, inner ring creep is more probable to occur in the main bearing seat. The creeping behaviour is investigated with finite element simulations and a measuring method is presented.