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.     

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.     

Characteristics of cementite formation in the surface layer of flake graphite cast iron formed by plasma transferred arc remelting

Using the plasma transferred arc (PTA) process to conduct surface remelting, the surface region of flake graphite cast iron can be hardened. To study the effect of silicon content on the microstructure of the hardened surface layer, flake graphite cast iron specimen of about 3.1 ～ 3.2 wt pet carbon and various silicon contents from 1.47 to 3.41 wt pet were selected for the PTA surface remelting experiment. The remelting current and voltage were set at 100 A and 32 V. The results show a rapidly solidified microstructure with large amounts of eutectic cementite. Reducing the silicon content increases the volume fraction of eutectic cementite and thus increases the hardness. The increase of cementite volume fraction with decreasing silicon content is linear. Reducing the silicon content is also effective in suppressing the formation of porosities.     

Structural and operating aspects of rapid solidification of surface layer of spheroidal graphite iron castings

The relationship between the surface layer microstructure of iron castings after gas tungsten arc weld (GTAW) surfacing and their service properties (friction wear resistance and microhardness) has been investigated. A structural analysis of the fused (remelted) zone, in particular evaluation of the interphase spacing in the cementite eutectic λ_av, is presented. Relationships between the GTAW process parameters, microstructure and microhardness of the fused zone have been developed and are presented. Formulae relating hardness to λ_αv and relating friction wear rate to λ_av and hardness are presented. IJCMR/426     

Analysis of ductile iron production on steel scrap base

The cast iron production (melting) process is commonly based on pig iron. However, economic and ecological factors necessitate that the melting process is more often based on steel scrap (with no pig iron at all). This kind of cast iron is called synthetic cast iron. This approach may, however, cause a decrease in the quality of the alloys obtained, particularly when ductile iron (DI) is produced. The following article presents an analysis of the product and some aspects of ductile iron produced exclusively on a base of steel scrap. The experiments and the charging materials are described. The results of the chemical analysis of the produced DI, the carburisation efficiency and microstructure of the obtained material are presented as well. The results of cast iron melted on a pig iron base are presented for comparison. A quantitative microstructure analysis is also presented.     

Effect of thermomechanical processing parameters on phase transformation and hardness of dual matrix ductile iron

The microstructural changes and hardness exhibited by ductile iron with dual matrix structure (DMS) are investigated. In particular, DMS microstructures are obtained by continuous cooling in the (ferrite+austenite) region followed by quenching to transform the austenite into martensite or by austempering at 375°C, to transform the austenite into ausferrite. Additionally, two deformation steps are applied in the austenite region. The structure was produced in a thermomechanical simulator equipped with a dilatometry system. The dilatometry is used to monitor the structure development throughout the thermomechanical processes. The structure was investigated using light optical microscopy and scanning electron microscopy. The influence of introducing ferrite to the microstructure and the deformation magnitude on the structure development and hardness properties are explored.     

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.     

Distribution of some active elements in primary graphite precipitates

The distributions of cerium and oxygen in the matrix and in graphite precipitates of a pure Fe–C–Ce cast iron sample have been studied using scanning Auger microscopy. It is shown that there is no accumulation of any element at the graphite-matrix interface. Cerium was detected in some cases in spheroidal graphite precipitates, most often associated with oxygen. Various non-spheroidal graphite precipitates proved to contain cerium and oxygen suggesting a correlation between cerium content and graphite degeneracy.     

Effect of elongated graphite on mechanical properties of hot-rolled ductile iron

The mechanical properties of hot-rolled and then annealed ductile iron were evaluated. The deformation of this two-phase material and the effects of the elongated graphite spheres on the mechanical properties and the development of anisotropy of mechanical properties were studied. An attempt was made to describe the anisotropy of tensile strength in terms of the deformation of graphite spheres and of the root curvature, area fraction, and the interbridges that result in disproportional changes of stress concentration, loading capacity, and the tendency to break the interbridges and link the neighboring deformed graphite spheres.     

Fluidity and solidification microstructures of flake graphite cast iron in thin sections

Fluidity determinations have been undertaken using a silica tube as the fluidity channel, metal being drawn in to the test tubes by means of a regulated vacuum system. Fe-Si and Ca-Si inoculants were added to a hypoeutectic cast iron melt which was then tested for fluidity. A linear relationship between fluidity length and tube diameter was obtained. Fluidity length increased with increasing suction pressure and with increasing temperature of the molten iron. Fluidity length is proportional to the square root of effective suction pressure. Structures within the fluidity specimens have been classified, from the tip backwards, into three zones, containing ledeburite, mottled and flake graphite structures. The length of each structural zone increases with increasing suction pressure. The total length of the ledeburite zone, plus the mottled zone, is constant for increasing tube diameter. Thus, the flake graphite zone increases linearly with tube diameter. The effects of inoculation on the structure and its fading were evaluated by the length of the flake-graphite zone.     

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.     

Influence of austenitising conditions and aluminium content on microstructure and properties of ductile irons

The microstructure and mechanical properties of ductile irons after casting are dependent on the heat treatment conditions. The shape, size, morphology and distribution of phases are related to the heat treatment variables. An attempt was made to determine the optimum treatment conditions. The objectives of this investigation were to study the influence of austenitising temperature and time and aluminium content on the microstructure and mechanical properties. Austenitising was carried out at different temperatures and holding times for a variety of experimental irons to achieve sufficient homogeneity for further isothermal heat treatment. Microscopic observation, hardness and electron-probe micro analysis (EPMA) were used to follow the phase transformation.     

QT-500球墨铸铁表面激光熔覆镍基合金的组织与性能

利用DL-T5000型二氧化碳激光器在QT-500球墨铸铁表面熔覆镍基合金,分析了激光熔覆层的显微组织,测试了其显微硬度及磨损性能。结果表明:所制得熔覆层组织致密、无裂纹,与基体形成了良好的冶金结合。从熔覆层表面到基体热影响区,组织依次为大量的树枝晶、等轴晶、树枝晶。熔覆层的硬度较基体提高了5倍,熔覆层的总磨损率大约为基体的1/6。熔覆层耐磨性能增强的主要原因是镍基合金与涂层元素镍、铬等固溶强化和碳化物等析出相的强化作用。     

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...     

激光扫描速度对球墨铸铁热轧辊表面铁基合金化层组织性能的影响

采用激光合金化技术在球墨铸铁QT600-3表面制备铁基合金化层,采用扫描电镜(SEM)、X射线衍射(XRD)、Raman光谱仪、显微硬度计和高温摩擦磨损试验等方法研究了不同激光扫描速度对铁基合金化层物相、微观结构、力学性能、常温和高温摩擦学性能的影响。研究结果表明,铁基合金化层与基体冶金结合良好、显微硬度高(高达830 HV0.1)、高温摩擦因数低至0.28、高温磨损率低至2.41×10^-6 g·N^-1·m^-1。合金化层显微组织为奥氏体树枝晶+共晶碳化物,且随着扫描速度增加,组织逐渐细化,合金化层平均厚度减小,裂纹率升高,显微硬度先增加后减小,高温耐磨性能逐渐提高。铁基合金化层的高温磨损机制以磨粒磨损为主,同时还存在着疲劳磨损和氧化磨损。     

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.     

Effect of Bi on graphite morphology and mechanical properties of heavy section ductile cast iron

To improve the mechanical properties of heavy section ductile cast iron, bismuth （Bi） was introduced into the iron. Five castings with different Bi content from 0 to 0.014 wt.% were prepared; and four positions in the casting from the edge to the center, with different solidification cooling rates, were chosen for microstructure observation and mechanical properties test. The effect of the Bi content on the graphite morphology and mechanical properties of heavy section ductile cast iron were investigated. Results show that the tensile strength, elongation and impact toughness at different positions in the five castings decrease with a decrease in cooling rate. With an increase in Bi content, the graphite morphology and the mechanical properties at the same position are improved, and the improvement of mechanical properties is obvious when the Bi content is no higher than 0.011wt.%. But when the Bi content is further increased to 0.014wt.%, the improvement of mechanical properties is not obvious due to the increase of chunky graphite number and the aggregation of chunky graphite. With an increase in Bi content, the tensile fracture mechanism is changed from brittle to mixture ductile-brittle fracture.     

Influence of microalloying and heat treatment on the kinetics of bainitic reaction in austempered ductile iron

The effect of different contents of alloying additions and austenitizing temperature on the transformation kinetics of austenite in a ductile iron austempered at 300 and 400 °C has been investigated in the present study. X-ray diffraction, optical microscopy, and hardness measurements were used to determine the transformation kinetics of low Ni iron, low Mo iron and low Ni, Mo iron during austempering at 300 and 400 °C for 1 to 480 min after austenitizing at 850 and 930 °C for 120 min. Nickel and molybdenum in used contents are shown to delay the bainitic transformation without the undesirable features. Decreasing the autenitizing temperature is shown to increase the driving force for stage I of reaction but to have only a small effect on stage II kinetics. This shifts the position of the processing window to short periods of time and leads to opening of the processing window, which is closed for higher autenitizing temperatures. A more uniform austempered microstructure can be obtained with a decrease of autenitizing temperature. Decreasing the autenitizing temperature has the disadvantage of reducing the austemperability.     

消除风电轴承座铸件夹渣缺陷的铸造技术研究

轴承座为大型风力发电机组的重要零部件之一,属较为典型的球墨铸铁铸件,其在铸造生产过程中极易出现夹渣缺陷。对其夹渣缺陷形成机理进行理论分析,通过优化设计和实践验证形成有效的避渣浇注系统方案,消除铸件高动载荷区和轴承安装配合面的夹渣缺陷。     

Computer model for the characterisation of the size distribution of spheroidal graphite in ductile cast iron

A computer model was developed to establish the relationship between the two-dimensional (2D) and the three-dimensional (3D) parameters, specifically the number of particles and particle size distribution. The computer experiments were performed for both monodispersed and lognormally polydispersed systems. The model was based on a random distribution of a number of spherical grains in a cubic unit, with no intersection between the grains. The cubic unit was cut by a random plane and the number of particles which appeared in the section and the area fraction were measured. This procedure was repeated until the average number of particles and the average area fraction became constant. Finally, the 2D size distribution of the particles over all sections was obtained.

It was concluded that the volume fraction is equal to the area fraction, irrespective of the particle size distribution, providing that the total number of measured 2D particles is large enough. As for the number of particles, an equation was found to calculate the number of 3D particles in a monodispersed system from the number of 2D particles and the volume fraction. However, in lognormally polydispersed systems the number of 3D particles could be calculated using the 3D mean and standard deviation, estimated from the 2D mean and standard deviation by a method specifically developed for the purpose.

Finally, the method was applied to two ductile cast iron specimens and the applicability of the present model was verified.