R. Dańko

Microstructure and Properties of Ductile Iron and Compacted Graphite Iron Castings

This book provides an overview of the surface effects at the interface boundary of metal/sand moulds, and their influence on the surface quality, microstructure and mechanical and anticorrosive properties of high-quality cast iron. It explores utilitarian aspects of the production of high-quality cast iron castings, including thin-walled castings of high-quality cast iron alloys, and examines problems related to the determination of moulding sands and reclaim quality, and their influence on castings. Presenting new material, this book takes into account the influence of metal quality, pouring temperature, solidification time, the quality of moulding sand with the reclaim application, as well the binders of moulding sands, on the formation of the degenerated graphite near surface layers. It also employs the latest research methods, such as a wavelength-dispersive spectrometer (WDS) analysis and thermodynamic calculations, which were carried out on the reactions occurring in the study area. Providing a valuable resource to academics and researchers interested in materials science, metal casting and metallurgy, this book is also intended for metal industry professionals

Effect of Reclamation on the Skin Layer of Ductile Iron Cast in Furan Molds

The paper presents the results of investigations of the influence of the quality of molding sand with furan resin hardened by paratoluenesulfonic acid, on the formation of microstructure and surface quality of ductile iron castings. Within the studies different molding sands were used: molding sand prepared with fresh sand and molding sands prepared with reclaimed sands of a different purification degree, determined by the ignition loss value. Various concentrations of sulfur and nitrogen in the sand molds as a function of the ignition loss were shown in the paper. A series of experimental melts of ductile iron in molds made of molding sand characterized by different levels of surface-active elements (e.g., sulfur) and different gas evolution rates were performed. It was shown that there exists a significant effect of the quality of the sand on the formation of the graphite degeneration layer.

Influence of a Reclaimed Sand Addition to Moulding Sand with Furan Resin on Its Impact on the Environment

Metalcasting involves having a molten metal poured in a hollow mould to produce metal objects. These moulds are generally made of sand and are chemically bonded, clay-bonded, or even unbounded. There are many binder systems used. Binders based on furfuryl resins constitute currently the highest fraction in the binders no-bake group. Moulding sand, after knocking out the cast, is partially reclaimed, and the remaining part, known as waste foundry sand is used or stored outside the foundry. In this case, the environment hazardous organic compounds and metals can be leached from the moulding sand, thus causing pollution of water and soil. Also during the casting moulds with molten metal, they emit pyrolysis gases containing many different compounds, often dangerous from the BTEX and PAH group, which has adverse impacts on the environment and workers. The article presents the results of research on the impact of the regenerate addition to the moulding sand matrix on emitted gases and the degree of threat to the environment due to leaching of hazardous components. Therefore, for the total assessment of the moulding sands harmfulness, it is necessary to perform investigations concerning the dangerous substances elution into the environment during their management and storage, as well as investigations concerning emissions of hazardous substances (especially from the BTEX and PAHs group) during moulds pouring, cooling, and casting knocking out. Both kinds of investigations indicated that reclaimed sand additions to moulding sands have significantly negative influence on the environment and working conditions.

Molds and Cores Systems in Foundry

Sand binding systems can have a significant impact on the nature of the casting skin formation. In particular, the binder containing elements such as S, O and N may adversely affect the structure of the layer. As in the case of spheroidal graphite cast iron (SGI) and compacted graphite cast iron (CGI) main factor causing the degeneration of graphite in the surface layer of the casting is sulfur, therefore these binding systems (binder) which contain sulfur have been thoroughly discussed here. The following are sand mold technologies: furan, acid catalyzed, phenolic, acid catalyzed, hot box, warm box and Shell (Croning) process. Sand molding with the use of furfuryl resins technology is presented in detail due to their widespread use in casting both cast iron and cast steel. To reduce the thickness of the surface layer, which may be the adverse effect of sulfur on the degeneracy of the graphite, S content in molding sand should be less than 0.15 % mass, and even below 0.07 % mass. Sand binding systems can have a significant impact on the nature of casting skin formation. In the case of green sand, moisture promotes the formation of the ferritic rim (Reisener, Br Foundryman 55:362–369, 1962; Matijasevic et al. Trans AFS 82:571–622, 1974; Narasimha and Wallace, AFS Trans 83:531–550, 1975). Research carried out for sand mold with sodium silicate and phenolic urethane has shown that SGI and CGI castings made in the first sand mold is pearlitic rim occurred, and in the second sand mold this occurrence is not found (Boonmee and Stefanescu, Foundry Trade J 186:225–228, 2012). Regarding the effect of the molding sand on the nature of the casting skin formation, they can be divided into molding sand: with binders containing sulfur (i.e. furfuryl alcohol and urea-formaldehyde resin) and the molding sand that are not bound with binders not containing sulfur (i.e. phenol-urethane resin ). From the point of view of the top layer the sulfur-containing molding sand is much more important, due to its adverse effect on the formation of spheroidal graphite.

Effect of Different Molding Materials on the Thin-Walled Compacted Graphite Iron Castings

This article addresses the effects of six mold materials used for obtaining thin-walled compacted graphite iron castings with a wall thickness of 3 mm. During this research, the following materials were analyzed: fine silica sand, coarse silica sand, cerabeads, molohite and also insulated materials in the shape of microspheres, including low-density alumina/silica ceramic sand. Granulometric and SEM observations indicate that the sand matrix used in these studies differs in terms of size, homogeneity and shape. This study shows that molds made with insulating sands (microspheres) possess both: thermal conductivity and material mold ability to absorb heat, on average to be more than five times lower compared to those of silica sand. In addition to that, the resultant peak of heat transfer coefficient at the mold/metal interface for microspheres is more than four times lower in comparison with fine silica sand. This is accompanied by a significant decrease in the cooling rate of metal in the mold cavity which promotes the development of compacted graphite in thin-walled castings as well as ferrite fractions in their microstructure.

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.

Influence of the Metal/Mold Processes on the Casting Mechanical Parameters and Corrosion Resistance

This chapter presents the issue related to the morphology of graphite, namely, flake/compacted/spheroidal graphite that is the main controlling factor of the mechanical and physical properties of cast irons. It is presented that flake graphite in the surface layer can be regarded as a local source of stress concentration and thus the crack growth resistance of the cast iron with flake graphite is extremely poor. It has also been shown that the presence of flake graphite in the surface layer has a significant influence on the mechanical properties of spheroidal graphite cast iron (SGI) and compacted graphite cast iron (CGI), including firstly its plasticity, which decreases with the increasing thickness of the flake graphite layer. Moreover it was shown that the presence of flake graphite makes the surface layer brittle, non-plastic, which results in forming cracks. No flake graphite in the surface layer causes the specimen to deform uniformly, providing the most favorable combination of mechanical properties. Finally, the results of SGI iron corrosion resistance, performed in 0.1 M sodium chloride solution, are shown. It has been indicated that the presence of a thick layer of flake graphite in the surface layer of the castings significantly increases the cathode current, which stimulates the cathodic reaction and oxygen reduction.

Methods of the Mold Sands Quality Assessment

This chapter on methods for assessing the quality of the matrix and molding sand presents the most important properties of these materials, from the point of view of the feasibility of using these molding sands for producing sound castings. In the case of matrix testing (both fresh and reclaimed), attention was drawn to the size and shape of the grain, which have a decisive influence on the permeability of the molding sand, on the content and the activity of the binder for the matrix used for the preparation of green sand, loss on ignition and the chemical nature of the matrix evaluated on the basis of the parameters such as ADV and pH. The next section presents complementary research methods of molding sand properties with reclaimed sand. Modern systems require evaluation and a careful approach to the factors that determine molding sand suitability for both technological use as well as the impact on the surrounding environment. In this context, a full evaluation of the quality of the reclaimed matrix goes beyond the usual instrumental methods and requires the addition of advanced apparatus research methods. This allows one to put the evaluation of the reclaimed matrix into perspective, both in terms of its use for the preparation of molding sand, as a substitute for fresh sand, and also in terms of environmental protection. Such modern methods may include, inter alia, an assessment of gas excretion in molding sand, identification of the type of emitted gases, the content of BTEX gases that are particularly dangerous to humans, and the evaluation of the surface of the casting made in molding sand with reclaimed sand.

Reclamation of Used Molding Sands

In this chapter, the main attention is focused on the theoretical and practical aspects of the reclamation of the molding and core sands. The definition and methods of reclamation characteristics for the initial and proper processes associated with the basic operations of the matrix from spent molding sands are shown. Within the theoretical description of the mechanical reclamation process the occurrence of these so-called elementary operations were highlighted, that result in the removal of waste of binding materials from the surface of the matrix (sand grains), and which include friction, abrasion and crushing. The next section discusses the most common, practical solutions for reclaimers used in methods for the dry reclamation—the vibrating reclaimer used for reclaiming the molding sands matrix, which has a good susceptibility to reclamation of spent furan sands, the pneumatic reclaimer, which is used to reclaim the molding sands matrix that are harder to reclaim, and the thermal regenerator, used in the case of the reclamation of molding sands with organic binders, where the effect of the removal of the binding material from the grains is up to 100 %. It also presents methods for combined pneumatic-thermal-pneumatic reclamation, used mostly for the reclamation of various sands from different technologies. The so-called unconventional methods of reclamation are mentioned at the end, where mechanical-cryogenic reclamation performed at about −80 °C is discussed.

Phenomena Model on the Mold/Casting Interface

Good quality casting that meets the customers requirements depends on many factors. Generally, these can be classified into two groups, related to metal and the casting mold. However, it may happen that the metal meets all the criteria and the mold is properly made but the quality of the casting can be unsatisfactory. The reason for this may be the phenomena on the interface mold/casting, which are not always taken into account. Processes taking place there are mainly due to the quality of the casting surface, which is very important; especially in the case of castings whose surface is exposed directly to adverse factors (wet corrosion, dry corrosion, wear, etc.). However, processes taking place on the interface mold/casting may also adversely affect the mechanical properties of the casting, e.g., tensile strength, fatigue limits, machinability. This is caused by the degeneration of nodular and compacted/vermicular graphite, or decarburization of the casting surface layer, or the appearance of areas of pearlitic/ferritic rim. Several mechanisms are responsible for the formation of abnormal surface layer on the CGI and SGI castings, which are associated with a reduction in the concentration of either Mg (reaction with S and/or O) or the cooling rate of the casting (formation of austenite layer).