M. Holtzer

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.

The influence of motorisation on the climate warming

The automotive industry is under continual pressure because of the harmful effects of cars on the environment. Highly developed countries must implement programs related to the efficient use of fuel. One way to do this is to reduce vehicles weight, where an important role is played by the foundry industry, searching for new construction materials, with a lower specific gravity and suitable mechanical properties. The paper discusses the materials for the automotive industry: spheroidal graphite cast iron (SGI), compacted graphite cast iron (CGI), grey cast iron (FGI), aluminium alloys and magnesium alloys, comparing their functional properties, important from the point of view of their use as automotive castings. An assessment was performed of the impact of the aforementioned casting materials on the environment based on life-cycle analysis of the product.

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.

The Influence of Mold/Metal Interactions on the Castings Microstructure—Bibliographical Research

The phenomena on the mold/metal interface largely determine the microstructure of the casting surface layer, quality of the surface and casting performance characteristics (mechanical properties, wear resistance , corrosion resistance, etc.). Processes that occur here are of particular importance in the case of cast iron with vermicular or spheroidal graphite performed in sand molds (graphite degradation and graphite depletion ). It is assumed that the direct cause of graphite degradation , resulting in a layer of flake graphite, is the Mg depletion at the surface layer of the casting and the main reason (although not the only) for the decrease of the Mg concentration is the reaction of sulfur and oxygen. To explain this phenomenon, Fe–Mg, Mg–S, Mg–O and Fe–S systems have been discussed. Performed thermodynamic calculations have shown that the magnesium and sulfur and oxygen reactions in liquid iron occur in the gas phase, wherein the thermodynamic reaction of Mg vapors with oxygen gas is more privileged than with the sulfur vapors. To clarify the mechanism of the formation of the surface layer on the casting Stefanescu et al. developed a thermo-diffusion model 2-D. However, this model does not take the kinetic agent and the degree of undercooling during solidification of the casting into account, which affects the amount of primary austenite formed, which may also result in a reduction of the Mg concentration. To reduce the degree of transition of sulfur from molding sand to casting surface layer, the use of protective coatings for molds and cores, containing desulfurization ingredients such as CaO, MgO, talc, or Mg-FeSi, which binds sulfur and supplements magnesium at the same time is preferred.

The Mold/Casting Interface Phenomena

Many of surface defects in casting are caused by interaction of the metal with the molding sand. These phenomena occurring on the mold/cast interface can be divided into three groups: metal penetration (mechanical, chemical, vapor state, water explosion penetration and eutectic exudation penetration); transition of elements from the molding sand into casting and from the casting into the mold and gas generation by thermal decomposition of the binder and/or coating. Transition of the casting elements such as P, N, H, C, S, Si from the molding sand to the surface layer of the casting and elements such as Mn, C from casting to the molding sand is also possible. Frequently, this can cause changes in the chemical composition of the surface layer of the casting, which may consequently lead to changes in the casting microstructure and properties. Gases emitted from the molding sand, core or protective coating during pouring liquid metal are often due to the cause of the so-called gas porosity. The rate of the evolution of gases from the mold and cores, and their volume depends strongly on the binder used. Air present in the mold cavity may also be the cause of porosity.

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).