In sand casting, the surface microstructure often differs from the bulk microstructure due to mold–metal interactions. In nodular and compacted graphite iron, these interactions manifest mostly by a variation in graphite morphology, often attributed to magnesium depletion due to the release of sulfur and oxygen from the molding materials. Additionally, previous research suggests that solidification time, surface condition, and metallostatic height also affect the degree of surface microstructure deviations, though the mechanisms remain poorly understood. Therefore, the present case study targeted an in-depth analysis of the casting skin phenomena in two cylinder blocks sourced from two different foundries to ascertain its occurrence, severity, and main characteristics. Interfaces with molding materials were evaluated under varying solidification times and metallostatic heights, and measurements of the degenerated graphite layer (DGL) thickness and nodularity were performed; average surface roughness was estimated through image analysis. Correlations between these variables and the DGL thickness were assessed. Nital and Motz’s etchants were also employed to examine the DGL characteristics in greater detail, while scanning electron microscopy and energy-dispersive x-ray spectroscopy further explored the presence of particles at the mold–metal interfaces. Results revealed a wide range of DGL structures without a clear correlation to the studied process variables. In fact, some trends observed were opposite to those reported in the literature. A deeper understanding of the key mechanisms is still needed as the casting skin continues to challenge foundries in daily production.

Porosity remains a persistent defect in cast iron components, posing challenges in quality and reliability. Although experts often infer causes from a pore's size, location, and surface features, these indicators are not sufficiently well known to identify the root mechanism consistently. This study introduces a novel experimental method to systematically correlate the type of gas and melt, the timing of pore formation, and its surface characteristics-an approach not typically addressed in conventional foundry analyses, but that could prove to be relevant to this practice. For this purpose, cast iron cylinders were melted under an inert atmosphere, where argon was carefully injected into the melt to form a stable bubble at constant pressure. Once fully solidified, the pore surfaces were examined via scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX). Key observations include the formation of graphite films in both hyper- and hypoeutectic melt conditions, although the dendrite tips were uncovered by graphite. Under an inert atmosphere, this indicates that film growth occurs directly from the liquid metal when graphite is stable, which implies external gases are, at the very least, not the sole driver behind graphite film growth. These insights contribute to a more systematic understanding of pore surface characteristics and their relationship to solidification conditions. The methodology presented has the potential to be extended to active gases, different cast alloys, and broader solidification conditions.

The aim of the present study is to elucidate the influence of individual microstructural parameters, such as pearlite fraction, nodularity, and eutectic cell size, on the tensile strength (UTS) of cast irons. The UTS model was built by integrating the rule of mixtures for each microstructural component, and the UTS was described as a function of the aforementioned factors. The UTS and the required microstructure parameters for the model calculation were obtained experimentally. In the model, two coefficients were introduced to quantify the influence of the eutectic cell size and the interaction terms for the mixed two components. These coefficients were determined through fitting the experimental data, and the model's accuracy was validated using data not included in the fitting process. The results exhibited reasonable agreement, confirming the model's reliability. The model, thus, offers insights into the influence of each microstructural factor on UTS and serves as a guide for designing alloys to achieve the desired UTS through microstructure modifications.

Thermal conductivity is an important property for cast components produced from different types of cast iron. Development of a general widely-accepted thermal conductivity model for compacted and lamellar graphite irons poses a research challenge. The present study extends the modeling approach introduced earlier for pearlitic lamellar graphite iron toward compacted graphite iron and ferritic lamellar graphite iron. The proposed thermal conductivity model of the bulk material is based on the alloy microstructure and Si segregation between eutectic cells and non-cell regions, at the main assumption that the heat paths in the eutectic cells are formed by connected graphite phases surrounded by ferrite phases. The overall thermal resistance of these heat paths is determined by the hydraulic diameter of the interdendritic region. The uncertainties both for the modeled and for experimentally derived thermal conductivities have been estimated. The importance of considering the Si segregation in the model has been discussed. For the investigated samples, the agreement between modeled and measured thermal conductivities has been achieved within 4% on the average, at the same value of the single fitting parameter found for pearlitic, pearlitic–ferritic lamellar, and compacted graphite iron alloys. The results contribute to the understanding of the material microstructure effects on the cast iron thermal conductivity.

Pore formation in cast iron castings is driven both by shrinkage and dissolved gases, where the latter stems from supersaturated gaseous species, such as nitrogen. During solidification, nitrogen partitions between the austenite and the liquid according to the ratio of its solubility in each phase. This ratio, known as the partition coefficient, is essential to characterize, as accumulation in the liquid phase can lead to critical supersaturation for pore formation. However, there is conflicting information in CALPHAD databases and literature regarding its partitioning behavior. This work evaluates nitrogen partitioning between the primary austenite and the liquid in a hypoeutectic lamellar cast iron alloy. To investigate this, a cylindrical specimen was produced and remelted under an inert atmosphere, allowing the austenite and the liquid to establish a solute equilibrium. After 6 days of holding at 1175 °C within the solid–liquid biphasic range, the specimen was quenched, and samples were extracted from the austenitic and liquid regions, which had transformed into martensite and ledeburite, respectively. The nitrogen concentration was measured by inert gas fusion (IGF), resulting in a nitrogen partition coefficient = 0.72±0.08, which represents partition in the opposite direction suggested by thermodynamic databases. The results indicate that there are opportunities to further explore nitrogen partitioning in other compositions and highlight the importance of selecting databases that more accurately represent the phenomena of interest. Moreover, a better understanding of nitrogen partitioning can enhance the control of porosity in cast iron processing.

Lamellar graphite iron (LGI) is used in the automotive industry as well as in a variety of other industries for geometrically complex components with demands on good balance between mechanical and thermal properties. Carbon content and cooling conditions influence the fraction and coarseness of the primary and eutectic microstructure that defines the mechanical and thermal properties. A wide range of carbon contents and cooling rates utilized in this work to produce LGI with ultimate tensile strength (UTS) that ranges from 200 to 350 MPa and thermal conductivity (lambda) from 31 to 63 W/m(-1)k(-1). The microstructure was extensively studied, and the results show that UTS and lambda are highly correlated with many different microstructural features. A relationship between the primary dendrites/eutectic interfacial area and maximum eutectic cell diameter is found, indicating the dependency of the eutectic cells diameter on the morphology of the pre-existing primary dendrites.

Silica sand is extensively used as a moulding sand for both mould and core making in the cast iron and steel casting industries. The pouring temperatures of cast iron and steel create a nonlinear distribution of temperatures across mould/core. The temperature fluctuations in mould/core establish different heat transport zones and result in a temperature-dependent undesirable expansion of silica. The expansion of silica is one of the primary sources for the formation of surface defects on castings. Additives are incorporated to mitigate the volumetric expansion of mould/core resulting from the granular expansion of silica sand. The paper aims to investigate the thermal dilation of unbonded silica sand integrated with different amounts of additive in the sand (0.8%, 1.0%, and 1.3%) using a horizontal dilatometer. The dilatometric investigations identified a decreasing trend in the thermal expansion behaviour of silica mixture with increasing content of additive inclusions in the mixtures. In theory, the additives in the sand mixtures decompose prior to the α↔β endothermic phase transition of quartz and provide intergranular spacing for the free expansion of silica. DSC and TGA were conducted to identify the phase change in silica and the degradation.

Spherical sheet steel molds filled with gray iron melts of varying chemical compositions and metallurgical conditions were air-cooled until solid, followed directly by austempering to preserve the austenite grain structure. The castings were studied using a combination of cooling curves and quantitative metallography, in order to clarify control of the austenite grain structure and its impact on the local microstructure. A novel method utilizing fast Fourier transform provided visual overview of macroscopic trends in the scale of the flake graphite structure. Castings inoculated with Sr-containing ferrosilicon featured finer eutectic cell structure but coarser equiaxed structure of austenite, emphasizing that melt treatments applied to control the graphite structure may have unintended effects on the austenite grain structure. In most non-inoculated castings, the microstructure was banded, with alternating layers of coarse and fine flake graphite with distance from the casting surface. The extent of the columnar zone of austenite grains showed no correlation with the graphite structure nor the volume fraction of dendrites. The volume-to-surface ratio of dendrites was more uniform in the columnar zone, but increased toward the center in the equiaxed zone. The casting with the highest carbon equivalent (4.34), featured zones containing finer dendrites and graphite. These zones appear to be gaps in the early solidification structure which filled later by secondary dendritic growth from surrounding austenite. This highlights that high carbon equivalent may lead to poor dendrite coherency which can make the microstructure less uniform and less predictable.

Low-austenitizing temperature practices resulted in substantial changes in both microstructure and mechanical properties of the fully ferritic as-cast Compacted Graphite Irons (CGI). The austempering processes were accomplished through first austenitizing at 850 °C for 60 min followed by quenching in a salt-bath at 275, 325, and 375 °C for times ranging from 30, 60, 90, and 120 min. In contrast with the austenitizing performed at 900 °C performed on the same material, the microstructure consisted of a notable volume fraction of proeutectoid ferrite, which was not observed under similar austempering temperature and time conditions. Lowering the austenitizing temperature to 850 °C resulted in decreased untransformed austenite. Depending on the austempering conditions, a notable improvement was achieved in both Brinell and Vickers hardness compared to the as-cast CGI. The ausferrite matrix led to remarkable increases in yield strength (YS), ultimate tensile strength (UTS), and a decrease in total elongation to failure. The highest YS and UTS values were achieved for specimens austempered at 275 °C while increasing the austempering temperature decreased both YS and UTS. Furthermore, the results showed that the austempering temperature had a more significant impact on YS and UTS than the austempering time. All austempered CGI specimens exhibited primarily brittle failure attributes, while ferritic CGIs showed a mixed failure mode.

Gas generation from molding materials creates a complex atmosphere in the mold–metal interface and is one of the primary causes of defects in cast components. Moisture, crystalline water, and decomposing binders are significant gas sources. The presence of volatiles and decomposing binder in the mold also affects the rate of heat absorption from the solidifying metal during the casting process. This work presents a measurement methodology to evaluate the rate and volume of gases generated from sand mixtures in combination with the temperature distribution and applied thermal analysis. The presented results show high reproducibility of the method. The thermal analysis results provide the start and end temperature of the binder decomposition reactions and the corresponding heat absorbed in this interval. The results obtained from the presented methodology can be used to validate the models/simulation tools developed to predict the gas evolution and related transport phenomena in the sand casting process.