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.

The occurrence of devious microstructure at the surface of cast iron has long been a subject of discussion but still lacks comprehensive understanding. In sand casting, the surface microstructure can differ from the bulk of the material; these surface deviations appear related to mold-metal interaction, often perceived as differences in graphite distribution and morphology at the surface of spheroidal and compacted graphite iron castings. Such deviations are mostly attributed to Mg depletion at the interface, with heightened sulfur and oxygen concentrations released from mold and cored materials as key contributing factors. Nevertheless, other factors, including variations in section thickness, pouring temperature, and metallostatic pressure, can influence the extent of the interaction. The present case study targeted an in-depth analysis of the casting skin phenomenon under different foundry conditions to ascertain its occurrence, severity, and main characteristics. Two cylinder blocks were sourced from two different foundries, one in its original as-cast state and the other after shot-blasting. Various mold-metal interfaces were investigated under different solidification times and metallostatic pressure conditions. The flake skin thickness at the interface was measured alongside nodularity and surface roughness. Etching based on Si segregation was also employed to understand the solidification behavior of the surface skin. Ultimately, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to investigate the existence of particles generated at the mold-metal interfaces.  

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.

Solidification of cast irons usually involves dendritic growth of austenite. This article presents a literature survey about the dendrites in cast irons, their consequences and how they may be manipulated. The literature review is supplemented with relevant micrographs from our research. While austenite usually transforms into ferrite or pearlite, the dendrites limit where liquid flows, where eutectic grows, and where segregated elements go. The amount and shape of dendrites show correlations with tensile strength in pearlitic gray and compacted graphite irons. There are also indications that a coarse dendrite grain structure may be beneficial to tensile strength. The dendrite grain structure depends on melting process parameters and shows sensitivity to melt treatment. The evolution of scale of dendrite arms and their spacing under isothermal condition is by now fairly well-understood; however, work remains to better understand its evolution during cooling and its interaction with the eutectic. The amount and shape of dendrites are less understood in irons of near-eutectic and hypereutectic composition, in particular mixtures of dendrites of distinct scales, associated with regions of distinct graphite morphology. While significant advances have been made in recent years, the role and control of dendrites remain a relatively unexplored area of research with potential to improve production and properties of cast irons.

Cast iron foundries are often afflicted by porosity defects, resulting in substantial economic, energetic and, by extension, environmental losses. In the current industrial context, where sustainability is key, efforts must be made to avoid these defects. As it is known by metallurgists, the morphological and surface characteristics can hint at the root cause of porosity defects; however, when it comes to the surface characteristics of pores, existing research lacks a systematic assessment concerning the nature of gases involved as well as the timing of pore formation. The present work aims to study the relationship between gases injected at predetermined moments during solidification and the pore surface characteristics. Cast iron cylinders were melted inside alumina crucibles under an inert atmosphere. During the solidification process, known gas mixtures were carefully injected from syringes into the melt with the objective of forming a stable bubble. Once the cooling was complete, the cylinders were sectioned, and the pore surfaces were analyzed with a Scanning Electron Microscope (SEM) and by Energy Dispersive X-ray Spectrometry (EDX). By gaining a deeper understanding of the processes generating various surfaces, foundries are better equipped to identify the root causes of pores and to strategize effectively for decreasing porosity defects in regular production. Future research may expand on this study by exploring a wider range of gases, base melt, and solidification conditions, to broaden the relevance of this study to different industrial contexts. 

Thermal conductivity is an important property for many iron cast components, and the lack of widely accepted thermal conductivity model for cast iron, especially grey cast iron, motivates the efforts in this research area. The present study contributes to understanding the effects alloy microstructure has on thermal conductivity. A thermal conductivity model for a pearlitic cast iron has been proposed, based on the as-cast alloy composition and microstructural parameters obtained at different solidification rates. According to the model, available parallel heat transfer paths formed by connected graphite flakes across eutectic cells are determined by the space between dendrite arms. The uncertainties both for model inputs and for validation measurements have been estimated. Sensitivity analysis has been conducted to result in better understanding of the model behaviour. The agreement between modelled and measured thermal conductivities has been achieved within 5% on the average for the investigated samples.

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.

An evaluation method for the initial penetration of molten cast iron into the sand mold was suggested based on the laboratory-scale penetration experiments for the cast iron. The horizontal penetration depth of the molten cast iron into the sand core was analyzed using the capillary model. The early stage of the penetration was discussed, and it was clarified that the penetration is not stopped by the solidification but is stopped by the decreasing of the equivalent pore radius. It was explained that the equivalent pore radius decreases with increasing the penetration depth, and the penetration is stopped when the critical pressure, i.e., the pressure required for the penetration, becomes higher than the pressure which is acting on the penetration front. Based on the analysis, an evaluation method of the penetration of depth at the early stage of the penetration was suggested. The analysis method was applied for the other type of metals (mercury and steel) as well, and reasonable results were obtained. A simplified finite-element model of liquid iron penetration into a sand core was developed, accounting for heat exchange between the melt and the porous medium, at different pore geometries.

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 between its solubility in each phase, which increases with decreasing temperature. This ratio is known as the partition coefficient. In the austenite, nitrogen mobility is very limited, which can create concentration gradients. Characterization of the partition coefficient is important because accumulation in the liquid phase facilitates reaching critical supersaturation for pore formation. However, there is conflicting information in CALPHAD databases and literature regarding its partitioning behavior. In this work, the partition of nitrogen between the primary austenite and the liquid has been assessed in a hypoeutectic cast iron alloy. To assess this issue, a sample was produced and remelted under an inert atmosphere, where austenite and the liquid established a solute equilibrium. After six days of holding, the sample was quenched and discs were extracted from the austenitic and liquid part, now martensite and ledeburite respectively. The nitrogen concentration was measured by inert gas fusion and the results point towards nitrogen accumulation in the liquid during solidification. The results indicate that there are opportunities to further explore the nitrogen partitioning in other compositions and stress the need to select the databases that more accurately portray the phenomena of interest.  Moreover, a better understanding of nitrogen partitioning can enhance the control of porosity in the processing of cast iron.

In lamellar and compacted graphite irons, graphite tends to grow as round units referred to as eutectic cells. The number of eutectic cells has importance for aspects such as solidification kinetics, material properties and avoidance of carbides. Scientific studies often measure the cell count on 2D sections using etching techniques which highlight the centers or boundaries of the cells. However, counting the cells depends on the etching procedure and relies heavily on interpretation thereof. This work presents two computer-aided methods for detection of eutectic cells on polished sections of the cast irons, both based on the observation that graphite particle sections in a eutectic cell tend to be elongated radially relative to its center. The first method utilizes the orientations of particle edges, while the other utilizes the dominant orientation of each particle section. The output is a field indicating areas more likely to include a eutectic cell center. The methods are complemented by automatic counting of cells based on the output fields. The performance of the computer-aided methods is evaluated on polished sections of three lamellar and one compacted graphite iron by comparison to etched sections of the same materials using Stead’s and Motz’s reagents.