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High alloyed by Si cast iron with spherical graphite (SGI) has a unique combination of mechanical properties at room temperature and high oxidation resistance at elevated temperature. The performance of this cast material can be improved by precise alloying and for the development of desired microstructural dispersity, using an effective melt inoculation in combination with an optimal cooling rate during solidification. There is consensus in the metalcasting community that high microstructural dispersity is desirable for good mechanical performance of high Si SGI at room temperature. However, it could not be the case for high-temperature applications. In this study, static high-temperature oxidation of heavy (100 mm), medium (18 mm), and thin (5 mm) wall castings from two SGI (base, referred to as SiMo and alloyed by 3% Al (SiMoAl) were investigated. Microstructural dispersity was characterized in 2D sections and in 3D space by µCT. Surface degradation was quantified by SEM and TEM analysis in cross sections after 100 h static oxidation in air at 800 °C. Carbon analysis was used to decouple the scale formation and decarburization (deC) processes which occurred simultaneously. Scale topology was determined by microcomputed tomography (µCT) analysis. Additional thermo-cycling tests of constrained specimens were performed to determine resistance to SGI failure during transient thermomechanical loading. These methods provided the quantification of the effects of microstructural dispersity parameters on high-temperature performance in SGI. The different trends were observed in the base SiMo and alloyed SiMoAl SGI. Methods for improving high-temperature performance by optimizing microstructural dispersity are suggested based on trends in surface degradation. [ABSTRACT FROM AUTHOR]

To improve the oxidation resistance of cast iron containing Si and Mo (HiSiMo), 1wt.% Cr and 3wt.% Al were added into the base alloy to improve high temperature oxidation resistance for exhaust manifold applications. Oxidation tests were performed in both dry and wet air (air + 10% H2O) at 800 °C for 1, 10, 24, and 50 h. The mass change and oxide thickness were measured to evaluate the oxidation resistance. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) and transmission electron microscopy (TEM) were employed to characterize the oxidized samples. The results showed that the additions of Al or Cr selectively oxidized to form a more oxidation resistant surface oxide layer, which reduced the mass gain during these experiments. [ABSTRACT FROM AUTHOR]

High alloyed Si ferritic ductile irons can offer potential benefits because they combine high strength, ductility at room temperature, and low oxidation rate at high temperature. However, there is one known drawback and that is these cast irons have limited performance during thermal cycling due to a significant drop of ductility at warm temperatures. This decrease in ductility has been linked to poisoning ferrite grain boundaries by Mg. Therefore, thermodynamic simulations were used to identify altering additions which were able to meditate this negative effect by forming intermetallic phases with Mg. To verify thermodynamic predictions, three alloys were cast including a base and two high Si ductile irons with additions of P and Sb. High-temperature performance of these alloys was experimentally verified including tensile properties at warm temperatures (350–550 °C), oxidation in air at temperatures (700–800 °C), and thermal cycling between 300 and 800 °C. SEM and TEM analyses confirmed that the studied additions reacted with Mg forming different compounds which could prevent poisoning ferrite grain boundaries and improve high-temperature performance of high Si ductile iron. [ABSTRACT FROM AUTHOR]

Silicon and molybdenum (SiMo) alloyed cast irons with spherical graphite are used for exhaust manifolds for service at high temperatures in air and combustion atmospheres containing water vapor. Analysis of degraded surfaces of in-service manifolds indicates existence of a combined oxidation and de-carburization (de-C) phenomena. Therefore, sequential high-temperature tests in air and combustion atmosphere with recording weight change together with carbon analysis at each time step were utilized to quantify the kinetics of both processes. The recorded weight change was related to weight gain due to oxidation and weight loss due to de-C. Carbon analysis was used to de-couple these two processes. SEM measured thicknesses of de-C layers were used to verify kinetics obtained from changes in carbon concentration during oxidation. It was shown that the oxidation and de-C kinetics have different sensitivities to testing temperature and the type of oxidizing atmospheres. At 700 °C and above, there are several significant mutual effects between scale formation and de-C processes which affect the kinetics of these processes and activation energy. The tested experimental methodology for decoupling these processes can be used for alloy optimization. [ABSTRACT FROM AUTHOR]

The complex nature of nodular graphite nuclei was studied in a partially solidified and quenched spheroidal graphite iron (SGI) that contained small graphite nodules. The reduced graphite to nuclei diameter ratio obtained by the accelerated solidification process increased the probability of nuclei exposure during metallographic cross sectioning. Ten thousand graphite nodules were analyzed to identify nucleation sites using an automated scanning electron microscopy-energy dispersive X-ray (SEM-EDX) system and the resultant ternary plot presents a chemistry of oxides, sulfides and nitrides located inside the graphite nodules. Focused ion beam milling was used to extract thin foil specimens and extensive high-resolution transmission electron microscopy (TEM) analysis of several representative nuclei was conducted. The nucleus of a graphite nodule was composed of a Mg – O core with Mg–Ca–S and Mg–Al–Si – N compounds attached on its sides. The crystallographic orientation relationships between different nucleating compounds and between graphite and nucleating compounds were analyzed using selected area diffraction patterns. For the first time, the direct observation of the nuclei interfaces revealed the presence of nano-structured boundary layers. The formation of a complex nodular graphite nucleus was thermodynamically simulated and the nucleation behavior of the complex compound was linked to the novel experimental results. Image 1 [ABSTRACT FROM AUTHOR]

High silicon molybdenum-alloyed cast irons (SiMo) with spherical graphite are widely used for cast thin-wall exhaust manifolds, worked in high-temperature oxidation environment; therefore, prevention of surface degradation during service is important for such common application. The base SiMo and two alloyed by 1 pct Cr and 3 pct Al cast irons were tested at high temperatures in air and a combustion atmosphere to study the microstructural processes of surface degradation. It was shown that surface degradation of SiMo cast irons involves several microstructural processes, including the formation of multilayered oxide scale and development of internal porosity, resulting in decarburization of graphite nodules. Monitoring weight change of the specimens and Leco C analysis were used for decoupling the rates of surface oxidation and metal matrix decarburization. SEM/EDX analysis was used to study the phase composition of scale and high-resolution TEM analysis was performed to determine the detailed information about the atomic structure of protection layers formed on scale/metal matrix interface. The results were used to qualify and link different surface degradation mechanisms. In the base SiMo cast iron, the protection efficiency of formed amorphous silica layers is limited by intensive decarburization at temperatures above 700 °C. It was shown that Cr and Al alloying prevents surface degradation and suppresses decarburization by developing thermally stable oxide films on metal-scale interface, thus allowed to increase a safe working temperature up to 800 °C in air and the combustion atmosphere. [ABSTRACT FROM AUTHOR]

High SiMo cast iron components in automotive exhaust systems are exposed to high-temperature oxidation over time. Quantitative analysis of formed oxide scale is therefore important for the assessment of a component durability. The brittle nature of multilayered scale and thermal stress limits capture of a true topology using traditional 2D destructive cut and polish methods. In this study, nondestructive high spatial-resolution 3D µCT analysis was performed on 2.90-mm-diameter oxidized specimens which permitted direct observation with 3.5 µm pixel resolution. The specimens were oxidized in three sequential time steps for a total 100 h at 700 °C and 800 °C in air and combustion gas atmospheres. A MATLAB-coded algorithm was used to quantify the topology, thickness variation in internal and external scale layers, and scale/metal interface unevenness. Scale topology was linked to oxidation temperature and gas atmosphere. A water vapor environment increases scale/metal interface unevenness and scale layer thickness irregularity which were related to an accelerated oxidation rate. [ABSTRACT FROM AUTHOR]

Inoculation of cast iron with spherical graphite (SGI) controls nucleation kinetics of graphite nodules and is used for improving casting performance and eliminating solidification micro-porosity. In this study, thermodynamic simulations were used to design inoculants. Thermal and chemical stability of different potential heterogeneous nuclei formed in the melt above the liquidus temperature and in the mushy zone during solidification was predicted. Several inoculation treatments of SGI were performed in laboratory heats. An automated SEM/EDX analysis was applied to examine the graphite nodule size distribution and the family of the non-metallic inclusions in the experimental castings. The data were used to reconstruct a relative graphite nodule nucleation rate in the castings. It was shown that the graphite nodule nucleation kinetics in the castings were significantly different from those predicted by classical nucleation models. In inoculated SGI, the observed bi-modal distribution of graphite nodules was related to a continuous nucleation that occurred towards the end of solidification. The origin of the continuous nucleation and the possibility of engineering graphite nodules nucleation kinetics to control SGI casting soundness by inoculation are discussed. [ABSTRACT FROM AUTHOR]

Important properties of cast iron, such as fatigue strength, wear resistance, and low-temperature toughness, relate to spatial distribution of graphite nodules. Characterization of spatial distribution can also provide insight into the solidification sequence in casting. An automated SEM/EDX analysis was utilized to distinguish graphite nodules from other structural features (pores and inclusions). The two-dimensional near-neighbor distance (NND) between nodule centers was calculated for three equal sets of nodule diameters (small, medium, and large) in each cast iron. Comparison of measured spatial distributions and ideal random distribution was executed by plotting the mean and variance ratios of NND on a spatial distribution quadrant. This method was used to clarify clustering or ordering tendencies of graphite nodules in studied cast irons. The suggested procedure was used to verify the effects of inoculation and the cooling rate on spatial distribution of graphite nodules. Inoculation of sand casting increased nodule counts, decreased mean NND, and eliminated clustering of small graphite nodules precipitated at the solidification end. Intensive surface cooling of a continuously cast bar significantly increased nodule count near the external surface and decreased NND without changing spatial distribution. The suggested analysis can be used as a tool for cast iron quality control and process development. [ABSTRACT FROM AUTHOR]

The shrinkage porosity of castings made from cast iron with spherical graphite (SGI) depends on a combination of intrinsic (density and volume of phases, solidification kinetics) and extrinsic conditions related to casting-mold thermo-mechanical interactions. Precipitation of graphite nodules increases the specific SGI volume, and control of the nucleation rate in solidified castings can be used for improving casting soundness. In this article, the method of structural reconstruction of solidification kinetics was used to link the nucleation rate of graphite nodules to experimentally observed shrinkage porosity in a specially designed test casting. An automated SEM/EDX system was used to determine the 'true' two-dimensional graphite nodule distributions in the casting sections. These two-dimensional distributions were converted into the volume particle distribution functions ( PDF), and the solidification kinetics were reconstructed by applying inverse simulations. The experiments were performed with variations in inoculation and pouring temperature. The shrinkage porosity was compared to the restored sequence of graphite nodule nucleation in the specific casting volumes. It is shown that the second nucleation wave in low-temperature poured and inoculated SGI eliminated interdendritic microporosity. The suggested method could be used in industry to improve the soundness of SGI castings. [ABSTRACT FROM AUTHOR]

Formation of low density graphite phase during solidification increases the volume of semi-solid cast iron with spheroidal graphite (SGI) and resulted pressure could help eliminate shrinkage micro-porosity in casting. Therefore, micro-porosity in casting can be linked to 3D graphite nodule structure. Two experimental SGI's with different levels of shrinkage micro-porosity in castings were intentionally produced by variation in inoculation parameters which influenced graphite nodule structure. 3D-morphological characteristics of graphite nodules and micro-porosity level were determined by μCT scanning. The obtained statistical data was discussed with respect to graphite nodule solidification mode and casting soundness. The capabilities of μCT scanning in predicting micro-shrinkage in SGI casting were compared to the other traditionally used techniques. • Two experimental cast irons with spherical graphite were prepared applying different inoculation methods. • Micro-CT scanning of experimental castings was performed targeting graphite nodule and micro-porosity. • Graphite nodule diameter and space distribution were statistically evaluated and linked micro-porosity. • Different methods of structure evaluation of cast iron with spherical graphite were compared. [ABSTRACT FROM AUTHOR]