The Impact of Fe-Mg Inoculation with Nickel Addition on the Microstructure of Thin-Walled Ductile Cast Iron ^†

Abstract

High-temperature-resistant materials, such as stainless steel and cast steel, are widely used in industrial applications. While cast steel has a lower casting ability, cast iron demonstrates a superior casting performance but suffers from structural instability in thin sections, in which its phase structure tends to shift from graphite to cementite. This limits its applicability in thin-walled components. This study aims to characterize thin-walled ductile cast iron with improved high-temperature resistance. The focus is on evaluating the effects of Fe-Mg inoculation with the addition of nickel on the microstructure and mechanical properties of the material. Gray cast iron was cast and inoculated with Fe-Mg and nickel. Mold designs incorporated thickness variations of two, three, four, and five mm. Chemical composition testing was performed in the liquid state using a CE meter and in the solid state using a spectrometer, following ASTM A536 standards for ductile cast iron. A microstructural analysis was conducted using a scanning electron microscope (SEM) JEOL JSM-IT500 SEM (JEOL Ltd., Tokyo, Japan), and hardness was measured using the Vickers method. The results demonstrated that Fe-Mg inoculation with the nickel addition improved the microstructure and hardness of the thin-walled ductile cast iron. These enhancements contribute to increased high-temperature resistance and structural integrity, providing significant benefits for industrial casting processes. The findings have implications for improving the quality of small and medium industry (SMI) products, including the development of advanced metal molds.

1. Introduction

The metallurgical properties of ductile cast iron have been a focal point of research due to its versatile applications in industrial casting. Various factors, including alloy composition, cooling rates, and microstructural modifications, significantly influence the mechanical and thermal properties of cast iron.

The metal casting industry has long relied on the versatile properties of gray cast iron, which is widely used due to its low melting temperature, minimal shrinkage, and resistance to deformation and corrosion. Despite its advantages, the material has significant limitations, particularly its low tensile strength and thermal fatigue resistance, which restrict its application in high-temperature environments such as exhaust manifolds [1]. With increasing demands for enhanced performance, researchers have explored innovative approaches to improve cast iron’s properties, such as adding alloys like nickel and Fe-Mg during the casting process [2].

The addition of alloying elements, such as nickel (Ni) and Fe-Mg, has been extensively studied for their effects on graphite morphology and matrix properties. Nickel is particularly known for enhancing the thermal and corrosion resistance of ductile iron, making it suitable for high-temperature applications [3]. Moreover, Fe-Mg inoculants promote the formation of nodular graphite, which improves mechanical properties such as ductility and toughness [4]. Studies by Mukherjee [5] demonstrated that the inclusion of Nickel mitigates the transition of graphite to cementite in thin-walled castings, a common challenge in achieving consistent microstructural integrity.

Microstructural analysis reveals that the cooling rate during casting plays a crucial role in determining the distribution and morphology of graphite and ferritic or pearlitic phases. Higher cooling rates often lead to a finer grain structure and an increased number of graphite nodules, as observed by Xavier [6]. Conversely, slower cooling rates can result in the precipitation of cementite and the formation of ledeburite structures, which are less desirable for mechanical performance [7]. The correlation between the cooling rate and mechanical properties underscores the importance of precise thermal management during the casting process.

The thickness of cast iron significantly affects its mechanical and thermal properties. Thin-walled sections are particularly challenging due to their propensity for cementite formation instead of graphite. Research has shown that the addition of nickel effectively suppresses cementite precipitation, enhancing the overall performance of thin-walled castings [8]. Zhang et al. [9] reported that inoculation with Fe-Mg combined with the addition of nickel resulted in higher nodule counts and improved nodularity, particularly in thin castings, which are more sensitive to rapid cooling.

Recent advancements in analytical methods have improved the understanding of microstructural transformations in ductile cast iron. Techniques such as SEM and spectroscopic analysis allow for detailed examinations of graphite morphology and phase distribution. For instance, the use of SEM in studying nodular cast iron has revealed the intricate relationships between alloying elements, cooling rates, and resulting microstructures [10]. These insights are critical for optimizing casting processes and ensuring the desired properties of the final product.

The practical implications of these findings are significant for the manufacturing of high-performance components in small and medium industries (SMIs). An improved understanding of alloying and inoculation effects enables the production of high-temperature-resistant components with enhanced mechanical properties. Lesz [11] highlighted the potential for such materials in exhaust manifolds and other applications requiring thermal stability. Furthermore, advancements in casting techniques, including the use of advanced molds and controlled inoculation processes, pave the way for more efficient and sustainable production practices.

Improving the microstructural and mechanical properties of thin-walled ductile cast iron is crucial for its application in high-temperature settings. Previous studies have highlighted the role of graphite nodularity in determining ductile cast iron’s mechanical characteristics [12]. The use of inoculants like Fe-Mg combined with nickel has shown promise in mitigating challenges such as graphite degeneration into cementite in thin castings [13]. However, further studies are needed to comprehensively understand the effect of these additives on the microstructure and mechanical properties, particularly in thin-walled applications.

Despite these advancements, there remain gaps in the literature regarding the long-term effects of nickel and Fe-Mg inoculation on the durability of ductile cast iron in high-temperature applications. Additionally, the interaction of multiple alloying elements in modifying microstructures requires further exploration, particularly in the context of thin-walled castings. Future studies should also address the environmental and economic aspects of alloying, focusing on sustainable practices for industrial-scale applications.

2. Materials and Methods

2.1. Materials

This study utilized ductile cast iron (FCD) prepared in accordance with ASTM A536 standards to investigate the effects of nickel (Ni) and Fe-Mg inoculation on the material’s microstructure and mechanical properties. The chemical composition of the specimens was measured using a spectrometer compliant with ASTM E415-8 standards [14].

2.2. Mold Preparation

Resin-coated sand (RCS) molds were used to cast specimens. The molds were fabricated at the Karya Mandiri Sentosa workshop, Klaten, Indonesia, using standard procedures to ensure consistency. The molds featured stepped designs with thickness variations of 2 mm, 3 mm, 4 mm, and 5 mm to evaluate the effect of casting thickness on the resulting material properties.

2.3. Casting and Inoculation Process

The casting process was performed at CV. Baja Tunggal, Klaten, using a medium-frequency induction furnace. Fe-Mg and Ni inoculants were added to the molten metal in predetermined amounts of 4% each by weight. The molten material was stirred to ensure homogeneity before pouring it into the prepared molds. A CE meter was used to measure the equivalent carbon content in the molten metal, ensuring it fell within the target range of 4–5%.

2.4. Sample Preparation

After casting, the specimens were machined to meet the dimensions specified for subsequent testing. The cutting process was conducted at the Karya Mandiri Sentosa workshop, following standardized procedures to minimize variability.

2.5. Chemical Composition Testing

The chemical composition of the cast specimens was analyzed in both liquid and solid states. A spectrometer was used for solid-state analysis, adhering to ASTM E415-8 standards. This ensured precise determination of alloying elements and their concentrations.

2.6. Microstructure Analysis

The microstructure of the cast specimens was examined using scanning electron microscopy (SEM) at the Engineering Materials Laboratory, Department of Mechanical Engineering, University of Muhammadiyah Surakarta. The SEM analysis followed ASTM E986 standards, providing detailed images of graphite morphology and matrix phases at various thicknesses. The atomic and weight percentages of chemical elements were also analyzed using energy-dispersive X-ray spectroscopy (EDS) integrated with SEM.

2.7. Hardness Testing

The hardness of the cast specimens was evaluated using the Vickers hardness test. This method was chosen for its precision in measuring the microhardness of small material sections. Hardness data were correlated with the microstructural characteristics to assess the effects of inoculation and casting thickness.

3. Results and Discussions

3.1. Chemical Composition Analysis

The chemical composition of the raw material is presented in

Table 1. Chemical composition results of raw material.

Element	W (%)
	Raw Material	Deviation Standard
Fe	93.25	0.076
Si	3.036	0.034
C	3.173	0.069
Ni	0.091	0.0018
Mn	0.199	0.0036
Mg	0.026	0.0048
Ti	0.057	0.0024
Cu	0.040	0.0017
S	0.029	0.0068
Cr	0.026	0.0037
P	0.028	0.0013
Al	0.023	0.0056
Nb	0.0079	0.0024
Mo	<0.0050	0.00000
Sn	0.0069	0.0024
Co	<0.0030	0.0000
W	<0.020	0.0029
B	<0.0010	0.0003
V	<0.0030	0.00000

, while

Table 2. Chemical composition results of inoculated nodular cast iron with 4% Ni and 4% Mg.

Element	W (%)
	+4% (Ni and Mg)	Deviation Standard
Fe	93.25	0.205
Si	4.068	0.058
C	3.263	0.1370
Ni	0.642	0.126
Mn	0.248	0.035
Mg	0.133	0.020
Ti	0.074	0.0043
Cu	0.034	0.0052
S	0.033	0.012
Cr	0.033	0.0058
P	0.032	0.0052
Al	0.023	0.0032
Nb	0.020	0.0065
Mo	0.011	0.0034
Sn	0.0069	0.0032
Co	0.0058	0.0018
W	<0.020	0.029
B	<0.0010	0.0017
V	<0.0030	0.00000

provides the composition of the specimens, analyzed to evaluate the effects of 4% nickel (Ni) and 4% Fe-Mg inoculation. The spectrometric analysis (ASTM E415-8 standard) revealed the presence of 19 alloying elements, with iron (93.25%), silicon (4.068%), and carbon (3.263%) as the dominant constituents. Compared to raw material values (Table 1), the inoculation process significantly increased the concentrations of nickel (from 0.091% to 0.642%) and magnesium (from 0.026% to 0.133%). Silicon, a key element for preventing carbide formation, exceeded the standard limit for nodular cast iron. This elevated silicon content contributed to decomposing carbide into iron and graphite, reducing the likelihood of ledeburite (Fe₃C) formation and enhancing the microstructural characteristics [10].

The increased nodular graphite count due to Fe-Mg inoculation, combined with nickel’s suppression of cementite, aligns with previous studies that demonstrate the synergy between these alloying elements in improving graphite morphology and thermal resistance [15]. The role of silicon as a carbide decomposition agent was particularly significant in ensuring a predominantly nodular graphite structure, which is essential for ductility and thermal fatigue resistance [16].

The chemical composition analysis reveals distinct differences between the raw material and the nodular cast iron inoculated with 4% nickel (Ni) and 4% magnesium (Mg), highlighting the effects of the inoculation process. The iron (Fe) content, which serves as the base matrix, remained constant at 93.25% in both materials. Silicon (Si), a crucial element for decomposing carbides (Fe₃C) into graphite, increased significantly from 3.036% in the raw material to 4.068% in the inoculated cast iron. This rise in silicon content enhances the material’s ability to form nodular graphite, improving its ductility and thermal resistance, a finding consistent with previous studies [15].

The carbon (C) content showed a slight increase from 3.173% to 3.263%, maintaining its role in contributing to the hardness and strength of the cast iron. Nickel (Ni), an essential alloying element for suppressing cementite formation and promoting graphite nodularity, exhibited a substantial increase from 0.091% to 0.642%. This adjustment significantly improves the material’s high-temperature resistance and nodular structure, aligning with findings by Yulianto [15]. Similarly, the magnesium (Mg) content rose from 0.026% to 0.133%, playing a vital role in spheroidizing graphite to enhance nodularity and mechanical performance [17].

Manganese (Mn) also increased moderately from 0.199% to 0.248%. While excessive Mn can lead to embrittlement, this controlled rise supports carbide formation without compromising the material’s ductility [18]. The titanium (Ti) content increased slightly from 0.057% to 0.074%, contributing to grain refinement and toughness. Sulfur (S), which can affect brittleness, rose marginally from 0.029% to 0.033% but remained within the acceptable limits for nodular cast iron.

Other elements, including phosphorus (P), chromium (Cr), and copper (Cu), showed minor variations that did not significantly affect the primary matrix but could influence secondary properties like corrosion resistance and hardness. Overall, the inoculation process with 4% Ni and 4% Mg successfully optimized the chemical composition, promoting the formation of a robust nodular graphite structure. These changes enhance the material’s mechanical properties, thermal resistance, and ductility, supporting its application in high-performance and high-temperature environments. These findings align with prior research emphasizing the importance of alloying elements in refining the microstructure and performance of ductile cast iron [19,20].

3.2. Microstructure Analysis

SEM revealed distinct microstructural changes influenced by the inoculation process and casting thickness. The microstructure predominantly consisted of nodular graphite embedded in a matrix of ferrite and pearlite. The nodular graphite was uniformly distributed, with darker spherical shapes evident under higher magnifications, as shown in Figure 1.

The microstructure exhibited variations across different casting thicknesses. In the two-mm-thick specimens, the graphite nodules were the most uniformly rounded, indicating enhanced inoculant effectiveness in finer sections. The ferrite phase surrounding the nodules provided additional ductility. In contrast, the five-mm-thick specimens displayed larger nodules with an increased pearlite content, contributing to higher hardness values but reduced ductility.

These findings are consistent with literature reports that correlate faster cooling rates in thinner castings with increased nodule count and nodularity [21]. The transition from graphite to cementite in thicker castings was mitigated by the high silicon content, which favored graphite formation even in slower cooling regions. The formation of a predominantly ferritic matrix in thinner sections can be attributed to the combined effects of Fe-Mg inoculation and the addition of nickel, which stabilized graphite structures during solidification [22].

The microstructure analysis using SEM revealed distinct differences between the raw cast iron and the inoculated nodular cast iron containing 4% nickel (Ni) and 4% magnesium (Mg). These differences were observed in the distribution and morphology of key phases: pearlite, graphite, and ferrite. The SEM images demonstrated that inoculation with 4% Ni and 4% Mg significantly altered the graphite phase, transforming it into a nodular shape that was consistently round and uniformly distributed. This morphological improvement enhances mechanical properties such as ductility and thermal resistance. In the raw material, graphite appeared irregular and less nodular, which correlates with a reduced mechanical performance.

In terms of the matrix composition, the inoculated cast iron exhibited a dominant pearlite phase in thicker specimens (e.g., five mm). Pearlite, characterized by its lamellar structure and dark gray coloration in SEM images, contributes to increased hardness and wear resistance. However, in thinner specimens (e.g., two mm), the matrix was predominantly ferritic. Ferrite, appearing bright in SEM images, enhances ductility but reduces hardness compared to pearlite. This variation suggests that cooling rates and thickness significantly influence the microstructural phases in inoculated cast iron.

The best-rounded graphite nodules were observed in the two-mm-thick specimens, in which the matrix showed a balanced distribution of ferrite and pearlite. Conversely, the five-mm-thick specimens displayed a higher concentration of pearlite, reflecting slower cooling rates that allow for greater carbide decomposition into iron and graphite, stabilized by the inoculants. These observations align with prior studies emphasizing the role of nickel in suppressing cementite formation and magnesium in promoting nodular graphite [5,23].

The inoculation process with 4% Ni and 4% Mg optimized the microstructure of nodular cast iron, improving graphite nodularity and tailoring the matrix composition to enhance specific mechanical properties. This transformation underlines the critical role of alloying elements and cooling rates in achieving desired phase structures in high-performance ductile cast iron.

3.3. Hardness Testing

Vickers hardness testing revealed a strong correlation between casting thickness and material hardness. The hardness values increased with thickness due to the higher pearlite content in the matrix, as shown in Figure 2. The five-mm specimens exhibited the highest hardness values, attributed to the greater presence of pearlite and reduced ferrite content.

While increased hardness is advantageous for wear resistance, it may compromise ductility, particularly in applications requiring thermal fatigue resistance. This balance highlights the importance of optimizing casting thickness for specific industrial applications. The observed trends align with studies that emphasize the trade-offs between hardness and ductility in cast iron alloys modified with inoculants [24].

The results underscore the effectiveness of combining Fe-Mg inoculation with the addition of nickel in enhancing the properties of thin-walled ductile cast iron. The uniformity of nodular graphite in thinner sections indicates the efficacy of the inoculants, particularly in mitigating cementite formation. The increase in hardness with thickness reflects the matrix’s transition from ferrite to pearlite, influenced by cooling rates and chemical composition.

The findings also highlight the critical role of silicon in decomposing carbides, preventing ledeburite formation, and supporting graphite nodularity. These mechanisms align with established principles in cast iron metallurgy, in which silicon and nickel are key modifiers of phase transitions during solidification [25].

Despite the improvements in microstructural uniformity and hardness, the study raises questions about the long-term stability of these properties in high-temperature applications. Further investigations into the thermal fatigue resistance of these castings, particularly under cyclic loading conditions, are necessary to validate their suitability for industrial use. Additionally, the environmental and economic aspects of using high-silicon and nickel inoculants warrant consideration for large-scale production.

4. Conclusions

This study concludes that the addition of 4% nickel (Ni) and 4% magnesium (Mg) significantly influenced the microstructure of nodular cast iron, particularly in promoting the formation of rounded graphite nodules. Among the specimens with varying thicknesses (two mm, three mm, four mm, and five mm), the two-mm-thick sample exhibited the most uniform and rounded graphite morphology. This sample also demonstrated a balanced matrix composition dominated by pearlite, contributing to enhanced mechanical properties. Conversely, the five-mm-thick sample, which showed a higher proportion of graphite, exhibited a matrix dominated by pearlite, indicative of the slower cooling rate. The chemical composition analysis revealed five dominant alloying elements: iron (Fe) at 93.25%, silicon (Si) at 4.068%, carbon (C) at 3.263%, nickel (Ni) at 0.642%, and manganese (Mn) at 0.248%. Silicon, at 4.068%, was particularly significant for its role in enhancing graphite formation and preventing carbide precipitation. These compositional adjustments ensured the formation of a nodular graphite structure, contributing to the improved mechanical performance and suitability of the cast iron for high-temperature and wear-resistant applications.