Nodular cast iron, also known as ductile iron, is a critical engineering material widely used in various industries due to its excellent combination of strength, ductility, and castability. The unique microstructure of nodular cast iron, characterized by graphite spheroids embedded in a metallic matrix, provides superior mechanical properties compared to traditional gray cast iron. With the rapid development of sectors such as transportation, mining machinery, and power generation, there is an increasing demand for large-scale, heavy-section castings that can withstand high temperatures and mechanical loads. Medium silicon molybdenum nodular cast iron has emerged as a promising material for such applications, offering good heat resistance and mechanical performance. However, producing thick-section castings with consistent microstructure and properties remains challenging, often leading to issues like coarse graphite, irregular graphite morphology, and increased ferrite content, which degrade mechanical strength.
Alloying is an effective strategy to refine graphite morphology and control the matrix structure in nodular cast iron. Nickel (Ni) is a notable alloying element that promotes graphitization, refines graphite nodules, and solid-solution strengthens the matrix, potentially enhancing the overall performance of nodular cast iron. This study investigates the influence of Ni content on the microstructure, mechanical properties, and heat resistance of medium silicon molybdenum nodular cast iron, with a focus on thick-section applications. By systematically varying Ni additions, we aim to understand its role in optimizing the material for high-temperature service. The findings contribute to the alloy design of heat-resistant nodular cast iron, providing insights for industrial production of large castings.
The base composition of the medium silicon molybdenum nodular cast iron was designed with 3.4% C, 3.0% Si, 0.65% Mo, and limited impurities (P < 0.04%, S < 0.01%). Nickel was added at six levels: 0%, 0.3%, 0.7%, 1.1%, 1.5%, and 1.9% by weight. The alloys were melted in a medium-frequency induction furnace using raw materials such as pig iron, ferrosilicon, ferromolybdenum, and electrolytic nickel. The molten metal was treated via the bottom-pouring method for nodularization and inoculation: a rare-earth silicon iron magnesium alloy served as the nodularizer, and 75SiFe alloy was used as the inoculant. After treatment, the metal was poured into resin sand molds to produce thick-section test blocks with a wall thickness of 75 mm, simulating the conditions of large castings. These blocks were machined into specimens for various analyses.
Microstructural examination was conducted using optical microscopy (Olympus GX71) and scanning electron microscopy (Thermo Scientific TM Apreo C). Graphite morphology was observed on polished samples, and the matrix structure was revealed by etching with 4% nital solution. Mechanical properties were evaluated through tensile testing on standard specimens (30 mm gauge length, 6 mm diameter) according to GB 1348-2009, using an MTS hydraulic universal testing machine at a crosshead speed of 5 mm/min. Hardness was measured with a Brinell hardness tester (310HBS-3000) under a load of 750 kgf and a 5 mm ball indenter. Heat resistance was assessed via cyclic oxidation tests: specimens (10 mm × 10 mm × 5 mm) were oxidized at 700°C, 800°C, and 900°C for 60 hours, with weight measurements taken every 5 hours to determine oxidation kinetics. The oxide scales were analyzed using X-ray diffraction (XRD) and SEM to identify phases and morphology.
The effect of Ni content on graphite morphology in the as-cast nodular cast iron is summarized in Table 1. With increasing Ni addition, the graphite nodules became slightly finer and more numerous, but the changes were marginal. All samples exhibited a nodularity of approximately 80%, with a graphite size rating of 6 and a nodularity grade of 3, indicating that Ni has a mild graphitizing effect but does not significantly alter the overall graphite characteristics. This can be attributed to Ni’s weak graphitizing power, estimated to be about one-fifth to one-third that of silicon. The consistency in graphite morphology across different Ni levels suggests that the inoculation and nodularization processes were effective, and Ni primarily influences the matrix structure rather than graphite formation.
| Ni Content (wt.%) | Graphite Nodule Size (μm) | Nodule Count per mm² | Nodularity Grade | Graphite Size Rating |
|---|---|---|---|---|
| 0 | 25-30 | 120-130 | 3 | 6 |
| 0.3 | 24-29 | 125-135 | 3 | 6 |
| 0.7 | 23-28 | 130-140 | 3 | 6 |
| 1.1 | 22-27 | 135-145 | 3 | 6 |
| 1.5 | 21-26 | 140-150 | 3 | 6 |
| 1.9 | 20-25 | 145-155 | 3 | 6 |
The matrix structure of the as-cast nodular cast iron consisted primarily of ferrite (white regions) with minor amounts of pearlite (dark regions) and trace cementite. As Ni content increased, the volume fraction of pearlite rose substantially, from 11% at 0% Ni to 31% at 1.9% Ni, while ferrite decreased correspondingly. This is because Ni retards the diffusion of carbon during the eutectoid transformation, promoting pearlite formation. Nickel atoms, with an atomic radius ratio to iron of approximately 1.071, substitute for iron in the lattice, increasing the activation energy for carbon diffusion. The diffusion activation energy (Q) can be expressed as:
$$ Q = Q_0 + \Delta Q_{\text{Ni}} $$
where \( Q_0 \) is the base activation energy for carbon diffusion in iron, and \( \Delta Q_{\text{Ni}} \) is the additional energy due to Ni alloying. For Ni-doped austenite, calculations suggest \( Q \approx 17.831 \, \text{eV} \), which hinders carbon migration from austenite to graphite nodules, thereby suppressing ferrite growth and enhancing pearlite formation. The effect of Ni on matrix constituents is quantified in Table 2.
| Ni Content (wt.%) | Ferrite Volume Fraction (%) | Pearlite Volume Fraction (%) | Cementite Volume Fraction (%) |
|---|---|---|---|
| 0 | 88 | 11 | 1 |
| 0.3 | 84 | 15 | 1 |
| 0.7 | 82 | 17 | 1 |
| 1.1 | 80 | 19 | 1 |
| 1.5 | 73 | 26 | 1 |
| 1.9 | 68 | 31 | 1 |
Mechanical properties of the nodular cast iron alloys were strongly influenced by Ni content. Tensile strength and hardness increased monotonically with Ni addition, while elongation decreased. The results are presented in Table 3. At 0% Ni, the tensile strength was 520 MPa, hardness 193 HBW, and elongation 18.5%. With 1.9% Ni, tensile strength reached 618 MPa, hardness 218 HBW, and elongation dropped to 12.0%. The enhancement in strength and hardness is attributed to Ni’s solid-solution strengthening effect and its role in promoting pearlite, which is harder and stronger than ferrite. The reduction in ductility is due to the increased pearlite content, as pearlite is more brittle than ferrite. The relationship between strength and Ni content can be modeled using a linear regression equation:
$$ \sigma_{\text{TS}} = \sigma_0 + k_{\text{Ni}} \cdot C_{\text{Ni}} $$
where \( \sigma_{\text{TS}} \) is the tensile strength, \( \sigma_0 \) is the base strength (520 MPa), \( k_{\text{Ni}} \) is a strengthening coefficient (approximately 50 MPa/wt.% Ni), and \( C_{\text{Ni}} \) is the Ni concentration. Similarly, hardness (HBW) follows a comparable trend, while elongation (ε) decreases exponentially:
$$ \epsilon = \epsilon_0 \cdot e^{-b \cdot C_{\text{Ni}}} $$
with \( \epsilon_0 = 18.5\% \) and \( b \approx 0.15 \, \text{wt.%}^{-1} \).
| Ni Content (wt.%) | Tensile Strength (MPa) | Hardness (HBW) | Elongation (%) |
|---|---|---|---|
| 0 | 520 | 193 | 18.5 |
| 0.3 | 540 | 198 | 17.0 |
| 0.7 | 560 | 203 | 15.5 |
| 1.1 | 580 | 208 | 14.0 |
| 1.5 | 600 | 213 | 13.0 |
| 1.9 | 618 | 218 | 12.0 |
Fracture surface analysis via SEM revealed that all specimens exhibited a mixed-mode failure, with predominant cleavage fractures (river patterns) and some ductile features (dimples and tear ridges). As Ni content increased, cleavage became more pronounced, correlating with the higher pearlite content. In nodular cast iron, graphite nodules act as stress concentrators and crack initiation sites. Under tensile loading, voids form around graphite, and the surrounding matrix deforms plastically. With more pearlite, the matrix’s ability to deform is reduced, leading to earlier crack propagation and lower elongation. This underscores the trade-off between strength and ductility in Ni-alloyed nodular cast iron.
The heat resistance of nodular cast iron was evaluated through oxidation tests at elevated temperatures. The oxidation kinetics followed a parabolic rate law, described by:
$$ x^2 = k_p \cdot t $$
where \( x \) is the oxide scale thickness, \( k_p \) is the parabolic rate constant, and \( t \) is time. As Ni content increased, the oxidation rate accelerated, resulting in thicker oxide scales. Table 4 summarizes the average oxide scale thickness after 60 hours of oxidation at different temperatures. At 700°C, the scale thickness increased from 51.22 μm at 0% Ni to 110.99 μm at 1.9% Ni. At 900°C, the thickness rose from 55.66 μm to 341.41 μm over the same Ni range. This indicates that Ni addition detrimentally affects oxidation resistance, contrary to its beneficial effects on mechanical properties.
| Ni Content (wt.%) | 700°C | 800°C | 900°C |
|---|---|---|---|
| 0 | 51.22 | 105.33 | 55.66 |
| 0.3 | 60.15 | 120.45 | 150.78 |
| 0.7 | 70.89 | 140.67 | 200.92 |
| 1.1 | 85.44 | 170.31 | 250.15 |
| 1.5 | 98.76 | 210.55 | 300.87 |
| 1.9 | 110.99 | 250.80 | 341.41 |
XRD analysis of the oxide scales identified Fe₂O₃, Fe₃O₄, FeO, SiO₂, and Fe₂SiO₄ as primary phases at 700°C and 800°C. At 900°C, additional phases such as NiO and NiFe₂O₄ were detected. Elemental line scanning across the oxide scale revealed a thin inner layer rich in Si and O (SiO₂), which acts as a diffusion barrier, followed by an intermediate layer containing FeSiO₄, and an outer layer dominated by iron oxides. The presence of Ni in the oxide scale, especially at high temperatures, increases cation vacancy concentrations in FeO and NiO, both p-type semiconductors. This enhances the outward diffusion of metal ions (Fe²⁺ and Ni²⁺) and electrons, accelerating oxidation. The oxidation mechanism can be described by the Wagner theory, where the parabolic rate constant \( k_p \) is proportional to the self-diffusion coefficient of cations:
$$ k_p \propto D_{\text{cation}} = D_0 \cdot e^{-\frac{E_a}{RT}} $$
with \( D_0 \) as the pre-exponential factor, \( E_a \) the activation energy for diffusion, R the gas constant, and T the absolute temperature. Ni alloying lowers \( E_a \) for cation diffusion in the oxide, increasing \( k_p \) and thus oxide growth.
Microstructural evolution during oxidation also involved decarburization, as evidenced by carbon enrichment in cracks within the oxide scale. This decarburization weakens the substrate and contributes to scale spallation at higher temperatures. The overall oxidation resistance of nodular cast iron is thus compromised by Ni addition, despite improvements in mechanical strength. This highlights the need for balanced alloy design when optimizing nodular cast iron for high-temperature applications, where both mechanical performance and environmental resistance are critical.
In summary, nickel alloying significantly influences the microstructure and properties of medium silicon molybdenum nodular cast iron. Ni promotes pearlite formation in the matrix, leading to higher strength and hardness but reduced ductility. Graphite morphology is slightly refined, but the nodularity remains largely unchanged. However, Ni adversely affects heat resistance by increasing oxidation rates and oxide scale thickness, particularly at temperatures above 800°C. These findings underscore the complex role of Ni in nodular cast iron, where benefits in mechanical properties must be weighed against degradation in oxidation resistance. For thick-section castings intended for high-temperature service, optimal Ni content should be determined based on specific application requirements, possibly complemented by other alloying elements or coatings to enhance heat resistance. Future research could explore synergistic effects of Ni with elements like Cr or Al to improve both mechanical and thermal performance of nodular cast iron.
The study demonstrates that nodular cast iron is a versatile material whose properties can be tailored through alloying. Understanding the effects of elements like nickel is essential for advancing the development of high-performance cast irons. As industries demand more durable and efficient components, continued innovation in nodular cast iron alloys will play a pivotal role in meeting these challenges. Further investigations into long-term stability, creep resistance, and thermal fatigue of Ni-alloyed nodular cast iron are recommended to fully exploit its potential in extreme environments.