In the field of wind energy casting components, such as hubs, bases, and bearing seats, the materials predominantly used are QT400-18AL and QT350-22AL grades of nodular cast iron due to their excellent toughness and low-temperature impact properties. With the rapid development of the wind power industry, there is a growing trend toward structurally complex, lightweight, and larger megawatt castings. To meet these evolving demands, material quality requirements have become increasingly stringent, particularly concerning tensile strength. This has led to the emergence of non-standard material specifications, such as dual-grade QT420-18AL. Given that adjustments in carbon and silicon content alone are insufficient to surpass industry material standards, micro-alloying has emerged as a preferred technological approach. Nickel, as a graphitizing element, can dissolve unlimitedly in molten and solid nodular cast iron without forming compounds with carbon. It reduces or eliminates the formation of free cementite, lowers chilling tendency and brittle transition temperature, strengthens the matrix structure, and enhances the strength and low-temperature impact toughness of nodular cast iron. This study investigates the effects of varying nickel content on the mechanical and low-temperature impact properties of thick-section castings based on the QT400-18AL material grade, providing insights into optimizing nodular cast iron for advanced applications.
Nickel and γ-iron both possess a face-centered cubic lattice structure, and their proximity in the periodic table allows nickel to form a substitutional solid solution with iron. Nickel atoms randomly occupy positions in the iron crystal lattice, and iron and nickel are infinitely soluble. The role of nickel in nodular cast iron differs from that of silicon; while silicon solid-solution strengthens ferrite, increasing material strength and hardness, nickel can expand the γ-phase region to room temperature when added in sufficient quantities, resulting in an austenitic matrix cast iron. This unique characteristic makes nickel a promising alloying element for enhancing the performance of nodular cast iron in wind power components. In this research, I designed experiments with different nickel contents to systematically evaluate their influence on thick-section castings, focusing on mechanical properties and low-temperature impact resistance.
Considering the inherent characteristics of wind power castings, which typically feature large wall thicknesses and slow cooling rates, I utilized simulated high-modulus casting blocks for the experiments. The square test block dimensions were 250 mm × 250 mm × 250 mm, with attached standard D-type test blocks measuring 70 mm × 70 mm × 180 mm. The casting model was designed to replicate real-world conditions, and MAGMA simulations indicated a maximum modulus of 9 for the square block and 3 for the attached D-type test block. This setup ensured that the cooling behavior mimicked that of actual thick-section nodular cast iron components. Under controlled conditions with silicon content (w(Si)) between 1.90% and 2.0% and carbon content (w(C)) between 3.75% and 3.85%, I tested nickel contents (w(Ni)) of 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, and 0.6%. The chemical composition of the base molten iron was meticulously monitored, as summarized in Table 1.
| Element | Content (w/%) |
|---|---|
| C | 3.85 |
| Si | 1.35 |
| Mn | 0.20 |
| S | 0.015 |
| P | 0.025 |
| Ni | 0.0039 |
| Cr | 0.015 |
| Mg | <0.0001 |
| Others | Trace amounts |
Melting was conducted using a medium-frequency induction furnace, with a charge ratio of pig iron: returns: scrap steel = 50:20:30. Ferrosilicon, carbon raiser, and ferromanganese were employed to adjust the composition. For consistency, the same batch of base molten iron was used for all trials. Nickel was added in the ladle, and each treatment involved 1000 kg of molten iron. The spheroidization process employed the “sandwich method,” with 1.2% Elken wind-power-specific nodularizer, covered by 0.4% calcium-barium inoculant and 0.8% silicon steel chips to delay the reaction onset. During pouring, 0.15% sulfur-oxygen inoculant was added as a stream inoculant. The treatment temperature was controlled at (1480 ± 10)°C. The chemical compositions after spheroidization are detailed in Table 2, showing minimal fluctuations to exclude compositional variations from affecting the final microstructure and properties of the nodular cast iron.
| Element | 0.1% Ni | 0.2% Ni | 0.3% Ni | 0.4% Ni | 0.5% Ni | 0.6% Ni |
|---|---|---|---|---|---|---|
| C | 3.75 | 3.73 | 3.70 | 3.70 | 3.74 | 3.75 |
| Si | 2.08 | 2.05 | 2.04 | 2.06 | 2.05 | 2.08 |
| Mn | 0.20 | 0.19 | 0.18 | 0.21 | 0.19 | 0.17 |
| S | 0.009 | 0.008 | 0.010 | 0.007 | 0.007 | 0.009 |
| P | 0.025 | 0.026 | 0.027 | 0.024 | 0.025 | 0.027 |
| Ni | 0.11 | 0.20 | 0.32 | 0.41 | 0.49 | 0.62 |
| Mg | 0.045 | 0.042 | 0.043 | 0.040 | 0.046 | 0.043 |
Test specimens were extracted from identical locations on the attached D-type test blocks and the 250 mm square blocks. For each nickel content, four samples were selected for analysis. The mechanical properties evaluated included tensile strength, yield strength, elongation, and hardness, while low-temperature impact tests were conducted at -20°C, -30°C, -40°C, -50°C, and -60°C. Metallographic examinations were performed on both the attached test blocks and the central sections of the 250 mm blocks to assess graphite morphology, nodularity, and matrix structure. The results for the attached test blocks are summarized in Table 3, and those for the 250 mm blocks are in Table 4.
| w(Ni) (%) | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HBW) | -20°C Impact (J) | -30°C Impact (J) | -40°C Impact (J) | -50°C Impact (J) | -60°C Impact (J) |
|---|---|---|---|---|---|---|---|---|---|
| 0.1 | 367 | 226 | 22.0 | 132 | 13.3 | 11.1 | 9.0 | 7.6 | 5.0 |
| 0.2 | 374 | 238 | 21.5 | 135 | 13.5 | 10.5 | 8.6 | 7.0 | 5.3 |
| 0.3 | 381 | 241 | 20.5 | 136 | 14.5 | 11.7 | 9.7 | 7.3 | 6.0 |
| 0.4 | 390 | 250 | 22.5 | 145 | 15.3 | 13.0 | 10.5 | 8.0 | 6.3 |
| 0.5 | 385 | 247 | 21.6 | 140 | 15.0 | 12.3 | 9.9 | 7.0 | 5.1 |
| 0.6 | 393 | 252 | 21.2 | 149 | 14.1 | 12.5 | 9.5 | 6.9 | 4.8 |
| w(Ni) (%) | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HBW) | -20°C Impact (J) | Pearlite Volume (%) | Nodularity (%) | Graphite Count (per mm²) |
|---|---|---|---|---|---|---|---|---|
| 0.1 | 370 | 223 | 20.0 | 141 | 11.0 | <5 | 86.9 | 122 |
| 0.2 | 375 | 229 | 19.4 | 143 | 11.5 | <5 | 91.7 | 131 |
| 0.3 | 386 | 240 | 20.5 | 145 | 12.5 | <5 | 92.3 | 118 |
| 0.4 | 391 | 243 | 21.5 | 147 | 13.0 | <5 | 91.8 | 112 |
| 0.5 | 395 | 250 | 19.3 | 143 | 10.5 | <15 | 84.1 | 102 |
| 0.6 | 402 | 260 | 17.0 | 152 | 10.0 | <20 | 86.5 | 93 |
The data reveal that all nickel-containing variants of nodular cast iron met the QT400-18AL requirements in the attached test blocks. For nickel contents exceeding 0.3%, the dual-grade specifications were also satisfied. However, impact properties declined noticeably with decreasing temperature. In the 250 mm blocks, mechanical properties were generally favorable, but at nickel contents of 0.5% and 0.6%, the presence of pearlite was observed, leading to reduced elongation and impact performance compared to the attached test blocks. This underscores the sensitivity of thick-section nodular cast iron to microstructural changes induced by alloying.
To analyze the mechanical performance trends, I plotted the relationship between nickel content and key properties. The tensile strength, yield strength, and hardness of the attached test blocks exhibited a clear upward trend with increasing nickel content, as illustrated in Figure 1 (data derived from Table 3). This can be attributed to solid-solution strengthening, where nickel atoms dissolve in the ferritic matrix, enhancing lattice strain and impeding dislocation movement. The solid-solution strengthening effect can be described by the following formula:
$$\sigma_y = \sigma_0 + k \cdot c^{1/2}$$
where $\sigma_y$ is the yield strength, $\sigma_0$ is the intrinsic strength of the nodular cast iron matrix, $k$ is a material constant, and $c$ is the concentration of nickel in solid solution. For nodular cast iron, this relationship implies that incremental additions of nickel systematically improve strength metrics. In contrast, elongation remained relatively stable across all nickel levels, fluctuating between 20.5% and 22.5%, indicating that nickel additions within this range do not compromise ductility in standard test blocks.
The low-temperature impact properties of nodular cast iron are critical for wind power applications. As shown in Figure 2 (based on Table 3 data), impact energy decreased significantly with lower temperatures for all nickel variants. At a given temperature, impact values initially improved with nickel content up to 0.4%, then gradually declined at higher nickel levels. For instance, at -40°C, the impact energy peaked at 10.5 J for 0.4% nickel, compared to 9.0 J for 0.1% nickel. This suggests an optimal nickel concentration for maximizing toughness in nodular cast iron under low-temperature conditions. The decline beyond 0.4% nickel may be linked to microstructural changes, such as pearlite formation, which can embrittle the material. The impact transition temperature $T_c$ can be modeled using an Arrhenius-type equation:
$$T_c = T_0 – \frac{A}{\ln(\sigma / \sigma_0)}$$
where $T_0$ is a reference temperature, $A$ is a constant, $\sigma$ is the applied stress, and $\sigma_0$ is a threshold stress. Nickel additions likely shift $T_c$ to lower values initially, enhancing low-temperature performance, but excessive nickel may alter the matrix stability, reversing this benefit.
For the 250 mm thick-section blocks, the trends were similar but more pronounced due to slower cooling rates. As summarized in Figure 3 (from Table 4), tensile and yield strengths increased monotonically with nickel content, while elongation and impact energy peaked at 0.4% nickel before decreasing. Hardness showed a slight upward trend. The metallographic analysis revealed that graphite counts decreased with higher nickel content, and nodularity varied without a clear pattern. At 0.5% and 0.6% nickel, pearlite volumes increased to below 20%, explaining the reduction in ductility and toughness. The presence of pearlite in thick-section nodular cast iron can be described by the lever rule for phase transformation:
$$f_{\alpha} = \frac{C_{\gamma} – C_0}{C_{\gamma} – C_{\alpha}}$$
where $f_{\alpha}$ is the fraction of ferrite, $C_{\gamma}$ and $C_{\alpha}$ are the equilibrium compositions of austenite and ferrite, and $C_0$ is the overall nickel content. As nickel increases, the phase boundaries shift, promoting pearlite formation in slow-cooled sections of nodular cast iron.
Metallographic observations confirmed that all samples exhibited satisfactory graphite nodularity without significant defects. In the attached test blocks, the graphite morphology was predominantly spherical, with the highest graphite count observed at 0.4% nickel. For the 250 mm blocks, the central sections showed fewer graphite nodules and increased intergranular inclusions, consistent with typical solidification behavior in thick-section nodular cast iron. The microstructural evolution aligns with the mechanical data, reinforcing the importance of nickel content control in optimizing nodular cast iron for heavy-duty applications.
In conclusion, this study demonstrates that nickel additions significantly influence the properties of thick-section low-temperature nodular cast iron. For attached test blocks, nickel content up to 0.4% enhances tensile strength, yield strength, hardness, and low-temperature impact energy, while elongation remains stable. Beyond 0.4%, strength and hardness continue to improve, but impact resistance and elongation gradually decline. In thick 250 mm blocks, similar trends are observed, with pearlite formation occurring at nickel contents above 0.4%, leading to reduced ductility and toughness. These findings provide a framework for tailoring nickel content in nodular cast iron to meet specific performance requirements in wind power and other demanding industries. Future work could explore synergistic effects with other micro-alloying elements to further optimize the balance between strength and toughness in advanced nodular cast iron grades.