In the context of advancing automotive industry demands for lightweight components, we focused on developing high-strength and high-elongation ductile iron castings, specifically targeting material grades such as QT700-10. Our approach involved optimizing chemical composition, cooling conditions, and nodularizing inoculation processes to enhance the microstructure, including ferrite distribution and pearlite morphology, thereby ensuring mechanical properties meet stringent standards while reducing micro-slag formation and improving performance stability. This article details our methodology, results, and analysis, emphasizing the role of innovative inoculants and process adjustments in achieving superior ductile iron castings.
The development of ductile iron castings with mixed matrix structures, comprising ferrite and pearlite, requires precise control over various parameters. We initiated the process by setting progressive targets: QT900-5, QT700-8, and QT700-10, with all test bars sampled from cast components having a wall thickness of 15 mm. Using medium-frequency induction furnaces for melting and green sand horizontal molding lines, we tailored the compositions and treatments for each grade. The chemical compositions for representative test bars are summarized in Table 1, highlighting key elements like carbon, silicon, and alloying additions such as copper and nickel, which influence the microstructure and properties of ductile iron castings.
| Test Bar | Material Grade | C | Si | Mn | P | S | Mg | Cu | Sn | Ni |
|---|---|---|---|---|---|---|---|---|---|---|
| #1 | QT900-5 | 3.58 | 2.51 | 0.21 | 0.02 | 0.01 | 0.039 | 0.72 | 0.01 | >1 |
| #2 | QT700-8 | 3.48 | 2.71 | 0.19 | 0.02 | 0.008 | 0.049 | 0.57 | 0.01 | 0.325 |
| #3 | QT700-10 | 3.49 | 2.8 | 0.25 | 0.02 | 0.007 | 0.043 | 0.53 | 0.01 | – |
For the nodularizing and inoculation treatments, we employed a three-step method where inoculants were directly placed over nodulizers in the ladle, covered with 0.5% silicon steel sheets. The process involved pre-heating the ladle, adding nodulizers, and pouring iron within 5 minutes, with the entire casting cycle completed within 8 minutes. Key parameters for the nodulizers and inoculants are outlined in Table 2. We utilized specific types such as ZM-N6013 and ZM-N6003 as low-rare earth nodulizers, and inoculants like ZM-IPE for strength enhancement, ZM-IFE for improved nucleation uniformity, ZM-IFA for increased graphite nodule count, and ZM-IFC for elongation improvement. These selections were critical in refining the microstructure of ductile iron castings and minimizing defects.
| Test Bar | Nodulizer Type | Nodulizer Addition (%) | In-Mold Inoculant Type | In-Mold Inoculant Addition (%) | Stream Inoculant Type | Stream Inoculant Addition (%) | Pouring Temperature (°C) | Pouring Time (min) |
|---|---|---|---|---|---|---|---|---|
| #1 | Mg5.5RE1.8 | 1.20 | ZM-IPE | 0.20 | ZM-IFA | 0.10 | 1380-1420 | ≤8 |
| #2 | ZM-N6013 | 1.05 | ZM-IFE | 0.20 | ZM-IFC | 0.10 | 1400-1440 | ≤8 |
| #3 | ZM-N6003 | 1.05 | ZM-IFE | 0.20 | ZM-IFC | 0.10 | – | ≤8 |
The mechanical properties and microstructural characteristics of the test bars are presented in Table 3. For instance, sample #1 (QT900-5) exhibited high tensile strength but limited elongation, whereas samples #2 and #3 showed progressive improvements in elongation while maintaining strength above 700 MPa. This underscores the importance of microstructural control in achieving balanced properties for ductile iron castings. The ferrite volume fraction increased from less than 5% in #1 to 30-45% in #3, accompanied by enhancements in graphite nodularity and pearlite refinement, which are vital for high-performance applications.
| Test Bar | Material Grade | Ferrite Volume Fraction (%) | Tensile Strength (MPa) | Elongation (%) | Brinell Hardness (HB) |
|---|---|---|---|---|---|
| #1 | QT900-5 | <5 | 932 | 5.0 | 334 |
| #2 | QT700-8 | 15-30 | 756 | 8.6 | 233 |
| #3 | QT700-10 | 30-45 | 722 | 10.2 | 251 |
In our analysis, we examined the distribution of ferrite and the structure of pearlite, as these factors significantly influence the mechanical behavior of ductile iron castings. For sample #3, which met the QT700-10 specifications, the microstructure revealed a typical “bull’s eye” pattern where ferrite enveloped graphite nodules, promoting ductility. The relationship between pearlite morphology and elongation can be described using a simplified formula for interlamellar spacing and its impact on toughness: $$ \lambda = \frac{k}{\sigma_y} $$ where $\lambda$ is the pearlite interlamellar spacing, $k$ is a material constant, and $\sigma_y$ is the yield strength. Reducing $\lambda$ through process optimization enhances elongation without compromising strength, as observed in samples with finer pearlite layers (e.g., spacing down to 0.1-0.2 μm).
Microdefects, such as inclusions and micro-slag, pose significant challenges to the stability of ductile iron castings. We compared two test bars from the same heat (3-1 and 3-2) with differing elongations (5.5% vs. 10.2%), attributing the variation to the size and location of slag inclusions. Energy-dispersive X-ray spectroscopy identified barium-rich and oxidation-related slags, which initiated brittle fracture paths. To mitigate this, we selected inoculants with lower melting points and more uniform phase distribution, such as ZM-IFC, which reduces slag formation tendency. The dissolution behavior of inoculants can be modeled using the heat of fusion equation: $$ Q = m \cdot \Delta H_f $$ where $Q$ is the energy required, $m$ is the mass, and $\Delta H_f$ is the enthalpy of fusion. For ZM-IFC, $\Delta H_f$ was measured at 285.8 J/g, compared to 352.8 J/g for conventional silicon-barium inoculants, indicating faster dissolution and lower energy consumption, thereby minimizing defect formation in ductile iron castings.
Furthermore, we investigated intergranular precipitates involving elements like nickel and molybdenum, which can adversely affect elongation if forming continuous networks. In samples with higher precipitate density, elongation decreased due to embrittlement. However, through compositional adjustments and cooling rate control, we achieved a more dispersed distribution, alleviating this issue. The role of alloying elements in precipitate formation can be expressed as: $$ C_{eq} = C + \frac{Si}{4} + \frac{Mn}{6} + \frac{Cu}{15} + \frac{Ni}{20} $$ where $C_{eq}$ is the carbon equivalent, influencing graphitization and microstructure in ductile iron castings. By optimizing $C_{eq}$ and other parameters, we balanced strength and ductility effectively.
In conclusion, our work demonstrates that enhancing the properties of as-cast high-strength ductile iron castings relies on meticulous microstructural engineering. Key strategies include adjusting chemical composition, refining pearlite morphology, and selecting advanced inoculants to reduce micro-slag. These approaches not only improve mechanical performance but also enhance production consistency for ductile iron casting applications in automotive and other industries. Future efforts will focus on further optimizing process variables to push the boundaries of ductile iron castings’ capabilities.