In the automotive industry, gray cast iron has been extensively utilized for critical components like engine blocks due to its superior castability, wear resistance, and damping capacity. As diesel engines evolve towards higher power outputs, the demand for enhanced material performance intensifies. Traditionally, two primary approaches have been pursued: developing high-strength gray cast iron or adopting ductile iron castings. While ductile iron castings offer excellent mechanical properties, they often present challenges in manufacturing consistency, higher costs, and complex processing. In contrast, gray cast iron exhibits better castability and, owing to its flake graphite structure that forms interconnected thermal pathways, provides superior thermal conductivity compared to ductile iron castings and compacted graphite iron. This study investigates the effects of low carbon equivalent (CE ≤ 3.9%) on the microstructure, mechanical properties, and thermal performance of gray cast iron, with implications for applications where a balance of strength and thermal management is crucial, potentially reducing the reliance on ductile iron castings in certain scenarios.
The experimental gray cast iron samples were prepared with carbon equivalents of 3.2%, 3.4%, 3.6%, and 3.8%, calculated using the formula CE = C + Si/3, where C and Si represent the mass percentages of carbon and silicon, respectively. These compositions were achieved through metal mold casting in a medium-frequency induction furnace. The molten iron was maintained at 1500°C for 10 minutes to ensure homogeneity, with alloying elements such as copper, molybdenum, and tin added to promote pearlite formation and inhibit ferrite. Inoculation treatment was performed at 1450°C before pouring into the metal mold to refine the graphite structure. The chemical composition ranges are summarized in Table 1, highlighting the controlled variations in key elements to achieve the desired carbon equivalents.
| Element | Range |
|---|---|
| C | 2.8 – 3.4 |
| Si | 1.2 |
| Mn | 0.3 – 0.6 |
| Cu | 0.3 – 0.6 |
| Mo | 0.2 – 0.5 |
| Sn | 0.8 – 1.2 |
| S | 0.05 – 0.09 |
Microstructural analysis was conducted using optical microscopy and scanning electron microscopy (SEM) to examine graphite morphology and pearlite distribution. Mechanical properties, including Brinell hardness and tensile strength, were evaluated with a Brinell hardness tester under a 3000 kgf load and an electronic universal testing machine at a strain rate of 1 mm/min, respectively. Thermal diffusivity was measured using a laser flash apparatus from room temperature up to 600°C, with the data analyzed to derive thermal conductivity relationships. These methods allowed for a comprehensive evaluation of how carbon equivalent influences material behavior, providing insights that could inform the design of high-strength gray iron as an alternative to ductile iron castings in thermal management applications.
The microstructure of the gray cast iron primarily consisted of flake graphite embedded in a pearlite matrix, with minimal ferrite content due to the addition of pearlite-promoting elements. As the carbon equivalent decreased from 3.8% to 3.2%, significant changes in graphite morphology were observed. Lower carbon equivalents resulted in finer, more curved graphite flakes, classified as type E graphite, whereas higher carbon equivalents led to coarser, straighter flakes akin to type C graphite. This refinement in graphite structure is critical because it reduces stress concentration points, thereby enhancing mechanical integrity. In contrast, ductile iron castings feature spheroidal graphite, which improves toughness but may not offer the same level of thermal conductivity due to the discontinuous graphite network. The quantitative analysis of graphite characteristics, as shown in Table 2, reveals a clear trend: both graphite length and content increase with rising carbon equivalent, underscoring the trade-off between graphite-induced thermal benefits and mechanical weaknesses.
| Carbon Equivalent (%) | Graphite Length (μm) | Graphite Content (%) |
|---|---|---|
| 3.2 | 55.38 | 7.0 |
| 3.4 | 70.25 | 7.5 |
| 3.6 | 90.41 | 8.0 |
| 3.8 | 111.93 | 8.5 |
The pearlite matrix also exhibited notable variations with carbon equivalent. At lower carbon equivalents, the pearlite interlamellar spacing decreased significantly, from an average of 1175 nm at CE 3.8% to 397 nm at CE 3.2%, resulting in a denser and more uniform distribution. This refinement contributes to higher strength and hardness, as pearlite acts as a strengthening phase in gray iron. The relationship between carbon equivalent and pearlite spacing can be modeled using a linear approximation, highlighting the microstructural control achievable through composition adjustments. For instance, the pearlite spacing (PS) in nanometers can be expressed as a function of carbon equivalent (CE):
$$ PS = 1850 \times CE – 5100 $$
with a coefficient of determination R² ≈ 0.95, indicating a strong correlation. Such microstructural optimization is essential for developing high-strength gray iron that can compete with ductile iron castings in terms of mechanical performance, while retaining the thermal advantages of flake graphite.
Mechanical properties demonstrated a pronounced improvement with decreasing carbon equivalent. The tensile strength increased from 251.1 MPa at CE 3.8% to 385.9 MPa at CE 3.2%, representing a 27% enhancement, while Brinell hardness rose from 210 HB to 245 HB, a 16.7% increase. These values meet or exceed the requirements for high-strength gray iron grades such as HT350, making it a viable candidate for demanding applications like cylinder blocks. The relationships between carbon equivalent and mechanical properties were quantified using polynomial regression models. For tensile strength (Rm in MPa):
$$ R_m = -5175 \times CE^2 + 3412 \times CE – 5238 $$
with R² = 0.967, and for Brinell hardness (HBW):
$$ HBW = -184 \times CE^2 + 1239 \times CE – 1835 $$
with R² = 0.923. These equations emphasize the inverse correlation between carbon equivalent and mechanical performance, suggesting that lower carbon equivalents facilitate a stronger matrix by reducing graphite size and optimizing pearlite. In comparison, ductile iron castings typically exhibit higher tensile strengths (e.g., 400-600 MPa) but may not provide the same level of thermal dissipation, highlighting the context-dependent advantages of gray iron.
Fracture analysis further elucidated the mechanical behavior. At lower carbon equivalents, the fracture surfaces displayed fewer and smaller voids resulting from graphite decohesion, along with increased river patterns indicative of higher energy absorption during crack propagation. This contrasts with higher carbon equivalent samples, where larger graphite flakes led to more extensive void formation and brittle fracture modes. The fracture toughness, though not directly measured, can be inferred to improve with graphite refinement, aligning with the goal of achieving high-strength gray iron that minimizes the need for ductile iron castings in components where thermal conductivity is paramount. The quality parameters, including degree of saturation (Sc), maturity (Rc), hardening (Hc), and quality factor (Qi), were evaluated to assess the overall material performance. For example, at CE 3.2%, Qi was 1.05, indicating an optimal balance of strength and machinability, whereas at CE 3.8%, Qi decreased to 0.91 due to reduced mechanical properties. These parameters are defined as:
$$ Sc = \frac{C_t}{4.265 – \frac{1}{3}(Si + P)} $$
$$ Rc = \frac{R_m}{1000 – 800 \times Sc} $$
$$ Hc = \frac{HBW}{530 – 344 \times Sc} $$
$$ Qi = \frac{Rc}{Hc} $$
where Ct is the actual carbon content, Si and P are silicon and phosphorus percentages, Rm is tensile strength, and HBW is Brinell hardness. The consistency in Qi values for CE 3.2% to 3.6% (around 1.05) underscores the stability of low carbon equivalent gray iron, whereas the drop at CE 3.8% reflects the detrimental effects of excessive graphite.
Thermal performance, a critical aspect for engine components, was evaluated through thermal diffusivity measurements. The results showed that thermal diffusivity decreased with increasing temperature for all carbon equivalents, but the rate of decrease was more pronounced at higher carbon equivalents. Specifically, at CE 3.8%, the thermal diffusivity declined by approximately 1.35 cm²/s per 100°C rise in temperature, whereas at CE 3.2%, the decrease was only 0.97 cm²/s per 100°C. This reduced temperature dependence at lower carbon equivalents is advantageous for applications involving thermal cycling, as it ensures more stable heat dissipation. The thermal diffusivity (α in cm²/s) as a function of temperature (T in °C) can be described by linear equations for each carbon equivalent:
For CE 3.2%: $$ \alpha = -0.0097 \times T + 0.1324 $$
For CE 3.4%: $$ \alpha = -0.0094 \times T + 0.1328 $$
For CE 3.6%: $$ \alpha = -0.0107 \times T + 0.1417 $$
For CE 3.8%: $$ \alpha = -0.0135 \times T + 0.1643 $$
with coefficients of determination R² ≥ 0.99, confirming the reliability of these models. The higher initial thermal diffusivity at elevated carbon equivalents is attributed to increased graphite content, which enhances thermal conduction paths; however, the greater sensitivity to temperature changes may limit performance in high-temperature environments. In contrast, ductile iron castings, with their isolated graphite spheres, typically exhibit lower thermal conductivity but better mechanical resilience, illustrating the trade-offs in material selection for thermal management systems.
The interplay between microstructure and properties underscores the potential of low carbon equivalent gray iron as a high-performance material. By refining graphite morphology and pearlite structure, it is possible to achieve a combination of strength and thermal conductivity that meets the demands of advanced automotive applications. For instance, in cylinder blocks, where both mechanical integrity and heat dissipation are critical, low carbon equivalent gray iron could serve as a cost-effective alternative to ductile iron castings, especially in scenarios where manufacturing simplicity and thermal performance are prioritized. Future research could explore hybrid approaches, such as incorporating elements that further enhance graphite nodularity or comparing the long-term durability against ductile iron castings under operational conditions.
In conclusion, reducing the carbon equivalent in gray cast iron from 3.8% to 3.2% leads to significant improvements in microstructure, mechanical properties, and thermal stability. The refinement of graphite and pearlite results in tensile strengths up to 385.9 MPa and enhanced hardness, while the thermal diffusivity shows reduced temperature dependence. Although ductile iron castings offer superior toughness and strength in many cases, the tailored properties of low carbon equivalent gray iron make it a compelling choice for applications requiring a balance of mechanical performance and thermal management. This study highlights the importance of compositional control in optimizing cast iron materials, potentially expanding the use of gray iron in high-power engine components and reducing the dependency on ductile iron castings where thermal conductivity is a key factor.