The evolution of the cast iron industry has seen a consistent rise in the adoption of ductile and compacted graphite irons. However, grey iron casting remains the dominant material, holding the largest market share due to its excellent castability and other favorable properties. In our foundry operations, grey iron casting constitutes the primary output, with grades predominantly clustered around high-strength specifications like HT280 and HT300. Our established production route relies on a high carbon equivalent, alloyed process, where the requisite strength and hardness of the grey iron casting are achieved through the addition of specific types and quantities of alloying elements.
In recent years, the escalating cost of alloying elements, particularly ferromolybdenum, has imposed significant financial pressure. Ferromolybdenum alone can account for over 60% of the total alloy cost in a typical grey iron casting production batch. This economic challenge has necessitated the exploration of alternative methodologies to maintain the critical performance metrics of grey iron castings while reducing or eliminating the dependence on costly alloys like molybdenum. One promising avenue is the adjustment of the fundamental composition ratio, specifically increasing the silicon-to-carbon (Si/C) ratio.
1. Foundry Production Conditions and Baseline Process
1.1 Melting Equipment and Raw Materials
The melting process for our grey iron casting production is conducted in medium-frequency induction furnaces, typically with an 8-ton capacity. The primary charge materials consist of pig iron, steel scrap, and returns (gating systems and rejected castings). Additional materials for composition adjustment include bulk ferrosilicon, ferromanganese, and carbon raiser.
1.2 Furnace Melting and Base Iron Composition
The standard charging procedure is followed: 5%–10% pig iron, 50%–60% steel scrap, with the remainder being returns. The materials are charged in the sequence: pig iron, returns, and finally steel scrap. During the melting process, ferrosilicon, ferromanganese, and carbon raiser are added to adjust the base molten iron to the target composition range, as summarized in Table 1.
| C | Si | Mn | P | S |
|---|---|---|---|---|
| 3.20–3.30 | 1.50–1.65 | 0.40–0.80 | < 0.06 | 0.08–0.12 |
1.3 Ladle Treatment and Alloying Principles
Following melting, a ladle treatment is performed. Inoculant and specific alloys are added to the bottom of the ladle before tapping. The tapping temperature is strictly controlled between 1,480 °C and 1,540 °C to ensure effective dissolution and assimilation of the additives. The inoculant used is a silicon-barium type.
For grades HT280 and above, a combination of alloys—electrolytic copper, tin granules, ferrochromium, and ferromolybdenum—is typically added to ensure consistent properties across varying section thicknesses of the grey iron casting. The primary functions of these elements in grey iron are detailed in Table 2.
| Element | Primary Functions in Grey Iron Casting |
|---|---|
| Copper (Cu) | Refines and homogenizes the pearlite and graphite structures; reduces chill tendency in thin sections; improves section sensitivity. |
| Tin (Sn) | Reduces or eliminates ferrite; stabilizes and refines pearlite; improves uniformity across different sections. |
| Chromium (Cr) | Refines graphite; refines and stabilizes the pearlitic matrix. |
| Molybdenum (Mo) | Refines graphite; increases the amount and refines the structure of pearlite; promotes carbide formation; enhances uniformity of microstructure in heavy sections. |
The final target composition for an HT280 grey iron casting, after ladle treatment, is shown in Table 3. Elements like C, S, Mn, and P are primarily controlled via the base iron composition and are not actively adjusted during the ladle treatment.
| Si | Cu | Cr | Sn | Mo |
|---|---|---|---|---|
| 1.70–2.00 | 0.30–0.50 | 0.20–0.40 | 0.03–0.05 | 0.10–0.20 |
1.4 Molding, Pouring, and Inoculation
Test bars and castings are produced using molds made of alkaline phenolic resin no-bake sand. During pouring, a secondary inoculation step is implemented using a stream inoculant. A silicon-zirconium inoculant with a grain size of 0.4–0.8 mm is employed at an addition rate of 0.08%–0.12%. It is uniformly dispersed onto the metal stream via a pneumatic dispensing device on the pouring line to ensure effective absorption. The pouring temperature is typically maintained between 1,360 °C and 1,400 °C to ensure complete mold filling.
2. Experimental Strategy for High Si/C Ratio Grey Iron Casting
The core objective of this exploration is to transition from a Cu-Cr-Mo-Sn alloyed grey iron casting to a Cu-Cr-Sn alloyed grey iron casting, maintaining the same carbon equivalent (CE). This shift involves eliminating the use of ferromolybdenum, which averages a reduction of approximately 0.15% Mo addition, projecting a cost saving of around 600 RMB per ton of grey iron casting produced.
The key modification is increasing the Si/C ratio from approximately 0.55 to 0.70. This is achieved by lowering the average carbon content by about 0.1% and simultaneously raising the average silicon content by about 0.4%. The comparative compositional ranges for the standard and high Si/C ratio grey iron casting are presented in Table 4.
| Material Designation | Base Iron C | Base Iron Si | Final Si | Cu | Cr | Sn | Mo |
|---|---|---|---|---|---|---|---|
| Cu-Cr-Mo-Sn Alloyed | 3.20–3.30 | 1.50–1.65 | 1.70–2.00 | 0.30–0.50 | 0.20–0.40 | 0.03–0.05 | 0.10–0.20 |
| Cu-Cr-Sn Alloyed (High Si/C ~0.70) | 3.10–3.20 | 1.50–1.65 | 2.10–2.20 | 0.30–0.50 | 0.20–0.40 | 0.03–0.05 | – |
Maintaining a constant carbon equivalent while increasing the Si/C ratio induces several interrelated changes in the solidification behavior, microstructure, and final properties of the grey iron casting. The carbon equivalent (CE) is commonly calculated using the formula:
$$ CE = \%C + \frac{\%Si + \%P}{3} $$
And the silicon-to-carbon ratio is simply:
$$ Si/C = \frac{\%Si}{\%C} $$
The microstructural and property implications are:
- Increased Primary Austenite: A higher Si/C ratio promotes the formation of more primary austenite dendrites during solidification, which can significantly strengthen the metallic matrix of the grey iron casting.
- Reduced Graphite Amount: The overall lower carbon content decreases the total volume fraction of graphite flakes. This mitigates the notch effect and the severing of the matrix by graphite, which is beneficial for strength.
- Pearlite Coarsening Tendency: The eutectoid transformation temperature is raised. Pearlite formed at a higher temperature tends to be coarser, which could negatively impact strength.
- Solid Solution Strengthening: A greater amount of silicon dissolves in the ferrite phase (including the ferrite within pearlite), leading to solid solution strengthening of the matrix in the grey iron casting.
The net effect on the mechanical properties of the grey iron casting is a complex balance of these competing factors.
3. Experimental Procedure and Results for High Si/C Grey Iron Casting
3.1 Procedure
Following the experimental plan, the carbon content of the base iron was adjusted to the range of 3.10%–3.20%. The amount of silicon-barium inoculant added during the ladle treatment was increased to compensate for the higher final silicon target, and the addition of ferromolybdenum was completely eliminated. All other alloy additions (Cu, Cr, Sn) remained unchanged.
Using standard ladle practice, one-ton heats were tapped to pour separately cast test bars and select production castings. The pouring temperature was controlled between 1,360 °C and 1,400 °C. Four separate trial heats were conducted. For each heat, the chemical composition, microstructure, and mechanical properties of the separately cast test bars were analyzed. Furthermore, critical sections of the actual grey iron castings were sectioned to evaluate the microstructure and mechanical properties at the casting body.
3.2 Results
3.2.1 Separately Cast Test Bar Properties
The results for the Cu-Cr-Sn alloyed grey iron casting with a high Si/C ratio (~0.7) are summarized in Table 5. All measured values for the test bars met the specified requirements for the HT280 grade of grey iron casting. The internal specification demands a tensile strength (Rm) ≥ 280 MPa and a Brinell hardness (HBW) between 190 and 260.
| Grade | Si/C | Composition (mass fraction, %) | Microstructure | Mechanical Properties | ||
|---|---|---|---|---|---|---|
| C / Final Si / Cu / Cr / Sn | Graphite | Matrix | Tensile (MPa) | Hardness (HBW) | ||
| HT280 | 0.70 | 3.11 / 2.18 / 0.47 / 0.29 / 0.047 | Type A, Size 4 | 98% Pearlite | 295 | 209 |
| HT280 | 0.67 | 3.15 / 2.11 / 0.45 / 0.25 / 0.040 | Type A, Size 4 | 98% Pearlite | 290 | 221 |
| HT280 | 0.68 | 3.18 / 2.17 / 0.43 / 0.30 / 0.043 | Type A, Size 4 | 98% Pearlite | 300 | 211 |
| HT280 | 0.69 | 3.12 / 2.16 / 0.40 / 0.24 / 0.043 | Type A, Size 4 | 98% Pearlite | 310 | 220 |
| Average | 0.69 | – / 2.15 / 0.44 / 0.27 / 0.04 | – | – | 299 | 215 |
3.2.2 Properties of the Casting Body (Bulk Properties)
A production grey iron casting was sectioned at a critical location—the main bolt boss area of a bearing cap, with a section thickness of approximately 50 mm. The results from the body samples, shown in Table 6, confirmed that the properties within the actual grey iron casting met the performance requirements, aligning with expectations.
| Sample ID | Grade | Microstructure | Mechanical Properties | ||
|---|---|---|---|---|---|
| Graphite | Matrix | Tensile (MPa) | Hardness (HBW) | ||
| 1# | HT280 | Type A, Size 4 | 98% Pearlite | 262 | 183 |
| 2# | HT280 | Type A, Size 4 | 98% Pearlite | 245 | 193 |
4. Comparative Analysis and Discussion
4.1 Compositional and Mechanical Performance Benchmarking
To establish a proper baseline, the performance data for the standard Cu-Cr-Mo-Sn alloyed grey iron casting (Si/C ~0.55) was collected and is presented in Table 7.
| Grade | Si/C | Composition (mass fraction, %) | Microstructure | Mechanical Properties | ||
|---|---|---|---|---|---|---|
| C / Final Si / Cu / Cr / Sn / Mo | Graphite | Matrix | Tensile (MPa) | Hardness (HBW) | ||
| HT280 | 0.58 | 3.25 / 1.89 / 0.46 / 0.25 / 0.046 / 0.15 | Type A, Size 4 | 98% Pearlite | 335 | 246 |
| HT280 | 0.57 | 3.24 / 1.85 / 0.46 / 0.28 / 0.047 / 0.12 | Type A, Size 4 | 98% Pearlite | 325 | 246 |
| HT280 | 0.55 | 3.26 / 1.79 / 0.47 / 0.29 / 0.045 / 0.12 | Type A, Size 4 | 98% Pearlite | 285 | 263 |
| HT280 | 0.55 | 3.30 / 1.80 / 0.47 / 0.30 / 0.047 / 0.13 | Type A, Size 4 | 98% Pearlite | 310 | 245 |
| Average | 0.56 | – / 1.83 / 0.47 / 0.28 / 0.046 / 0.13 | – | – | 314 | 250 |
To further enrich the comparison and isolate the effect of molybdenum, additional verification trials were conducted using the Cu-Cr-Sn alloy composition but at the standard low Si/C ratio (~0.55). The metal from these verification heats was poured into test bars and then returned to the furnace to prevent the production of off-specification grey iron castings. The results are shown in Table 8.
| Grade | Si/C | Composition (mass fraction, %) | Microstructure | Mechanical Properties | ||
|---|---|---|---|---|---|---|
| C / Final Si / Cu / Cr / Sn | Graphite | Matrix | Tensile (MPa) | Hardness (HBW) | ||
| HT280 | 0.57 | 3.25 / 1.85 / 0.48 / 0.26 / 0.045 | Type A, Size 4 | 98% Pearlite | 275 | 200 |
| HT280 | 0.57 | 3.26 / 1.85 / 0.45 / 0.26 / 0.042 | Type A, Size 4 | 98% Pearlite | 295 | 210 |
| HT280 | 0.54 | 3.30 / 1.79 / 0.45 / 0.25 / 0.041 | Type A, Size 4 | 98% Pearlite | 280 | 208 |
| HT280 | 0.56 | 3.26 / 1.83 / 0.44 / 0.29 / 0.042 | Type A, Size 4 | 98% Pearlite | 285 | 195 |
| Average | 0.56 | – / 1.83 / 0.46 / 0.27 / 0.043 | – | – | 284 | 203 |
The performance comparison of the three distinct grey iron casting compositions is synthesized in Table 9 and can be analyzed as follows:
| Grey Iron Casting Type | Avg. Si/C | Avg. Rm (MPa) | Avg. HBW | Key Alloy Difference |
|---|---|---|---|---|
| Cu-Cr-Mo-Sn (Baseline) | 0.56 | 314 | 250 | Contains ~0.13% Mo |
| Cu-Cr-Sn (Low Si/C) | 0.56 | 284 | 203 | No Mo |
| Cu-Cr-Sn (High Si/C ~0.7) | 0.69 | 299 | 215 | No Mo, High Si/C |
- Effect of Molybdenum at Constant Low Si/C: Comparing the first two rows in Table 9, at an identical Si/C ratio of ~0.55, the presence of molybdenum in the baseline grey iron casting provides a significant strengthening effect. The average tensile strength is 10.6% higher (314 MPa vs. 284 MPa) and the hardness is 23% higher (250 HBW vs. 203 HBW). This confirms that simply eliminating molybdenum from the standard low Si/C composition poses a performance risk, as the strength occasionally approaches the minimum specification limit (e.g., 275 MPa in one trial).
- Effect of High Si/C Ratio (Molybdenum-Free): The crucial finding is observed by comparing the last two rows. At the same carbon equivalent but with the Si/C ratio increased from ~0.56 to ~0.69, the tensile strength of the molybdenum-free grey iron casting increased by approximately 5.3% (from 284 MPa to 299 MPa). The hardness also increased by about 5.9% (from 203 HBW to 215 HBW). Most importantly, the high Si/C ratio, Cu-Cr-Sn alloyed grey iron casting consistently achieved an average tensile strength of 299 MPa, which is well above the 280 MPa requirement for HT280. This demonstrates that the microstructural modifications induced by the high Si/C ratio effectively compensate for the strength loss resulting from the removal of molybdenum.
4.2 Microstructural Comparison
A comparative analysis of the microstructures reveals distinct differences. The standard Cu-Cr-Mo-Sn grey iron casting exhibits a typical, well-refined pearlitic matrix with a uniform distribution of Type A graphite. In contrast, the high Si/C ratio Cu-Cr-Sn grey iron casting shows noticeably shorter and more segmented graphite flakes. This reduced graphite length diminishes the stress-concentrating effect and the severing of the metallic matrix, contributing positively to strength. While the pearlite content remains similarly high (>98%) in both, the grain structure of the high Si/C ratio material appears more refined. This refinement, coupled with the solid solution strengthening from the higher silicon content in the ferrite, counteracts the potential coarsening of pearlite that can occur at the raised eutectoid temperature.
5. Conclusions for Grey Iron Casting Production
- The conventional Cu-Cr-Mo-Sn alloying approach for producing high-grade (e.g., HT280) grey iron castings involves significant alloy addition costs. Directly reducing these alloy additions, particularly molybdenum, while maintaining a low Si/C ratio carries a substantial risk of failing to meet mechanical property specifications.
- Maintaining a constant carbon equivalent while strategically increasing the silicon-to-carbon (Si/C) ratio presents a viable pathway for cost reduction in grey iron casting production. The increase in Si/C ratio itself induces microstructural changes that enhance mechanical performance.
- For a Cu-Cr-Sn alloyed grey iron casting, increasing the Si/C ratio from approximately 0.55 to 0.70 results in an average tensile strength improvement of about 5%. This enhancement is sufficient to ensure the grey iron casting meets the HT280 grade requirements without the use of ferromolybdenum. While not as potent as direct molybdenum alloying, the high Si/C strategy offers a clear economic advantage.
- In the context of rising production costs for alloyed grey iron castings, the strategy of increasing the Si/C ratio to enable a reduction in expensive alloy additions represents a practical and effective direction for foundries seeking to optimize their cost structure without compromising the integrity and performance of their grey iron casting products.
The successful application of this principle requires precise control over melting and inoculation practices to manage the inherent changes in solidification and microstructure. However, the potential for significant cost savings makes the high Si/C ratio approach a compelling consideration for the future of economical, high-performance grey iron casting manufacturing.