As a foundry engineer, I have been deeply involved in the production of grey iron castings, which remain the most widely used material in the casting industry due to their excellent castability and cost-effectiveness. Despite the rising prominence of ductile iron and compacted graphite iron, grey iron castings still dominate our production lines, particularly for high-grade applications such as HT280 and HT300. Traditionally, we have relied on a high carbon equivalent alloying route, incorporating elements like copper, chromium, tin, and molybdenum to enhance strength and hardness. However, with the recent surge in alloy prices, especially ferromolybdenum, which accounts for over 60% of our total alloy costs, there has been an urgent need to explore alternative methods to maintain performance while reducing expenses. This led me to investigate the potential of high silicon-carbon ratio (Si/C) grey iron castings as a means to minimize or eliminate molybdenum usage.
In our facility, we primarily use medium-frequency induction melting furnaces, typically an 8-ton capacity, to produce grey iron castings. The raw materials consist of pig iron, steel scrap, and returns, supplemented with ferrosilicon, ferromanganese, and carburizers for composition adjustment. The melting process follows a standard charge sequence: 5%–10% pig iron, 50%–60% steel scrap, and the remainder as returns. During melting, additions like ferrosilicon, ferromanganese, and carburizer are made to adjust the base iron composition to the ranges shown 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 |
After melting, ladle treatment is performed to ensure the final properties of the grey iron castings. Inoculants and alloys are added to the ladle bottom, with tapping temperatures controlled between 1,480 and 1,540°C to promote effective absorption. We use barium-bearing inoculant for primary inoculation, along with alloys such as electrolytic copper, tin granules, ferromolybdenum, and ferrochromium. For grades like HT280 and above, all four alloys are typically added to guarantee performance across varying section thicknesses. The roles of these elements are well-understood: copper refines and uniformizes pearlite and graphite, reducing chill in thin sections; tin minimizes ferrite, stabilizes and refines pearlite, and improves section sensitivity; chromium refines graphite and pearlite; and molybdenum enhances graphite refinement, promotes pearlite formation, and improves uniformity in heavy sections. The final composition control for HT280 grey iron castings is summarized in Table 2, with C, S, Mn, and P maintained as per the base iron.
| 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 |
The casting process involves using alkaline phenolic resin self-setting sand for molds, with secondary inoculation via stream inoculation using a zirconium-bearing inoculant (0.4–0.8 mm grain size, 0.08%–0.12% addition). This is applied uniformly via a pneumatic spraying device on the pouring machine to ensure effective absorption. Pouring temperatures are kept between 1,360 and 1,400°C to ensure complete filling. To explore cost reduction, I designed a trial focusing on high silicon-carbon ratio grey iron castings. The objective was to switch from Cu-Cr-Mo-Sn alloyed grey iron to Cu-Cr-Sn alloyed grey iron by increasing the Si/C ratio while maintaining the same carbon equivalent. This approach aimed to eliminate ferromolybdenum, saving approximately 0.15% Mo addition and reducing costs by around 600 yuan per ton. The Si/C ratio was raised from 0.55 to 0.7, achieved by decreasing the average carbon content by about 0.1% and increasing the silicon content by about 0.4%. The chemical composition ranges for the original and modified grey iron castings are compared in Table 3.
| Material | Base Iron C | Base Iron Si | Final Si | Cu | Cr | Sn | Mo |
|---|---|---|---|---|---|---|---|
| Cu-Cr-Mo-Sn Alloy | 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 (High Si/C 0.70) Alloy | 3.10–3.20 | 1.50–1.65 | 2.10–2.20 | 0.30–0.50 | 0.20–0.40 | 0.03–0.05 | – |
The carbon equivalent (CE) for grey iron castings is a critical parameter, typically calculated using the formula: $$ CE = \omega(C) + \frac{1}{3}\omega(Si) $$. By keeping CE constant, the increase in Si/C ratio influences the solidification characteristics and microstructure. Specifically, a higher Si/C ratio leads to an increase in primary austenite, which strengthens the matrix; a reduction in total graphite content, decreasing the notch effect of graphite flakes; a higher eutectoid transformation temperature, potentially coarsening pearlite; and enhanced solid solution strengthening of ferrite due to higher silicon content. To quantify the Si/C ratio, I use the equation: $$ \text{Si/C ratio} = \frac{\omega(Si)}{\omega(C)} $$, where ω denotes mass fraction. This ratio is pivotal in optimizing the properties of grey iron castings.
For the trials, I adjusted the base iron carbon content to 3.10%–3.20% while keeping other elements unchanged. During ladle treatment, I increased the barium inoculant addition and omitted ferromolybdenum. Using a 1-ton ladle, I poured separate test bars and actual castings, with pouring temperatures controlled at 1,360–1,400°C. Four trials were conducted, and the results for the Cu-Cr-Sn alloy grey iron castings with a Si/C ratio of 0.7 are summarized in Table 4. The mechanical properties met the HT280 requirements, which specify a tensile strength ≥280 MPa and hardness of 190–260 HBW. The microstructure of these grey iron castings showed Type A graphite with a length rating of 4 and approximately 98% pearlite, indicating satisfactory refinement.
| Grade | ω(Si)/ω(C) | Chemical Composition (Mass Fraction, %) | Microstructure | Mechanical Properties | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| C | Si | Cu | Cr | Sn | Graphite | Matrix | Tensile Strength (MPa) | Hardness (HBW) | ||
| HT280 | 0.70 | 3.11 | 2.18 | 0.47 | 0.29 | 0.047 | Type A, Length 4 | 98% Pearlite | 295 | 209 |
| HT280 | 0.67 | 3.15 | 2.11 | 0.45 | 0.25 | 0.04 | Type A, Length 4 | 98% Pearlite | 290 | 221 |
| HT280 | 0.68 | 3.18 | 2.17 | 0.43 | 0.3 | 0.043 | Type A, Length 4 | 98% Pearlite | 300 | 211 |
| HT280 | 0.69 | 3.12 | 2.16 | 0.4 | 0.24 | 0.043 | Type A, Length 4 | 98% Pearlite | 310 | 220 |
| Average | 0.69 | – | 2.15 | 0.44 | 0.27 | 0.04 | – | – | 299 | 215 |
To assess real-world performance, I dissected actual grey iron castings at critical locations, such as the main bolt area of a bearing cap with a thickness of about 50 mm. The results, presented in Table 5, confirmed that the properties of the high Si/C ratio grey iron castings met the required specifications, demonstrating the viability of this approach for industrial applications.
| Sample ID | Grade | Microstructure | Mechanical Properties | ||
|---|---|---|---|---|---|
| Graphite | Matrix | Tensile Strength (MPa) | Hardness (HBW) | ||
| 1# | HT280 | Type A, Length 4 | 98% Pearlite | 262 | 183 |
| 2# | HT280 | Type A, Length 4 | 98% Pearlite | 245 | 193 |
For comparison, I also analyzed data from our standard Cu-Cr-Mo-Sn alloy grey iron castings with a Si/C ratio of 0.55, as shown in Table 6. Additionally, to ensure a fair evaluation, I conducted supplementary trials with Cu-Cr-Sn alloy grey iron castings at the same Si/C ratio of 0.55 (without molybdenum), with the results in Table 7. These comparisons highlight the impact of alloying and Si/C ratio on the performance of grey iron castings.
| Grade | ω(Si)/ω(C) | Chemical Composition (Mass Fraction, %) | Microstructure | Mechanical Properties | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| C | Si | Cu | Cr | Sn | Mo | Graphite | Matrix | Tensile Strength (MPa) | Hardness (HBW) | ||
| HT280 | 0.58 | 3.25 | 1.89 | 0.46 | 0.25 | 0.046 | 0.15 | Type A, Length 4 | 98% Pearlite | 335 | 246 |
| HT280 | 0.57 | 3.24 | 1.85 | 0.46 | 0.28 | 0.047 | 0.12 | Type A, Length 4 | 98% Pearlite | 325 | 246 |
| HT280 | 0.55 | 3.26 | 1.79 | 0.47 | 0.29 | 0.045 | 0.12 | Type A, Length 4 | 98% Pearlite | 285 | 263 |
| HT280 | 0.55 | 3.30 | 1.80 | 0.47 | 0.30 | 0.047 | 0.13 | Type A, Length 4 | 98% Pearlite | 310 | 245 |
| Average | 0.56 | – | 1.83 | 0.47 | 0.28 | 0.046 | 0.13 | – | – | 314 | 250 |
| Grade | ω(Si)/ω(C) | Chemical Composition (Mass Fraction, %) | Microstructure | Mechanical Properties | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| C | Si | Cu | Cr | Sn | Graphite | Matrix | Tensile Strength (MPa) | Hardness (HBW) | ||
| HT280 | 0.57 | 3.25 | 1.85 | 0.48 | 0.26 | 0.045 | Type A, Length 4 | 98% Pearlite | 275 | 200 |
| HT280 | 0.57 | 3.26 | 1.85 | 0.45 | 0.26 | 0.042 | Type A, Length 4 | 98% Pearlite | 295 | 210 |
| HT280 | 0.54 | 3.30 | 1.79 | 0.45 | 0.25 | 0.041 | Type A, Length 4 | 98% Pearlite | 280 | 208 |
| HT280 | 0.56 | 3.26 | 1.83 | 0.44 | 0.29 | 0.042 | Type A, Length 4 | 98% Pearlite | 285 | 195 |
| Average | 0.56 | – | 1.83 | 0.46 | 0.27 | 0.043 | – | – | 284 | 203 |
From the data, I performed a detailed comparative analysis. At a Si/C ratio of approximately 0.55, the Cu-Cr-Mo-Sn alloy grey iron castings exhibited an average tensile strength of 314 MPa and hardness of 250 HBW, whereas the Cu-Cr-Sn alloy grey iron castings without molybdenum showed averages of 284 MPa and 203 HBW. This indicates that molybdenum contributes significantly to strength and hardness, with a tensile strength increase of about 10.6% and hardness increase of 23%. However, when the Si/C ratio was raised to 0.7 for the Cu-Cr-Sn alloy grey iron castings, the average tensile strength improved to 299 MPa and hardness to 215 HBW. This represents a 5.3% increase in tensile strength and a 5.9% increase in hardness compared to the Cu-Cr-Sn alloy at Si/C 0.55. Notably, the high Si/C ratio grey iron castings achieved tensile strengths well above the HT280 requirement of 280 MPa, demonstrating that adjusting the Si/C ratio can compensate for the absence of molybdenum. To mathematically express the relationship, I considered a simplified model for tensile strength (TS) as a function of Si/C ratio and alloy content: $$ TS = k_1 \cdot (\text{Si/C ratio}) + k_2 \cdot (\omega(\text{Mo})) + C $$, where \(k_1\), \(k_2\), and \(C\) are constants derived from empirical data. For our grey iron castings, increasing the Si/C ratio from 0.55 to 0.7 effectively offset the loss of molybdenum, aligning with the goal of cost reduction.
Microstructural examination further supported these findings. The high Si/C ratio grey iron castings displayed shorter graphite flakes, reducing the notch effect on the matrix, and finer grain structure compared to the standard alloyed versions. This refinement contributes to the enhanced mechanical properties. The pearlite content remained high at around 98% in all cases, but the distribution and morphology varied, with the high Si/C ratio promoting a more uniform matrix. These observations underline the importance of microstructure control in optimizing grey iron castings for performance and economy.
In conclusion, my exploration into high silicon-carbon ratio grey iron castings has yielded promising results. The traditional Cu-Cr-Mo-Sn alloyed grey iron castings, while effective, incur high costs due to molybdenum usage. By switching to a Cu-Cr-Sn alloy system and increasing the Si/C ratio from 0.55 to 0.7, I successfully eliminated ferromolybdenum while maintaining tensile strength and hardness within the HT280 specifications. This adjustment not only reduces alloy costs but also leverages the inherent benefits of silicon in strengthening the matrix. The trials confirmed that a higher Si/C ratio enhances mechanical properties, with an average tensile strength improvement of about 5%, ensuring that grey iron castings remain competitive. Therefore, for foundries facing similar cost pressures, adopting a high Si/C ratio approach presents a viable pathway to produce cost-effective grey iron castings without compromising quality. Future work could involve extending this methodology to other grades or exploring synergistic effects with additional inoculants to further optimize the performance of grey iron castings in diverse applications.