In the evolving landscape of the cast iron industry, while the proportion of ductile iron and compacted graphite iron continues to rise, gray iron remains the dominant material due to its excellent casting properties and widespread applicability. In our production facility, gray iron castings constitute the majority of output, primarily focusing on high-grade specifications such as HT280 and HT300. We employ a high carbon equivalent alloying process, incorporating specific types and quantities of alloys to enhance the strength and hardness of the castings. However, recent years have seen a significant increase in alloy costs, particularly for ferromolybdenum, which now accounts for over 60% of total alloy expenses. This economic pressure has driven us to explore alternative methods, such as optimizing the silicon-to-carbon (Si/C) ratio, to reduce or eliminate the use of ferromolybdenum while maintaining performance standards.
Our production setup utilizes medium-frequency induction melting furnaces, typically of 8-ton capacity. Raw materials include pig iron, steel scrap, and returns, supplemented with ferrosilicon, ferromanganese, and carbon additives to adjust the molten iron composition. The standard charging sequence involves adding pig iron (5-10%), followed by returns and steel scrap (50-60%), with simultaneous addition of silicon and manganese alloys and carbon raisers during melting to achieve the target base iron composition, as controlled within 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 |
Post-melting, we conduct ladle treatment by adding inoculants and specific alloys to the bottom of the ladle. The tapping temperature is maintained between 1480-1540°C to ensure effective absorption. We use barium-bearing silicon inoculant, along with alloys like electrolytic copper, tin granules, ferromolybdenum, and ferrochromium. For grades HT280 and above, all four alloys are typically incorporated to guarantee performance across varying section thicknesses. Copper refines and homogenizes pearlite and graphite, reducing chill in thin sections and improving uniformity in heavy sections. Tin minimizes or eliminates ferrite, stabilizing and refining pearlite while enhancing section sensitivity. Chromium refines graphite and pearlite, and molybdenum promotes graphite refinement, increases pearlite content, and improves uniformity in thick sections. The final chemical composition for HT280 gray iron castings is controlled as per Table 2, with C, S, Mn, and P levels adhering to base iron specifications without further adjustment during 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 |
For mold preparation, we use alkaline phenolic resin self-setting sand. During pouring, secondary inoculation is performed with a stream inoculant, specifically zirconium-bearing silicon inoculant of 0.4-0.8 mm grain size, added at 0.08-0.12% via a pneumatic dispensing device on the pouring machine to ensure uniform absorption. Pouring temperatures are controlled between 1360-1400°C to achieve complete mold filling.
To address the high cost of alloys in gray iron castings, we developed a trial process focusing on high Si/C ratio gray iron. Under constant carbon equivalent conditions, we transitioned from a Cu-Cr-Mo-Sn alloyed gray iron to a Cu-Cr-Sn composition, eliminating ferromolybdenum usage. This change reduces molybdenum addition by an average of 0.15%, projecting cost savings of approximately 600 yuan per ton. The Si/C ratio was increased from 0.55 to 0.7, involving a decrease in carbon content by about 0.1% and an increase in silicon content by approximately 0.4%. The chemical composition ranges for the original and modified gray iron castings are detailed in Table 3.
| Material | Base C | Base Si | Final Si | Cu | Cr | Sn | Mo |
|---|---|---|---|---|---|---|---|
| Cu-Cr-Mo-Sn Gray Iron | 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) Gray Iron | 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 the same carbon equivalent while increasing the Si/C ratio induces several microstructural and property changes in gray iron castings. The carbon equivalent (CE) can be expressed as: $$ CE = C + \frac{1}{3}Si $$ where C and Si are the mass percentages of carbon and silicon, respectively. The Si/C ratio is defined as: $$ \text{Si/C ratio} = \frac{\text{Si}}{\text{C}} $$ With a higher Si/C ratio, the volume of primary austenite increases, strengthening the matrix. Reduced total carbon content decreases graphite quantity, mitigating the embrittling effect of graphite flakes on the matrix. The eutectoid transformation temperature rises, potentially coarsening pearlite, which could negatively impact strength. Additionally, increased silicon dissolved in ferrite strengthens it, including the ferrite within pearlite.
We conducted four trials by adjusting the base iron carbon content to 3.10-3.20% while keeping other components unchanged. During ladle treatment, we increased the barium-silicon inoculant addition and omitted ferromolybdenum. Using a 1-ton ladle, we poured separate test bars and actual castings at controlled temperatures of 1360-1400°C. Test bars and castings were analyzed for chemical composition, metallographic structure, and mechanical properties. For castings, key sections were dissected to assess本体 performance.
The results for Cu-Cr-Sn alloyed gray iron castings with a Si/C ratio of 0.7 are summarized in Table 4. All values meet the HT280 grade requirements, which specify a tensile strength ≥280 MPa and hardness between 190-260 HBW for gray iron castings.
| Grade | Si/C | 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, length 4 | 98% Pearlite | 295 | 209 |
| HT280 | 0.67 | 3.15 | 2.11 | 0.45 | 0.25 | 0.040 | Type A, length 4 | 98% Pearlite | 290 | 221 |
| HT280 | 0.68 | 3.18 | 2.17 | 0.43 | 0.30 | 0.043 | Type A, length 4 | 98% Pearlite | 300 | 211 |
| HT280 | 0.69 | 3.12 | 2.16 | 0.40 | 0.24 | 0.043 | Type A, length 4 | 98% Pearlite | 310 | 220 |
| Average | 0.69 | – | 2.15 | 0.44 | 0.27 | 0.043 | – | – | 299 | 215 |
Dissection of castings, specifically at the main bolt position of a housing with approximately 50 mm thickness, yielded the results in Table 5. The本体 properties of gray iron castings align with expectations, confirming that performance requirements are satisfied.
| Sample | Grade | Graphite | Matrix | Tensile (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, Table 6 presents data for Cu-Cr-Mo-Sn alloyed gray iron castings with a Si/C ratio of 0.55, highlighting the baseline performance with molybdenum addition.
| Grade | Si/C | 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, 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 |
To further validate the impact of Si/C ratio, we conducted additional trials on Cu-Cr-Sn gray iron with a Si/C ratio of 0.55 (without molybdenum), as shown in Table 7. These castings were remelted after testing to avoid any performance issues in production.
| Grade | Si/C | 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, 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 |
Comparative analysis reveals that at a Si/C ratio of approximately 0.55, Cu-Cr-Mo-Sn gray iron exhibits 10.6% higher tensile strength than Cu-Cr-Sn gray iron. Molybdenum’s role in refining pearlite is critical; without it, tensile strength may fall short, making direct elimination risky in production. Hardness values for molybdenum-containing gray iron are 23% higher. However, under constant carbon equivalent, increasing the Si/C ratio from 0.55 to 0.7 in Cu-Cr-Sn gray iron boosts tensile strength by 5.3% and hardness by 5.9%. Notably, the Cu-Cr-Sn gray iron with a Si/C ratio of 0.7 achieves an average tensile strength of 299 MPa, well above the HT280 requirement for gray iron castings.
Metallographic examination shows that high Si/C ratio gray iron castings have shorter graphite flakes, reducing the embrittling effect on the matrix. Pearlite content remains similar, but grain refinement is more pronounced in high Si/C ratio gray iron. The microstructure of gray iron castings is crucial for performance, and optimizing the Si/C ratio enhances these characteristics.
In summary, the current HT280 grade Cu-Cr-Sn-Mo gray iron involves high alloy additions, leading to elevated costs. Direct reduction of alloys risks performance deficiencies. By maintaining the same carbon equivalent and switching to a Cu-Cr-Sn composition with an increased Si/C ratio from 0.55 to 0.7, we can eliminate ferromolybdenum, enhance mechanical properties, and reduce costs. This approach results in an average tensile strength improvement of about 5%, sufficient to meet HT280 specifications. Although not as effective as direct molybdenum alloying, it offers a viable path for cost reduction in gray iron castings. Given the high cost pressures in gray iron production, increasing the Si/C ratio while reducing alloy content represents a promising strategy for foundries. Further optimization of gray iron casting processes through Si/C ratio adjustments can lead to sustainable manufacturing of high-quality gray iron components.
The exploration of high silicon-carbon ratio in gray iron castings demonstrates that microstructural control via composition tuning is key to achieving desired properties. Gray iron, as a versatile material, benefits from such innovations, ensuring its continued relevance in industrial applications. Future work may focus on refining the Si/C ratio for other grades of gray iron and exploring interactions with other alloying elements to further enhance the performance of gray iron castings.