In the evolving landscape of the foundry industry, gray iron casting remains a dominant force due to its excellent castability, machinability, and cost-effectiveness. Despite the rising prominence of ductile iron and compacted graphite iron, gray iron castings continue to hold the largest market share, particularly for applications requiring high strength and wear resistance. At our production facility, we primarily focus on high-grade gray iron castings, such as HT280 and HT300, utilizing a high carbon equivalent alloying route. This involves the addition of various alloys to enhance mechanical properties. However, the recent surge in alloy prices, especially ferro-molybdenum, which constitutes over 60% of our total alloy cost, has necessitated the exploration of alternative strategies. One promising approach is adjusting the silicon-to-carbon (Si/C) ratio in gray iron castings, aiming to reduce or eliminate the use of costly alloys like molybdenum while maintaining performance standards. This article delves into our investigation of high Si/C ratio gray iron castings, presenting a comprehensive analysis of the process, results, and implications for cost reduction and quality enhancement.
The production of gray iron castings involves precise control over metallurgical parameters. Our operations employ medium-frequency induction melting furnaces, typically of 8-ton capacity. Raw materials include pig iron, steel scrap, and returns, supplemented with ferroalloys like ferrosilicon and ferromanganese, as well as carburizers for composition adjustment. The melting process follows a standard sequence: first, 5%–10% pig iron is charged, followed by 50%–60% steel scrap, and the remainder as returns. During melting, additives such as ferrosilicon, ferromanganese, and carburizer are introduced to achieve the target base iron composition, as detailed in Table 1.
| Element | Control Range |
|---|---|
| C | 3.20–3.30 |
| Si | 1.50–1.65 |
| Mn | 0.40–0.80 |
| P | < 0.06 |
| S | 0.08–0.12 |
After melting, ladle treatment is critical. Inoculants and alloys are added to the bottom of the ladle to fine-tune the final properties for different grades of gray iron casting. The tapping temperature is maintained between 1,480°C and 1,540°C to ensure effective absorption. We use silicon-barium inoculants, along with alloys such as electrolytic copper, tin granules, ferro-molybdenum, and ferro-chromium. For grades like HT280 and above, all four alloys are typically incorporated to ensure consistent performance across varying section thicknesses. The roles of these elements are multifaceted: copper refines and homogenizes 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 chemical composition for HT280 gray iron castings is controlled within the ranges shown in Table 2, with elements like C, S, Mn, and P adhering to the base iron specifications without further adjustment during ladle treatment.
| Element | Control Range |
|---|---|
| Si | 1.70–2.00 |
| Cu | 0.30–0.50 |
| Cr | 0.20–0.40 |
| Sn | 0.03–0.05 |
| Mo | 0.10–0.20 |
The casting process utilizes alkaline phenolic resin no-bake sand molds. During pouring, a secondary inoculation is applied using a silicon-zirconium inoculant with a grain size of 0.4–0.8 mm, added at 0.08%–0.12% via a pneumatic dispensing system on the pouring machine. This ensures uniform absorption and effective modification. Pouring temperatures are controlled between 1,360°C and 1,400°C to guarantee complete mold filling and sound gray iron casting integrity.
To address the cost challenges, we developed a trial process focusing on high Si/C ratio gray iron castings. The objective was to switch from a Cu-Cr-Mo-Sn alloyed gray iron to a Cu-Cr-Sn alloyed gray iron, while maintaining the same carbon equivalent. Carbon equivalent (CE) is a key parameter in gray iron casting, often calculated using the formula: $$CE = C + \frac{Si}{3} + \frac{P}{3}$$. By increasing the Si/C ratio from approximately 0.55 to 0.7, we aimed to reduce the carbon content by about 0.1% and increase silicon by about 0.4%, thereby eliminating the need for ferro-molybdenum, which averages 0.15% addition, leading to an estimated cost saving of 600 yuan per ton. The chemical composition ranges for the conventional and high Si/C ratio gray iron castings are compared in Table 3.
| Material Type | 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) | 3.10–3.20 | 1.50–1.65 | 2.10–2.20 | 0.30–0.50 | 0.20–0.40 | 0.03–0.05 | – |
Increasing the Si/C ratio in gray iron castings, while keeping carbon equivalent constant, induces several microstructural and property changes. The volume of primary austenite increases, strengthening the matrix. Reduced overall carbon content decreases graphite quantity, mitigating the notch effect of graphite flakes on the matrix. However, the eutectoid transformation temperature rises, potentially coarsening pearlite, which could negatively impact strength. Additionally, higher silicon content in solid solution strengthens ferrite, including that within pearlite. These effects can be modeled using relationships such as the Hall-Petch-type equation for strength: $$\sigma_y = \sigma_0 + k_y \cdot d^{-1/2}$$, where $d$ is the grain size, and $\sigma_0$ and $k_y$ are material constants. For gray iron casting, the Si/C ratio influences $d$ and thus mechanical properties.
The trial involved adjusting the base iron carbon content to 3.10%–3.20% while maintaining other elements. During ladle treatment, the silicon-barium inoculant addition was increased, and ferro-molybdenum was omitted. We conducted four separate trials, each producing separately cast test bars and actual castings. The test bars and castings were poured at temperatures of 1,360°C–1,400°C, with secondary inoculation applied. The results for the high Si/C ratio Cu-Cr-Sn gray iron castings are summarized in Table 4, showing compliance with HT280 requirements (tensile strength ≥ 280 MPa, hardness 190–260 HBW).
| Trial | Si/C Ratio | C (mass %) | Si (mass %) | Cu (mass %) | Cr (mass %) | Sn (mass %) | Graphite Structure | Matrix | Tensile Strength (MPa) | Hardness (HBW) |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 0.70 | 3.11 | 2.18 | 0.47 | 0.29 | 0.047 | Type A, Length 4 | 98% Pearlite | 295 | 209 |
| 2 | 0.67 | 3.15 | 2.11 | 0.45 | 0.25 | 0.040 | Type A, Length 4 | 98% Pearlite | 290 | 221 |
| 3 | 0.68 | 3.18 | 2.17 | 0.43 | 0.30 | 0.043 | Type A, Length 4 | 98% Pearlite | 300 | 211 |
| 4 | 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 |
To assess real-world performance, we dissected actual gray iron castings, sampling from critical locations such as the main bolt area of a bearing cap (approximately 50 mm thick). The results, presented in Table 5, confirm that the high Si/C ratio gray iron castings meet the required specifications in service conditions.
| Sample ID | Graphite Structure | Matrix | Tensile Strength (MPa) | Hardness (HBW) |
|---|---|---|---|---|
| 1# | Type A, Length 4 | 98% Pearlite | 262 | 183 |
| 2# | Type A, Length 4 | 98% Pearlite | 245 | 193 |
For comparison, we evaluated conventional Cu-Cr-Mo-Sn gray iron castings with a Si/C ratio around 0.55, as well as Cu-Cr-Sn gray iron castings with the same lower Si/C ratio (without molybdenum). The data are compiled in Tables 6 and 7, respectively. The performance metrics reveal that molybdenum-containing gray iron castings exhibit higher strength and hardness due to pearlite refinement. However, the high Si/C ratio approach compensates for the absence of molybdenum.
| Trial | Si/C Ratio | C (mass %) | Si (mass %) | Cu (mass %) | Cr (mass %) | Sn (mass %) | Mo (mass %) | Tensile Strength (MPa) | Hardness (HBW) |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 0.58 | 3.25 | 1.89 | 0.46 | 0.25 | 0.046 | 0.15 | 335 | 246 |
| 2 | 0.57 | 3.24 | 1.85 | 0.46 | 0.28 | 0.047 | 0.12 | 325 | 246 |
| 3 | 0.55 | 3.26 | 1.79 | 0.47 | 0.29 | 0.045 | 0.12 | 285 | 263 |
| 4 | 0.55 | 3.30 | 1.80 | 0.47 | 0.30 | 0.047 | 0.13 | 310 | 245 |
| Average | 0.56 | – | 1.83 | 0.47 | 0.28 | 0.046 | 0.13 | 314 | 250 |
| Trial | Si/C Ratio | C (mass %) | Si (mass %) | Cu (mass %) | Cr (mass %) | Sn (mass %) | Tensile Strength (MPa) | Hardness (HBW) |
|---|---|---|---|---|---|---|---|---|
| 1 | 0.57 | 3.25 | 1.85 | 0.48 | 0.26 | 0.045 | 275 | 200 |
| 2 | 0.57 | 3.26 | 1.85 | 0.45 | 0.26 | 0.042 | 295 | 210 |
| 3 | 0.54 | 3.30 | 1.79 | 0.45 | 0.25 | 0.041 | 280 | 208 |
| 4 | 0.56 | 3.26 | 1.83 | 0.44 | 0.29 | 0.042 | 285 | 195 |
| Average | 0.56 | – | 1.83 | 0.46 | 0.27 | 0.043 | 284 | 203 |
A comparative analysis of the three types of gray iron castings reveals significant insights. For a Si/C ratio of 0.55, Cu-Cr-Mo-Sn gray iron castings exhibit approximately 10.6% higher tensile strength and 23% higher hardness than Cu-Cr-Sn gray iron castings, underscoring the strengthening effect of molybdenum. However, when the Si/C ratio is increased to 0.7 in Cu-Cr-Sn gray iron castings, tensile strength improves by about 5.3% and hardness by 5.9% compared to the low Si/C ratio Cu-Cr-Sn variant. This enhancement is sufficient to meet HT280 requirements, with an average tensile strength of 299 MPa. The relationship between Si/C ratio and mechanical properties can be expressed using empirical formulas. For instance, tensile strength ($\sigma_t$) might correlate with Si/C ratio ($R_{Si/C}$) as: $$\sigma_t = \alpha \cdot R_{Si/C} + \beta$$, where $\alpha$ and $\beta$ are constants derived from regression analysis of gray iron casting data. Similarly, hardness (HB) could follow: $$HB = \gamma \cdot R_{Si/C} + \delta$$.
Microstructural examination further elucidates the benefits of high Si/C ratio gray iron castings. In conventional molybdenum-containing gray iron castings, graphite appears as Type A with moderate length, and pearlite is finely dispersed. In high Si/C ratio gray iron castings without molybdenum, graphite is shorter, reducing its notch effect, and the pearlitic matrix shows refined grain boundaries, contributing to strength. This refinement can be quantified using the ASTM grain size number $G$, where $G$ increases with finer grains. The Hall-Petch relationship, as mentioned earlier, links this to yield strength. For gray iron casting, the interplay between graphite morphology and matrix structure is crucial, and the Si/C ratio directly influences both.
The economic implications are substantial. By eliminating ferro-molybdenum and adjusting the Si/C ratio, we achieve significant cost savings without compromising the quality of gray iron castings. A detailed cost analysis can be modeled using the formula: $$C_{total} = C_{raw} + C_{alloy} + C_{processing}$$, where $C_{alloy}$ decreases due to reduced alloy usage. For a production volume of 10,000 tons of gray iron castings annually, the savings could exceed 6 million yuan, highlighting the scalability of this approach.
In conclusion, our exploration of high silicon-to-carbon ratio gray iron castings demonstrates a viable pathway to cost reduction while maintaining performance. The traditional Cu-Cr-Mo-Sn alloyed gray iron castings, though effective, incur high costs due to expensive alloys like molybdenum. By switching to a Cu-Cr-Sn composition and increasing the Si/C ratio from 0.55 to approximately 0.7, we successfully produced gray iron castings that meet HT280 specifications, with average tensile strength of 299 MPa and hardness of 215 HBW. This adjustment enhances matrix strengthening through increased primary austenite and reduced graphite content, offsetting the absence of molybdenum. The microstructural benefits include shorter graphite flakes and refined grains, improving mechanical properties. Therefore, optimizing the Si/C ratio presents a promising strategy for foundries seeking to reduce alloy dependency and lower production costs for high-grade gray iron castings. Future work could involve extending this approach to other grades, such as HT300 gray iron castings, and exploring dynamic relationships between Si/C ratio, cooling rates, and section sensitivity in complex gray iron casting geometries.