In the pursuit of manufacturing high-strength and high-ductility casting parts such as QT600-10, the traditional pathways often involve the addition of relatively expensive alloying elements like Copper (Cu), Tin (Sn), or Nickel (Ni), or the implementation of high-silicon solid solution strengthening. These methods, while effective, present limitations in terms of cost or application constraints, particularly for casting parts subjected to impact loads at low temperatures. Our research explores a novel, cost-effective production technology for QT600-10 casting parts by leveraging the micro-alloying effect of Antimony (Sb). This paper presents a detailed investigation into the influence of controlled Sb additions on the microstructure, mechanical properties, and the overall production process stability for these critical casting parts.
The foundation for producing sound casting parts lies in meticulous raw material selection and charge calculation. Based on cost considerations and extensive production experience, we designed a charge mix that maintains performance while minimizing expense. The primary charge composition is summarized in Table 1.
| Charge Component | Pig Iron | Steel Scrap | Returns |
|---|---|---|---|
| Charge Ratio (wt.%) | 10 | 30 | 60 |
The chemical composition is the most critical factor determining the final properties of the casting parts. Precise control over each element is essential for achieving the desired pearlitic-ferritic matrix in QT600-10.
Carbon Equivalent (CE) and Carbon (C): The Carbon Equivalent, calculated as $$CE = \%C + \frac{1}{3}\%Si$$, plays a pivotal role. A higher CE improves graphite nodule count, ductility, toughness, and castability, reducing shrinkage defects. However, it can lower strength and hardness. For our target QT600-10 casting parts, an optimal CE range of 4.4 to 4.6 wt.% was identified. Within this, the carbon content is controlled between 3.6 and 3.8 wt.% to provide a solid base for strength and hardness in the final casting parts.
Silicon (Si): Silicon is a potent graphitizer and ferrite promoter. Its influence on the mechanical properties of casting parts is significant, as illustrated by its relationship with tensile strength (Rm), elongation (A), and hardness (HB). For a pearlitic-ferritic matrix, Si content must be kept relatively low to avoid excessive ferrite formation and suppression of pearlite. In our process for QT600-10 casting parts, the Si content is tightly controlled within the range of 1.2 to 1.3 wt.%.
Manganese (Mn) and Antimony (Sb): Manganese is added to stabilize pearlite and enhance strength, typically maintained at 0.40-0.50 wt.%. The key innovation in this study is the micro-addition of Antimony. Sb is known to segregate at the graphite/matrix interface, forming a Sb-rich layer. This layer alters the transformation kinetics, hindering the diffusion of carbon to graphite ($$ \gamma \rightarrow \alpha + G $$) and thereby promoting the pearlitic transformation ($$ \gamma \rightarrow P(\alpha + Fe_3C) $$). Furthermore, it refines the pearlite structure and can improve graphite nodularity. However, excess Sb can lead to degenerate graphite forms. To study its effect, we produced three sets of attached test bars (S1, S2, S3) with varying Sb contents, as detailed in Table 2.
| Sample ID | C (wt.%) | Si (wt.%) | Mn (wt.%) | P (wt.%) | S (wt.%) | Sb (wt.%) |
|---|---|---|---|---|---|---|
| S1 | 3.60 – 3.80 | 1.20 – 1.30 | 0.40 – 0.50 | <0.06 | <0.030 | 0.004 |
| S2 | 0.008 | |||||
| S3 | 0.010 |
Spheroidization and Inoculation Treatment: The production of high-quality ductile iron casting parts relies on effective spheroidization and inoculation. We employed a rare-earth-containing spheroidizer (with Mg as the primary agent) using the sandwich process in the ladle. Inoculation was performed in three stages to ensure a high nodule count and prevent undercooled structures: 0.3 wt.% inoculant in the ladle bottom, 0.3-0.4 wt.% added during tapping, and a final 0.15 wt.% added during pouring via a funnel. This robust treatment is crucial for achieving the required microstructure in heavy-section casting parts.
The microstructural analysis of the produced casting part samples reveals the profound impact of Sb micro-alloying. All samples exhibited a nodularity greater than 90% and a graphite size of approximately grade 6. However, the key difference lay in the matrix structure.
The visual analysis of the microstructure clearly shows the evolution. As the Sb content increases, the volume fraction of pearlite rises significantly from 40% in S1 to 55% in S2, and further to 65% in S3. Concurrently, the graphite nodules appear more rounded and their count increases, while their size slightly decreases, primarily falling within the 20-40 μm range. This can be attributed to the Sb-rich layer at the interface, which acts as a diffusion barrier for carbon atoms, $$ J_C \propto -D_C \frac{\partial C}{\partial x} $$, thereby impeding graphite growth and refining the nodules. The relationship between Sb content and pearlite fraction can be modeled as: $$ P_c = f_{base} + k_{Sb} \cdot [Sb] $$ where \( P_c \) is the pearlite content, \( f_{base} \) is the base pearlite content without Sb, \( k_{Sb} \) is a rate constant, and \( [Sb] \) is the antimony concentration.
The mechanical properties of the casting parts were evaluated according to standard procedures, and the results are summarized in Table 3. The data demonstrates a clear trend: Sb addition significantly enhances the strength and hardness of the casting parts.
| Sample ID | Tensile Strength, Rm (MPa) | Yield Strength, Rp0.2 (MPa) | Elongation, A (%) | Hardness, HBW |
|---|---|---|---|---|
| S1 (0.004% Sb) | 556 | 329 | 15.5 | 160 |
| 573 | 345 | 14.5 | 183 | |
| 566 | 332 | 14.0 | 183 | |
| S2 (0.008% Sb) | 643 | 376 | 11.0 | 203 |
| 636 | 371 | 12.0 | 204 | |
| 634 | 376 | 12.0 | 207 | |
| S3 (0.010% Sb) | 689 | 399 | 9.0 | 221 |
| 701 | 411 | 8.5 | 223 | |
| 682 | 380 | 9.0 | 216 |
Sample S2, with 0.008 wt.% Sb, consistently meets the QT600-10 standard requirements (Rm ≥ 600 MPa, A ≥ 10%). The strengthening mechanism is twofold: first, the significant increase in the strong, hard pearlite phase; second, the solid solution strengthening provided by Sb atoms. The atomic radius of Sb (≈1.6 Å) is much larger than that of Fe or C, causing lattice strain when dissolved in the ferrite/pearlitic matrix, which impedes dislocation motion according to the relationship: $$ \Delta \tau_{ss} = G \cdot \epsilon^{3/2} \cdot \sqrt{c} $$ where \( \Delta \tau_{ss} \) is the increase in critical resolved shear stress, \( G \) is the shear modulus, \( \epsilon \) is the lattice strain, and \( c \) is the solute concentration. However, when the Sb content reaches 0.010 wt.% (S3), the elongation drops below 10%, failing the specification despite the higher strength. This indicates an optimal window for Sb addition in these casting parts.
The effectiveness of Sb can be compared to traditional alloying elements. The potency of Sb in promoting pearlite is remarkable; approximately 0.006-0.015 wt.% Sb can increase pearlite content from 60% to 90%. This effect is estimated to be about 5 times stronger than Manganese and 20 times stronger than Copper on a weight-percent basis for pearlite promotion in casting parts. The primary advantage, however, is economic. Replacing costly Cu or Sn with a minute amount of Sb substantially reduces the raw material cost for producing these high-performance casting parts. A simplified cost impact analysis is presented in Table 4.
| Alloying Strategy | Typical Addition (wt.%) | Relative Material Cost Factor* | Key Effect on Casting Parts |
|---|---|---|---|
| Copper (Cu) | 0.8 – 1.0 | High | Pearlite promotion, solid solution strengthening |
| Tin (Sn) | 0.05 – 0.10 | Very High | Strong pearlite promotion, risk of embrittlement |
| Antimony (Sb) | 0.004 – 0.008 | Low | Strong pearlite promotion, graphite refinement, cost-effective |
*Relative cost factor is a qualitative comparison based on typical market prices of these metals as alloying additives.
The relationship between tensile strength, pearlite content, and Sb concentration can be expressed through a combined model: $$ R_m = R_{m0} + \alpha \cdot P_c + \beta \cdot [Sb]_{ss} $$ where \( R_{m0} \) is the base strength, \( \alpha \) is the strengthening coefficient from pearlite, \( P_c \) is the pearlite content (itself a function of [Sb]), and \( \beta \cdot [Sb]_{ss} \) represents the solid solution strengthening contribution from dissolved Sb. For the foundry engineer, controlling the Sb content within the narrow window of 0.006 to 0.008 wt.% is critical for achieving the perfect balance of strength and ductility in QT600-10 casting parts.
In conclusion, our study successfully demonstrates a low-cost, stable production technology for QT600-10 casting parts using Sb micro-alloying. By precisely controlling the base chemistry (C: 3.6-3.8%, Si: 1.2-1.3%, Mn: 0.4-0.5%, CE: 4.4-4.6%) and adding 0.006-0.008 wt.% Antimony, in conjunction with a robust spheroidizing and triple-inoculation process, casting parts with a pearlitic-ferritic matrix containing approximately 55% pearlite can be reliably produced. This microstructure delivers the required mechanical properties: tensile strength exceeding 600 MPa and elongation greater than 10%. The Sb addition not only promotes pearlite formation but also refines and rounds the graphite nodules. The significant cost advantage of using微量 Sb over traditional alloying elements like Cu or Sn makes this technology highly attractive for the commercial production of demanding casting parts. Future work may focus on further optimizing the inoculation practice in synergy with Sb and modeling the exact kinetics of the Sb-rich layer formation to refine process control for even more consistent quality in heavy-section casting parts.