In the automotive industry, brake discs are critical components responsible for converting kinetic energy into thermal energy through friction during braking. This process generates substantial heat, leading to thermal stresses that can cause cracking and failure. Therefore, brake discs must exhibit excellent thermal conductivity, high strength, wear resistance, and damping capacity. Grey iron casting has been widely adopted for brake discs due to its favorable combination of these properties, along with ease of machining and low production costs. Traditionally, grey iron casting for brake discs is produced using pig iron and scrap steel; however, the use of specific waste materials, such as high carbon steel wires from recycled tires, presents an innovative and sustainable approach. This study explores the microstructure and performance of grey iron casting brake discs manufactured with waste high carbon steel wires, aiming to demonstrate their viability and superior characteristics compared to conventional materials.
The motivation for utilizing waste high carbon steel wires stems from the abundance of such materials in tire recycling processes. These wires, typically with carbon content around 0.70–0.75%, offer a high-carbon feedstock that can influence the graphite morphology and matrix structure in grey iron casting. Previous research indicates that using clean scrap steel can enhance the formation of fine A-type graphite and improve mechanical properties, but the effects of high-carbon scrap specifically are less documented. In this work, we investigate how varying the proportion of waste high carbon steel wires in the charge affects the graphite distribution, pearlite content, hardness, tensile strength, wear resistance, and cyclic lifespan of grey iron casting brake discs. Our findings reveal that this approach not only meets but exceeds the standards for EN-GJL-200 grey iron, providing a cost-effective and environmentally friendly solution for automotive applications.
To begin, the chemical composition of the grey iron casting was designed based on the EN-GJL-200 standard, which specifies ranges for carbon, silicon, manganese, chromium, and other elements. The target composition is summarized in Table 1. This design ensures adequate fluidity, castability, and the desired mechanical properties for brake discs. The carbon equivalent (CE) is a key parameter in grey iron casting, calculated using the formula:
$$CE = C + \frac{Si}{3} + \frac{P}{3}$$
For our compositions, CE values range from approximately 3.8 to 4.0, indicating good graphitization potential. The use of waste high carbon steel wires as the primary raw material introduces inherent carbon, reducing the need for excessive carburizers and potentially refining the graphite structure.
| Element | Range |
|---|---|
| C | 3.2–3.4 |
| Si | 1.7–1.9 |
| Mn | 0.7–0.8 |
| Cr | 0.20–0.25 |
| Cu | ≤0.3 |
| P | ≤0.1 |
| S | 0.04–0.08 |
| Fe | Balance |
The raw materials consisted of waste high carbon steel wires (composition: 0.70–0.75% C, 0.15–0.30% Si, 0.40–0.60% Mn, ≤0.20% P, ≤0.20% S), along with returns such as gates, risers, and machining chips. Alloying elements were adjusted using ferromanganese, ferrochromium, and pure copper. Carburization was achieved with a calcined coal-based carburizer (90% fixed carbon). Two distinct charges were prepared: Group A with 70% waste high carbon steel wires and Group B with 80%, to study the effect of wire proportion on the grey iron casting properties. The actual chemical compositions after melting are shown in Table 2, confirming that the targets were met with minor variations.
| Group | C | Si | Mn | Cr | Cu | P | S | Other Elements | Waste Wire % |
|---|---|---|---|---|---|---|---|---|---|
| A | 3.260 | 1.77 | 0.780 | 0.240 | 0.235 | 0.041 | 0.043 | Ni: 0.004, Mo: 0.000 | 70 |
| B | 3.289 | 1.75 | 0.760 | 0.236 | 0.275 | 0.016 | 0.077 | Ni: 0.001, Mo: 0.001 | 80 |
Melting was conducted in a 2-ton medium-frequency induction furnace, with a tapping temperature of 1,550°C. Inoculation plays a crucial role in grey iron casting to promote graphite nucleation and control microstructure. We employed two-stage inoculation: primary inoculation with 75 ferrosilicon (3–8 mm grain size) added at 0.2–0.25% of the ladle weight, and secondary inoculation with a silicon-barium-calcium-aluminum alloy (1–3 mm grain size) added at 0.1–0.2% of the mold weight. The pouring temperature ranged from 1,360 to 1,460°C. Brake disc castings had a maximum outer diameter of 288 mm, with wall thicknesses varying from 8 to 21.9 mm. For each ladle, a separately cast test bar (Φ30 mm × 200 mm) was poured to evaluate tensile properties, and three castings per group were sampled for comprehensive analysis.
Microstructural examination was performed on longitudinal sections of the grey iron casting brake discs. Samples were etched with 4% nital to reveal graphite and matrix features. The graphite morphology, as observed under optical microscopy, is depicted in the images (though not referenced by number). Both groups exhibited predominantly A-type graphite, which is desirable for grey iron casting due to its uniform distribution and beneficial effects on mechanical properties. In Group A, A-type graphite accounted for 98.6% of the total graphite area, with an average flake length of 0.1162 mm, corresponding to a graphite size grade of 5 according to ASTM standards. The flakes were finely dispersed and exhibited considerable curvature. In Group B, the proportion of A-type graphite increased to 99.1%, with an average length reduced to 0.1071 mm, also grade 5. The graphite became even finer, more uniformly distributed, and more curved as the waste wire content increased. This refinement can be attributed to the enhanced nucleation sites provided by the carburizer and the inherent microheterogeneities from the high-carbon steel wires. The relationship between graphite length and inoculant effectiveness can be expressed as:
$$L_g = k \cdot \frac{1}{N_n}$$
where \(L_g\) is the average graphite length, \(k\) is a constant related to cooling rate, and \(N_n\) is the number of nucleation sites. Higher \(N_n\) from increased wire and carburizer usage leads to shorter graphite, improving the strength of the grey iron casting.
The matrix structure of the grey iron casting consisted primarily of pearlite, with minimal ferrite and negligible carbides. Quantitative analysis revealed that Group A had 98.6% pearlite, 0.7% ferrite, and less than 1% carbides, while Group B showed 99.2% pearlite, 0.5% ferrite, and similarly low carbides. The pearlite interlamellar spacing was measured as 0.98 μm for Group A and 0.82 μm for Group B, both classified as grade 2. This spacing refinement contributes significantly to hardness and strength. The Hall-Petch type relationship for pearlitic grey iron casting can be approximated as:
$$\sigma_y = \sigma_0 + \frac{k_y}{\sqrt{d}}$$
where \(\sigma_y\) is the yield strength, \(\sigma_0\) is the friction stress, \(k_y\) is a constant, and \(d\) is the interlamellar spacing. Finer spacing enhances strength, which is evident in our results. The microstructural data are summarized in Table 3.
| Group | Pearlite Content (%) | Ferrite Content (%) | Carbide Content (%) | Pearlite Spacing (μm) | Spacing Grade |
|---|---|---|---|---|---|
| A | 98.6 | 0.7 | <1 | 0.98 | 2 |
| B | 99.2 | 0.5 | <1 | 0.82 | 2 |
Mechanical properties were assessed through hardness and tensile testing. Brinell hardness was measured at 12 locations on each brake disc casting, as illustrated in the hardness test position diagram. The results, presented in Table 4, demonstrate uniform hardness distribution across the castings. Group A had an average hardness of HB 205, while Group B reached HB 209, with all values exceeding HB 202. This uniformity is advantageous for grey iron casting brake discs, ensuring consistent wear resistance and thermal stability. The tensile strength, derived from separately cast test bars, was 269 MPa for Group A and 275 MPa for Group B, both above the EN-GJL-200 requirement of 220 MPa. The improvement with higher wire content aligns with the microstructural refinements. The correlation between hardness and tensile strength in grey iron casting can be approximated by the empirical formula:
$$\sigma_u \approx 0.1 \times HB \times 9.80665 \quad \text{(in MPa)}$$
where \(\sigma_u\) is the tensile strength. For Group B, using HB 209 gives an estimated strength of 205 MPa, but our measured value is higher, indicating additional strengthening from fine graphite and pearlite.
| Group | Hardness Measurement Points (HB) | Average Hardness (HB) | Tensile Strength (MPa) |
|---|---|---|---|
| A | 209 | 205 | 269 |
| 211 | |||
| 207 | |||
| 212 | |||
| 203 | |||
| 211 | |||
| 204 | |||
| 209 | |||
| 202 | |||
| 211 | |||
| 207 | |||
| 212 | |||
| B | 211 | 209 | 275 |
| 204 | |||
| 211 | |||
| 210 | |||
| 207 | |||
| 205 | |||
| 204 | |||
| 210 | |||
| 211 | |||
| 204 | |||
| 205 | |||
| 204 |
Wear performance was evaluated using a bench test machine under conditions of 150 kN load and 400 rpm for 50 minutes. Both groups exhibited some wear, with surface features including microcracks, pits, and minor cracks. Group A showed a few larger cracks and pits, while Group B had reduced pit counts and essentially no large cracks, though microcracks persisted. This indicates that increasing the waste high carbon steel wire content enhances the wear resistance of grey iron casting brake discs. The wear volume loss can be modeled using the Archard wear equation:
$$V = k \frac{W \cdot s}{H}$$
where \(V\) is the wear volume, \(k\) is the wear coefficient, \(W\) is the load, \(s\) is the sliding distance, and \(H\) is the hardness. Higher hardness and refined microstructure reduce \(k\), leading to better wear resistance. For grey iron casting, the presence of fine, curved graphite flakes also acts as solid lubricants and crack arrestors, further improving wear behavior.
Cyclic life testing simulated real-world braking conditions according to ZF ATE N 550 03.05 specifications, with an inertia of 83 kg·m² and a maximum speed of 185 km/h. Each test involved multiple brake stops per cycle, and up to 100 cycles were conducted. The results are summarized in Table 5. In Group A, one brake disc survived 100 cycles without macroscopic cracks, while two failed at 91 and 80 cycles, respectively. In Group B, all three discs completed 100 cycles without visible macroscopic cracks. This demonstrates a significant improvement in cyclic lifespan with higher waste wire usage. The fatigue life of grey iron casting can be related to the graphite morphology and matrix strength. Fine A-type graphite disperses stress concentrations, and high pearlite content resists crack propagation. The fatigue limit \(\sigma_f\) for grey iron casting often correlates with tensile strength:
$$\sigma_f \approx 0.4 \times \sigma_u$$
For Group B, \(\sigma_f\) is around 110 MPa, sufficient to withstand cyclic thermal and mechanical stresses.
| Group | Disc Number | Cycles Completed | Brake Stops | Result |
|---|---|---|---|---|
| A | 1 | 100 | 400 | No macroscopic cracks |
| 2 | 91 | 364 | Macroscopic cracks appeared | |
| 3 | 80 | 320 | Macroscopic cracks appeared | |
| B | 1 | 100 | 400 | No macroscopic cracks |
| 2 | 100 | 400 | No macroscopic cracks | |
| 3 | 100 | 400 | No macroscopic cracks |
The enhanced properties of grey iron casting produced with waste high carbon steel wires can be attributed to several metallurgical factors. First, the high carbon content in the wires, combined with carburizer addition, creates numerous micro-regions with varying carbon concentrations. This heterogeneity promotes the formation of abundant nucleation sites for graphite, leading to fine A-type flakes. The absence of graphite inheritance from pig iron, as often seen in traditional grey iron casting, further enhances graphitization and reduces graphite size. Second, trace elements in the waste wires, such as nitrogen (around 0.003–0.005%) and molybdenum, contribute to pearlite stabilization and refinement. Nitrogen, also introduced via carburizer, forms nitrides that pin grain boundaries and refine the matrix. The alloying elements chromium and copper further increase pearlite content and reduce interlamellar spacing. The overall effect is a stronger, harder, and more wear-resistant grey iron casting. Compared to conventional grey iron casting using mixed scrap or pig iron, our approach yields higher hardness (by approximately HB 10–24) and tensile strength (by about 19 MPa), as reported in prior studies.
From a production perspective, the use of waste high carbon steel wires in grey iron casting offers economic and environmental benefits. It reduces reliance on virgin materials and utilizes a by-product from tire recycling. In practical trials at an industrial facility, brake discs manufactured with this method consistently met EN-GJL-200 standards, with tensile strengths above 220 MPa and successful cyclic testing up to 95 cycles (380 brake stops) without failure. The process has been scaled up for mass production, demonstrating its feasibility for automotive components. The consistent quality of grey iron casting from this method underscores its reliability for high-performance applications.
In conclusion, this research validates the effectiveness of using waste high carbon steel wires in producing high-quality grey iron casting brake discs. The microstructure is characterized by fine, uniformly distributed A-type graphite and a pearlite-rich matrix with fine interlamellar spacing. Mechanical properties, including hardness and tensile strength, exceed standard requirements and improve with increasing wire content. Wear resistance and cyclic lifespan are enhanced, making these grey iron casting discs suitable for demanding automotive environments. Future work could explore optimization of wire proportions, inoculation practices, and the effects of other waste streams on grey iron casting properties. Overall, this approach contributes to sustainable manufacturing while advancing the performance of grey iron casting components.