In the field of heavy machinery, high manganese steel castings play a critical role due to their exceptional wear resistance and toughness under impact loads. Components such as track shoes and bucket teeth are essential parts of excavators, often subjected to harsh service conditions involving heavy loads and abrasive wear. However, a persistent issue in producing these high manganese steel castings is sand burning, where molten metal adheres to the mold core, leading to defects that compromise quality and increase production costs. Traditionally, chromite sand has been used as a core sand material to prevent such defects, but its high cost and import dependency drive the search for alternatives. In this study, we explore the use of pearl sand as a substitute for chromite sand in high manganese steel castings, focusing on its performance, economic benefits, and practical applicability.
High manganese steel castings, like those for track shoes and bucket teeth, feature complex internal geometries that require multiple cores for shaping. The core sand must withstand high temperatures and resist chemical reactions with the molten metal. Previously, silica sand was employed, but its acidic nature conflicts with the basic oxides in high manganese steel, such as MnO, resulting in severe sand burning. This not only complicates cleaning but also affects heat treatment, potentially leaving carbides at grain boundaries that initiate cracks. Chromite sand, being alkaline, offers better resistance but is expensive and scarce. Pearl sand, a neutral material derived from bauxite, presents a promising alternative due to its high refractoriness and cost-effectiveness. Our investigation aims to validate pearl sand’s efficacy in high manganese steel castings, comparing it with chromite sand through experimental trials and theoretical analysis.
Pearl sand is a synthetic molding sand produced from high-alumina bauxite ore through a high-temperature sintering process. The manufacturing involves melting the bauxite in an electric furnace, blowing the molten material into droplets with compressed air, and cooling it to form spherical grains. This results in a neutral pH material with excellent thermal stability and recyclability, making it suitable for various casting alloys, including high manganese steel castings. The spherical shape of pearl sand particles enhances flowability and compaction, reducing porosity in cores. Chemically, pearl sand contains 78–85% Al2O3, 8–15% SiO2, and trace amounts of TiO2 and Fe2O3, as summarized in Table 1. Its thermal properties, such as high refractoriness (up to 1790°C) and low thermal expansion, are advantageous for preventing sand burning in high manganese steel castings.
| Al2O3 | SiO2 | TiO2 | Fe2O3 | Others |
|---|---|---|---|---|
| 78–85 | 8–15 | ≤3.5 | ≤4 | Trace |
To assess pearl sand’s suitability for high manganese steel castings, we compared its physical and thermal properties with those of chromite sand and silica sand, as shown in Table 2. Pearl sand has a lower bulk density (1.9–2.0 t/m³) compared to chromite sand (2.8 t/m³), meaning less material is required for the same volume. Its thermal conductivity is moderate, but its neutral pH and high Al2O3 content minimize chemical interactions with the basic oxides in high manganese steel. This reduces the risk of low-melting-point compound formation, a common cause of chemical sand burning. Additionally, pearl sand’s spherical particles allow for better compaction, decreasing the interstitial spaces where metal penetration can occur.
| Property | Silica Sand | Chromite Sand | Pearl Sand |
|---|---|---|---|
| pH | 7–8 | 7.8 | 7.6 |
| Refractoriness (°C) | 1710 | 1830 | 1790 |
| Bulk Density (t/m³) | 1.4–1.5 | 2.8 | 1.9–2.0 |
| Thermal Conductivity (W/m·K) | 0.7–0.8 | 0.65 | 0.5–0.6 |
| Thermal Expansion (%) | 1.5 | 0.3–0.4 | 0.13 |
| Specific Heat (J/kg·K) | 1130 | 1214 | 2210 |
Our experimental trials focused on high manganese steel castings for track shoes and bucket teeth, which are prone to sand burning due to their intricate cores. Track shoes weigh approximately 800–1200 kg, with thermal sections around 70 mm, while bucket teeth range from 350–450 kg, featuring thermal sections up to 170 mm. We replaced chromite sand with pearl sand in all cores for these components. The core distribution, as illustrated in the figure, included multiple cores such as No. 1 for bucket teeth and various cores for track shoes. After casting, we evaluated the surfaces for sand burning defects. Results showed that pearl sand performed effectively in track shoe cores, where thermal sections were below 100 mm, achieving sand burning resistance comparable to chromite sand. However, in bucket teeth cores with thermal sections exceeding 150 mm, some sand burning occurred, indicating a limitation for larger thermal masses.
The mechanism of sand burning involves both mechanical penetration and chemical reactions. Mechanical penetration occurs when molten metal enters the sand pores under metallostatic pressure. The critical pressure for penetration, \( P_{\text{crit}} \), can be expressed as:
$$ P_{\text{crit}} = \frac{2\sigma \cos \theta}{r_{\text{sand}}} \times 10^{-5} \, \text{N/cm}^2 $$
where \( \sigma \) is the surface tension of the molten metal, \( \theta \) is the contact angle, and \( r_{\text{sand}} \) is the radius of the sand pores. Pearl sand’s spherical particles enable higher compaction, reducing \( r_{\text{sand}} \) and increasing \( P_{\text{crit}} \), thus resisting metal penetration. For chemical sand burning, silica sand reacts with FeO in high manganese steel to form low-melting-point ferrosilicates:
$$ \text{SiO}_2 (s) + 2\text{FeO} (l) \rightarrow 2\text{FeO} \cdot \text{SiO}_2 (l) $$
Pearl sand’s high Al2O3 content and neutral nature prevent such reactions, as Al2O3 does not readily form compounds with iron oxides. However, in thermal sections above 100 mm, prolonged heat retention may cause sintering, leading to sand burning. This highlights the importance of thermal management in high manganese steel castings when using pearl sand.
Economically, pearl sand offers significant advantages over chromite sand. As shown in Table 3, for a 1-ton track shoe, pearl sand reduces material usage by 80 kg due to its lower bulk density. With chromite sand priced at 4580 yuan/t and pearl sand at 3850 yuan/t, the cost savings per casting are substantial. Additionally, the spherical shape of pearl sand reduces resin and hardener consumption in core making, as detailed in Table 4. The resin usage for pearl sand is 1.1% of sand weight, compared to 1.73% for chromite sand, further lowering costs. Overall, as summarized in Table 5, substituting chromite sand with pearl sand saves approximately 592 yuan per track shoe casting, making it a viable option for high-volume production of high manganese steel castings.
| Sand Type | Unit Price (yuan/t) | Sand Used per Casting (kg) | Total Cost (yuan) |
|---|---|---|---|
| Chromite Sand | 4580 | 350 | 1603 |
| Pearl Sand | 3850 | 270 | 1040 |
| Difference | – | -80 | 563 |
| Sand Type | Resin (% of sand) | Hardener (% of resin) | Resin Used (kg) | Hardener Used (kg) | Resin Cost (yuan/kg) | Hardener Cost (yuan/kg) | Total Cost (yuan) |
|---|---|---|---|---|---|---|---|
| Chromite Sand | 1.73 | 25 | 6.06 | 1.2 | 7 | 12 | 57 |
| Pearl Sand | 1.1 | 25 | 2.97 | 0.59 | 7 | 12 | 28 |
| Difference | -0.63 | 0 | -3.09 | -0.61 | 0 | 0 | 29 |
| Cost Component | Chromite Sand (yuan) | Pearl Sand (yuan) | Savings (yuan) |
|---|---|---|---|
| Sand Cost | 1603 | 1040 | 563 |
| Resin/Hardener Cost | 57 | 28 | 29 |
| Total Cost | 1660 | 1068 | 592 |
In conclusion, pearl sand is an effective and economical alternative to chromite sand for core making in high manganese steel castings, particularly for components with thermal sections below 100 mm. Its neutral chemical properties, spherical particle shape, and lower cost contribute to reduced sand burning and overall production expenses. For high manganese steel castings like track shoes, the savings can be significant, enhancing competitiveness in the market. Further research is needed to optimize its use in larger thermal sections, but current results support its adoption in industrial applications. This study underscores the potential of pearl sand to address longstanding challenges in high manganese steel casting production, aligning with the industry’s goal of cost reduction without compromising quality.
The application of pearl sand in high manganese steel castings not only mitigates sand burning but also promotes sustainability by utilizing domestically available bauxite resources. As the demand for durable components like track shoes and bucket teeth grows, innovations in core sand materials will continue to drive advancements in high manganese steel casting technology. We recommend further trials on other high manganese steel casting types to expand the applicability of pearl sand, ensuring its role in future foundry practices.