In the field of sand casting, the production of high-quality castings, particularly those made from duplex stainless steels, often requires the use of feeding aids to prevent shrinkage defects. Traditionally, metallic pads have been employed to extend the feeding range of risers, but these must be removed after casting, leading to significant material waste and energy consumption. This issue is especially pronounced in duplex steel castings, where the high alloy content amplifies costs. To address this, we explored the use of exothermic pads as a replacement for metallic pads in sand casting processes. Exothermic pads, made from materials that generate heat upon reaction, maintain localized liquid metal longer, thereby enhancing feeding efficiency without the need for post-casting removal. In this article, I will detail our investigation into the application of exothermic pads in sand casting, focusing on material selection, design simulations, production trials, and economic benefits, with an emphasis on improving sustainability in sand casting operations.
Sand casting is a widely used manufacturing method for producing complex metal components, but it faces challenges in ensuring sound casting integrity, particularly in thick sections. Duplex steels, such as ASTM A890 5A, are prone to shrinkage porosity due to their solidification characteristics. Metallic pads, while effective, contribute to high metal consumption and additional processing steps. Our objective was to evaluate whether exothermic pads could serve as a viable alternative, reducing resource usage while maintaining casting quality. We selected an EX5 BLEND exothermic material for its superior thermal properties, as it demonstrated extended holding times at elevated temperatures compared to conventional exothermic materials. This characteristic is crucial in sand casting, where controlled solidification is key to defect-free castings.
The thermal performance of exothermic materials is fundamental to their effectiveness in sand casting. For instance, the heat generation can be modeled using the following equation for exothermic reactions: $$ Q = m \cdot \Delta H $$ where \( Q \) is the total heat released, \( m \) is the mass of the exothermic material, and \( \Delta H \) is the enthalpy of reaction. In sand casting, this heat input helps maintain a thermal gradient, prolonging the liquid state in critical areas. We compared the combustion curves of standard exothermic materials and EX5 BLEND, noting that the latter sustained a temperature of 1,147 °C for approximately 5.70 minutes, versus only 2.25 minutes for conventional materials. This extended duration is vital for feeding in sand casting, as it aligns with the solidification time of duplex steels, reducing the risk of shrinkage defects.
To design the exothermic pads for sand casting, we first established the dimensions based on empirical data and simulation inputs. For a valve body casting with a contour size of 830 mm × 500 mm × 440 mm and a rough weight of 540 kg, we initially designed metallic pads for the flanges with dimensions of 270 mm × 260 mm × 90 mm, resulting in a total theoretical weight of 70 kg for two pads. The total pour weight with metallic pads was 1,050 kg, yielding a process yield of 51.4%, calculated as: $$ \text{Process Yield} = \frac{\text{Casting Weight}}{\text{Total Pour Weight}} \times 100\% $$ Using solidification simulation software, we verified that this design met the feeding requirements for sand casting. Subsequently, we designed exothermic pads of the same dimensions, incorporating the thermal properties of EX5 BLEND into the simulation model. The results indicated that the exothermic pads could achieve similar feeding performance, with a reduced total pour weight of 980 kg and an improved process yield of 55.1%. This enhancement underscores the potential of exothermic pads in optimizing sand casting processes.
The production of exothermic pads for sand casting involved mixing powdered exothermic material with a binder in specified proportions, then molding them using a core box. This prefabrication step ensured consistent pad geometry and easy integration into the sand mold. During mold assembly in sand casting, the exothermic pads were positioned at designated locations, with vent pipes installed to facilitate gas escape during the exothermic reaction. After applying a refractory coating and drying, the mold was ready for pouring. In contrast, metallic pads required post-casting removal through cutting or sawing, adding to energy consumption and labor costs. The use of exothermic pads in sand casting simplified this process, as the reacted material could be easily removed during shakeout, minimizing secondary operations.
Post-casting inspection in sand casting revealed that both metallic and exothermic pad approaches produced castings free from internal defects, as confirmed by radiographic testing. Visual examination showed smooth surface finishes with no signs of gas porosity or sand adhesion in the exothermic pad areas. The table below summarizes a comparative analysis of key parameters in sand casting using metallic versus exothermic pads:
| Parameter | Metallic Pads | Exothermic Pads |
|---|---|---|
| Pad Dimensions (mm) | 270 × 260 × 90 | 270 × 260 × 90 |
| Total Pad Weight (kg) | 70 | ~5 (material weight) |
| Total Pour Weight (kg) | 1,050 | 980 |
| Process Yield (%) | 51.4 | 55.1 |
| Energy for Removal | High (cutting/sawing) | Low (shakeout) |
| Material Cost | High (metal consumption) | Moderate (exothermic material) |
From an economic perspective, the adoption of exothermic pads in sand casting offers substantial cost savings. The reduction in metal usage directly decreases material expenses, while the elimination of energy-intensive removal processes lowers operational costs. For duplex steel castings in sand casting, where alloy costs are significant, this translates to a notable reduction in overall production expenses. Moreover, the environmental impact is reduced due to decreased waste and energy consumption, aligning with sustainable practices in sand casting industries.
In terms of thermal dynamics, the efficiency of exothermic pads in sand casting can be further analyzed through heat transfer models. The temperature maintenance in the feeding zone can be described by the equation: $$ T(t) = T_0 + \frac{Q}{c_p \cdot m} \cdot e^{-k t} $$ where \( T(t) \) is the temperature at time \( t \), \( T_0 \) is the initial temperature, \( c_p \) is the specific heat capacity, \( m \) is the mass of metal, and \( k \) is a cooling constant. For EX5 BLEND, the prolonged heat release ensures that \( T(t) \) remains above the solidus temperature for an extended period, critical for effective feeding in sand casting. This mathematical approach reinforces the reliability of exothermic pads in complex sand casting applications.
Our trials in sand casting also highlighted the importance of proper pad installation and venting to prevent defects. Inadequate venting could lead to gas entrapment, compromising casting quality. Therefore, we recommend thorough design validation using simulation tools before implementation in sand casting projects. The success of exothermic pads in this study demonstrates their versatility across various sand casting scenarios, particularly for high-value materials like duplex steels.
In conclusion, the integration of exothermic pads into sand casting processes for duplex steel castings presents a viable alternative to traditional metallic pads. Through careful design and simulation, we achieved improved process yields and significant cost reductions while maintaining high casting quality. The use of EX5 BLEND exothermic material proved effective in extending feeding times, reducing metal consumption, and minimizing energy usage in sand casting. As the sand casting industry continues to evolve, adopting such innovative approaches can enhance efficiency and sustainability. Future work could focus on optimizing exothermic pad compositions for specific sand casting applications, further pushing the boundaries of this technology.