In the field of heavy-duty mining equipment, the production of high manganese steel casting components is critical due to their exceptional wear resistance and toughness. However, the inherent challenges in casting high manganese steel, such as high linear shrinkage (2.4% to 3%) and low thermal conductivity (approximately one-fourth to one-sixth that of carbon steel), often lead to defects like shrinkage porosity, hot tearing, and misruns in complex geometries. This study focuses on optimizing the casting process for a intricate high manganese steel casting used in electric shovels for mining applications. The component features internal cavities and uneven wall thicknesses, making it prone to defects during solidification. We employed numerical simulation tools, including ADSTEFAN and ProCAST, to analyze the initial casting process, identify defect-prone areas, and propose an optimized design. By adjusting riser dimensions and chill configurations, we aimed to enhance the quality and reliability of high manganese steel casting.
The initial casting process for the high manganese steel casting involved a sand casting method with a bottom gating system. The casting, with an outer diameter of 1638 mm and varying thicknesses (e.g., 279 mm at the rim and 584 mm at the hub), included nine uniformly distributed through-holes that complicated feeding and solidification. To address this, the initial design incorporated a top riser A at the hub and nine inverted conical top risers B at the rim, along with 23 chills placed around the rim’s upper and lower surfaces. The pouring temperature was set at 1420°C with a filling time of 100 seconds, using resin sand for the mold and chromite sand for the face coat. The material properties of ZGMn13, as listed in Table 1, were derived from literature and Lever rule calculations, with a liquidus temperature of 1393°C and a solidus temperature of 1187°C.
| Property | Value |
|---|---|
| Density (kg/m³) | 7800 |
| Thermal Conductivity (W/m·K) | 15.2 |
| Specific Heat Capacity (J/kg·K) | 550 |
| Linear Shrinkage (%) | 2.4–3.0 |
| Young’s Modulus (GPa) | 200 |
| Poisson’s Ratio | 0.3 |
Numerical simulation of the initial process revealed critical insights into the temperature distribution and defect formation. The governing equations for heat transfer and solidification in high manganese steel casting include the energy conservation equation:
$$ \frac{\partial (\rho c_p T)}{\partial t} = \nabla \cdot (k \nabla T) + Q $$
where \( \rho \) is density, \( c_p \) is specific heat, \( T \) is temperature, \( t \) is time, \( k \) is thermal conductivity, and \( Q \) represents latent heat release during phase change. The latent heat is modeled using the Lever rule for microsegregation. Simulation results showed that the rim areas cooled rapidly due to chill effects, leading to premature solidification compared to the risers. This resulted in inadequate feeding and potential defects, as summarized in Table 2.
| Location | Niyama Value (℃¹/²·min¹/²·cm⁻¹) | Defect Risk |
|---|---|---|
| Rim Area | < 1.0 | High Shrinkage Porosity |
| Hub Corner | 1.2 | Moderate Hot Tearing |
| Riser B Regions | > 2.0 | Low Risk |
The Niyama criterion, defined as \( G / \sqrt{\dot{T}} \), where \( G \) is the temperature gradient and \( \dot{T} \) is the cooling rate, was applied with a critical threshold of 1.0 °C¹/²·min¹/²·cm⁻¹ for high manganese steel casting. Values below this indicated a high propensity for shrinkage defects. Additionally, hot tearing susceptibility was assessed using a thermal stress model based on the strain accumulation during solidification:
$$ \sigma = E \alpha \Delta T $$
where \( \sigma \) is stress, \( E \) is Young’s modulus, \( \alpha \) is the coefficient of thermal expansion, and \( \Delta T \) is the temperature difference. The initial process exhibited significant stress concentrations at the hub and rim junctions, increasing the risk of cracks.
To address these issues, we optimized the casting process for high manganese steel casting by modifying riser sizes and chill arrangements. Riser A at the hub was reduced in volume to minimize material usage while maintaining feeding efficiency, and its insulation was upgraded from a 30 mm thick insulating board (thermal conductivity 0.34 W/m·K) to a 60 mm thick riser insulating brick (thermal conductivity 0.24 W/m·K). Riser B at the rim was enlarged to improve feeding, with a height-to-diameter ratio of 2. The chills were reconfigured to a ring-shaped design placed only at the lower rim to promote directional solidification. The optimized parameters are compared in Table 3.
| Parameter | Initial Process | Optimized Process |
|---|---|---|
| Riser A Diameter (mm) | 880 (outer) | 650 (outer) |
| Riser A Height (mm) | 850 | 650 |
| Riser B Top Diameter (mm) | 240 | 260 |
| Riser B Height (mm) | 320 | 500 |
| Chill Configuration | 23 individual chills | Ring chill at lower rim |
Simulation of the optimized high manganese steel casting process demonstrated improved temperature fields and reduced defect risks. The ring chill enhanced cooling at the rim base, establishing a bottom-up solidification sequence. This is described by the modified Fourier number for transient heat conduction:
$$ Fo = \frac{\alpha t}{L^2} $$
where \( \alpha \) is thermal diffusivity, \( t \) is time, and \( L \) is characteristic length. Higher \( Fo \) values in the rim area indicated faster solidification, reducing the time for defect formation. The Niyama criterion predictions showed that shrinkage porosity in the rim was eliminated, as values exceeded 1.5 °C¹/²·min¹/²·cm⁻¹. Hot tearing tendency decreased significantly, with stress levels dropping by approximately 30% in critical regions, as calculated using the von Mises criterion:
$$ \sigma_{v} = \sqrt{\frac{(\sigma_1 – \sigma_2)^2 + (\sigma_2 – \sigma_3)^2 + (\sigma_3 – \sigma_1)^2}{2}} $$
where \( \sigma_1, \sigma_2, \sigma_3 \) are principal stresses. The optimized process for high manganese steel casting also resulted in a more uniform temperature gradient, minimizing thermal stresses.
Experimental validation was conducted through actual casting trials of both initial and optimized high manganese steel casting components. Non-destructive testing using an X-ray linear accelerator (DZ-9/3000) with a sensitivity of 2 mm confirmed the simulation findings. The initial casting exhibited multiple cracks in the rim area, correlating with predicted shrinkage zones. In contrast, the optimized high manganese steel casting showed no defects in the rim, with only minor cracks in the hub region, consistent with reduced hot tearing predictions. This alignment between simulation and experiment underscores the effectiveness of the optimization for high manganese steel casting.
In conclusion, the optimization of the casting process for complex high manganese steel casting components through numerical simulation and practical adjustments proved highly successful. By enlarging riser B, implementing a ring chill, and refining riser A, we achieved a directional solidification pattern that mitigated shrinkage and hot tearing defects. The use of advanced software tools enabled precise defect prediction, leading to a reliable manufacturing process for high manganese steel casting. Future work could focus on further reducing hub cracks by improving sand core compliance and optimizing fillet radii. This study highlights the importance of integrated simulation and experimentation in enhancing the quality of high manganese steel casting for demanding industrial applications.