In the field of industrial applications, high manganese steel casting remains a cornerstone material due to its exceptional impact resistance and abrasion performance, widely used in crushing machinery across cement, mining, and power sectors. Despite the availability of alternative materials like high-chromium cast iron, high manganese steel casting continues to be the preferred choice for components such as liners and hammers, owing to its balance of safety and cost-effectiveness. However, traditional sand casting methods coupled with water toughening treatment often lead to issues like shrinkage porosity and elongated production cycles, driven by the material’s pasty solidification characteristics. To address these challenges, I have explored the sand-lined metal mold process, which leverages a metal mold coated with a thin resin sand layer to enhance heat transfer and control solidification. This study compares the microstructure and properties of high manganese steel liners produced by sand-lined metal mold casting against conventional water glass sand casting, focusing on reducing defects and improving performance.
The high manganese steel casting composition used in this investigation is as follows (in mass %): 1.0% C, 13% Mn, 0.5% Si, 2.0% Cr, with S ≤ 0.04 and P ≤ 0.07. Melting was conducted in a 500 kg medium-frequency induction furnace, utilizing industrial raw materials. The process involved melting scrap steel and pig iron first, followed by the addition of high-carbon ferromanganese, high-carbon ferrochromium, and electrolytic manganese. After melting, the chemical composition was verified using a Labspark750B spectrometer, and the molten steel was subjected to slag removal, aluminum deoxidation, and pouring at temperatures of 1500°C ± 20°C for tapping and 1400°C ± 20°C for casting. Two casting methods were employed: water glass sand molding and sand-lined metal mold casting, with both processes using the same batch of molten steel to ensure consistent conditions.
For the sand-lined metal mold process, the design emphasized optimizing heat exchange to increase solidification rates. The resin sand layer thickness was critical, set at 3–5 mm for general surfaces and 5–10 mm at corners, depending on the liner dimensions. The metal mold, made from vermicular or nodular graphite cast iron to withstand thermal fatigue, had an average thickness of 60 mm and a mold-to-casting weight ratio of 3.5. The casting system integrated gating and risering, with the upper mold (liner back) using water glass sand and the lower mold (working surface) employing the sand-lined metal mold. Riser sleeves were pre-formed with insulation, and the mold featured a射砂口 at 120 mm diameter, inclined at 5°, along with multiple排气 holes fitted with plugs to facilitate gas escape during sand hardening. The self-curing resin sand consisted of two low-nitrogen resins (A and B) mixed in proportions of 0.5%–2.0% each, combined with 96%–98% sand and a catalyst to control hardening time to approximately 10 minutes. Blow molding was performed at 0.4 MPa for 30 seconds, followed by a 15-minute hardening period before demolding.
Post-casting, the liners were naturally cooled for 5–10 hours, then cleaned, and subjected to water toughening heat treatment: 650°C for 1 hour followed by 1070°C for 4–6 hours, with water quenching. Samples for microstructural analysis and property testing were extracted from the maximum thickness region (100 mm) of the waveform liners using wire electrical discharge machining, with specimens taken in different orientations for both as-cast and heat-treated conditions. Microstructural observation was conducted on a Zeiss Axio Imager.A1M optical microscope, with specimens prepared standardly and etched in 4% nital. Impact toughness was measured on a JB-300B impact tester with U-notched specimens (10 mm × 10 mm × 55 mm, 2 mm depth), and hardness was assessed using Brinell methods. Impact abrasion wear tests were performed on an MLD-10 tester under non-lubricated conditions, with an upper specimen (10 mm × 10 mm × 30 mm) impacting a lower high-chromium cast iron ring (HRC 58 hardness) at 3.0 N·m impact energy, 150 impacts/min, and 80–120 mesh quartz sand flow of 0.5 kg/min over 120-minute cycles. Weight loss was recorded after each cycle to evaluate wear resistance.
The microstructural analysis revealed significant differences between the two high manganese steel casting processes. In conventional water glass sand casting, the as-cast structure exhibited numerous granular carbides at grain boundaries and interiors, often forming continuous networks, which dissolved after heat treatment to yield a single austenite phase. In contrast, the sand-lined metal mold casting resulted in an as-cast microstructure with minimal carbides, displaying distinct dendritic patterns that aligned directionally; these features vanished post-heat treatment, leaving a homogeneous austenitic structure. This reduction in carbides is attributed to the accelerated solidification rate in the sand-lined metal mold, which suppresses carbide precipitation. The relationship between solidification rate and carbide formation can be expressed using a kinetic equation for carbide precipitation: $$ \frac{dC}{dt} = k (C_s – C) $$ where \( C \) is the carbon concentration, \( C_s \) is the saturation concentration, and \( k \) is the rate constant, highlighting how faster cooling in high manganese steel casting reduces time for diffusion and carbide growth.
| Sample Type | Condition | Average Hardness (HB) | Impact Toughness \( a_k \) (J/cm²) | Measured Impact Toughness (J/cm²) |
|---|---|---|---|---|
| Sand-Lined Metal Mold | As-Cast | 218 | 61 | 55, 94, 42 |
| Sand-Lined Metal Mold | Heat-Treated | 198 | 214 | 212, 216 |
| Water Glass Sand Mold | As-Cast | 230 | 21 | 18, 17, 28 |
| Water Glass Sand Mold | Heat-Treated | 197 | 190 | 165, 215 |
Mechanical properties further underscore the advantages of high manganese steel casting via sand-lined metal mold. As shown in the table, hardness values were similar across conditions, with a slight decrease post-heat treatment due to carbide dissolution. However, impact toughness differed markedly: the as-cast sand-lined metal mold specimens achieved an average of 61 J/cm², with a peak of 94 J/cm², significantly higher than the 21 J/cm² for conventional sand casting. After heat treatment, both methods showed improved toughness, converging to comparable levels. The enhanced as-cast toughness in sand-lined metal mold high manganese steel casting is linked to reduced carbide content and fewer defects like shrinkage porosity, which can be modeled using the Hall-Petch relationship for strength and toughness: $$ \sigma_y = \sigma_0 + k_y d^{-1/2} $$ where \( \sigma_y \) is yield strength, \( \sigma_0 \) and \( k_y \) are constants, and \( d \) is grain size, implying that finer microstructures from rapid solidification contribute to better performance.
Wear resistance, a critical aspect of high manganese steel casting, was evaluated through impact abrasion tests, with results summarized in the following table. The sand-lined metal mold liners demonstrated lower weight loss in both as-cast and heat-treated states compared to heat-treated water glass sand cast liners, indicating superior abrasion resistance. Specifically, the as-cast sand-lined metal mold showed a 36% reduction in wear, while the heat-treated version had a 28% reduction. This improvement stems from the combination of higher impact toughness and reduced internal defects, which enhance the material’s ability to withstand impact loads. The wear rate can be described by an empirical formula for abrasion: $$ W = k_w \cdot H^{-n} \cdot a_k^{m} $$ where \( W \) is wear rate, \( H \) is hardness, \( a_k \) is impact toughness, and \( k_w, n, m \) are constants, illustrating how optimized mechanical properties in high manganese steel casting prolong service life.
| Sample Type | Condition | Weight Loss (g) – Test 1 | Weight Loss (g) – Test 2 | Average Weight Loss (g) |
|---|---|---|---|---|
| Sand-Lined Metal Mold | As-Cast | 0.260 | 0.290 | 0.292 |
| Sand-Lined Metal Mold | Heat-Treated | 0.269 | 0.316 | 0.275 |
| Water Glass Sand Mold | Heat-Treated | 0.453 | 0.296 | 0.374 |
Discussion of these results highlights the efficacy of the sand-lined metal mold process in high manganese steel casting. The accelerated solidification not only minimizes carbide formation but also enhances density by reducing shrinkage defects, as quantified by the solidification time equation: $$ t_s = \frac{(T_p – T_s)^2}{\pi \alpha} \left( \frac{V}{A} \right)^2 $$ where \( t_s \) is solidification time, \( T_p \) is pouring temperature, \( T_s \) is solidus temperature, \( \alpha \) is thermal diffusivity, and \( V/A \) is volume-to-surface area ratio. In sand-lined metal mold casting, the higher cooling rate decreases \( t_s \), leading to a finer microstructure and improved integrity. Moreover, the directional solidification promoted by this method aligns with optimal risering, further mitigating porosity. The impact of these microstructural improvements on wear resistance is profound, as the absence of continuous carbide networks and the presence of a tough austenitic matrix allow for better work hardening under impact, a hallmark of high manganese steel casting.
In conclusion, the sand-lined metal mold process significantly enhances the performance of high manganese steel casting by refining microstructure, boosting impact toughness, and improving abrasion resistance. The as-cast properties achieved through this method are sufficient for applications in medium and small ball mills, potentially eliminating the need for heat treatment and thereby saving energy and costs. This advancement in high manganese steel casting not only addresses longstanding issues like shrinkage defects but also opens avenues for more efficient production, underscoring the importance of process optimization in industrial applications. Future work could focus on scaling this approach and exploring its integration with other alloy modifications to further push the boundaries of high manganese steel casting capabilities.