High manganese steel casting is widely used in mining machinery due to its excellent mechanical properties and cost-effectiveness. It is commonly employed in components such as excavator teeth, ball mill liners, and railway tracks, where high wear resistance is required. However, under non-intensive impact loads, high manganese steel casting may not exhibit sufficient work hardening, leading to reduced wear performance. Various methods have been proposed to address this issue, including grain refinement through cold rolling and heat treatment, as well as the addition of elements like chromium, molybdenum, and vanadium to enhance hardness and wear resistance. In this study, we investigate the effects of different heat treatment processes on the microstructure and properties of high manganese steel casting, focusing on the role of low-temperature pre-isothermal treatment in refining the austenite grain structure and improving impact toughness.
The Fe-Mn-C ternary phase diagram, as shown in the referenced image, indicates that when high manganese steel casting is heated to temperatures between 450°C and 800°C, a eutectoid reaction occurs, leading to the formation of pearlite. This transformation can serve as a nucleation site for new austenite grains during subsequent austenitization, potentially refining the original austenite structure. Our research aims to optimize the heat treatment process to achieve a fully austenitic microstructure with fine grains, thereby enhancing the mechanical properties of high manganese steel casting.
We used a finished ZGMn13 high manganese steel casting from a industrial source as the research material. The chemical composition of the high manganese steel casting was determined through chemical analysis and is presented in Table 1. The composition falls within the range specified by the national standard for high manganese steel castings, with carbon content controlled between 0.90% and 1.05% to balance hardness and impact absorption. Manganese, as an austenite-stabilizing element, helps inhibit carbide precipitation in the as-cast state and promotes dissolution during heat treatment.
| Element | C | Si | Mn | P | S | Mo | Fe |
|---|---|---|---|---|---|---|---|
| Content | 0.98 | 0.59 | 13.42 | 0.036 | 0.005 | 0.92 | Balance |
The as-cast microstructure of the high manganese steel casting was examined using scanning electron microscopy (SEM) after etching with 4% nitric alcohol solution. The results revealed an austenitic matrix with numerous carbides dispersed at grain boundaries and within grains. These carbides exhibited blocky, granular, and acicular morphologies. Energy-dispersive X-ray spectroscopy (EDS) analysis indicated that the granular carbides were rich in molybdenum and manganese, corresponding to alloy cementite of the form (Fe, Mo, Mn)₃C, while the acicular and blocky carbides were manganese-rich (Fe, Mn)₃C.
To optimize the heat treatment process, we designed four different schemes, as summarized in Table 2. All samples were heated at a rate of 100°C/h to ensure uniform temperature distribution. After holding at the specified temperatures, the samples were rapidly quenched in circulating water to prevent carbide reprecipitation. The cooling water temperature was maintained below 60°C to minimize the risk of embrittlement.
| Scheme No. | Heat Treatment Process | Sample ID |
|---|---|---|
| 1 | 1100°C × 2 h + Water Quench | HT1100 |
| 2 | 600°C × 2 h + 1100°C × 2 h + Water Quench | HT600-1100 |
| 3 | 600°C × 2 h + 1050°C × 2 h + Water Quench | HT600-1050 |
| 4 | 600°C × 2 h + Water Quench | HT600 |
Microstructural analysis was performed using optical microscopy and SEM on polished and etched specimens. The hardness of the high manganese steel casting was measured using the Brinell hardness test, and impact toughness was evaluated through Charpy impact tests at room temperature. The results were analyzed to correlate the microstructure with mechanical properties.
The as-cast high manganese steel casting exhibited a microstructure consisting of austenite and carbides. After heat treatment, all samples showed a complete dissolution of carbides, resulting in a single-phase austenitic structure. However, the grain size varied depending on the heat treatment process. Samples subjected to direct austenitization at 1100°C (HT1100) displayed coarse austenite grains, whereas those with a pre-isothermal treatment at 600°C (HT600-1100 and HT600-1050) showed finer grains. The HT600-1050 sample, in particular, demonstrated the most refined microstructure, with uniform austenite grains.
The formation of pearlite during the pre-isothermal treatment at 600°C was confirmed in the HT600 sample, where the microstructure revealed black blocky carbides and light-colored pearlitic regions. This pearlite acts as a nucleation site for new austenite grains during subsequent austenitization, leading to grain refinement. The austenite grain size (d) can be described by the equation: $$d = k \cdot t^n \cdot \exp\left(-\frac{Q}{RT}\right)$$ where k is a constant, t is time, n is the time exponent, Q is the activation energy, R is the gas constant, and T is temperature. This equation highlights the influence of temperature and time on grain growth, supporting the observed refinement in pre-isothermal treated samples.
The mechanical properties of the high manganese steel casting are summarized in Table 3. The hardness values ranged from 190 HB to 194 HB, with the as-cast sample showing slightly higher hardness due to carbide reinforcement. The impact toughness, however, significantly improved after heat treatment, with the HT600-1050 sample achieving the highest impact absorbed energy of 249.4 J. This enhancement is attributed to the fine austenite grain structure, which increases the energy required for crack propagation.
| Sample Condition | Hardness (HB) | Impact Absorbed Energy (J) |
|---|---|---|
| As-Cast | 194.0 | 120.0 |
| HT1100 | 190.5 | 178.0 |
| HT600-1100 | 192.3 | 210.5 |
| HT600-1050 | 194.1 | 249.4 |
| HT600 | 193.0 | 150.0 |
The relationship between grain size and impact toughness can be expressed using the Hall-Petch equation: $$\sigma_y = \sigma_0 + \frac{k_y}{\sqrt{d}}$$ where $\sigma_y$ is the yield strength, $\sigma_0$ is the friction stress, $k_y$ is the strengthening coefficient, and d is the grain diameter. Although this equation primarily applies to strength, it indirectly relates to toughness, as finer grains impede crack propagation, thereby improving impact resistance. In high manganese steel casting, the refined austenite grains in the HT600-1050 sample contribute to its superior toughness.
Further analysis of the carbide dissolution kinetics was conducted based on the diffusion-controlled process. The dissolution time (t) for carbides can be estimated using the equation: $$t = \frac{r^2}{D}$$ where r is the carbide radius and D is the diffusion coefficient. For high manganese steel casting, the diffusion coefficient of carbon in austenite is given by: $$D = D_0 \exp\left(-\frac{Q_d}{RT}\right)$$ where $D_0$ is the pre-exponential factor and $Q_d$ is the activation energy for diffusion. At 1050°C, the diffusion rate is sufficient to dissolve carbides within the 2-hour holding time, ensuring a homogeneous austenitic structure.
In conclusion, the optimal heat treatment process for high manganese steel casting involves a pre-isothermal treatment at 600°C for 2 hours, followed by austenitization at 1050°C for 2 hours and rapid quenching. This process effectively dissolves carbides and refines the austenite grain structure, resulting in a high manganese steel casting with enhanced impact toughness and balanced hardness. The findings demonstrate the importance of microstructural control in improving the performance of high manganese steel casting for mining applications. Future work could explore the effects of additional alloying elements or alternative heat treatment cycles on the properties of high manganese steel casting.