High manganese steel castings are widely utilized as wear-resistant materials in various industrial applications, including mining, metallurgy, and power generation, due to their exceptional toughness and work-hardening capabilities. However, under certain severe operating conditions, the performance of conventional high manganese steel castings may fall short of requirements, necessitating enhancements in mechanical properties and wear resistance. The quality and microstructure of high manganese steel castings are critical determinants of their overall performance, with control over non-metallic inclusions and grain refinement being pivotal for improving these attributes. In this study, we investigate the effects of microalloying with tungsten (W) and rare earth (RE) elements on the formation, morphology, and distribution of inclusions, as well as the microstructure of high manganese steel castings. Our aim is to elucidate the mechanisms underlying grain refinement and inclusion modification, thereby providing insights for optimizing the production of high-performance high manganese steel castings.
The presence of non-metallic inclusions in high manganese steel castings significantly impacts their mechanical properties and service life. In particular, coarse inclusions such as sulfides, phosphides, and oxides can act as stress concentrators, leading to crack initiation and propagation. Phosphorus (P) is a particularly detrimental element in high manganese steel castings, as it tends to segregate at grain boundaries during solidification, forming low-melting-point phosphide eutectics that embrittle the material. Similarly, manganese sulfide (MnS) inclusions can form in the interdendritic regions, further compromising the integrity of high manganese steel castings. Traditional melting practices often struggle to reduce the levels of these impurities effectively, highlighting the need for innovative approaches to modify inclusion characteristics and distribution.
Microalloying and inoculation treatments have emerged as promising strategies to enhance the properties of high manganese steel castings. The addition of strong carbide-forming elements like tungsten can promote the formation of high-temperature stable carbides that serve as heterogeneous nucleation sites during solidification, leading to grain refinement. Meanwhile, rare earth elements exhibit a high affinity for oxygen and sulfur, enabling them to purify the melt by forming stable compounds that float out as slag. Additionally, residual RE inclusions can act as nuclei for favorable phase transformations, further improving the microstructure of high manganese steel castings. In this work, we explore the synergistic effects of W and RE additions on the inclusion behavior and microstructural evolution in high manganese steel castings, with a focus on practical implications for industrial applications.
The solidification process of high manganese steel castings involves complex phase transformations and segregation phenomena. As the melt cools below the liquidus temperature, austenite begins to precipitate, with carbon and alloying elements partitioning between the solid and liquid phases. Phosphorus, having limited solubility in austenite, tends to accumulate at grain boundaries and interdendritic regions, resulting in the formation of brittle phosphide networks. Similarly, MnS inclusions nucleate and grow in the residual liquid, often coalescing into larger aggregates that weaken the material. These issues are exacerbated in thick-section high manganese steel castings, where slower cooling rates promote greater segregation. Therefore, controlling the solidification behavior is essential for producing high-quality high manganese steel castings.
Alloying with tungsten is motivated by its strong interaction with carbon and its ability to form stable carbides such as W2C, which has a melting point of approximately 2785°C. The precipitation of W2C during solidification can provide nucleation sites for austenite, reducing grain size and improving the homogeneity of high manganese steel castings. Moreover, tungsten atoms can segregate to dislocations, enhancing the strength through solute drag effects. The interaction energy between tungsten and iron dislocations is significant, as quantified in Table 1, which summarizes the interaction energies for various alloying elements. The negative values indicate attractive interactions, with tungsten exhibiting a strong tendency to lock dislocations, thereby increasing the yield strength of high manganese steel castings.
| Element | Δμs (10−19 J·mol−1) | Δμm (10−19 J·mol−1) | Δμ600 (10−19 J·mol−1) |
|---|---|---|---|
| Co | -0.13 | +0.06 | -0.05 |
| Cr | -0.11 | +0.00 | -0.09 |
| Mn | -1.16 | -0.11 | -0.22 |
| Ni | -0.13 | +0.03 | -0.77 |
| Nb | -1.46 | -0.06 | -1.22 |
| Mo | -1.17 | +0.19 | -0.79 |
| V | -0.73 | -0.05 | -0.62 |
| W | -1.31 | +0.26 | -0.85 |
The interaction energy components include Δμs (due to atomic size mismatch), Δμm (due to modulus mismatch), and Δμ600 (the combined effect at 600°C). The data indicate that tungsten ranks among the most effective elements for strengthening iron-based alloys, supporting its selection for microalloying high manganese steel castings. The formation of W2C can be described by the reaction: $$2W + C \rightarrow W_2C$$ with a standard free energy change that is highly negative at elevated temperatures, favoring precipitation during solidification. The nucleation rate of W2C can be modeled using classical nucleation theory: $$I = I_0 \exp\left(-\frac{\Delta G^*}{kT}\right)$$ where \(I\) is the nucleation rate, \(I_0\) is a pre-exponential factor, \(\Delta G^*\) is the critical Gibbs free energy for nucleation, \(k\) is Boltzmann’s constant, and \(T\) is temperature. The presence of such carbides facilitates heterogeneous nucleation of austenite, refining the grain structure of high manganese steel castings.
Rare earth elements, such as cerium (Ce) and lanthanum (La), are added as inoculants to modify inclusions in high manganese steel castings. RE elements react with oxygen and sulfur to form stable oxysulfides and other compounds, which can be removed from the melt or remain as fine, dispersed particles. The deoxidation and desulfurization reactions can be represented as: $$2RE + 3O \rightarrow RE_2O_3$$ $$RE + S \rightarrow RES$$ These reactions reduce the content of harmful elements and alter the morphology of inclusions, making them more spherical and less detrimental to the properties of high manganese steel castings. Furthermore, RE elements can reduce phosphorus content through reactions like: $$3RE + P \rightarrow RE_3P$$ although this is less common due to kinetic limitations. The effectiveness of RE treatment depends on the amount added and the initial impurity levels in the high manganese steel castings.
In our experimental investigation, we produced high manganese steel castings with and without W and RE additions. The base composition was similar to standard high manganese steel, with high carbon and manganese contents. The chemical compositions of the tested high manganese steel castings are listed in Table 2. We used a medium-frequency induction furnace to melt the charge materials, including scrap steel, ferromanganese, and other alloys. The melt was deoxidized with aluminum, and RE was added during tapping for modification. The casting temperature was maintained at 1480°C, and Y-block samples were poured for analysis.
| Sample | C | Mn | Si | P | S | W | RE |
|---|---|---|---|---|---|---|---|
| 1 | 1.12 | 12.23 | 0.42 | 0.02 | 0.03 | 0.00 | 0.00 |
| 2 | 1.14 | 12.01 | 0.40 | 0.02 | 0.01 | 1.09 | 0.30 |
The microstructure of the high manganese steel castings was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), coupled with energy-dispersive spectroscopy (EDS) for compositional analysis. Samples were sectioned from the Y-blocks, polished, and etched with 5% nital solution to reveal the microstructure. The inclusion characteristics and phase distributions were evaluated to assess the effects of W and RE additions.
The as-cast microstructure of conventional high manganese steel castings (Sample 1) exhibited coarse austenitic grains with continuous networks of carbides and inclusions along the grain boundaries. In contrast, the modified high manganese steel castings (Sample 2) showed significant grain refinement and a more dispersed distribution of carbides and inclusions. The addition of W and RE resulted in a finer grain size, with carbide particles distributed both intra-granularly and inter-granularly, but in a non-continuous manner. This microstructural improvement is attributed to the nucleation effects of W2C and the inclusion modification by RE.
EDS analysis confirmed that the matrix of modified high manganese steel castings had lower levels of sulfur and phosphorus, indicating effective purification. However, some particles rich in W, S, and P were detected, suggesting the presence of complex inclusions. TEM analysis identified these particles as W2C, with adjacent regions containing MnS and phosphide eutectics. The sequence of inclusion precipitation during solidification can be summarized as: W2C → MnS → phosphide eutectic, based on their respective melting points and thermodynamic stability. The precipitation temperature for W2C is above 1300°C, for MnS around 1375°C, and for phosphide eutectic between 950°C and 1005°C. This sequence allows W2C to act as a nucleation site for subsequent phases, thereby controlling their distribution in high manganese steel castings.
The grain refinement mechanism in high manganese steel castings with W addition can be explained by the enhanced nucleation rate due to heterogeneous sites. The number of nuclei per unit volume, \(N_v\), can be related to the undercooling, \(\Delta T\), and the interfacial energy, \(\sigma\), by: $$N_v = N_0 \exp\left(-\frac{A}{\Delta T^2}\right)$$ where \(N_0\) is a constant and \(A\) is a parameter dependent on \(\sigma\). The presence of W2C particles reduces the interfacial energy barrier for austenite nucleation, increasing \(N_v\) and refining the grain structure. Additionally, the solute drag effect of W atoms slows down grain growth, further contributing to the fine microstructure in high manganese steel castings.
The role of RE in inclusion modification is quantified by the reduction in inclusion size and the change in morphology. The number density of inclusions, \(N_i\), decreases after RE treatment, while the average size, \(d_i\), may increase slightly due to coalescence, but the overall volume fraction decreases. The desulfurization efficiency, \(\eta_S\), can be calculated as: $$\eta_S = \frac{S_0 – S_f}{S_0} \times 100\%$$ where \(S_0\) and \(S_f\) are the initial and final sulfur contents, respectively. In our high manganese steel castings, \(\eta_S\) exceeded 80%, leading to a significant reduction in MnS inclusions. The remaining RE-containing inclusions are fine and globular, reducing stress concentration and improving the toughness of high manganese steel castings.
The distribution of phosphide eutectics is also altered in modified high manganese steel castings. Instead of continuous networks, they appear as isolated clusters associated with W2C particles. This redistribution mitigates the hot brittleness caused by phosphorus segregation. The effectiveness of this approach depends on the solidification conditions and the concentration of alloying elements. We observed that the optimized addition levels of 1% W and 0.3% RE provided the best balance between grain refinement and inclusion control in high manganese steel castings.
In conclusion, the microalloying of high manganese steel castings with tungsten and rare earth elements significantly improves their microstructure and inclusion characteristics. The precipitation of W2C during solidification promotes grain refinement by acting as heterogeneous nucleation sites, while RE elements purify the melt and modify harmful inclusions into less detrimental forms. These changes enhance the mechanical properties and service life of high manganese steel castings, making them more suitable for demanding applications. Future work should focus on optimizing the processing parameters and exploring the effects of other alloying elements to further advance the performance of high manganese steel castings.