Non-metallic inclusions formed during the smelting and pouring of molten steel are critical factors leading to crack initiation in steel castings. Among these, MnS inclusions significantly degrade mechanical properties such as strength, toughness, fatigue resistance, and corrosion performance. This article systematically investigates the morphological characteristics, compositional features, and formation mechanisms of MnS inclusions in low-alloy steel castings through optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX).
1. Classification and Characteristics of MnS Inclusions
Two distinct MnS inclusion types were identified in steel castings:
| Type | Morphology | Size Range | Core Composition |
|---|---|---|---|
| Type I | Irregular flocculent | 20–100 μm | MnO-SiO2 composite oxides |
| Type II | Spherical | 5–50 μm | Mn-Si-O complex oxides |
Both types exhibit a core-shell structure with MnS precipitates surrounding oxide nuclei. The thermodynamic driving force for MnS precipitation can be expressed as:
$$
\Delta G = -RT \ln \left( \frac{[Mn][S]}{K_{sp}} \right)
$$
where \( K_{sp} \) represents the solubility product of MnS, and \([Mn]\), \([S]\) denote the activity of dissolved Mn and S in molten steel.
2. Formation Mechanism of MnS Inclusions
The formation process involves three stages:
- Oxide Nucleation: Primary oxides (MnO-SiO2 or Mn-Si-O) form during deoxidation.
$$
2[Mn] + [O] \rightarrow 2MnO_{(s)}
$$ - MnS Heterogeneous Precipitation: Dissolved Mn and S segregate at oxide interfaces.
$$
[Mn] + [S] \rightarrow MnS_{(s)}
$$ - Coarsening: Ostwald ripening dominates inclusion growth:
$$
r(t)^3 – r_0^3 = \frac{8\gamma DC_0}{9RT} t
$$
where \( \gamma \) is interfacial energy, \( D \) the diffusion coefficient, and \( C_0 \) the solute concentration.
3. Compositional Analysis
EDX mapping reveals elemental distribution in Type I inclusions:
| Region | Element | Atomic % |
|---|---|---|
| Core | O | 58.94 |
| Si | 19.66 | |
| Mn | 20.85 | |
| Shell | Mn | 63.2 |
| S | 36.8 |
4. Process Optimization for Inclusion Control
Key parameters influencing MnS formation in steel casting:
| Parameter | Optimal Range | Effect on MnS |
|---|---|---|
| Pouring Temperature | 1520–1550°C | Reduces oxide nucleation |
| S Content | <0.015 wt% | Limits sulfide availability |
| Cooling Rate | >15°C/s | Suppresses segregation |
5. Industrial Implications
In steel casting production, implementing these strategies reduces MnS inclusion density by 40–60%, improving ultimate tensile strength (UTS) and impact toughness:
$$
\sigma_{UTS} \propto \frac{1}{\sqrt{d_{inclusion}}}
$$
where \( d_{inclusion} \) represents the average inclusion diameter. Field trials demonstrate 18% improvement in fatigue life when MnS content decreases below 0.03 vol%.
6. Conclusion
This study establishes the correlation between oxide nucleation and MnS precipitation in steel casting processes. The proposed formation mechanism provides theoretical guidance for optimizing deoxidation practices, controlling cooling rates, and adjusting alloy compositions to minimize harmful inclusions. Future work should focus on real-time inclusion monitoring during steel casting operations to validate these findings.