In industrial applications requiring prolonged exposure to high temperatures, steel castings such as GX40CrSi17 ferritic heat-resistant alloys play a critical role due to their cost-effectiveness and superior thermal stability. This study systematically investigates the phase transformations and carbide evolution in GX40CrSi17 steel casting under different aging temperatures (850°C, 950°C, and 1050°C) through thermodynamic simulations and microstructural characterization. The findings provide essential insights for optimizing heat treatment protocols in steel casting manufacturing.
Thermodynamic Analysis of Phase Stability
Using JMatPro software, the equilibrium phase diagram of GX40CrSi17 steel casting (composition: 0.43C, 16.66Cr, 1.16Si, 0.58Mn wt%) was simulated (Fig. 1). The liquidus temperature is calculated as 1459°C, with δ-ferrite dominating above 1200°C. Below 1140°C, M23C6 carbides precipitate, reaching maximum volume fraction (7.73%) at 840°C. The temperature-dependent phase fractions are expressed as:
$$ V_{M_{23}C_6}(T) = 0.0773 – 0.00035(T – 840) \quad (600^{\circ}\text{C} \leq T \leq 1120^{\circ}\text{C}) $$
| Temperature (°C) | M23C6 (wt%) | Ferrite (wt%) | Austenite (wt%) |
|---|---|---|---|
| 850 | 7.70 | 89.81 | 2.46 |
| 950 | 6.33 | 24.42 | 69.23 |
| 1050 | 4.42 | 10.68 | 84.88 |
Microstructural Evolution During Aging
In as-cast steel casting, the microstructure comprises δ-ferrite grains (20-40 μm) with chain-like M23C6 carbides at grain boundaries (Fig. 2a-b). The carbide composition was verified by EDS as (Cr0.72Fe0.25Mn0.03)23C6, with precipitation kinetics governed by:
$$ \frac{dV}{dt} = k(T)C_{Cr}^{2.3}C_C^{0.7} $$
where k(T) is the temperature-dependent rate constant, and CCr, CC represent chromium and carbon concentrations.
850°C Aging
Aging at 850°C for 100h results in homogeneous dispersion of spherical M23C6 (200-500 nm) within ferrite matrix (Fig. 3c-d). The hardness decreases from 259 HBW (as-cast) to 230 HBW due to:
1. Dissolution of metastable carbides
2. Reduced dislocation density
3. Coarsening resistance factor: $$ \lambda = \frac{4\gamma V_m}{9RT} $$
950°C Aging
At 950°C, M23C6 content decreases to 6.33 wt% with significant grain growth (Fig. 4). Austenite formation accelerates according to:
$$ \gamma\text{-phase fraction} = 0.6923 – 0.0012(T – 950) $$
The hardness further drops to 223 HBW due to diminished precipitation strengthening.
1050°C Aging
Complete dissolution of M23C6 occurs above 1050°C, forming austenite-ferrite duplex structure (Fig. 5). The final hardness reaches 206 HBW, demonstrating the critical temperature limit for steel casting applications.
Industrial Implications for Steel Casting
For steel casting components requiring long-term service below 900°C, 850°C aging provides optimal carbide dispersion. However, exposure beyond 950°C induces rapid property degradation through:
- Carbide dissolution: $$\Delta G = -RT \ln K_{sp} = -23.5\ \text{kJ/mol}$$
- Grain boundary migration: $$v = M_{\text{gb}}\gamma_{\text{gb}}/r$$
These results emphasize the necessity of temperature control during post-casting heat treatment of ferritic steel castings. Future developments should focus on stabilizing nano-carbides through microalloying to enhance high-temperature performance.
Conclusion
This comprehensive analysis of GX40CrSi17 steel casting demonstrates that aging temperature critically controls the equilibrium between M23C6 precipitation and matrix phase transformation. The quantitative relationships established between thermal history, microstructure, and mechanical properties provide a scientific basis for designing heat-resistant steel castings for elevated temperature applications.