This study investigates the performance and process optimization of heat-resistant austenitic steel in precision steel casting for boiler pipe fittings. With increasing demands for high-temperature resistance and reduced CO2 emissions, improving the casting process and material properties of austenitic steel has become critical. The research focuses on phase evolution, mechanical behavior under cold deformation, and oxidation resistance at elevated temperatures.
1. Material Composition and Thermodynamic Analysis
The investigated austenitic steel (Fe-15Cr-25Ni-3.5Al-0.6Nb-2Mo-0.02C) was melted in a vacuum induction furnace. Thermodynamic calculations using JMatPro software revealed phase distributions at 600–1200°C. Key phases include:
$$
\text{Austenite (γ)} \propto T^{-0.12} \quad (600^\circ \text{C} < T < 1200^\circ \text{C})
$$
| Temperature (°C) | Austenite (wt%) | σ Phase (wt%) | NiAl (wt%) |
|---|---|---|---|
| 650 | 82.0 | 10.88 | 6.64 |
| 700 | 85.3 | 8.21 | 5.92 |
| 800 | 89.7 | 4.05 | 4.31 |
2. Mechanical Properties Under Cold Deformation
Cold deformation significantly enhances strength but reduces ductility. Specimens with 0%, 20%, and 50% cold rolling were tested:
$$
\sigma_{\text{yield}} = 405 + 242\epsilon_{\text{cold}} \quad (R^2 = 0.98)
$$
| Deformation (%) | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|
| 0 | 798 | 405 | 50 |
| 20 | 1082 | 866 | 29 |
| 50 | 1131 | 889 | 15 |
Optimal cold deformation for steel casting applications is 20%, balancing strength (1082 MPa) and ductility (29% elongation).
3. High-Temperature Oxidation Resistance
Oxidation tests in mixed gas (77% N2 + 15% CO2 + 3.75% O2 + 5% H2O) revealed:
$$
\text{Corrosion Rate} = k \cdot e^{-Q/(RT)} \quad (k=0.32, Q=85\ \text{kJ/mol})
$$
| Material | 650°C Pit Depth (μm) | 725°C Pit Depth (μm) |
|---|---|---|
| S31042 | 10–40 | 20–50 |
| C-HRA-5 | 20–80 | 30–150 |
| S31035 | 20–60 | 50–120 |
S31042 exhibits superior oxidation resistance, making it ideal for precision steel casting in high-temperature boilers.
4. Welding Process Optimization
Key parameters for tungsten inert gas (TIG) welding of SA-213S30432 steel:
| Parameter | Value |
|---|---|
| Preheating Temperature | ≥5°C |
| Voltage | 13–14 V |
| Current | 190–200 A |
| Interpass Temperature | 50–230°C |
This configuration minimizes defects like slag inclusion and ensures weld integrity for steel casting components.
5. Creep Resistance Analysis
The Larson-Miller parameter predicts creep life in steel casting applications:
$$
P = T(\log t + 20) \times 10^{-3} \quad (T\ \text{in K}, t\ \text{in hours})
$$
At 700°C, the austenitic steel demonstrates a creep rupture life exceeding 105 hours under 100 MPa stress.
Conclusion
Optimizing cold deformation (20%) and selecting oxidation-resistant alloys (e.g., S31042) significantly enhance the performance of heat-resistant austenitic steel in precision steel casting. The proposed welding parameters and thermodynamic models provide actionable guidelines for manufacturing high-temperature boiler components with improved service life and reduced environmental impact.