The influence of normalizing temperature (850–1000 °C) on the microstructure and mechanical properties of G18CrMo2-6 steel casting was systematically investigated. The results reveal that the microstructure primarily consists of granular bainite, composed of bainitic ferrite (BF) and martensite-austenite (M-A) islands, across all tested temperatures. The evolution of M-A island characteristics and grain refinement mechanisms critically govern the strength and toughness of the steel casting.
Microstructural Evolution
The quantitative analysis of M-A island distribution and BF grain size is summarized in Table 1. The relationship between normalizing temperature and microstructural parameters follows:
| Normalizing Temperature (°C) | M-A Island Density (counts/mm²) | Average M-A Size (μm) | BF Grain Size (μm) |
|---|---|---|---|
| 850 | 662 | 2.37 | 21.54 |
| 880 | 631 | 2.28 | 21.91 |
| 910 | 197 | 2.20 | 21.87 |
| 940 | 115 | 2.06 | 22.60 |
| 970 | 193 | 2.38 | 32.04 |
| 1000 | 348 | 2.55 | 54.92 |
The phase transformation kinetics can be described by modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation:
$$ f(t) = 1 – \exp(-kt^n) $$
where \( f(t) \) represents transformed fraction, \( k \) is temperature-dependent rate constant, and \( n \) is Avrami exponent. For steel casting undergoing bainitic transformation, \( n \) typically ranges between 1.5 and 2.5.
Mechanical Behavior
The mechanical properties demonstrate strong temperature dependence (Figure 1). The optimum combination of tensile strength (942 MPa) and impact energy (28 J) occurs at 940°C, correlating with minimized M-A island fraction and refined microstructure. The yield strength follows Hall-Petch relationship:
$$ \sigma_y = \sigma_0 + k_y \cdot d^{-1/2} $$
where \( \sigma_0 \) is lattice friction stress, \( k_y \) is strengthening coefficient, and \( d \) is effective grain size. The deterioration of properties above 940°C results from coarsening effects:
$$ d = d_0 \exp\left(-\frac{Q}{RT}\right) $$
where \( d_0 \) is initial grain size, \( Q \) is activation energy for grain growth, \( R \) is gas constant, and \( T \) is absolute temperature.
Fracture Mechanism
The transition from ductile to brittle fracture modes corresponds to M-A island characteristics. The critical stress intensity factor \( K_{IC} \) shows inverse proportionality with M-A island size:
$$ K_{IC} \propto \frac{1}{\sqrt{D_{MA}}} $$
where \( D_{MA} \) represents average M-A island diameter. This relationship highlights the importance of microstructural control in steel casting applications requiring high fracture toughness.
Industrial Implications
For steel casting manufacturers, the recommended normalizing protocol involves:
- Controlled heating rate: 50–100°C/h
- Optimal austenitizing: 940°C ±10°C for 2 hours
- Air cooling with regulated airflow uniformity
This regimen ensures homogeneous microstructure while maintaining production efficiency. Advanced steel casting techniques combined with precision thermal processing can achieve yield strength exceeding 650 MPa with Charpy impact values >25 J at –20°C.
Conclusion
The mechanical performance of G18CrMo2-6 steel casting is predominantly governed by M-A island morphology and distribution. Optimal normalizing at 940°C produces superior strength-toughness balance through:
- M-A island refinement (≤2.1 μm)
- Dislocation density enhancement in BF matrix
- Suppressed carbide precipitation
These findings provide critical processing guidelines for heavy-section steel castings in nuclear power applications, demonstrating how microstructural engineering enhances component reliability in extreme service conditions.