Microstructural Evolution and Tensile Behavior of Grey Cast Iron Under Low-Frequency Thermal Fatigue Loading

By /

This study investigates the degradation mechanisms of grey cast iron subjected to cyclic high-temperature oxidation through systematic analysis of tensile strength, microstructural evolution, and elemental composition. The experimental results reveal critical relationships between thermal fatigue resistance and microstructural characteristics under low-frequency thermal loading conditions.

1. Thermal Cycling Protocol

Specimens of HT200 grey cast iron (composition in Table 1) underwent cyclic thermal treatments simulating industrial service conditions:

$$T(t) = \begin{cases}
25^\circ\text{C} \to 900^\circ\text{C} \ (10^\circ\text{C/min}) & \text{Heating phase} \\
900^\circ\text{C} \ (60\text{-min hold}) & \text{Oxidation phase} \\
900^\circ\text{C} \to 25^\circ\text{C} \ (10^\circ\text{C/min}) & \text{Cooling phase}
\end{cases}$$

Table 1. Chemical composition of HT200 grey cast iron (wt%)
C Si Mn P S Fe
3.58 1.59 0.89 0.087 0.076 Bal.

2. Tensile Strength Degradation

The tensile strength evolution at different thermal cycles follows distinct temperature-dependent patterns:

Table 2. Tensile strength variation with thermal cycles (MPa)
Cycles 400°C 600°C 800°C
10 >125 74.95 29.86
30 85.95 55.65 24.69
50 45.96 38.09 19.81

The strength reduction follows linear relationships with carbon content:

$$\sigma_{400} = \begin{cases}
22.10C + 47.19 & \text{(Pearlite matrix)} \\
313.3C – 404.6 & \text{(Ferrite matrix)}
\end{cases}$$

3. Microstructural Transformations

Key microstructural changes during thermal cycling include:

  1. Graphite fragmentation: Original A-type graphite (length: 200-400 μm) breaks into sub-100 μm fragments
  2. Matrix evolution: Pearlite → Ferrite transition completes after 30 cycles
  3. Secondary precipitation: Fe3C forms at grain boundaries post 30 cycles
Table 3. Carbon depletion during thermal cycling
Cycles 0 10 30 50
C (wt%) 3.58 3.08 1.55 1.44

4. Oxidation-Induced Degradation

The oxidation process follows parabolic kinetics:

$$\frac{dx}{dt} = \frac{k_p}{x}$$

Where $k_p$ represents temperature-dependent oxidation rate constant. Graphite flakes act as oxygen diffusion channels, accelerating formation of Fe-Si-Mn oxides at flake/matrix interfaces.

5. Fracture Mechanism Analysis

Three-stage fracture evolution occurs:

  1. Microcrack initiation at graphite tips
  2. Crack propagation through oxide-weakened matrix
  3. Final fracture via interconnected cracks

The graphite exposure ratio ($R_{\text{exp}}$) increases with thermal cycles:

$$R_{\text{exp}} = 1 – e^{-0.15N}$$

Where N = number of thermal cycles

6. Thermal Fatigue Life Prediction

The Coffin-Manson relationship modified for grey cast iron:

$$\frac{\Delta \varepsilon}{2} = \varepsilon_f'(2N)^c + \frac{\sigma_f’}{E}(2N)^b$$

Where material constants for HT200 are:

  • Fatigue ductility coefficient $\varepsilon_f’$: 0.25
  • Fatigue strength coefficient $\sigma_f’$: 850 MPa
  • Fatigue ductility exponent $c$: -0.6
  • Fatigue strength exponent $b$: -0.09

7. Conclusion

This comprehensive analysis demonstrates that grey cast iron’s thermal fatigue resistance is governed by synergistic effects of:

  1. Carbon depletion rate
  2. Graphite morphology evolution
  3. Matrix phase transformation
  4. Oxidation-induced interfacial weakening

The derived quantitative relationships enable lifetime prediction and material optimization for grey cast iron components in thermal fatigue applications.

Scroll to Top