In recent years, stringent emission control requirements have driven the need for improved fuel efficiency and reduced exhaust emissions in internal combustion engines, leading to more severe service conditions for key components. This often results in fatigue failures during engine start-up and shutdown cycles. Lightweight design and fatigue durability are critical optimization focuses, making it essential to fully understand the stress-strain response behavior of materials used in high-temperature engine components under both static and cyclic loading across the service temperature range. Such understanding is vital for accurate fatigue durability assessment of these parts.
High-silicon molybdenum spheroidal graphite cast iron is commonly used for manufacturing exhaust manifolds and turbocharger housings in internal combustion engines. For instance, the spheroidal graphite cast iron grade QTRSi4Mo1, with approximately 4% Si and 1% Mo, is employed in exhaust manifolds that can experience operating temperatures up to 760°C. While international studies have extensively investigated the mechanical properties of various types of spheroidal graphite cast iron at room and elevated temperatures, there is limited reporting on the stress-strain response behavior of high-silicon molybdenum spheroidal graphite cast iron across a broad temperature spectrum, particularly under the thermal cycles encountered during engine start-stop operations.
This study focuses on the as-cast spheroidal graphite cast iron QTRSi4Mo1. We conducted static tensile tests and strain-controlled low-cycle fatigue tests from room temperature up to 760°C. The primary aim is to elucidate the influence of temperature on the tensile properties and cyclic stress-strain response characteristics of this material, thereby providing support for the fatigue durability design and service life evaluation of high-temperature components in internal combustion engines.
Experimental Materials and Methods
The material under investigation is a ferritic spheroidal graphite cast iron, QTRSi4Mo1, widely used in engine exhaust manifolds and combustion chambers. Its chemical composition is detailed in Table 1. Both tensile and fatigue specimens were extracted from cast blocks measuring 50 mm × 50 mm × 300 mm. The geometry of the tensile specimen is shown schematically, featuring a gauge length designed for high-temperature testing. Similarly, the strain-controlled fatigue specimen has a specific geometry to ensure proper strain measurement and failure within the gauge section.
| C | Si | Mo | Mg | Mn | S | P | Fe |
|---|---|---|---|---|---|---|---|
| 3.18 | 4.43 | 1.23 | 0.03 | 0.31 | 0.008 | 0.027 | Bal. |
Static tensile tests were performed on an RDJ30 mechanical high-temperature endurance testing machine. Tests were conducted at temperatures of 25°C (room temperature), 200°C, 300°C, 400°C, 500°C, 600°C, 700°C, and 760°C. A constant tensile strain rate of $$2.5 \times 10^{-4} \, \text{s}^{-1}$$ was maintained for all tests to ensure consistent deformation conditions. The measured properties included yield strength, ultimate tensile strength, percentage elongation after fracture, and percentage reduction of area.
Low-cycle fatigue tests were carried out using an MTS 809 axial/torsional servo-hydraulic testing system. To minimize the influence of surface machining scratches, the gauge section of each specimen was progressively polished with P600, P1000, and P2000 grit wet sandpaper. Subsequently, specimens were ultrasonically cleaned in acetone for 20 minutes to remove machining oils and residues. Testing was performed under strain control at temperatures of 25°C, 200°C, 400°C, 500°C, and 760°C. A fully reversed strain ratio (R = -1) was applied using a symmetric triangular waveform. The strain rate was set at $$5 \times 10^{-3} \, \text{s}^{-1}$$. Detailed testing parameters for various strain amplitudes at each temperature are summarized in Table 2. Fatigue life, denoted as $$N_f$$, was defined as the number of cycles to specimen fracture or a rapid 5% drop in the maximum tensile stress.
| Temperature (°C) | Strain Amplitude (%) | Strain Ratio (R) | Loading Rate (s⁻¹) |
|---|---|---|---|
| 25 | 0.25 | -1 | $$5 \times 10^{-3}$$ |
| 0.3 | |||
| 0.4 | |||
| 0.5 | |||
| 200 | 0.25 | ||
| 0.3 | |||
| 0.4 | |||
| 400 | 0.25 | ||
| 0.3 | |||
| 0.4 | |||
| 500 | 0.15 | ||
| 0.25 | |||
| 0.4 | |||
| 760 | 0.15 | ||
| 0.3 | |||
| 0.4 |
Tensile Mechanical Behavior of Spheroidal Graphite Cast Iron QTRSi4Mo1 at Various Temperatures
The tensile properties of the spheroidal graphite cast iron QTRSi4Mo1 from room temperature to 760°C are listed in Table 3. To illustrate the temperature dependence, the data were normalized relative to the room temperature values, as depicted graphically. The yield strength and ultimate tensile strength of this spheroidal graphite cast iron decrease progressively with increasing temperature. The relationship between yield strength and temperature can be described by a bilinear function, as shown in Figure 1, which was derived through linear fitting. The temperature-dependent yield strength expression is given by:
$$ \sigma_{0.2}(T) = \begin{cases} 481.80 – 0.2624T & \text{for } T \leq 419^\circ\text{C} \\ 806.15 – 1.0365T & \text{for } T > 419^\circ\text{C} \end{cases} $$
The transition point of this piecewise function occurs at approximately 419°C. In contrast, ductility indicators such as percentage elongation and percentage reduction of area exhibit a trend of initial decrease followed by an increase with rising temperature. Minimum values are observed in the range of 300°C to 400°C, indicating that the spheroidal graphite cast iron QTRSi4Mo1 exhibits a certain degree of brittleness near this temperature range. This phenomenon aligns with literature reports of 400°C embrittlement in some spheroidal graphite cast irons and correlates well with the transition temperature in the yield strength equation. Above 500°C, the material’s ductility significantly improves. This embrittlement around 400°C is a critical consideration as it may lead to reduced fatigue life under thermal or thermomechanical fatigue conditions.
| Temperature (°C) | Yield Strength (MPa) | Ultimate Tensile Strength (MPa) | Elongation (%) | Reduction of Area (%) |
|---|---|---|---|---|
| 25 | 500 | 627 | 13.5 | 7.7 |
| 200 | 451 | 588 | 8.7 | 7.7 |
| 300 | 406 | 558 | 8.6 | 4.2 |
| 400 | 386 | 525 | 11.4 | 11.6 |
| 500 | 306 | 325 | 18.9 | 21.1 |
| 600 | 144 | 157 | 31.3 | 34.8 |
| 700 | 63 | 70 | 36.8 | 44.7 |
| 760 | 39 | 42 | 38.4 | 45.8 |
Cyclic Stress-Strain Response Behavior of Spheroidal Graphite Cast Iron QTRSi4Mo1
The cyclic stress amplitude responses of the spheroidal graphite cast iron QTRSi4Mo1 under different temperatures and strain amplitudes are illustrated in Figures 2a through 2e. At room temperature, the material exhibits a characteristic sequence of cyclic hardening, followed by softening, and then secondary hardening, as shown in Figure 2a. The intermediate softening stage is less pronounced at smaller strain amplitudes. At 200°C, a similar pattern of hardening → softening → hardening is observed (Figure 2b). However, unlike at room temperature, the initial rapid hardening phase reaches the maximum stress amplitude, and the subsequent secondary hardening stress amplitudes remain lower than the peak achieved during the initial hardening.
Figures 2c and 2d depict the cyclic stress amplitude evolution at 400°C and 500°C, respectively. In both cases, the spheroidal graphite cast iron displays cyclic hardening without any apparent softening stage. The difference lies in the nature of hardening: at 400°C, a monotonic hardening occurs, whereas at 500°C, the stress amplitude increases rapidly, saturates, and then drops sharply prior to failure due to micro-crack initiation and propagation. For all applied strain amplitudes at 760°C, the material exhibits clear cyclic softening behavior, as seen in Figure 2e. This is attributed to the more dominant creep effects at 760°C, where microstructural evolution leads to gradual cyclic softening during high-temperature low-cycle fatigue.
The cyclic stress-strain curve represents the material’s stress-strain response under cyclic loading, independent of the loading path. It can be effectively described by the Ramberg-Osgood equation:
$$ \varepsilon_a = \varepsilon_a^e + \varepsilon_a^{in} = \frac{\sigma_{\text{max}}}{E} + \left( \frac{\sigma_{\text{max}}}{K’} \right)^{1/n’} $$
Here, $$ \varepsilon_a $$, $$ \varepsilon_a^e $$, and $$ \varepsilon_a^{in} $$ represent the total strain amplitude, elastic strain amplitude, and inelastic (plastic) strain amplitude, respectively. $$ \sigma_{\text{max}} $$ is the maximum tensile stress during stable cycling, $$ E $$ is the elastic modulus, $$ K’ $$ is the cyclic strength coefficient, and $$ n’ $$ is the cyclic strain hardening exponent. By fitting the isothermal low-cycle fatigue test data using the least squares method, we obtained the parameters $$ K’ $$ and $$ n’ $$ for different temperatures, as listed in Table 4.
| Temperature (°C) | Elastic Modulus, E (GPa) | Cyclic Strength Coefficient, K’ (MPa) | Cyclic Strain Hardening Exponent, n’ |
|---|---|---|---|
| 25 | 166 | 1231.18 | 0.1417 |
| 200 | 127 | 637.67 | 0.0716 |
| 400 | 114 | 648.19 | 0.0517 |
| 500 | 111 | 618.17 | 0.0778 |
| 760 | 51 | 79.26 | 0.0516 |
Figures 3a through 3e show the hysteresis loops at half-life cycle and the cyclic stress-strain curves for the spheroidal graphite cast iron under various temperature and strain amplitude conditions. The hysteresis loops exhibit tension-compression asymmetry; under symmetric applied strain amplitudes, the compressive load component is greater than the tensile component. As the applied strain amplitude increases, the area enclosed by the hysteresis loop also increases, indicating greater inelastic strain energy dissipation during fatigue. At a given temperature, the cyclic stress-strain curve calculated using the parameters from Table 4 aligns well with the locus of hysteresis loop tips, confirming that the Ramberg-Osgood equation accurately describes the cyclic stress-strain response of this spheroidal graphite cast iron across different temperatures.
Although the Ramberg-Osgood relation effectively represents the steady-state stress amplitude versus strain amplitude relationship, it does not inherently describe the shape of the cyclic hysteresis loop. To investigate the Masing behavior, hysteresis loops at different strain amplitudes for a given temperature were translated such that their compressive tips coincided at the origin of the coordinate system. It was found that only at temperatures of 500°C and 760°C did the ascending branches of the hysteresis loops for different strain amplitudes superimpose well, exhibiting typical Masing characteristics, as illustrated in Figure 4. This indicates that the cyclic stress-strain response of the spheroidal graphite cast iron QTRSi4Mo1 shows Masing behavior at temperatures above 500°C, while non-Masing behavior is observed at lower temperatures. This transition is likely related to temperature-induced changes in the material’s microstructure, such as dislocation mobility, precipitation behavior, and phase stability, which differ across the temperature range studied.
Discussion on the Temperature-Dependent Mechanical Behavior
The observed tensile properties highlight a critical temperature-dependent ductility trough for this spheroidal graphite cast iron. The embrittlement around 300-400°C is a well-documented phenomenon in some ferritic cast irons and is often associated with dynamic strain aging or precipitation effects that impede dislocation motion, reducing ductility. The recovery of ductility above 500°C coincides with increased thermal activation, which aids dislocation climb and cross-slip, enhancing plastic deformation capability. The continuous decrease in strength with temperature is expected due to thermal softening mechanisms.
The complex cyclic hardening-softening-hardening sequences at lower temperatures (25°C and 200°C) suggest competing microstructural processes. Initial cyclic hardening likely results from dislocation multiplication and interaction. Subsequent softening may be attributed to the rearrangement of dislocations into lower-energy configurations or the breakdown of initial barriers. The secondary hardening phase could be related to strain-induced precipitation or further dislocation interactions. The absence of a softening stage at 400°C and 500°C, where only hardening is observed, implies that dislocation accumulation and interaction dominate the cyclic response throughout life at these intermediate temperatures. The pure cyclic softening at 760°C is strongly influenced by time-dependent deformation mechanisms like creep and dynamic recovery, which promote microstructural softening and outweigh any hardening contributions.
The tension-compression asymmetry in the hysteresis loops is a common feature in cast irons and can be attributed to the different behavior of the spheroidal graphite nodules and the metal matrix under tensile and compressive stresses. Under compression, the matrix may experience closure of micro-voids or different stress states around the graphite nodules, leading to a higher flow stress in compression for the same strain magnitude.
The transition from non-Masing to Masing behavior with increasing temperature is significant for constitutive modeling and fatigue life prediction. Masing behavior implies that a single master curve can describe the cyclic stress-strain response for all strain amplitudes, simplifying analysis. Non-Masing behavior requires more complex models that account for changing cyclic deformation mechanisms with strain amplitude. For this spheroidal graphite cast iron, the emergence of Masing characteristics above 500°C suggests a more homogeneous and temperature-activated deformation process, possibly where thermal recovery processes standardize the dislocation structures irrespective of the strain amplitude.
Implications for Component Design and Fatigue Assessment
Understanding these temperature-dependent properties is crucial for the design and life assessment of engine components like exhaust manifolds. The embrittlement region (300-400°C) may represent a critical temperature window where components are more susceptible to crack initiation under thermal cycling, potentially reducing thermomechanical fatigue life. Design strategies might aim to minimize dwell times or stress concentrations in this temperature range during operation.
The cyclic stress-strain data and the fitted Ramberg-Osgood parameters provided in Table 4 can be directly used in strain-based fatigue life prediction methods, such as the Universal Slopes method or local strain approaches. The clear identification of cyclic hardening/softening regimes helps in selecting appropriate cyclic plasticity models for finite element simulations of components under cyclic thermal-mechanical loading. For accurate high-temperature life prediction, models must incorporate the cyclic softening observed at 760°C, possibly integrating creep-fatigue interaction rules.
The different cyclic responses also inform material selection and processing. If a component is expected to experience significant low-cycle fatigue in a specific temperature range, the material’s behavior in that range should be a primary consideration. For instance, the strong cyclic hardening at 400-500°C might be beneficial for mitigating ratcheting under asymmetric stress cycles, while the softening at 760°C necessitates careful design to limit strain accumulation.
Conclusion
Based on static tensile and isothermal low-cycle fatigue testing of spheroidal graphite cast iron QTRSi4Mo1 from room temperature to 760°C, the following conclusions are drawn concerning the effects of temperature on its mechanical properties:
- The percentage elongation and reduction of area of the spheroidal graphite cast iron QTRSi4Mo1 initially decrease and then increase with rising temperature, exhibiting a ductility minimum and associated brittleness in the range of 300°C to 400°C. Conversely, both yield strength and ultimate tensile strength decrease monotonically with increasing temperature.
- The cyclic stress-strain response of this spheroidal graphite cast iron at room temperature and 200°C displays a complex sequence of initial hardening, followed by softening, and then secondary hardening. At medium temperatures (400°C and 500°C), the response is characterized by cyclic hardening without a distinct softening stage. At the highest temperature investigated (760°C), the material exhibits clear cyclic softening behavior.
- The hysteresis loops demonstrate tension-compression asymmetry across all tested temperatures. The cyclic stress-strain response exhibits non-Masing characteristics at lower temperatures (≤400°C) but transitions to Masing behavior at higher temperatures (≥500°C). The Ramberg-Osgood equation provides an accurate description of the cyclic stress-strain relationship for this spheroidal graphite cast iron throughout the studied temperature range.
These findings underscore the importance of considering the specific service temperature environment when evaluating the fatigue performance and designing components made from spheroidal graphite cast iron. The identified behavioral transitions, particularly the embrittlement range and the shift in cyclic response and Masing behavior, provide essential data for developing more accurate constitutive models and life prediction methodologies for high-temperature automotive components subjected to complex thermomechanical loading.