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High-temperature alloy pipes are widely used in high-temperature and high-pressure environments such as aero-engines, gas turbines, and nuclear reactors due to their excellent high-temperature strength, oxidation resistance, and creep resistance. However, their mechanical properties (such as strength, plasticity, and creep resistance) change significantly with temperature. Studying the variation of mechanical properties of high-temperature alloy pipes at different temperatures is of great significance for material selection, structural design, and safety assessment.
1. Mechanical Properties in the Room Temperature to Medium Temperature Range (<800°C)
In the room temperature to medium temperature range (usually below 800°C), high-temperature alloy pipes mainly exhibit high yield strength and tensile strength while maintaining good plasticity and toughness. In this stage, the strengthening mechanism of the alloy mainly relies on solid solution strengthening (such as elements like tungsten and molybdenum in nickel-based alloys) and precipitation strengthening (such as γ' phase Ni₃(Al,Ti)).
Strength Variation: As temperature increases, atomic thermal vibration intensifies, dislocation movement resistance decreases, leading to a gradual decrease in yield strength and tensile strength. For example, the yield strength of Inconel 718 alloy is approximately 1000 MPa at room temperature, but may drop to approximately 700 MPa at 600°C.
Plasticity Change: In the mid-temperature range, the alloy generally exhibits good plasticity, with high elongation and reduction of area, making it suitable for withstanding certain plastic deformations.
Toughness Performance: Due to the minimal impact of low-temperature brittleness, the alloy maintains good impact toughness at room to mid-temperature temperatures. However, near 800°C, some alloys may exhibit a slight tendency towards a ductile-brittle transition.
2. Mechanical Properties in the High-Temperature Range (800–1100°C)
When the temperature further increases to 800–1100°C, the mechanical properties of high-temperature alloy pipes change significantly, mainly manifested in decreased strength, dominant creep behavior, and increased importance of oxidation resistance.
Strength Decrease: In this temperature range, grain boundary diffusion and dislocation climb accelerate, leading to a significant decrease in the alloy's yield strength and tensile strength. For example, the yield strength of GH4169 alloy at 1000°C may only be 30–40% of that at room temperature.
Creep Behavior: At high temperatures, creep (slow plastic deformation) becomes the main failure mechanism of alloys. The creep rate is affected by stress, temperature, and grain boundary structure, and generally follows the Arrhenius equation. Creep resistance can be improved by optimizing alloy composition (e.g., adding refractory metals such as Re and Ru) and heat treatment processes (e.g., directional solidification).
Plastic Changes: Despite the decrease in strength, some high-temperature alloys (such as nickel-based single-crystal alloys) can still maintain a certain degree of plasticity at high temperatures to adapt to changes in thermal expansion and mechanical loads.
3. Mechanical Properties in the Ultra-High Temperature Range (>1100°C)
At extreme temperatures exceeding 1100°C, the mechanical properties of high-temperature alloy pipes face severe challenges, mainly manifested as:
A sharp decrease in strength: Extremely high atomic diffusion rates, accelerated grain boundary slip and grain growth lead to a significant reduction in the load-bearing capacity of the alloy. For example, the strength of nickel-based alloys at 1200°C may be only 10–20% of that at room temperature.
Accelerated Oxidation and Corrosion: At high temperatures, the formation rate and stability of the oxide film (such as Cr₂O₃, Al₂O₃) on the alloy surface become critical. If the oxide film is damaged, the base metal will be rapidly consumed, further weakening mechanical properties.
Shortened Creep Fracture Life: At ultra-high temperatures, even at low stress, alloys may experience creep fracture within a short time. Therefore, cooling structures (such as film cooling) or more advanced single-crystal alloys (such as the CMSX series) are required to improve temperature resistance.
4. Key Factors Affecting Changes in Mechanical Properties
The mechanical properties of high-temperature alloy pipes are affected by a variety of factors, including:
Alloy Composition: For example, the content of γ' phase stabilizing elements (Al, Ti) in nickel-based alloys directly affects high-temperature strength.
Microstructure: Grain size, grain boundary state, and second-phase distribution are crucial for creep and fatigue performance.
Heat treatment process: Solution treatment and aging treatment can optimize precipitation strengthening effect, thereby improving high-temperature performance.
