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High-Temperature Mechanical Properties of Materials: Key Knowledge for Machinery Industry
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In industrial fields such as aerospace, energy, and chemical engineering, numerous mechanical components operate under high-temperature conditions for extended periods—including engines, boilers, and oil refining equipment. These components impose stringent requirements on the high-temperature mechanical properties of materials. Accurately evaluating materials, using them rationally, and developing new high-temperature-resistant materials have become crucial tasks for the advancement of these industries and materials science research. This article elaborates on essential knowledge related to the high-temperature mechanical properties of materials, providing valuable insights for the machinery sector.
1. Definition of "High Temperature" for Metallic Materials
The classification of "high" or "low" temperature is relative to the melting point of the metal. A common criterion is the "homologous temperature" T/Tm (where Tm denotes the material's melting point). When T/Tm > 0.4-0.5, the temperature is considered high for that specific material.
Practical application examples:
The operating temperature of civil aircraft engines approaches 1500°C, while that of military aircraft engines reaches around 2000°C.
Localized operating temperatures of aerospace vehicles can soar to 2500°C.
For high-temperature and high-pressure pipelines in chemical equipment, even if the applied stress is lower than the yield strength of the material at the operating temperature, continuous plastic deformation may occur over long-term use, leading to gradual pipe diameter expansion and potential rupture.
2. Effects of Temperature and Time on Material Properties
The mechanical properties of materials are significantly influenced by both temperature and load duration under high-temperature conditions, differing markedly from room-temperature mechanical properties.
2.1 Temperature Effect
Generally, as temperature increases, the strength of metallic materials decreases while their plasticity increases.
2.2 Load Duration Effect
When σ < σs (yield strength), creep may occur during long-term service, potentially resulting in fracture.
With prolonged load duration, the tensile strength of steel at high temperatures decreases.
Under short-term high-temperature tension, material plasticity increases; however, under long-term load, the plasticity of metallic materials decreases significantly, notch sensitivity increases, and brittle fracture often occurs.
The combined effect of temperature and time also influences the fracture path of materials.
2.3 Equal-Strength Temperature (TE)
As temperature rises, both grain strength and grain boundary strength decrease. Due to the irregular atomic arrangement at grain boundaries, diffusion occurs more easily along these boundaries, causing grain boundary strength to decline more rapidly. The temperature at which the strength of grains equals that of grain boundaries is defined as the equal-strength temperature (TE).
When materials operate above TE, the fracture mode transitions from the common transgranular fracture to intergranular fracture.
TE is not a fixed value but is significantly affected by the strain rate. Since grain boundary strength is much more sensitive to strain rate than grain strength, TE increases with increasing strain rate.
In summary, the investigation of material mechanical properties at high temperatures must incorporate both temperature and time as critical factors.
3. Creep Phenomenon in Metallic Materials
3.1 Definition of Creep
Creep refers to the phenomenon where metals undergo slow plastic deformation under long-term constant temperature and constant load conditions—even when the stress is lower than the yield strength at that temperature. Fracture caused by creep deformation is known as creep fracture. While creep can occur at low temperatures, it becomes particularly noticeable when the homologous temperature exceeds 0.3. For instance:
Creep effects must be considered for carbon steel above 300°C and alloy steel above 400°C.
3.2 Creep Process of Metals
The creep curve of metals typically consists of three stages (under constant stress and temperature):
Primary Creep Stage (Transient Creep Stage): Characterized by a high initial creep rate that gradually decreases over time, reaching a minimum at the end of this stage.
Secondary Creep Stage (Steady-State Creep Stage): The creep rate remains nearly constant during this stage. The creep rate of metals is generally defined as the steady-state creep rate ε from this stage.
Tertiary Creep Stage (Accelerated Creep Stage): The creep rate increases progressively with time, ultimately leading to creep fracture.
