The Elevated Temperature Tensile Test is a test that applies tension to metallic materials at a specified temperature above room temperature to determine their mechanical properties. For equipment operating at high temperatures for long periods, such as boilers, pressure vessels, steam turbines, and aero-engines, the elevated temperature tensile test is the most important basis for determining the allowable stress of materials. The following is a systematic analysis of the elevated temperature tensile test:
1. Why Conduct Elevated Temperature Tensile Testing?
The properties of metallic materials at high temperatures differ significantly from those at room temperature:
- Decreased strength: As temperature rises, atomic activity intensifies, softening the material, resulting in a significant reduction in yield strength and tensile strength.
- Changed ductility: Elongation generally increases (the material becomes tougher), but may decrease in certain temperature ranges, such as the blue-brittle region.
- Creep behavior: Under high temperature and constant load, the material deforms slowly over time (creep).
Therefore, when designing high-temperature equipment, room-temperature data cannot be used; high-temperature mechanical property data corresponding to the service temperature must be adopted.
2. Test Principle and Equipment
The basic principle of the elevated temperature tensile test is the same as that of the room-temperature tensile test (pulling the specimen to fracture and recording force and elongation), but it is more complex in terms of equipment and control:
A. Core Equipment: High-Temperature Furnace
- Heating method: Usually resistance wire heating or infrared heating.
- Temperature control requirement: Temperature must be strictly controlled. Standards stipulate that the deviation of the test temperature is generally required to be within ±2℃ or ±3℃.
- Uniform temperature zone: A zone with uniform temperature (uniform temperature zone) must be provided inside the furnace, and the gauge length section of the specimen must be completely located within this zone.
B. Specimen Heating and Soaking
- Heating: Place the specimen in the furnace and heat it to the specified temperature.
- Soaking: After reaching the target temperature, the test shall not start immediately; a soaking period (usually 15–30 minutes) is required.
- Purpose: To ensure uniform temperature between the core and the surface of the specimen, and to eliminate thermal stress caused by heating.
C. Extensometer
A high-temperature resistant extensometer (usually equipped with ceramic or sapphire rods) must be used.Ordinary metal extensometers will soften or oxidize and fail at high temperatures.
3. Key Measured Indicators
The elevated temperature tensile test mainly measures the following indicators (these data will be included in the high-temperature property table of materials):
- High-temperature yield strength (ReH or Rp0.2):Since materials generally do not exhibit a distinct physical yield point at high temperatures (no serrated plateau), the proof strength, non-proportional extension (Rp0.2) is usually determined, i.e., the stress at which 0.2% permanent deformation occurs.
- High-temperature tensile strength (RmH):The maximum stress of the material before fracture at high temperature.
- High-temperature elongation after fracture (A) and reduction of area (Z):Measures of the material’s ductility at high temperatures.
4. Relevant Standards
- Chinese Standard: GB/T 4338-2006 Metallic materials — Tensile testing at elevated temperature
- International Standards: ISO 6892-2, ASTM E21
5. Detailed Test Procedure
- Specimen preparation:Similar to room-temperature specimens, but generally requires a lower surface roughness to reduce the influence of high-temperature oxide scale.
- Specimen clamping:Mount the specimen on the grips of the testing machine, then raise the high-temperature furnace to enclose the specimen.
- Extensometer installation:Attach the high-temperature resistant extensometer to the gauge length section of the specimen.
- Heating and soaking:Set the target temperature (e.g., 350℃, 500℃, 600℃), heat and soak for sufficient time.
- Loading test:Apply tensile force at the rate specified in the standard.The loading rate for elevated temperature tests is generally slower than that at room temperature to simulate the creep behavior of materials at high temperatures.
- Recording and calculation:Record the force-elongation curve and calculate yield strength and tensile strength.
- Cooling and measurement:After the test, allow the specimen to cool, then measure the gauge length after fracture and the cross-sectional area.
6. Special Considerations
- Oxidation protection:Metals are prone to oxidation at high temperatures, resulting in poor fracture quality or weight loss. For some oxidizable alloys (e.g., titanium alloys, nickel-base alloys), the test may need to be performed under vacuum or inert gas protection.
- High-temperature creep effect:At high temperatures, the specimen elongates gradually even under constant tensile force. Therefore, when determining yield strength, an extensometer must be used to monitor deformation in real time, instead of only reading load values as in room-temperature tests.
- Measurement after specimen cooling:For some materials that undergo phase transformation at high temperatures, their microstructure after cooling may differ from that at high temperatures. However, standards generally specify that elongation after fracture be measured at room temperature (unless high-temperature measurement is required by special agreement).
Summary
The elevated temperature tensile test is a “thermometer” for the design of high-temperature equipment. Its function: To tell engineers how much pressure a steel plate can withstand at 500℃ without deformation or fracture. Core challenges: Precise temperature control and elimination of thermal stress. Mnemonic: Furnace temperature must be accurate, soaking time sufficient, extensometer heat-resistant, loading steady and consistent.
