ASTM E21 is one of the main international standards used to evaluate the mechanical properties of metals at high temperatures. Particularly for highly critical industries such as aerospace, energy and automotive, understanding how metals react at elevated temperatures is critical to ensuring safe and durable performance. In this guide, we will explore in detail what ASTM E21 requires, focusing on measured parameters, test procedures and technical requirements.
What is ASTM E21?
ASTM E21 defines procedures for tensile tests on metallic materials exposed to high temperatures. These tests are essential to study how the mechanical properties of metals change when subjected to heat, ensuring accurate design of components intended to operate in high temperature environments.
Why are high temperature tests important?
When metals are subjected to high temperatures, the strength and ductility can change significantly. Materials that show excellent mechanical properties at room temperature may become more brittle or lose their load-bearing capacity under intense heat. This is why hot tensile testing is essential: it ensures that metal components designed to operate at high temperatures retain their reliability.
Typical values determined according to ASTM E21
High temperature tensile tests according to ASTM E21 allow several key parameters to be determined:
- Tensile strength: is the maximum stress that the material can withstand before breaking.
- Yield point: represents the stress level at which the material begins to deform plastically.
- Elastic limit offset: defines the stress required to produce a specific amount of permanent deformation, as in the case of offset Rp0.1.
- Strain at failure: measures the deformation that the material undergoes until failure.
- Striction: represents the decrease in the cross-sectional area of the specimen after fracture, an indicator of the residual ductility of the material.
Requirements for testing machines according to ASTM E21
To achieve accurate and compliant results, ASTM E21 specifies a number of requirements for the testing machines used:
- Specimen alignment: the specimen must be perfectly aligned to ensure uniform load distribution.
- Strain control: the maximum bending strain must be within 10% of the axial strain.
- Thermocouples: they must be calibrated regularly, usually once a year, in accordance with the ASTM E220 standard. These thermal sensors must be extremely accurate to ensure an accurate temperature reading.
High temperature testing: temperature management
In the context of hot tensile testing, temperature is a crucial factor. According to ASTM E21, it is essential to use precision thermocouples to monitor and maintain the specimen temperature within very narrow margins:
- ±3 °C (±5 °F) for temperatures up to 980 °C (1800 °F).
- ±6 °C (±10 °F) for temperatures above 980 °C (1800 °F).
High temperature tensile tests according to ASTM E21 refer to any test performed above 38 °C (100 °F), making them a complement to the room temperature tests governed by ASTM E8/E8M.
Strain measurement according to ASTM E21
Strain measurement is an essential step in tensile testing. To ensure that the data are reliable, ASTM E21 recommends the use of a strain gauge conforming to ASTM E83, which provides sensitivity and accuracy. To measure the elastic limit offset (e.g. Rp0.1), a class B-2 strain gauge should be used.
In addition, it is important that the stress-strain curve of the material remains uniform, not varying more than 10% from the nominal modulus of elasticity.
Specimen types according to ASTM E21
For high temperature testing, ASTM E21 specifies several standardised specimen shapes:
- Round specimens with threaded ends
- Round specimens with shoulder ends
- Flat specimens with locking pins
These specimens must be prepared with high precision to ensure reliable and reproducible results.
Test speed control
Another fundamental technical aspect is the control of the deformation speed. In the initial phase of the test, ASTM E21 specifies a speed of 0.005 ± 0.002 in./in./min or mm/min for the determination of the yield point and elastic limit offset. Subsequently, the speed can be increased to 0.05 ± 0.01 in./in./min to obtain a complete stress-strain curve.
Measurement of strain at failure and section reduction
Measurements of strain at failure and section reduction are generally performed at room temperature, even if the test was conducted hot. The distance between the marks applied on the specimen is measured accurately, usually 0.01 inch (0.3 mm). The area reduction is calculated by measuring the minimum diameter of the fractured specimen.
Conclusions and industrial applications
High temperature tensile tests according to ASTM E21 are indispensable for the design of metal components that must withstand thermal stress conditions. The data collected are crucial for sectors such as:
- Aerospace industry
- Energy
- Automotive
In these sectors, the strength and durability of metals at high temperatures are decisive for safety and operational efficiency. Test results can influence the choice of materials and production technologies, improving the quality and durability of metal components.
FAQ
What does ASTM E21 measure?
ASTM E21 measures the mechanical properties of metals subjected to tensile stress at high temperatures, such as tensile strength and yield point.
What temperatures are considered high according to ASTM E21?
According to ASTM E21, all tests performed above 38 °C (100 °F) are considered high temperature.
Which specimens are used for ASTM E21 tests?
Round or flat metal specimens prepared according to the specifications of ASTM E8/E8M are used.
What is the strain rate for ASTM E21 tests?
When determining the yield point and elastic limit offset, the rate is 0.005 ± 0.002 in./in./min.
How is the temperature measured during testing?
The temperature is measured with calibrated thermocouples, adhering to ASTM E220 tolerances.
Why is section reduction important in tensile testing?
Section reduction indicates the residual ductility of the material, a key parameter in assessing its ability to withstand thermal stress conditions.
Learn more about our testing solutions
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