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ASTM E21 – Elevated Temperature Tension Test on Metals

The ASTM E21 standard describes the single-axis elevated temperature tension test/hot tensile test on metallic materials. The test is used to determine characteristic values including tensile strength, yield point, offset yield, strain at break and reduction of area.

These mechanical material properties characterize the material behavior of metals in terms of strength and ductility. In addition, determining the temperature dependence of metals in their many different applications is of important significance. To investigate this dependence between metals and elevated temperature, various tensile tests must be performed at different hot temperature levels.

Characteristic values Testing machine Temperature Strain measurement Specimens Test speed Strain at break and reduction of area Related products for hot tensile testing

The most important characteristic values determined according to ASTM E21

ASTM E21: Testing machine requirements

ASTM E21 requires optimal and precise testing machine alignment when applying the load on the gripped specimen. The testing machine and specimen grips must load a precisely machined specimen in such a way that the maximum bending strain does not exceed 10% of the axial strain. It should be noted that the verification of these conditions under elevated temperature is not possible in practice and can therefore be performed at room temperature. In this context, reference is made to ASTM E1012.

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Elevated temperature tension testing on metals to ASTM E21 with high specimen throughput

ASTM E21: Temperature testing requirements

The hot tensile test according to ASTM E21 is performed at elevated temperatures. As differentiated from the ASTM E8/E8M standard, it is understood to mean a temperature above room temperature, that is, higher than 38 °C or 100 °F. The temperature is measured using thermocouples with sufficient sensitivity and reliability.

The following limits must be maintained when measuring the temperature according to ASTM E21:

  • Specified temperatures less than or equal to 980 °C: ±3 °C, (< 1800 °F: ±5 °F)
  • Specified temperatures greater than 980 °C: ±6 °C, (> 1800 °F: ±10 °F).

The thermocouples that are used to measure the temperature must be checked on an annual basis, and if necessary, must be re-calibrated according to ASTM E220.

Strain measurement to ASTM E21

When selecting an extensometer, it is recommended to use the ASTM E83 standard to meet the sensitivity and accuracy requirements. To determine the offset yield (Rp0.1 and higher), a Class B-2 extensometer is required. It is important to note that requirement compliance should be verified under elevated temperature conditions. One of these tests can be the determination of the slope of the stress-strain curve, which should be within 10% of the modulus of elasticity (nominal value) of the tested material.

For strain measurement, ASTM E21 explicitly addresses the positioning of specimen markings of the knife edges of the extensometer and of the temperature sensors. If for these specifications, there is a need to deviate from the standard, this must be reported and in the case of acceptance tests, it must be agreed on in advance.

The ASTM E21 standard requires that the selected extensometer is able to determine the strain rates. In addition, it is important to note that the determination of the modulus of elasticity is not included in this standard and that test conditions under rapid heating or high strain rates are not considered. It is important to note, however, that the characteristic values indicated in units of inches and pounds are in accordance with the standard.

ASTM E12: Specimen shapes

The specimen shapes used for ASTM E21 are specified in the ASTM E8 and ASTM E8M standard—these specimens are also fundamentally suited for tensile tests at elevated temperatures.

Typical specimen shapes for hot tensile testing to ASTM E21 are:

  • Round specimens with threaded ends
  • Round specimens with shoulder ends
  • Flat specimens with locking pins

Test speeds according to ASTM E21

When determining yield point and offset yield values, the ASTM E21 standard requires that a strain rate of 0.005 ± 0.002 in./in./min or mm/mm/min (corresponding to 0.000083 ± 0.000033 /sec) be maintained within the reduced parallel section.

After the yield point and offset yield have been determined, the test speed can be increased to 0.05 ± 0.01 in./in./min or mm/mm/min (corresponding to 0.00083 ± 0.00017 / sec). The test speed (strain rate or crosshead speed) must be recorded.

After yield point and offset yield determination, and when it has been confirmed that the crosshead speed is stable within the above tolerances, the crosshead speed in in./min or mm/min can also be calculated from the reduced parallel section (inches or mm) by multiplying by 0.05. This applies to all specimen shapes described in ASTM E8/E8M.

Example: If the reduced parallel section (previously also referred to as test length) is 4 inches long, then a crosshead speed of 0.2 in./min must be set; and for a reduced parallel section with a length of 100 mm, a crosshead speed of 5 mm/min. must be set.

If a complete stress-strain curve is to be recorded, then the test speed of 0.005 ± 0.002 in./in./min. or mm/mm/min. can be maintained throughout the entire tensile test.
If determination of the yield point and offset yield is not required, the use of an extensometer is also not absolutely necessary. The crosshead speed then has to be set so that a strain rate of 0.05 ± 0.01 in./in./min or mm/mm/min can be achieved during the tensile test.

In order to take the behavior of the test arrangement into account in the initial part of the stress-strain curve for determining yield point and offset yield, the set value for the strain rate may be corrected slightly upwards. A specific reference is provided in ASTM E21. Beyond the yield point and offset yield, the uncorrected set value must be applied again.

Strain at break and reduction of area

Manual strain at break measurement is carried out at room temperature. The measurement of the distance between the previously applied gauge marks should be made to the nearest 0.01 inch or 0.3 mm.

Manual measurement to determine the reduction of area is also performed at room temperature. The ends of the fractured round specimen must be fit together carefully and the minimum diameter is measured to the nearest 0.01 inch or 0.3 mm. If the fracture cross section is not round, make sufficient measurements to establish the reduction of area.

The contents of the test report are listed and described in detail. It is important that the test report reflects the test and measurement conditions as accurately as possible. In the case of strain at break, explicit reference is made to the need for specification after measurement at break and measurement after break. Also which initial gauge length is referred to: for rectangular specimens (e.g., 2 in. or 50 mm) and very important for round specimens with 4x diameter or 5x diameter.

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