Hot tensile testing

In the field of metallic materials, a high temperature tensile test is defined as any test conducted at a temperature above 35°C. The hot tensile test is critical for determining the mechanical properties of metals by simulating operating conditions in high thermal stress environments to measure key parameters such as yield strength and tensile strength.

International standards for hot tensile testing: UNI EN ISO 6892-2 and ASTM E21

The global reference standards for performing these tests are ISO 6892-2 and ASTM E21. The influence of test speed on the mechanical properties of the material is much more pronounced at high temperatures than at room temperature testing. To compensate for this sensitivity, both guidelines prescribe performing the test under strain control.

The methods provided for strain rate management are of two types:

The machine uses the strain gauge reading to constantly adjust the crosshead displacement and keep the imposed strain rate fixed.

Also known as estimated strain rate, an average strain rate is set based on the length of the calibrated stretch adjusted by the displacement of the moving crosshead.

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Thermal tolerances and strain measurement

In addition to velocity, temperature critically affects the mechanical behavior of the specimen. To ensure accurate results, regulations impose strict limits on thermal stability along the calibrated stretch.
Strain measurement then requires the use of a strain gauge that meets specific regulatory requirements.

Thermal tolerances in hot tensile testing

According to EN ISO 6892-2:

for test temperatures up to 600°C the allowable tolerance is ±3°C, while the temperature difference on the specimen is maximum 3°C
for test temperatures between 600°C and 800°C the allowable tolerance is ±4°C, while the temperature difference on the specimen is maximum 4°C
for test temperatures between 800°C and 1000°C the allowable tolerance is ±5°C, while the temperature difference on the specimen is maximum 5°C
for test temperatures between 1000°C and 1100°C the allowable tolerance is ±6°C, while the temperature difference on the specimen is maximum 6°C

Instead, according to ASTM E21 for test temperatures up to 980°C the allowable tolerance is ±3°C, while for values above 980°C the limit expands to ±6°C.
Thermal uniformity is ensured by employing three thermocouples in direct contact with the specimen, which independently control three separate zones of the furnace.

Deformation measurement

The strain gauge for strain measurement must comply with specific regulatory requirements: class 1 according to ISO 9513 standard for ISO 6892-2 tests and class B-2 according to ASTM E83 standard for ASTM E21 tests. The strain gauge must record at least the first portion of the tensile curve until yield point is reached. Once the elastic limit has been exceeded, the measurement can theoretically continue by capturing the displacement of the crossmember, a method that introduces errors, however, related to the deformations of the entire test system. To obtain complete and accurate curves up to failure, the use of noncontact optical technologies, such as laser systems based on the principle of image contrast, is optimal. A laser beam projects a pattern onto the metal surface, and a set of cameras captures the images from the preload, following the actual deformation without any mechanical interference.

Configuration of the grasping system

Properly gripping the specimen inside the high-temperature furnace requires specially designed equipment. The gripping system includes a number of dedicated elements: extensions, threaded specimen inserts, flat specimen inserts with centering pins, and ball joints designed to ensure the optimal alignment prescribed by ASTM E1012. All components of the loading train are made of nickel-based superalloys, such as Mar-M 246, selected for their excellent viscous creep resistance properties at extreme temperatures.

Hot tensile testing with TEC Eurolab

TEC Eurolab’s mechanical testing laboratory performs NADCAP-accredited hot tensile tests, covering a temperature range up to 1200°C with load cells up to 100 kN. To ensure the highest fidelity of the data, especially in the plastic phase beyond yielding, tests are conducted employing non-contact optical extensometry (laserXtens system), eliminating the mechanical influences typical of physical probes at high temperatures. The setup is configurable for different specimen geometries, including cylindrical threaded and flat specimens, supporting the Aerospace, Automotive and Energy industries in the qualification of superalloys and structural materials according to methods A and B of current standards.

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