Industry Standard – Metals – ASTM E21 – Elevated Temperature Tension Tests of Metallic Materials

What is Elevated Temperature Tension Testing (ASTM E21)?

Elevated temperature tensile testing, as defined by ASTM E21, is a standardized method for determining the mechanical properties of metallic materials at temperatures above room temperature. This test is crucial for evaluating materials used in high-heat applications such as jet engines, gas turbines, nuclear reactors, and industrial furnaces. Unlike room temperature tests, ASTM E21 specifically addresses the challenges of thermal expansion, oxidation, and time-dependent deformation (creep) that occur when metals are subjected to sustained loads at high temperatures. The primary goal is to provide a comparative measure of a material’s strength and ductility under these demanding conditions.

What is a typical Elevated Temperature Tensile Test Result?

The results from an ASTM E21 test provide key mechanical properties that are often significantly different from their room-temperature counterparts:

① Yield Strength ( Rp0.2 ): The stress at which a material exhibits a permanent strain of 0.2%. At elevated temperatures, this value is typically lower than at room temperature due to increased atomic mobility.

② Tensile Strength ( Rm ): The maximum stress the material can withstand before necking or fracture. This property also generally decreases with increasing temperature.

③ Elongation ( A ): The percentage increase in the gauge length after fracture, indicating the material’s ductility. Ductility can either increase or decrease with temperature depending on the material and the specific temperature range.

④ Reduction of Area ( Z ): The percentage decrease in the cross-sectional area at the point of fracture, another critical measure of ductility, especially for forged or cast products.

⑤ Strain Rate Sensitivity: A unique aspect of high-temperature testing is that the measured strength is highly dependent on the rate at which the load is applied. ASTM E21 mandates a specific strain rate during the yield determination phase to ensure consistency.

⑥ Fracture Mode: The description of how the specimen failed (e.g., ductile cup-and-cone, brittle intergranular, or shear), which can be influenced by temperature and environmental effects like oxidation.

Discovery and Evolution of High-Temperature Testing

The development of steam turbines and internal combustion engines in the early 20th century highlighted the inadequacy of room temperature data for high-heat applications. Early attempts at high-temperature testing suffered from poor temperature control, uneven heating, and undefined loading rates, leading to inconsistent results. As gas turbine technology matured in the mid-20th century, the need for a standardized approach became critical. ASTM E21 was established to provide a rigorous framework for these tests, harmonizing practices across the industry. Over decades, revisions have addressed the challenges of testing advanced superalloys, emphasizing precise temperature uniformity, soaking times, and strain rate control to differentiate between short-term tensile properties and long-term creep behavior.

The standard defines specific mechanical properties for material acceptance:

Thermal Management: Mandating specific soaking times and multi-point temperature monitoring to ensure thermal equilibrium.

Strain Rate Protocol: Specifying a single, tightly controlled strain rate for the yield phase to minimize creep effects, unlike the more flexible rates allowed in room temperature testing.

High-Temperature Instrumentation: Requiring specialized extensometers and gripping fixtures designed to function reliably in a hot furnace environment.

Oxidation Considerations: Acknowledging that the test atmosphere (usually air) can affect results through surface oxidation, and providing guidance for inert or vacuum environments if needed.

The key contents covered by the standard include:

Specimen geometry and preparation (referencing ASTM E8/E8M for dimensions).

Furnace design and calibration procedures to ensure temperature uniformity (typically within ±1% or specific degree limits).

Heating rates and soaking times to achieve thermal equilibrium.

Extensometer requirements, including classes of accuracy suitable for elevated temperatures.

Procedures for attaching extensometers to hot specimens.

Definitions of test speeds (strain rates) for elastic and plastic regions.

Calculation methods for yield strength, tensile strength, and ductility at temperature.

Reporting requirements, including detailed temperature histories and strain rates.

Referenced ASTM Standards

ASTM E4: Practices for Force Verification of Testing Machines.

ASTM E6: Terminology Relating to Methods of Mechanical Testing.

ASTM E8/E8M: Test Methods for Tension Testing of Metallic Materials (Room Temperature).

ASTM E29: Practice for Using Significant Digits in Test Data.

ASTM E83: Practice for Verification and Classification of Extensometer Systems.

ASTM E220: Test Method for Calibration of Thermocouples.

ISO 6892-2: Metallic materials — Tensile testing — Part 2: Method of test at elevated temperature (International counterpart).

ASTM E139: Standard Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials.

ISO 204: Metallic materials — Creep testing — Tensile method.

ASME Boiler and Pressure Vessel Code, Section II, Part D: Provides design allowables based on data generated from tests like ASTM E21.

Environment:

Temperature: From just above room temperature up to the limits of the furnace and thermocouples (commonly 1800°F/1000°C or higher).

Atmosphere: Typically ambient air at atmospheric pressure. However, for oxidation-sensitive materials, the standard allows for testing in a vacuum or inert gas capsule, with necessary corrections for pressure differentials.

Safety: Requires precautions for handling hot equipment, potential for specimen ejection upon fracture, and use of protective shielding.

Test Procedure:

Specimen Preparation: Machine a specimen from the material in its final condition (e.g., heat-treated). Ensure tight tolerances on straightness and surface finish.
Instrumentation: Attach a calibrated high-temperature extensometer to the specimen’s reduced section. Install multiple thermocouples along the gauge length.
Furnace Setup: Place the specimen in the furnace, ensuring the thermocouples are in intimate contact and shielded from radiation.
Heating and Soaking: Heat the furnace to the target temperature at a controlled rate. Hold (soak) for at least 20 minutes to achieve thermal equilibrium.
Alignment Check: Carefully install the specimen into the testing machine grips, taking care to avoid introducing bending stresses.
Testing: Begin the test by applying load at a rate that maintains a strain rate of 0.005 ± 002 mm/mm/min in the reduced section until the yield strength is determined. Then, increase the crosshead speed to complete the test to fracture.
Data Recording: Continuously record force and extension. Monitor and log the specimen temperature throughout.
Post-Test Measurement: After the specimen cools, measure the final gauge length and necked area to calculate elongation and reduction of area.
Reporting: Compile a detailed report including all material information, test conditions, calculated properties, and a description of the fracture.