Liquid nitrogen
Environmental Chamber 3009-008
A furnace or high-temperature chamber around the specimen, with the load train passing through it. Soak time and through-thickness uniformity matter more than reaching the set point.
SpecificationsTesting standard
ASTM E21
Standard Test Methods for Elevated Temperature Tension Tests of Metallic Materials
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM E21 covers the procedure and equipment for determining tensile strength, yield strength, elongation and reduction of area of metallic materials at elevated temperatures. It explicitly excludes modulus of elasticity and proportional limit, and does not cover rapid heating or rapid strain rates.
At a glance
- Test type
- Tensile — the specimen is pulled apart
- Published by
- ASTM
- Edition
- E21-20
- Material
- Metals, alloys & welds
- Runs on
- Series 7200 and Series 9000
What the test does
A machined specimen is measured at room temperature, fitted with thermocouples attached to it rather than to the furnace, and assembled into a load train whose grips stay outside the hot zone with pull rods carrying load through it. It is heated to the test temperature, the gradient along the reduced section verified, and held until the specimen is at temperature throughout — the soak time being recorded. It is then pulled at the specified rate to fracture, giving tensile strength, yield strength, elongation and reduction of area, each reported against the temperature.
What it measures, and why it matters
How much a metal's tensile behaviour changes when it is hot, which is the question behind every component that runs above ambient — turbine parts, pressure vessels, exhaust systems, furnace hardware. The standard describes the result as a useful estimate of the ability to withstand applied tensile forces, and notes that conventional relationships give some indication of behaviour in compression and shear. The exclusions are as informative as the inclusions: modulus of elasticity and proportional limit are not determined, and rapid heating and rapid strain rates are not covered.
What it will and will not report
The exclusions are as informative as the inclusions, and they are stated in the scope rather than buried.
- Reported
- Tensile strength, yield strength, elongation and reduction of area
- Not determined
- Modulus of elasticity and proportional limitExcluded in the scope. Measuring a modulus at temperature is a harder problem than this method sets out to solve.
- Not covered
- Rapid heating or rapid strain ratesThis is a slow, soaked, quasi-static test. Anything faster is a different discipline.
- Temperature control
- Held within the tolerance over the reduced sectionA gradient along the gauge means part of the specimen is being tested at a temperature nobody recorded.
- Soak
- Held long enough for the specimen to reach temperature throughoutReaching the set point on a thermocouple is not the same as the specimen being at temperature.
- Attach thermocouples to the specimen, not the furnace
- DakFurnace air temperature and specimen temperature are different numbers, and the difference grows with section.
Elevated-temperature tension gives a useful estimate of how a metal withstands applied tensile forces at temperature, and by conventional relationships some indication of behaviour in compression and shear. It is an estimate, and the standard describes it as one.
Test speed
- Rate
- As specified for the property being determined
- Reported
- Rm, yield, elongation and reduction of area, each with the test temperature
- Temperature
- Recorded throughout, not only at the start
- Report the soak time with the temperature
- DakTwo laboratories at the same set point and different soak times are not running the same test.
Calculations
Tensile strength at temperatureRm
Rm = Fm / S₀
- Fm
- maximum force at the test temperature
- S₀
- original cross-sectional area, measured at room temperature
On the room-temperature area by convention. The specimen has expanded at temperature, and the convention ignores that so results stay comparable.
Reduction of areaZ
Z = (S₀ − Su) / S₀ × 100
Measured after the specimen has cooled, on a section that was hot when it necked.
Why no modulus—
Excluded from the scope
Extensometry at temperature, thermal drift in the gauge and the specimen's own expansion all sit inside the small strains a modulus needs. The standard declines rather than pretending.
How the test runs
- 01Machine the specimen to the specified geometry and measure it at room temperature.
- 02Attach thermocouples to the specimen itself over the reduced section.
- 03Assemble the load train with the grips outside the furnace.
- 04Fit a high-temperature extensometer to the gauge length.
- 05Heat to the test temperature and verify the gradient along the reduced section is within tolerance.
- 06Soak until the specimen is at temperature throughout, and record the soak time.
- 07Load at the specified rate, recording force and extension.
- 08Hold the temperature within tolerance for the whole of the test.
- 09Continue to fracture and record the maximum force.
- 10Cool, then measure the final gauge length and minimum section.
- 11Report every value against the test temperature and the soak time.
Grips and fixtures for this method
TJ-144
Heavy Duty Hydraulic Grips
Grips that stay outside the hot zone, with pull rods carrying load through it — a wedge at temperature loses its grip as it and the specimen both expand.
SpecificationsWhat the report has to contain
- Reference to ASTM E21 and the edition
- Material, heat and condition
- Specimen geometry and room-temperature dimensions
- Test temperature, tolerance and how it was measured
- Soak time before loading
- Rate of testing
- Tensile strength, yield strength, elongation and reduction of area
- How strain was measured
- Temperature record through the test
- Any deviation of the gradient along the reduced section
What the machine must be capable of
A furnace or chamber that holds temperature within tolerance across the reduced section, not merely at a set point, and a load train that passes through it with the grips outside. Grips inside the hot zone expand with the specimen and lose their bite, so the specimen draws out rather than stretching. A high-temperature extensometer is needed because elongation and reduction of area are reported. The frame itself is unexceptional; everything difficult about this test is thermal, and the furnace is where the money and the uncertainty both go — a frame adequate for room-temperature work becomes an elevated-temperature machine by what is built around it.
What goes wrong in practice
Measuring furnace temperature instead of specimen temperature, which is a different number and diverges further the thicker the section. Loading before the specimen has soaked through, so its interior is cooler than its surface and the result belongs to no single temperature. Reporting a value without the temperature and the soak time, which makes it unreproducible. A gradient along the reduced section, which means part of the gauge was tested at a temperature nobody recorded. And expecting a modulus, which the scope excludes for the good reason that extensometer drift, thermal expansion and uncertain contact with a hot specimen all sit inside the very small strains a modulus is computed from.
ASTM E21 or ISO 6892-2
| ASTM E21 | ISO 6892-2 | |
|---|---|---|
| Family | ASTM | ISO |
| Scope | Elevated temperature tension | Elevated temperature tension |
| Modulus | Excluded | Handled differently |
| Cite | Where the specification names it | Where the specification names it |
Two routes to the same property. Which applies is contractual, and a result should name its standard because the temperature tolerances and soak requirements are not identical.
Questions we are asked about this test
What is ASTM E21?
It is the ASTM method for tension testing metallic materials at elevated temperature, covering tensile strength, yield strength, elongation and reduction of area. It is a slow, soaked, quasi-static test. The current designation is ASTM E21-20, and a work item proposes extending its scope below room temperature.
Why does it exclude modulus of elasticity?
Because measuring one at temperature is a harder problem than this method sets out to solve. A modulus needs strain resolved over a very small window, and at temperature the extensometer itself drifts, the specimen expands, and the gauge's contact with a hot specimen is less certain. All of that sits inside the strains a modulus is computed from. The standard declines to report it rather than reporting it badly.
Why is the soak time as important as the temperature?
Because reaching a set point on a controller is not the same as the specimen being at temperature throughout. Heat has to conduct into the section, and a thick specimen lags its surface considerably. Two laboratories at the same nominal temperature with different soak times are not running the same test, which is why the soak is specified and why it belongs in the report.
Why must thermocouples attach to the specimen?
Because furnace air temperature and specimen temperature are different numbers, and the difference grows with section size and with how fast the furnace is working. A thermocouple in the chamber tells you what the furnace is doing; one on the specimen tells you what the metal is doing, and only the second is the test temperature.
Why do the grips stay outside the furnace?
Because a wedge grip at temperature expands along with the specimen and loses its bite, so the specimen draws out of the jaws instead of stretching. Keeping the grips cool and passing load through pull rods into the hot zone means only the specimen and the rods are at temperature, which is also why the gradient along the reduced section has to be checked — the rods are conducting heat away at both ends.
Why is strength calculated on the room-temperature area?
By convention, and it is worth knowing it is one. The specimen has thermally expanded at the test temperature, so its actual cross-section is slightly larger than the one used in the arithmetic. Every laboratory ignores that the same way, which keeps results comparable — but it means the reported stress is not the true stress in the hot specimen.
Does it cover rapid heating or high strain rates?
No, and the scope says so. This is a slow test with a controlled soak. Rapid heating changes the microstructure the specimen is being tested in, and high strain rates change how the material responds at temperature. Both are real disciplines with their own methods; neither is covered here.
Running ASTM E21 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Moderate to high — as the alloy and section require, though most metals weaken with temperature | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 over the working range | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Strain measurement | An extensometer of the class the method specifies | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | A furnace around the specimen with the load train passing through it, and grips kept outside the hot zone | Our self-tightening serrated wedge grips, with V-jaws for round specimens, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.