Testing standard
Metallic materials — Tensile testing — Part 2: Method of test at elevated temperature
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 6892-2 is the tensile test for metals at elevated temperature. The specimen sits inside a split furnace with thermocouples on its parallel length, is soaked until the temperature is stable and uniform, then pulled to fracture entirely at temperature. It reports the same properties as the room-temperature test — proof strength, tensile strength, elongation, reduction of area — measured where the component will actually work.
A test piece is mounted in pull rods passing through a split furnace closed around it, thermocouples touching its parallel length. The furnace is brought to temperature and the piece soaked until stable and uniform. It is then pulled along its axis at a controlled rate until it fractures, entirely at temperature.
The properties are those of a room-temperature tensile test — proof strength at a specified non-proportional extension, upper and lower yield where they appear, tensile strength, elongation after fracture, reduction of area — but measured at the temperature the component will see. Metals lose strength and change in ductility as they heat, by no fixed proportion between alloys, so a hot proof strength is what lets a designer size a boiler part, turbine casing or pressure vessel against its service temperature. Hot ductility flags alloys that embrittle in a particular window.
The specimen is much as in Part 1. Everything difficult about this method is the temperature and how it is proved rather than assumed.
Rp0,2 = stress at 0.2 % plastic extension, at the test temperature
The equation is unchanged from Part 1. What changes is that every quantity in it belongs to the temperature at which it was measured, and quoting one without the temperature makes it meaningless.
Rm = F_m / S₀
S₀ is measured cold. The specimen expands in the furnace, and the method does not attempt to correct for that — which is a convention rather than an oversight, and it is why S₀ must be recorded as a room-temperature dimension.
A controlled-temperature enclosure around the load train. This method's accuracy rests on the specimen's own temperature being stable and uniform, which is a property of the enclosure and its control rather than of the frame.
Specifications
Grips outside the hot zone, loading through pull rods that pass into it — so the clamping is done cold on material that has not lost strength.
SpecificationsHot tensile forces are far below room-temperature ones for the same section: a 6 mm round of creep-resistant steel usually peaks under 20 kN, so frames of 50 kN to 250 kN with a split multi-zone furnace cover most of this work. Force measurement is calibrated to ISO 7500-1 class 1 or better, and extensometry to ISO 9513 class 1 or better for proof strengths, class 2 being accepted for the higher-extension properties.
The method is rate-sensitive, and slower than at room temperature. Method A is strain-rate based with a ±20 % tolerance: 0.000 07 s⁻¹, the rate the standard recommends, or 0.000 25 s⁻¹ through the yield and proof-strength region, with about 0.001 4 s⁻¹ for the tensile-strength and elongation portion under crosshead-position control. Method B uses the wider conventional strain-rate ranges and tolerances.
Temperature control is the defining requirement. Testing is above 35 °C, and both the deviation from the specified temperature and the variation along the test piece are held to a few degrees — tightest at the low end, near ±3 °C at or below 600 °C, and widening in steps as the test temperature rises; the band that applies to a given temperature is tabulated in the standard. Beyond 1 100 °C the deviations are agreed between the parties, so that is the top of the tabulated range rather than an equipment ceiling. Sensors resolve to 1 °C or better, one at each end of the parallel length, with a third at the centre once the gauge length exceeds 50 mm.
The load train has to work hot: threaded or pin-loaded high-temperature pull rods carry the specimen through the furnace, and extension is read by a side-entry extensometer or by rods brought out of the hot zone. Cold wedge grips cannot survive there, and a specimen held outside the furnace would be pulled across an unknown temperature profile, belonging to no single temperature.
The commonest fault is a thermal gradient along the parallel length that stays within tolerance at the control thermocouple but not at the specimen ends. Fracture migrates to the hottest zone, strength reads low, and scatter between identical pieces is wide; trimming the furnace zones is the fix, not a better load cell.
Short soaks come next. An operator working a batch starts the pull when the controller settles rather than when the specimen and load train have; the still-expanding pull rods then show as a slow force drift that corrupts the origin of the curve and the proof strength read from it.
Oxidation is slower to show: at the upper end of the range a reactive alloy scales during a long soak, so the section is no longer what was measured cold and thermocouple contact degrades as scale builds under the junction.
| ISO 6892-1 | ISO 6892-2 | |
|---|---|---|
| Temperature | Ambient, 10–35 °C | Elevated, in a furnace |
| Extensometer | ISO 9513 Class 1 | High-temperature or non-contact |
| Dominant difficulty | Rate control through yield | Proving temperature uniformity |
| Result quoted with | The method | The method AND the temperature |
Metals lose strength and change ductility as they heat, and by no fixed proportion between alloys — so a room-temperature figure cannot be derated to a service temperature by a rule of thumb. That is the whole reason Part 2 exists.
It is the international standard for tensile testing of metallic materials at elevated temperature. The specimen is enclosed in a furnace, soaked until its temperature is stable and uniform, and pulled to fracture entirely at temperature. It reports the same properties as the room-temperature test, measured where the component will actually operate.
Because metals lose strength and change in ductility as they heat, by no fixed proportion between alloys. Two steels with the same room-temperature proof strength can differ substantially at 500 °C. A designer sizing a boiler part, turbine casing or pressure vessel needs the property at the service temperature, which is what this method supplies.
Because furnace air temperature is not specimen temperature. The specimen is connected to cold pull rods that draw heat out of its ends, so its parallel length can sit well below the furnace set point and can carry a gradient along it. Thermocouples on the parallel length measure the thing that actually governs the result.
The hottest section is the weakest, so that is where the specimen necks and fails — and the reported properties then belong to that section's temperature rather than to the nominal one. The result looks entirely normal and is quietly wrong, which is why uniformity is proved by soaking and by multiple thermocouples rather than assumed.
Cold, at room temperature, and the method makes no correction for thermal expansion. That is a convention rather than an oversight — it keeps S₀ a measurable, repeatable quantity — but it does mean the room-temperature dimension has to be recorded as such.
Either a high-temperature contacting device with ceramic or refractory rods reaching into the furnace, or a non-contact system viewing the specimen through a window. An ordinary clip-on gauge cannot survive the hot zone, and taking strain from crosshead travel is worse here than at ambient because the load train itself expands as it heats.
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 | Hot tensile forces are far below room-temperature ones for the same section: a 6 mm round of creep-resistant steel usually peaks under 20 kN, so 50 kN to 250 kN frames fitted with a split three-zone furnace cover the great majority of elevated-temperature work. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ISO 7500-1 Class 1 | ISO 7500-1 Class 0.5 — a class tighter than the method asks |
| Strain measurement | An extensometer to ISO 9513 Class 1, gauge length Extensometer gauge length of at least 10 mm; test-piece geometry follows ISO 6892-1, so proportional pieces normally use Lo = 5.65 root So, giving 50 mm on a 10 mm round. All dimensions are taken at room temperature, and the standard sets out four ways of establishing the extensometer gauge length (at room temperature, at test temperature, reduced, or corrected). | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | High-temperature threaded or pin-loaded pull rods inside a split multi-zone furnace, with a side-entry or rod-type extensometer | Our a fixture built for this method, built to the specimen |
| Environment | Furnace above 35 °C, held within ±3 °C up to 600 °C, ±4 °C to 800 °C, ±5 °C to 1 000 °C and ±6 °C to 1 100 °C, after a soak until the test piece temperature is stable | 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.
