Why Impact Toughness Governs Line Pipe Safety
Impact toughness is the ability of a steel to absorb energy and deform plastically before it fractures under a sudden load. For a buried or subsea pipeline the property is decisive, because a pipe that fails in a brittle manner can propagate a running crack over hundreds of metres, while a pipe with adequate toughness arrests the crack instead. Because toughness falls sharply as temperature drops, the test is always defined together with a test temperature, and the standard and the purchase order together fix both the temperature and the acceptance level. The applicable requirements for API 5L pipe come from API 5L itself, from ISO 3183 for the equivalent international grade, and from the test method standards that the test is performed to.
The Charpy V-Notch Test
The standard tool for routine acceptance testing is the pendulum impact bend test on a simply supported notched bar, described in ISO 148-1 and, for steel products, in the mechanical testing sections of ASTM A370. The procedure is straightforward and highly repeatable.
A full size specimen measures 10 mm by 10 mm by 55 mm with a 45 degree V-notch, a notch depth of 2 mm and a notch root radius of 0.25 mm.
The specimen is cooled to the specified test temperature in a bath or chamber and transferred to the machine within the time allowed by the method, so that the temperature of the steel rather than of the room decides the result.
A pendulum of known energy strikes the specimen behind the notch and fractures it. The energy absorbed in the fracture is read directly from the machine, in joules.
After fracture, the shear area is often measured on the fracture surface and lateral expansion is recorded at the notch root. A high proportion of fibrous, shear fracture surface indicates ductile behaviour.
When the wall of the pipe is thinner than 10 mm, sub-size specimens with reduced width are machined so that the specimen represents the full wall thickness. The absorbed energy of a sub-size specimen is lower than that of a full size specimen simply because there is less material to fracture, so the specified values are scaled accordingly and the specimen size must always be reported with the result.
Test Frequency, Location and Direction
Impact tests are taken from the pipe body in the transverse direction, and welded pipe is also tested in the weld and in the heat affected zone. Sampling frequency is linked to the heat or to the lot, as defined in the standard for the product specification level that applies. Product specification level 1 does not require impact testing unless the order asks for it; product specification level 2 does require the test, which is one of the main technical differences between the two levels. Test results are reported as a set, normally three specimens, with a minimum average value and a minimum individual value.
Drop Weight Tear Testing for Large Diameter Pipe
Charpy testing measures the energy absorbed by a small, notched specimen, but it does not fully reproduce the behaviour of a pressurised pipe containing a running crack. For large diameter, high pressure transmission lines, a drop weight tear test is therefore specified in addition. A full thickness, notched plate is struck by a falling weight and broken, and the proportion of the fracture surface that is shear rather than cleavage is measured. A high minimum shear area, commonly expressed as a percentage of the fracture face, demonstrates that the steel will resist crack propagation. The test method is described in ASTM E436 and is invoked through the annexes of API 5L when the pipe size and application call for it.
What Moves the Transition Temperature
| Factor | Effect on impact toughness |
|---|---|
| Carbon content | Increasing carbon raises the ductile to brittle transition temperature and lowers the maximum absorbed energy |
| Grain size | Grain refinement lowers the transition temperature and is the most reliable way to improve toughness |
| Microstructure | Coarse pearlite bands and free ferrite reduce toughness, while fine acicular ferrite and bainitic structures improve it |
| Sulphur and inclusions | Elongated manganese sulphide inclusions split the steel and are controlled by low sulphur practice and calcium treatment |
| Rolling and cooling practice | Controlled rolling and accelerated cooling refine the structure and raise the absorbed energy at low temperature |
| Wall thickness | Heavier walls cool more slowly and tend to have a coarser structure, so thick pipe usually has tighter requirements than thin pipe of the same grade |
FAQ
Q: What unit is used for impact toughness?
Absorbed energy is reported in joules. When the result is normalised by the cross-sectional area at the notch, it is expressed in joules per square centimetre or kilojoules per square metre, which allows specimens of different size to be compared.
Q: At what temperature is an API 5L pipe impact tested?
The test temperature is specified by API 5L and tightened by the purchase order, and it is normally related to the lowest expected operating or installation temperature of the pipeline rather than to room temperature.
Q: Can a sub-size specimen be converted to a full size value?
No single conversion factor is accepted for all steels. The specified acceptance values for sub-size specimens are set out with the specimen size in the standard, and the report must state the size used.
Q: Why is a drop weight tear test needed when Charpy results are already acceptable?
The two tests measure different things. Charpy indicates the energy absorbed by a small specimen, while the drop weight tear test measures the resistance of full thickness material to a running crack. Large diameter, high pressure lines commonly require both.
Q: How does sulphur content affect the result?
High sulphur forms elongated sulphide inclusions that act as internal crack paths, lowering the absorbed energy and raising the transition temperature, especially in the transverse direction. Low sulphur practice with inclusion shape control is the normal remedy.
Q: What should be recorded on the test certificate?
The grade and product specification level, heat number, specimen orientation and size, test temperature, absorbed energy of each specimen, the mean value and, where required, shear area or lateral expansion, all traceable to the pipe lengths tested.
