Flattening Test on Tubes

The flattening test on tubes is a destructive testing method in which metal tubes of circular cross-section are compressed between two parallel platens down to a specified gap, so that the material's plastic deformation capability, its ductility behaviour and — particularly in welded tubes — the integrity of the weld zone can be assessed. During the test the most highly stressed regions around the outer circumference of the tube (the 12, 3, 6 and 9 o'clock positions) are subjected to high tensile and compressive stresses; a ductile material withstands this deformation without cracking, whereas a material with a brittle structure, excessive hardness, coarse grain, inclusions or weld defects develops surface or edge cracks as flattening progresses. The method therefore does not produce a numerical strength value but a pass/fail type conformity decision on whether the product has "acceptable ductility or not".

In practice a short ring specimen is cut transversely from the tube and prepared with clean, burr-free edges (typically 50 mm long or a defined multiple of the diameter). In welded tubes the position of the weld seam is critical: the specimen is placed so that the weld seam is most often at 90° (horizontal) relative to the direction of compression and, where required, at 0° (vertical); this allows the behaviour of the weld under both tensile and compressive loading to be examined. The specimen is placed on a hydraulic or mechanical press table and the tube is progressively crushed as the platens are brought together.

The platen gap at which flattening is stopped (the H value) is not arbitrary but calculated from a relationship given in the standards: H = (1 + e) · t / (e + t/D). Here t is the wall thickness, D the outside diameter of the tube and e a deformation constant determined by the material and product type (for low-carbon welded steels, for example, a typical value is of the order of 0.09). Some specifications instead require the specimen to be flattened completely until the inner surfaces meet. The essential equipment consists of a press of sufficient capacity, two mutually parallel and sufficiently hard compression platens and a distance measuring arrangement for the gap; the deformation rate is controlled so that loading is steady and slow rather than a sudden impact.

Examined at application-specific depth, the real strength of the method is that it screens weld quality in welded tube production quickly and economically. If the weld seam opens, separates or cracks in the heat-affected zone (HAZ) under flattening, this indicates insufficient melting, lack of fusion, excessive hardening or unsuitable heat treatment. In seamless tubes the result provides information about the overall ductility of the material and the quality of the inner and outer surfaces (scaling, laminations, inclusions). The method does not give a strength figure as a tensile test does; instead it offers a practical indicator of manufacturing and forming capability.

Evaluation is carried out once the specified gap H has been reached, by examining the outer surface and the edges of the specimen with the unaided eye (with a magnifier where necessary). The general acceptance criterion is that no cracks, tears or weld separations are observed in the predefined regions up to the specified degree of flattening. Non-capillary discontinuities limited to edge effects may be tolerated under some specifications, whereas cracks originating in the weld or the parent material are typically grounds for rejection. The report documents the specimen dimensions, the tube diameter and thickness, the weld position, the applied H value or degree of flattening, the observation result (crack present/absent and its location) and the final pass/fail decision. In this respect the flattening test is valuable as a simple, repeatable and low-cost verification step in batch acceptance and the quality assurance flow of tube production.