Hashemi - The University of Birjand , Department of Mechanical Engineering Birjand , IRAN
Jalai - Sadid pipe and Equipment Company, Tehran ,IRAN

In this paper the results of an experimental work on spiral welded gas pipeline steel of grade API X70 aimed at material characterisation are presented. The X70 steel is the main pipe material used in Sadid Pipe and Equipment Company for pipeline manufacture and largely utilised in gas transportation systems in Iran. The experimental programme consisted of tensile and instrumented Charpy impact testing in the pipe axial nd circumferential directions. Because of space limitation, tensile test results are given here and the data from dynamic impact experiments will be presented elsewhere. To conduct the tensile experiments, cylindrical tensile bars were machined from the original pipe. These were not flattened, which is common practice in pipeline material testing and introduces pre-strains in test specimens. The tensile samples were tested at room temperature under low strain rate of 0.05 mm/s on a Zwick testing machine with 600kN loading capacity. Axial displacements and load data were captured in each test. These data were subsequently used to determine the nominal and true stress-strain characteristics of X70 steel. The experiment showed that all tensile specimens suffered considerable ovalisation resulting in a deformed oval cross section whose minor axis was in the pipe thickness orientation. This directional behaviour was due to high rolling schedule of the pipe material during forming processes. Similarly, different values for tensile strength, ductility, strength coefficient and strain hardening exponent were obtained in the two perpendicular (axial and hoop) directions. Optical microscopy of the broken specimens revealed mode I fracture (flat tearing) at the centre and mode III fracture (slant shearing) near the edges of fractured specimens. Furthermore, splits (separations parallel to the pipe surface) were observed on the fracture surface of all test samples. Despite this, the material yield strength measured experimentally in both axial and circumferential directions of the pipe fulfilled the specified minimum yield strength (SMYS) of 485MPa for API X70 pipeline steel.

Ductile fracture, Gas transportation pipeline, X70 steel, Anisotropy