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Thermo-mechanical control processing is used to produce API pipeline steels. To design a proper thermo-mechanical cycle, it is needed to determine the critical temperatures including non-recrystallization temperature (Tnr) and austenite to ferrite transformation start and finish temperatures (Ar3 and Ar1). In this research, average schedule and continuous cooling torsion after a real schedule were used to determine critical temperatures of API X65 steel for the first time in Iran. This steel is imported from abroad and is extensively used in Iran for large diameter, high-pressure gas transportation pipelines and for oil transmission networks. It was found that the average schedule was a proper method to determine Tnr; while, continuous cooling torsion was proper to determine Ar3 and Ar1. The obtained results were compared with Boratto and Ouchi experimental relations with the purpose of evaluating the reliability of these relations for determination of Tnr and Ar3 critical temperatures. The obtained 4 percent relative error from both relations showed the need of conducting the experimental studies. With regard to the lack of experimental data, the obtained results can be used to design the optimum thermo-mechanical control process through the selection of proper temperature ranges for rough and finish rolling stages in domestic manufacturing of the test steel.

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Fractography of drop weight tear test (DWTT) specimens has received great attention by researchers in recent years due to the complex fracture surface of this test specimen. In this research, macroscopic characteristics of fracture surface of spiral seam weld in API X65 pipeline steel are investigated for the first time using chevron-notched DWTT specimensTest specimens were machined from an actual steel pipe of API X65 grade with an outside diameter of 1219mm and wall thickness of 14.3mm. Then chevron notch of 5.1, 10 and 15mm depth was placed in the center of each specimen and test samples were fractured under dynamic loading of 7m/s. Fractography of the fracture surface of test specimen with 5.1mm notch depth (as typical of test samples) showed that cleavage flat fracture initiated from the notch root (where stress intensity factor was high). Cleavage fracture changed immediately to ductile shear fracture, deviated to one side of specimen and grew extensively in heat affected zone, and finally terminated in base metal. Delaminations were observed in shear fracture area almost parallel to crack growth direction. After that, shear lips and inverse fracture appeared in hammer impacted area. By calculating the percent shear area from standard formulations, it was found that test specimen had above 95% shear area, and ductile fracture was the dominant fracture mode implying the fitness of tested steel for application in high-pressure gas transportation pipelines.

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One of the most important purposes of the drop weight tear test (DWTT) is to achieve the value of fracture energy for better evaluation of tested steel properties. In the present research, experimental and numerical measurement of fracture energy in drop weight tear test specimen with chevron notch on API X65 steel has been carried out. The purpose of the determination of this energy is to estimate the strength of material due to fracture. The test specimen was cut from an actual spiral seam welded steel pipe of API X65 grade with an outside diameter of 1219mm and wall thickness of 14.3mm and then it has been machined to standard size. Then chevron notch with a length of 5.1 was placed in the middle of the specimen and the specimen was fractured under dynamic loading with an initial impact velocity of 6.3m/s. The maximum force of 229kN and 225kN were achieved for experimental and numerical data, respectively by drawing force-displacement and energy-displacement curves. The fracture energy of the test sample for experimental and numerical data was obtained as 7085J and 6800J, respectively by evaluation of the area under the force-displacement curve. Based on the results of experimental curves, about %59 of fracture energy was used for crack propagation and the remaining was used for crack initiation and plastic deformation of test sample near anvils and striker regions. In the end, drawing a linear curve for fracture energy of specimen based on the hammer velocity showed that the slope of this curve could be a good criterion for estimating the energy loss and fracture behavior of the test specimen.