Fatigue Study of Pipeline Steels

Two types of pipeline steels, Alloy B [Fe-0.05C-1.52Mn-0.12Si-0.092Nb, weight percent (wt.%)] and Alloy C [Fe-0.04C-1.61Mn-0.14Si-0.096Nb, wt.%)], were tested. Vickers hardness and nanoindentation tests were used to obtain the hardness and elastic modulus. Compact-tension (CT) specimens were employed for fatigue experiments. Different frequencies (10 Hz, 1 Hz, and 0.1 Hz) and different stress ratios [0.1 and 0.5, stress ratio (R) is the ratio between P_min. [minimum applied load] and P_max.[maximum applied load]. R = P_min./P_max.] were used, and the tests were done in air, at room temperature. The effects of frequencies and different R ratios on crack-growth rates were compared. It is concluded that a higher R ratio leads to a greater fatigue-crack-growth rate (FCGR), while frequency does not have much influence. Moreover, Alloy B tends to have a better fatigue resistance than Alloy C under various test conditions. The microstructures of two alloys were investigated by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Fracture surfaces show transgranular patterns, and fatigue striation was observed under the scanning electron microscope (SEM).

Another type of pipeline steel, X70 [Fe-0.053C-1.52Mn-0.25Cr-0.19Si-0.089Nb, weight percent (wt.%)], was also studied. Fatigue tests were performed at different load levels, and comparisons were made between different parts of weld and base metals. Fracture surfaces were observed by SEM to identify fatigue and fracture mechanisms.

Neutron-scattering-diffraction experiments were performed to study the deformation behavior around the crack tip of X52 [Fe-0.071C-1.06Mn-0.24Si-0.026Nb, weight percent (wt.%)] and X70 pipeline steel. Both the hydrogen-charged sample and as-received sample were used to detect the influence of hydrogen. Preliminary results are presented in this thesis.