Variation of the Fracture Toughness of a High-Strength Pipeline Steel under Cathodic Protection

Abstract

Crack-tip opening displacement (CTOD) fracture toughness measurements have been performed on longitudinal, through-wall thickness, three-point bend samples from an X-80 pipeline steel. Samples were subjected to cathodic protection in an NS-4 test solution prior to and during fracture toughness determination. Variation of the precharge time and the crosshead speed for CTOD measurement indicated that precharge times > 7 days and crosshead speeds giving rates of increase of stress intensity factor < 0.2 MPa m0.5/s were required to obtain worst-case conditions. Under the worst-case conditions, the CTOD fracture toughness for the X-80 material was reduced by 75%, to ≈ 0.17 mm, from a value of ≈ 0.70 mm in air. This reduction in fracture toughness was found to be similar at pH values down to 2.5. Although the samples were not taken with the fracture in an orientation representative of a typical pipeline defect, the reduction in fracture toughness was representative of the behavior of X-80. Comparison with literature data for conventional pipeline steels indicates that they suffer a similar loss in fracture toughness under worst-case conditions. The level of fracture toughness reduction for high-strength steels is of greater concern because they will operate under higher stresses. However, tolerable defect size calculations indicate that hydrogen-induced stress cracking (HSC) will not be a failure mechanism for X-80 steel parent material without some additional influence.