Variation of the fracture toughness of a high-strength pipeline steel under cathodic protection

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 m(0.5)/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 approximate to 0. 17 mm, from a value of approximate to 0.70 mm in air This reduction infracture 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 infracture 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.