Influence of Line-Pipe Steel Metallurgy on Ductile Tearing of Stress-Corrosion Cracks During Simulated Hydrostatic Testing

Hydrostatic testing is one method to confirm the integrity of pipelines containing colonies of stress-corrosion cracks. Although this technique is widely used, a concern of the pipeline industry is the potential for ductile tearing damage of subcritical flaws in pipes with cracks in the base metal and in the welds. The objectives of the current study were to determine the amount of ductile tearing and crack tip blunting that may occur at the crack tips of flaws that survive a hydrotest and to evaluate the influence of metallurgy on the extent of ductile tearing. In this research, stress-corrosion cracks (SCC) were grown in a near-neutral-pH environment in compact tension (CT) specimens made from two heats of X-65 line-pipe steel and one heat of X-52 line-pipe steel with an electric resistance weld (ERW). Simulated hydrostatic tests were performed on these specimens at loads that corresponded to hoop stresses at and above the specified minimum yield strength (SMYS) of the pipe steel, resulting in applied J-integral values near and above J(Q). Some specimens ruptured; some did not fail. Crack tip blunting and the extent of tearing were evaluated. Based on curve fits of the data collected from the CT specimens, the CorLAS™ software was utilized to predict the maximum amount of tearing for cracks of varying flaw dimensions and hydrostatic pressures.