Toward a fracture mechanics based prediction of failure under high temperature hydrogen attack

High temperature hydrogen attack (HTHA) is a severe degradation of mechanical properties of steel components operating in hydrogen at high pressure and temperature. Hydrogen solute atoms react with carbides forming methane bubbles, typically at grain boundaries. These pressurized bubbles grow and coalesce to form microcracks which can compromise the structural integrity of the steel component. From an engineering design perspective, the interaction of HTHA damage with pre-existing cracks is most detrimental. This work investigates the growth and coalescence of a cluster of methane bubbles in the neighborhood of an axial crack on the inner diameter surface of a pressure vessel. For a given crack depth and hydrogen pressure and operating temperature, simulations were carried out to predict bubble growth and coalescence leading to microcrack formation by combined creep and grain boundary diffusion. Important effects of stress triaxiality ahead of the crack tip on bubble growth are included. By considering the time for microcrack formation as time to crack propagation and failure—a conservative measure of failure time for a component—Nelson-type curves were constructed for the case of 2.25Cr–1Mo steel indicating time to failure on a hydrogen pressure vs temperature diagram for different crack sizes. The proposed flaw-sensitive design curves offer a promising initial step toward improving the empirical API Nelson-curve diagram while maintaining its simplicity.