Microstructural aspects of sulfide stress cracking in an API X-80 pipeline steel

Crack growth in an API X-80 exposed to sour gas environments was investigated using modified wedge-opening-loaded (MWOL) specimens; The MWOL specimens were tested in the as-received condition and after annealing followed by water spraying to simulate improperly welded regions. It was found that water-sprayed MWOL specimens were susceptible to stress sulfide cracking in a NaCl-free NACE solution. Crack growth was relatively slow when subjected to an initially applied K-I of 30 MPa root m. Under these conditions, crack growth rates continually decreased until crack arrest was exhibited at a threshold K-I(K-ISSC) of 26 MPa root m. The exhibited crack growth rates were related to the facility with which nucleated microcracks joined the main crack front. Apparently, preferential nucleation and growth of microcracks within the main crack tip plastic zone accounted for the exhibited embrittlement. In particular, favorable microcrack growth followed a path consisting of fractured (cut) carbide regions, as well as various interfaces, including globular inclusions and grain boundary precipitates.