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 inves tigated using modified wedge-opening-loaded (MWOL) specimens; The MWOL specimens were tested in the as-received condition and after annealin g followed by water spraying to simulate improperly welded regions. It was found that water-sprayed MWOL specimens were susceptible to stres s sulfide cracking in a NaCl-free NACE solution. Crack growth was rela tively slow when subjected to an initially applied K-I of 30 MPa root m. Under these conditions, crack growth rates continually decreased un til crack arrest was exhibited at a threshold K-I(K-ISSC) of 26 MPa ro ot m. The exhibited crack growth rates were related to the facility wi th which nucleated microcracks joined the main crack front. Apparently , preferential nucleation and growth of microcracks within the main cr ack tip plastic zone accounted for the exhibited embrittlement. In par ticular, favorable microcrack growth followed a path consisting of fra ctured (cut) carbide regions, as well as various interfaces, including globular inclusions and grain boundary precipitates.