Grain Boundary Graphitization in P1 (C-1/2Mo) Alloy Pipe Available to Purchase

Failure analysis performed on SA-335, Grade P1, Carbon-1/2Molybdenum (P1) steel pipe confirmed that failure in cold-formed bends was due to graphitization, but revealed that the graphite had developed in the affected material in a manner that has received little attention in the technical literature. In particular, the graphite developed in arrays of small micro-nodules preferentially concentrated within grain boundaries that were oriented normal to the hoop stress. The failure in question occurred after approximately 275,000 hours of operation at an average operating temperature of approximately 830F. Because of the preferential alignment of the graphitization, the material toughness in the tangential (hoop) direction was significantly degraded. While there is ample documentation of graphitization in P1 material after prolonged exposure to elevated temperatures, the observed morphology of volumetric grain boundary graphitization is very unusual. In response to this unique form of graphitization, cryo-cracking, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction were carried out to confirm that the small nodular “particles” within the grain boundaries were in fact graphite and not some other type of material contamination. Mechanical tests were also performed to assess the degree of material degradation, and the results have been compared to the damage levels observed in metallographic samples using a five-level damage ranking system developed for the purpose of surveying the multiple bends involved in the study. Many of the same factors that in previous studies have been identified as important influences on the formation of graphite in steel were observed in the samples examined as part of this failure analysis. These included the original steel making practice (i.e., high levels of aluminum), the original forming process (i.e., cold bending), and the service conditions (i.e., time and temperature).