Abstract
Federal rule changes governing natural gas pipelines have driven operators in the United States to establish materials verification programs (MVP) for pipelines that lack reliable records. One component of a successful MVP is the use of nondestructive testing (NDT) to assess pipe properties in situ. Pacific Gas and Electric Company (PG&E) augments traditional NDT with quantitative analysis of pipe microstructures. The microstructures are acquired non-destructively by surface replication, and are quantitatively evaluated to assess the ferrite grain size and fraction of pearlite. To support interpretation of the data, PG&E has compiled a database of steel line pipes for which microstructures from both destructive cross-sections (cutouts) and in situ surface replicas have been evaluated, and for which mechanical test data are available. The database is currently comprised of more than fifty pipe samples with manufacturing dates between 1931 and 2017, and a variety of grades and seam types.
This work presents a comparative analysis of the different methods used by PG&E to quantify pipe microstructures, and shows that microstructures from surface replicas are generally consistent with those from cutout cross-sections. In addition, semi-quantitative correlations between micro structure and mechanical properties are presented. The data suggest a correlation between grain size and the ductile-brittle Charpy V-notch (CVN) transition temperature, and an inverse correlation between fraction of pearlite and the upper-bound of the CVN upper-shelf energy. In addition, the analysis shows that grain size can provide a lower bound on the manufacturing year (vintage) due to the advent of steelmaking practices that produce refined grain structures. By themselves, the observed trends can provide semi-quantitative bounds for these properties; however, the predictive value of the microstructural assessments may be improved by the development of more sophisticated algorithms that consider additional information, such as composition and hardness.
