HA-FCG of pipeline steels with varying amounts of polygonal ferrite (PF) tested at a hydrogen gas pressure of 5.5 MPa, loading frequency of 1 Hz, and stress ratio of 0.5. AF = acicular ferrite, B = bainite, P = pearlite, and XX = all secondary potential constituents. It has been found that the percent polygonal ferrite in pipeline steels correlates well with the HA-FCGR for these tests. Reprinted from Amaro et al., J. Pressure Vessel Technol. 140, 021403 (2018).111