A study of Hydrogen-Induced Cracking mechanism in Old and New Generation Pipeline Steels

The characteristics of hydrogen-induced cracking (HIC) in old generation mid-strength (OG) and new generation high-strength (NG) API 5L pipeline steels were studied. The HIC was induced by cathodic charging and the crack growth was monitored by straight beam ultrasonic inspection. The microstructure, HIC crack profiles and fracture surfaces were examined by optical, scanning electron microscopy, and electron backscatter diffraction (ESBD). The hydrogen microprint technique (HMT) was used to visualize the hydrogen permeation sites in the microstructure. The results showed that both in OG and NG steels, the HIC started as randomly scattered individual cracks, identified as Stage I, with the maximum growth rate of 0.35 cm 2 /h, followed by crack interconnection, identified as Stage II, where the crack growth rate decreased below 0.75 cm 2 /h to finally became near zero. The HIC growth rate in Stage I correlates well with H-solubility separately for each steel type but it correlated very well with the density of H-reversible traps for both OG and NG steels, independent of the microstructure. HMT showed that the hydrogen path in OG steels was mainly through pearlite islands, while in NG steel it was through blocky martensite/austenite, but is more intense around NMI inclusions of cuboidal shapes. EBSD analysis showed that HIC paths for NG steels are preferably along high-angle grain boundaries and HIC crack propagated through deformed or recovered grains and high-angle grain boundaries facilitated HIC regardless of the microstructure. The results showed that the kinetics of HIC depend on the synergistic effect of microstructural features such as microstructure, grain size, inclusion content and density of random high angle grain boundaries.