Effect of manganese content and microstructure on the susceptibility of X70 pipeline steel to hydrogen cracking

The influence of composition and microstructure on susceptibility to hydrogen induced cracking (HIC) was investigated in high strength pipeline steels, with Mn contents of 1.2% (standard, X70), and 0.5% (medium, MX70). The HIC resistance of the simulated coarse grained heat affected zone microstructures and normalized X70 transfer bar was also investigated. Notched and fatigue pre-cracked samples were charged with hydrogen prior to three point bend tests.The conditional fracture toughness JQ was determined. The results are discussed in relation to grain size, microstructure, composition and the type and distribution of non-metallic inclusions and precipitates.

► Increasing the hydrogen content decreases the fracture toughness (JQ) linearly. ► X70 strip exhibits higher JQ than MX70 strip before and after hydrogen charging. ► The normalized X70 transfer bar shows much lower JQ than the hot rolled strips. ► The fracture morphology changed from ductile to brittle with increasing hydrogen. ► Simulated CGHAZ structures show low JQ which decreases with increasing hydrogen.