Impact of welding simulated heat treatment on hydrogen embrittlement behavior of high-strength fine-grained steels

•HE sensitivity of S690QL structural steel and X80 pipeline steel in both as delivered and welding simulated heat treatment conditions have been investigated by SSRT with in-situ hydrogen charging.•Hydrogen-induced property degradation is particularly associated with the brittle nature of quenched lath microstructure with the raised fraction of HAGBs, as well as grain coarsening.•S690QL picks up higher levels of diffusive hydrogen than X80 and reveals larger fraction of cleavage dominated fracture surfaces at the same in-situ charging condition.•X80 steel developed lath bainitic microstructure with micro-alloying induced precipitation during welding simulated heat treatment, which conserves good mechanical properties and retains an improved HE resistance. Hydrogen embrittlement (HE) sensitivity of S690QL structural steel and X80 pipeline steel in both as delivered and welding simulated heat treatment conditions have been investigated by slow strain rate test (SSRT) with in-situ hydrogen charging. Both investigated steel grades show low strength losses, however, high ductility losses, represented by losses in total elongation and area reduction after in-situ hydrogen charging at a constant current density. The resulting diffusive hydrogen contents after charging are associated with the material microstructure, which are 2–3 ppm for S690QL and 1–2 ppm for X80. The lath martensite dominated microstructure in heat-treated S690QL specimens revealed extremely high HE sensitivity as demonstrated by fracture occurring within the elastic deformation regime. Hydrogen induced damage was found associated with the brittle nature of the quenched lath martensite, the severe grain coarsening, and the large fraction of high angle grain boundaries. The X80 pipeline steel reveals a transition from granular bainite dominated microstructure to lath bainite/martensite dominated microstructure after welding simulated heat treatment. The uptake of diffusive hydrogen during the in-situ charging process of X80 specimens was less compared to S690QL specimens, which was also desorbed at higher temperatures as characterized by hydrogen thermal desorption analysis. The results suggest a controlled bainitic/martensitic microstructure transformation with designed precipitation hardening would contribute to both strength enhancement and the suppression of hydrogen mobility.