Effect of gaseous hydrogen charging on the tensile properties of standard and medium Mn X70 pipeline steels

The tensile properties of two X70 steels with high (1.14 wt-) and medium (0.5 wt-) Mn contents have been investigated by testing at 25°C of tubular specimens charged with an internal gas pressure of 10 MPa of hydrogen or argon. The hydrogen-charged samples were additionally tested at 50 and 100°C. Tensile testing showed that the equiaxed ferrite–pearlite microstructure of higher Mn steel was most sensitive to hydrogen embrittlement and that the banded ferrite–pearlite microstructure of the higher Mn strip was more susceptible to hydrogen embrittlement than the medium Mn strip. The more highly banded ferrite–pearlite microstructure in the higher Mn steel provided numerous sites for concentration of hydrogen to levels that promoted crack initiation and growth. Test temperatures up to 100°C reduced the yield and tensile strengths, increased the total elongation and decreased the extent of hydrogen embrittlement because of enhanced dislocation mobility and less effective hydrogen trapping.