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.