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.