NG-18-37-R01 Susceptibility of High-strength Pipeline Steels to Hydrogen Cracking

The susceptibility of heat-treated, high-strength line pipe steels to hydrogen stress cracking (HSC) has been studied under laboratory conditions simulating cathodic protection in the field and also under actual field conditions. The rate at which atomic hydrogen enters steel during cathodic protection in soil environments also was determined. Generally, for steel of a given strength level, the sensitivity to HSC decreases as the rate of which hydrogen enters the steel decreases. The data obtained from this study have shown that steels with yield strengths up to 130 ksi could be used, without encountering hydrogen-stress-cracking failures under normal operating conditions, for line pipe transmission of clean (sweet) natural gas.
The factors that influence hydrogen-stress cracking in hard spots in line pipe have been studied. Analysis of the operating conditions at the time of failure has not identified specific operating conditions that cause the failures, Analysis of the stresses in hard-spot regions, as the result of internal gas pressure and residual stresses, has shown that the effective hoop-tensile stress in some regions of the hard spot can be significantly greater than the stress predicted by the Barlow formula. The stress distribution is believed to be an important factor contributing to HSC in hard spots. Although all the parameters that contribute to HSC in hard spots have not been identified, methods to minimize or prevent the occurrence of HSC in hard spots have been developed.