Hydrogen Embrittlement Testing of High Strength Low Carbon Martensitic Steels

High strength carbon steels are being used as the external reinforcement for a novel composite pipeline. This system, known as X200, combines a thin wall corrosion resistant alloy with the reinforcement to produce a high pressure pipeline which is manufactured on site giving a lower cost alternative to conventional thick-wall pipelines. Despite the widely known susceptibility of high strength ferritic-bainitic carbon steels to hydrogen embrittlement, there is little published data on the hydrogen embrittlement behaviour of martensitic carbon steels. Therefore, a series of tests was performed to establish the risk of cracking in a buried condition where cathodic protection is applied. Tests were performed to establish the plateau hydrogen content and the hydrogen flux under various charging conditions, and then a series of slow strain rate and constant load tests performed under polarised conditions to establish any reduction in ductility over samples tested in air. SEM analysis of the fracture surfaces was performed to identify any change in the fracture morphology. A stressed full ring test of a sample of X200 was also tested under simulated cathodic protection conditions. The results of this testing are discussed and the implications for X200 as a buried pipeline system evaluated.