Safety evaluation of pipeline steels under SNG containing H₂

Synthetic natural gas(SNG) is an important developing direction of the clean utilization of coal. The present pipeline steel may cause hydrogen-induced delayed cracking when used to transit SNG due to the hydrogen partial pressure in SNG perhaps up to 0.72 MPa. Therefore,the safety assessment of pipeline steel for long-term service is very important and necessary. In this work,hydrogen damage and hydrogen-induced delayed cracking in X-70 pipeline steel and 20#steel at different hydrogen contents were studied by using autoclave constant load test and electrochemical hydrogen charging methods. The service safety of the steels in SNG was evaluated. There is no hydrogen damage in the two kinds of steel after being placed in the autoclave in which the total pressure 12 MPa(10 MPa N₂+ 2 MPa H₂) is kept for one month. Hydrogen-induced delayed cracking does not occur in the U-bending specimen. And the constant load samples do not fail after they are kept at the yield stress for one month. The hydrogen content entering into X-70 and 20^# samples is much lower than the threshold hydrogen content for hydrogen-induced delayed cracking and that for hydrogen damage.Therefore,both X-70 steel and 20^# steel have high safety factors during the long-term service of transiting SNG.