HYDROGEN INDUCED CRACKING (HIC) - LABORATORY TESTING ASSESSMENT OF LOW ALLOY STEEL LINEPIPE

摘要

As a first step, a literature survey was undertaken on Hydrogen Induced Cracking (HIC) phenomena. Material and metallurgical parameters were investigated; in addition the various testing methodologies, environments and pass/fail criteria used for HIC fit-for-purpose (FFP) testing were assessed. The literature sources suggested a variety of methodologies that can be used for FFP HIC testing. However, a universally applicable FFP test method was not identified. Therefore, an experimental study was launched to establish a suitable methodology and better define the key parameters involved in HIC phenomena. The use of "sweet service grade" C-Mn linepipe in mild sour service could provide economic benefits, but would require an assessment of the HIC performance to be undertaken for the intended service conditions. Therefore, five different sweet service line-pipe steels were used for these studies. Experiments consisted of HIC exposure tests based on the NACE TM0284-2003 Standard Test Method. Parameters studied were pH (3.5 to 5.5), H_2S partial pressure (3 to 100mbar), and time of exposure (96 hours to 3 months). Characterisation included hydrogen content measurement for diffusible hydrogen (glycerol method). HIC cracking was evaluated by ultrasonic testing and by metallographic examination in accordance with the NACE TM0284-2003 Standard Test method.From the HIC exposure tests, a good correlation was found between diffusible hydrogen and HIC. Above 1ppm diffusible hydrogen, all specimens exhibited severe HIC. For specimens tested at a level of 10mbar H_2S no HIC was observed after 96 hours exposure. But tests with longer exposure (4 weeks) did result in HIC. This suggests that the low severity conditions require longer exposures to reach equilibrium levels of internal hydrogen. This must be taken into account for FFP HIC testing in mildly sour environment. Considering the impact of the environment, H_2S partial pressure seemed to have a much greater impact than pH on the extent of HIC.These results suggest that a "regions of environmental severity" diagram, similar to that developed for sulphide stress cracking in the 1990s and later incorporated into NACE MR0175/ISO 15156, can be developed for the HIC resistance of suitable sweet service grade steels. This relates the HIC resistance to the two-dimensional environmental severity defined by the partial pressure of H_2S and the pH. This diagram will enable the mapping of HIC according to cracking susceptibility and help in defining the 'HIC sour service' limits for line-pipe steels.