TWI Industrial Member Report Summary 648/1998
By D C Buxton
The document NACE MR0175 provides material requirement details for resistance to sulphide stress cracking (SSC). The standard is used widely for metallic equipment and facilities exposed to hydrogen sulphide (H 2S)-containing hydrocarbons. Where an environment is defined as sour, the maximum hardness of carbon and carbon-manganese steels is limited to 22HRC (approximately 250HV). Guidelines on materials requirements for safe application of carbon and low alloy steels are also given in EFC document No.16. The EFC document allows for a relaxation of requirements for moderately sour environments and for environments with high pH values, say about 5.
The 0.5%Cr pipeline steel used in this work has been developed to provide increased corrosion resistance in CO 2-containing environments, an area of concern particularly in the oil and gas industry. Corrosion rates obtained for 0.5%Cr seamless pipe in CO 2 media may be less than half the value for a carbon-manganese steel. Many CO 2-containing environments also contain moderate levels of H 2S. There remains, however, a lack of data on welded linepipe and similar steels, including specifically 0.5%Cr material. Further, little information exists on material behaviour in environments of increasing H 2S activity, as might occur following reservoir souring.
In previous work, SSC tests were performed on 0.5%Cr specimens taken from bead-in-groove runs. Maximum heat affected zone (HAZ) hardness levels above and below the NACE limit were produced, and it was concluded both that the NACE hardness criterion is applicable to welded 0.5%Cr steel, and that the material transformation and SSC behaviour is similar to that of carbon-manganese steel.
The work described in this report investigates the dependence of SSC behaviour of welded 0.5%Cr steel on the environment pH and H 2S partial pressure. Also, the effect on SSC behaviour of pre-exposure to low levels of H 2S is studied.