Sulfide stress cracking susceptibility of the heat-affected zone of an 9% Ni steel welded joint

Abstract

A 9% Ni steel (ASTM A333 Gr. 8) was selected for the first time for use in CO2 injection units (CO2-IUs) in the Brazilian pre-salt; however, because of the presence of H2S, it is susceptible to sulfide stress cracking (SSC), especially in welded joints. In this regard, the susceptibility to SSC of the subzones of the heat-affected zone (HAZ) of a 9% Ni steel (BM) welded pipe joint was evaluated. The thermal cycles of the HAZ were simulated numerically and physically. HAZ subzones were evaluated through optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers microhardness, and slow strain rate testing. The BM and subcritical HAZ showed similar microstructures (fine ferrite and carbide/retained austenite (γret) aggregates) and behaviors in aggressive environment, with plastic elongation ratios (REs) of 0.15 and 0.17, respectively. The intercritical HAZ (ICHAZ; fine ferrite and martensite) had the highest microhardness and γret content, with a decrease in RE (0.04); fine-grain HAZ (FGHAZ; martensite and bainite) showed the smallest RE (0.01) and brittle fracture. The coarse-grain HAZ (CGHAZ; coarse martensite and coalesced bainite) showed the lowest plastic region and brittle fracture, with RE of 0.05 and the highest time-to-failure ratio. The high microhardness of the ICHAZ, FGHAZ, and CGHAZ (>250 HV, NACE 0175 / ISO 15156) made them more susceptible to SSC than the BM. These results lead to the conclusion that both the 9% Ni steel and its welded joints are prone to SSC, and the use of this steel in CO2-IUs presents an operational failure risk.