Higher Hardenability Low Alloy Steels for H2S-Resistant Oil Country Tubulars

Abstract Nine higher hardenability low alloy steels were evaluated for potential application as hydrogen sulfide resistant oil country tubulars. The hardenability of the steels was measured using dilatometry to obtain continuous cooling transformation (CCT) diagrams. Sulfide stress cracking (SSC) resistance of the steels was determined by the double cantilever beam (DCB) test method which measures a crack arrest fracture toughness, KIssc. Two modified AISI 4130 steels containing 0.75% Mo-0.02% Nb and 1.0% Ni-0.75% Mo-0.02% Nb, and a 0.20% C-2.25% Cr-1.0% Mo steel were evaluated for application as coupling stock for C-90 and C-10 casing pipe. These steels were cooled to simulate the water quenching of pipe with a wall thickness of 25 mm or 63 mm (1 in. or 2.5 in.). The Ni-Mo-Nb modified 4.130 steel had the best SSC resistance, followed by the 2.25% Cr-1% Mb and modified 4130 steels, whose relative ranking depended on the cooling rate. Another six 0.4% C-Cr-Mo or Cr-Ni-Mo steels were evaluated for possible application as bainitic, air cooled and tempered 90 ksi grade casing pipe. These steels were cooled to simulate the normalizing of pipe with a wall thickness of 19 mm (0.75 in.). All six of the 0.4% C steel were fully bainitic on cooling and had good SSC resistance. A 0.5% Mn steel had a coarser microstructure and lower SSC resistance than the other steels which contained 1.0% Mn. Addition of 0.03% Nb slightly raised SSC resistance. Increasing chromium content from 1% to 1.5%, or nickel content from zero to one percent did not significantly affect SSC resistance.