Abstract: Process for manufacturing a pressure vessel intended to work under pressure between −40° C. and 200° C. in the presence of H2S under conditions defined by the NACE MR 0175-97 standard, in which: components of the vessel are manufactured from a steel of chemical composition: 0.03%≦C≦0.15%; 0%≦Si≦0.5%; 0.4%≦Mn≦2.5%; 0.5%≦Ni≦3%; 0%≦Cr≦1%; 0%≦Mo≦0.5%; 0%≦Al≦0.070%; 0%≦Ti≦0.04%; 0%≦B≦0.004%; 0%≦V≦0.02%; 0%≦Nb≦0.05%; Cu≦1%; S≦0.015%; P≦0.03%; the balance being iron and impurities resulting from the smelting operation; CET=C+(Mn+Mo)/10+(Cr+Cu)/20+Ni/40<0.35; 800/500 cct<10 s; the components being quenched and tempered; the structure is martensitic or martensitic-bainitic with less than 10% ferrite; the temper temperature is <680° C. Optionally, a stress-relieving operation is carried out at a temperature ≧595° C.

Abstract: The agent in question is cobalt added in contents of between 0.05% and 2% to steels containing from 0.05% to 0.6% of carbon and less than 10% of alloying elements taken from silicon, manganese, nickel, chromium and molybdenum, to produce optionally normalized blocks, plates, bars or pieces of large size, with improved hydrogen cracking resistance and with improved weld-ability and suitability for thermal cutting. The invention also relates to a process for employing the agent and to the pieces thus obtained.