Manufacture and Application for Low Carbon Nb-Cr (Near Stoichiometry) Htp Linepipe Steel

by Jitendra Patel

Publisher - CBMM Technology Suisse, International Metallurgy Ltd

Category - General Novel

As carbon content is lowered and the chemical composition approaches stoichiometry (Nb:C = 7.74:1) larger volume fractions of niobium carbide are dissolved which maximizes solute drag effects during high temperature processing and subsequently increases precipitation hardening potential during low temperature controlled rolling of austenite and on eventual transformation to ferrite. These effects make the steels less reliant on vanadium and molybdenum alloying. Steels with ultra-low carbon contents (<0.04 percent) and increased niobium contents were applied first in Canada in 1971. Thereafter usage was sporadic until the concept was refined by Sumitomo Steel Corporation in 1983 and was applied in earnest for construction of the Pemex Cantrell X-70 sour gas pipeline in 1998. The High Temperature Processing (HTP) concept involves moderate deformation regimes, compared with the severe low temperature reduction schedules applied to conventional Nb-V alloyed steels. This results in less severe textures which improves H2S (sour service) resistance and improves ductile fracture propagation results. In addition the steel is more economical than those relying extensively on vanadium or molybdenum strengthening. The paper chronicles the evolution of the technology during the past 40 years and summarises metallurgical, welding, toughness and application experience.

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