Low Cr Bearing Steel for Sweet and Sour Services

摘要

Low Cr bearing steels have been development as cost effective steel to API L80 13Cr steel and to meet "Fitness for Purpose", and have been used in CO_2 environments. These steels are often used in CO_2 environments contaminated by H_2S and outside of tubing which consist of low Cr steel is exposed in completion fluids. It is well known that sulfide stress cracking (SSC) and stress corrosion cracking (SCC) relates to hardening and Cr depletion due to heat treatment, respectively. Therefore, the effect of heat treatments, especially tempering temperature on sweet corrosion, SSC and SCC of 3 mass% Cr steel was investigated in CO_2, H_2S and completion fluid environments, respectively. The 3 mass% Cr steel did not show the influence of heat treatment to CO_2 corrosion in the CO_2 environment at 60°C and 100°C. It is thought that Cr enrichment in dark gray phase(outer phase) in corrosion products contributes to CO_2 corrosion resistance of Cr bearing steel. It was confirmed that the SSC susceptibility of 3 mass% Cr steel related to hardness. NACE tensile test result showed that SSC resistance of 3 mass% Cr steel was better than that of API L80 13Cr Steel. Hot Cl~- SCC in the 3 mass% Cr steel did not occur even in as quenched and tempered at the temperature between 500°C and 700°C.