Caustic cracking of 9Cr 1Mo steel at 300°C

Abstract

The stress corrosion cracking of correctly heat-treated 9Cr-1Mo in caustic solutions at 300°C has been examined using a bursting tube version of the slow strain rate technique. Some susceptibility was detected at all concentrations examined (1–8 M), the more concentrated solutions (3, 4 and 8 M) which favoured less protective oxide formation, producing the more severe cracking. The cracking in these strong solutions proceeded at velocities up to 0.1 mm h −1 and produced wide oxide filled cracks. This cracking is not strongly dependent on potential, although more anodic potentials promote thicker oxide formation while cathodic potentials favour general dissolution. Normalized and tempered, annealed, and a simulated heat-affected zone microstructure were all equally susceptible to cracking. Some constant pressure tests were performed which suggested cracking could occur at stresses below the yield stress. In more dilute solutions susceptibility was much less, and the crack velocity was about 0.01 mm h −1, i.e. ten times slower than in 8 M NaOH. The relationship between slow strain rate tests, conventional tests and practical experience are discussed. The behaviour of 9Cr 1Mo in caustic below 2 1 2 M places it in the low susceptibility category and failures under realistic conditions are unlikely.