Effect of Lubricants on the SCC Susceptibility of Bolting Material SCM 435 for Use in High Temperature Water

Abstract

SCM 435 is a martensitic low alloy steel that is often used as a bolting material when high strength is required in high temperature domains up to approximately 600K. When bolts made of this steel are used, the threads are coated with a lubricant. Bolts used in wet steam (or hot pure water) sometimes suffer damage from stress corrosion cracking (SCC). In such environments, the SCC behavior will be influenced by the gases generated from the lubricant and the decomposition products soluble to water, together with the lubricant itself. This study was conducted to determine the effects of the lubricants and the S content of the steel on the stress corrosion cracking susceptibility of SCM 435, a bolting material, in high temperature deaerated pure water. The results are as follows.
(1) No SCC was observed in SCM 435 after the test at 473K regardless of type of lubricants. Pb₃O₄ induced SCC in the steel at 523K although Ni and graphite did not, suggesting that Pb₃O₄ is stronger SCC promoter than the other two lubricants.
(2) Corrosion pits ware observed at the crack initiation sites, suggesting that SCC process contains the following sub-processes; corrosion pit formation and growth to a critical size, followed by SCC initiation and propagation. The transition from pit to SCC occurred at K₁SCC=18MPa·m^0.5 in this case.
(3) The number of corrosion pits decreased with decreasing S content in the steels. Thus if we assume that the SCC process mentioned above, decrease in S content will be very effective to mitigate SCC in SCM 435.