Stress Corrosion Cracking of CrNi Martensitic Stainless Steel in Concentrated MgCl₂ Solutions Available to Purchase

The stress corrosion cracking (SCC) of CrNi martensitic stainless steels (SSs) was studied chiefly in 42% MgCl₂ solution, with a particular emphasis on intercritical heat treatment. Martensitic SSs containing a few percent of Ni have been known to crack readily in concentrated MgCl₂ solutions. However, as is demonstrated in this study, when the Cr content exceeds a certain value (~15%), the cracking sensitivity of these steels can be improved considerably by an intercritical heat treatment to the extent that they do not crack in boiling 42% MgCl₂ solution. Intercritical tempering results in the formation of a mixed microstructure consisting of tempered martensite and fresh martensite. The attained cracking resistance is not impaired by a second tempering unless the tempering temperature is high enough beyond the A_Cl, temperature to destroy the microstructure achieved by the first intercritical tempering. Thus, there is a range of temperatures in the first and second temperings for these steels to become immune to cracking. The appropriate temperature range becomes narrower as the Cr content decreases. The intercritically tempered steel exhibits a much faster repassivation rate than the normally tempered steel in the solution only when external stress is applied. Further work is necessary to elucidate the mechanism of the improvement achieved by intercritical tempering.