Grain size effects on the SCC susceptibility of a nitrogen steel in hot NaCl solutions

The effect of grain size on the exhibited stress corrosion cracking (SCC) resistance of an austenitic stainless steel containing 0.97 wt.% nitrogen was investigated in this work. In order to vary the grain size, the as-received steel (AR) was heat treated (HT) at 550 and at 1100 °C for 1 h. Cert specimens of the AR and HT N-containing steels were then machined and tested in a Cortest machine using slow strain rates ranging from 7.95 × 10−6 s−1 to 7.06 × 10−7 s−1. The corrosion environment was hot water, as well as a 30% NaCl solution at 90 °C. In all the steel conditions, SCC was directly related to active pitting development. In all the cases, pits were found to preferentially form at Mn–Al inclusions, but were not able to grow too deep probably as a result of active repassivation reactions. It was found that the drop in the exhibited mechanical properties of the steels such as yield strength, UTS and % elongation was particularly enhanced for the coarse grained steel HT at 1100 °C. A SCC susceptibility index confirmed that this was indeed the case. Apparently, a drop in the yield strength tends to reduce the mechanical stability of the passive layer promoting its breakdown. Finally, quasi-cleavage and the lack of appreciable plastic deformation in some of the fracture surfaces indicated that the steel underwent some degree of embrittlement when exposed to these aqueous environments.