AEM of Grain-Boundary Precipitation in Type 304 Stainless Steels Containing N

A systematic analytical electron microscope (AEM) study was conducted on a series of 18Cr-8Ni stainless steels in order to determine the effect of N additions on sensitization. This phenomenon is associated with intergranular carbide (Cr23C6) precipitation, and the subsequent Cr depletion of the surrounding region, and leads to susceptibility to intergranular corrosion and intergranular stress corrosion cracking. Sensitization can be retarded by reducing the C content of the steels, but this causes an unacceptable loss of mechanical properties. A better solution is to replace much of the C by N.A series of steels with varying N (0.04-0.25 wt%) and C (0.02-0.07 wt%) contents were studied in the AEM, and the results compared with electrochemical potentiokinetic reactivation (EPR) test data used to measure the degree of sensitization (DOS). EPR tests indicate that N additions up to 0.16 wt% retard sensitization, but above 0.16 wt% promote it, a result which does not agree with thermodynamic calculations of carbide precipitation which show that N slows the Cr23C6 growth kinetics, even at levels above 0.16 wt%.