TEM observation of grain boundary precipitates and Cr-depleted zone in 18 mass%Cr steels

18 mass%Cr steels containing small amount of C and N individually were water-quenched or air-cooled after solution treatment. Grain boundary precipitates and Cr-depleted zone in the steels were observed by FE-TEM/EDX. In C-containing steels, M7C3-type carbides were formed densely at the grain boundaries. Both the size of the carbides and the degree of Cr depletion observed near the grain boundary carbides in the air-cooled sample were larger than those in the water-quenched samples. In N-containing steels, Cr2N-type nitrides were formed at the grain boundaries. The size and the density of the nitrides were very different in each grain boundary and smaller than those of the carbides in C-containing steels. A Cr-depleted zone was not detected clearly when the grain boundary nitrides were small. Near some grain boundaries, where the large nitrides are formed separately, the degree of Cr depletion at the grain boundary apart from the nitrides was larger than that at the interface between the nitride and matrix. From his fact it is deduced that the growth of the nitrides and the formation of the Cr-depleted zone are controlled by Cr diffusion from the matrix to the nitrides via grain boundaries. The difference in precipitation behavior between grain boundary carbides and nitrides may indicate that the nitrides hardly nucleate at grain boundaries and that the nucleation behavior of the nitrides depends on the grain boundary structure more sensitively than that of the carbides. The degree of corrosion at each grain boundary observed after EPR tests was strongly related to the degree of Cr depletion determined by TEM. This result suggests that the corrosion behavior of the whole specimen is presented by the average of the corrosion susceptibility of each grain boundary.