MS-4-P-1949 TEM Observations of Nano-Scale MX Precipitates in Crept Super304H Austenitic Steel

Super304H austenitic steels containing small amount of Cu and Nb elements are widely used as superheated tubes in 600°C USC power plants. The precipitation of Cu-rich phases in Super304H austenitic steels has been extensively investigated and it has been well recognized that nano-scale Cu-rich precipitates can improve creep strength of the Super304H steel. However, the precipitation of nano-scale MX phases received little attention until now. In this work, the precipitation of nano-scale MX Phases in the Super304H steel crept at 650°C/447 hours has been observed by TEM. Based on the TEM observations, the precipitation mechanism of nano-scale MX Phases was discussed additionally.

The chemical composition of the Super304H austenitic steel is 0.08C, 0.23Si, 0.80Mn, 0.027P, 0.001S, 9.5Ni, 18.5Cr, 2.81Cu, 0.51Nb, 0.11N, 0.0034B (in mass %) with the balance of Fe. The TEM samples were cut from the steel crept at 650°C/447 hours and were prepared by twin-jet electro-polishing in a 5 vol.% perchloric acid and 95 vol.% ethanol solution at about 243 K and at 60 V. TEM observations were conducted on JEOL 2100F machine operating at 200 kV. Fig. 1 is a TEM micrograph of crept Super304H steel, showing high density of nano-scale circular shaped Cu-rich precipitates with a weak contrast and small amount of cubical-shaped precipitates with nano-scale diameter and dislocations with a dark contrast in the austenitic matrix. Fig. 2(a) is a HRTEM micrograph along the [011] direction of the austenitic matrix, showing a character of Moire fringes of the cubical-shaped precipitate. The FFT diffractogram as shown in Fig. 2(b) and EDS result in Fig. 2(c) acquired from the precipitate indicate that the cubical-shaped precipitate is fcc-structured NbC with a lattice constant of about 0.4454 nm and a cubic/cubic crystallographic relationship with the austenitic matrix. The interface of nano-scale NbC with the austenitic matrix is the (111) plane. Fig. 3 are TEM micrographs, which showing the precipitation of nano-scale MX phases at different locations in the austenitic matrix. Fig. 3(a) shows that MX phases precipitate together with the Cu-rich phases owing to a relatively high concentration of Nb near the Cu-rich precipitates in the matrix. Moreover, MX phases often precipitate along dislocation line as shown in Fig. 3(b), because small carbon or nitrogen atoms are easily clustered at dislocation core. Fig. 3(c) shows MX phases precipitating along glide dislocations, which interacting with the Cu-rich precipitates. These TEM observations confirm an interaction of nano-scale MX and Cu-rich precipitates with glide dislocations in the austenitic matrix, which obviously enhances creep strength of the Super304H austenitic steel.