Microstructural characterisation and microanalysis of creep resistant steels

Steels for high temperature applications require good creep resistance which is controlled by the chemistry and microstructure of the materials. This paper focuses on the microstructural characterisation of a creep resistant steel using electron microscopy. The existence of various primary carbides, e.g. NbC, M7C₃ and M₂3C₆ was confirmed by electron diffraction. The primary chromium carbides transformed from M7C₃ to M₂3C₆ during creep while the niobium carbides were nearly unaltered. In addition, secondary precipitates (M₂3C₆) were observed within the matrix after creep. The size and distribution of the secondary carbides were analysed by a 80 mm² windowless X-Max ^N SDD at 3 kV on an SEM. Scanning transmission electron microscopy (STEM) observations showed the appearance of fine NbC, G phase (Ni16Nb ₆Si7) and (Nb, Ti)(C, N) particles.