The microstructure of 25Cr-35Ni-0.4C refractory steels consists of an austenitic matrix and eutectic carbides precipitated in the interdendritic regions. In-depth studies of the morphology and chemical composition of these carbides are extremely important for industry, since the microstructural components of these steels are responsible for their hot mechanical properties. In this context, it is reported here on the microstructural characterization of ASTM A297 Grade HP 40 steels modified with niobium and zirconium, using scanning electron microscopy, microanalysis and X-ray diffraction, and determination of the time to rupture at 1100ºC under a constant stress of 17 MPa.