In the present work was evaluated the microstructure of ASTM A335 P92 steel when this material is exposed to two corrosive mixtures O2 - H2O, O2 - N2 - H2O. In addition, the hardness and microhardness of this steel were studied. The equipment used to perform the metallography tests was an Olympus GX 71 microscope. The microhardness tests were performed on an Innovatest 400TM Series microdurometer, 230 V / 50Hz / 3A. The load applied for the microhardness analysis was equal to 50 gf. The hardness tests were performed using a Wilson hardness tester, using an applied load of 60 kgf. The tempered martensite steel microstructure was observed before exposure to corrosive environments. When this material was exposed to corrosive environments, the steel showed changes in its microstructure tending the tempered martensite to transform into a ferritic matrix. Moreover, changes in the microhardness and hardness of the material directly related to the changes in microstructure were observed as a result of the presence of oxides and carbides.