High-temperature oxidation resistance in static air of OCr18Ni9Ti stainless steel (substrate for short), and that of the substrates after hafnizing process and hafnizing+carburizing process were investigated. The results show that the Hf-alloyed layer of 35 gm in thickness is continuous and dense with no holes. It combines with the matrix as a metallurgical bond, whose surface phase is Hf2Fe, HfC and Hf; however, the Hf+C layer is 100 gm in thickness, many granular and short rod carbide particles with sizes 1 similar to 2 mu m distribute dispersively, and the main types are MC, M7C3 and M23C6. The oxidation rate of the Hf-alloyed sample at 1050 degrees C and 1100 degrees C is just 1/3 and 1/8 of that of the substrate, respectively, while that of the Hf+C layer sample is its 1/8 and 1/25, respectively. The oxidation rate increases as the temperature rises, the rate and oxidation mass is in the order of Hf+C layer sample < Hf-alloyed sample < substrate, and the oxidation mass obeys the parabolic law. After hafnizing and carburizing followed by high-temperature oxidation, the surface alloying element content declines, Hf content increases, surface flaking improves, oxidation defects decrease and holes turn smaller. Hf can solidify the surface oxide layer and improve solid solution strengthening. Carbide particles including HfC and oxides such as HfO2 can reduce the diffusion rate of oxygen ions and improve the oxidation resistance markedly.
