Austenite Formation in the Oxidized Layer of Ultra-High-Strength 13Ni15Co10Mo Maraging Steel

Maraging steels are precipitation hardening alloys that can achieve an ultra-high yield strength (~3 GPa), however associated with low toughness. During exposure to high temperatures, an oxidation process occurs on the surface of these steels, generally, the oxides formed are hematite and/or magnetite. The aim of this study was to investigate oxidation on a maraging 13Ni15Co10Mo at annealing temperature of 900 °C. The bulk microstructure was investigated by several complementary techniques and the oxidized surface was characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD). The results showed that the bulk microstructure, at annealed condition, consists of a lath martensite with a hardness of round 400 HV. The most external and oxidized surface contains the oxides hematite, magnetite and kamiokite. Finally, the presence of austenite was detected in the first 2 μm below the surface. Chemical microanalysis indicated that the austenite is stable at room temperature in this region due a composition gradient that makes this region rich in nickel and cobalt. The composition gradient is due atom diffusion during oxides formation. Austenite near to the surface is very convenient as it could avoid crack initiation and propagation, improving toughness.