CATASTROPHIC OXIDATION OF AISI M4 ALLOY

In the annealing of some pure metals and various alloys, the phenomenon of accelerated oxidation is known, which is due to the fact that the resulting surface oxides fail to inhibit or completely stop the oxidation process. Vanadium ferrous alloys are rapidly oxidized during high-temperature annealing in oxygen atmospheres. The rate of oxidation is so great that it is considered a catastrophic oxidation. In this work, the AISI M4 alloy was investigated. The mechanism and model of the oxidation of alloy AISI M4 were developed on the basis of an analysis of the oxidation in a temperature range of 700–1000 °C. The holding time was 6 h. The alloy was analysed with an optical microscope and scanning electron microscopy equipped with EDS, WDS and EPMA detectors. It was found that below 850 °C, molten oxide is formed, which is predominantly of V2O5 composition. Above 850 °C, the oxide layer is multi-layered. The oxide layer consists of complex oxides, which formed with other elements. The X-ray analysis confirmed the presence of the following oxides in the oxide layer: Fe2O3, Cr2O3, FeVO4, V2O3 and Fe2WO6; and the second oxide may additionally be: V2O5, Cr2O5 and FeWO4.

Microstructure and Oxidation Behavior of Anti-Oxidation Coatings on Mo-Based Alloys through HAPC Process: A Review

Mo and Mo-based alloys are important aerospace materials with excellent high temperature mechanical properties. However, their oxidation resistance is very poor at high temperature, and the formation of volatile MoO3 will lead to catastrophic oxidation failure of molybdenum alloy components. Extensive research on the poor oxidation problem has indicated that the halide activated pack cementation (HAPC) technology is an ideal method to solve the problem. In this work, the microstructure, oxide growth mechanism, oxidation characteristics, and oxidation mechanism of the HAPC coatings were summarized and analyzed. In addition, the merits and demerits of HPAC techniques are critically examined and the future scope of research in the domain is outlined.

Oxidation Behavior of Quaternary Co-20Re-25Cr-3Si Alloy at Laboratory Air

The iso-thermal oxidation of the quaternary Co-20Re-25Cr-3Si alloy at 1,000–1,200 C under laboratory air for 24 h has been studied. The oxidation kinetics at 1,000–1,200 C follow parabolic law perfectly and the value of oxidation activation energy is 220.74 kJ/mol. Unlike the experimental condition of 0.1 MPa pure oxygen, under laboratory air Co-20Re-25Cr-3Si alloy presents a good oxidation resistance due to the formation of a dense and continuous layer of Cr2O3 in contact with the alloy. A 79% drop in oxygen pressure has changed the oxidation mode from catastrophic oxidation to mild oxidation. The thickness of the protective Cr2O3 layer decreases when temperature increases. Therefore, Co-20Re-25Cr-3Si alloy shows the best oxidation resistance at 1,000 C. Moreover, from another perspective, compared with Co-17Re-23Cr-3Si alloy, the addition of about 2% Cr can also improve the oxidation resistance of the alloy greatly by forming the protective Cr2O3 layer.

Effect of SiO2–Al2O3 Glass Composite Coating on the Oxidation Behavior of Ti60 Alloy

A SiO2–Al2O3 glass composite coating was prepared on Ti60 alloy via air spraying slurry and then a suitable baking process. It was composed of potassium silicate glass, alumina and quartz powders. The high temperature oxidation performance of the alloy with and without coating was evaluated in static air at both 800 °C and 900 °C. The results show that catastrophic oxidation occurs for Ti60 bare alloy. It had a mass gain of about 2 mg/cm2 after oxidation at 800 °C and 17 mg/cm2 at 900 °C for 100 h. On the contrary, the oxidation resistance of alloy coated with composite coating was much improved with the mass gain about 0.36 mg/cm2 and 0.95 mg/cm2 at 800 °C and at 900 °C, respectively. The microstructure evolution of the composite coating and the alloy was analyzed by scanning electron microscope and electron probe microanalyzer. The effect of the composite coating on the oxidation performance of the alloy is discussed especially in terms of oxygen diffusion and interfacial reaction.

Phosphorus Removal Options at Induction Melting of Steel

The main aspects of effective dephosphorization of steel under conditions of induction melting are presented. Regularities of scale growth on the surface of iron, as well as the conditions of its catastrophic oxidation, are considered. An industrial experiment was conducted to remove phosphorus from steel intended for brake discs.