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Properties and Applications of Molybdenum
Published in C. K. Gupta, Extractive Metallurgy of Molybdenum, 2017
Molybdenum dioxide, MoO2, is a dark brown powder produced by thermally reducing the trioxide with hydrogen at 450 to 470°C. Its density is 6.34 g/cm3 and its heat of formation is 141 kcal/mol. The reaction between the trioxide and hydrogen at temperatures higher than 450 to 470°C leads to the formation of metallic molybdenum. At around 1770°C, MoO2 readily oxidizes in the absence of air to form volatile MoO3 and metallic molybdenum (3MoO2 → 2MoO3 + Mo). In the presence of air, MoO2 readily oxidizes at elevated temperatures to form MoO3. Molybdenum dioxide is virtually insoluble in water. It is inert to aqueous solutions of alkali hydroxides, nonoxidizing acids, and molten salts. Nitric acid oxidizes MoO2 to MoO3.
Influence of processing conditions on the properties of thermal sprayed coating: a review
Published in Surface Engineering, 2021
Molybdenum (Mo) is principally utilised as an alloying element to steel to enhance the mechanical characteristics such as strength, creep resistance, fracture toughness, etc., by the development of dispersed carbides. However, Mo begins to oxidise significantly in an air environment at a temperature of 300°C. Oxidation becomes quick at a temperature of 500°C and the rate of attack becomes very rapid when the temperature reaches about 1200°C [162]. At a temperature less than 400°C an adherent relatively stable MoO2 scale is developed and oxidation takes place as per the parabolic law under diffusion-controlled transport through the growing scale. But, molybdenum dioxide (MoO2) is unstable to further oxidation to molybdenum trioxide (MoO3), which has a significant vapour pressure. However, the vapour pressure of MoO3 becomes progressively significant at a temperature above 500°C, based upon on the total partial pressure of oxygen [163].
A novel perception toward welding of stainless steel by activated TIG welding: a review
Published in Materials and Manufacturing Processes, 2021
Dipali Pandya, Amarish Badgujar, Nilesh Ghetiya
A layer of oxide flux acts as an insulating barrier to the arc current. At the center of the weld pool, the heat is sufficient enough to dissolve the flux. Hence, by the insulating effect of activated flux, the diameter of arc at the weld pool surface decreases. The density of heat at the center of the weld pool increases at a specific current due to the insulating effect of the activated flux. The high heat input leads to an increase in pressure and magnetic pinch forces in the weld pool; thus, strong convective downwards flow in the weld pool can be observed in stainless steel.[64] Some of the elements that can be used as activated flux are Manganese Oxide (MnO2), Titanium Dioxide (TiO2), Molybdenum Dioxide (MoO3), Silicon Dioxide (SiO2), and Aluminum Oxide (Al2O3). The size of the particles can lie between 30 µm and 60 µm75.
Features of electron beam welding of molybdenum alloy ZrMo2А
Published in Welding International, 2020
E. V. Terentev, A. P. Sliva, A. L. Goncharov, M. V. Goryachkina, A. V. Teternteva, I. O. Skuratov
In the welding of specimen No. 1 at a welding speed of 60 m/h, a seam was formed with a width of 2.7 mm at the apex and 0.9 mm at the root. An undercut with a depth of 0.5 mm formed on the root side. A crack formed along the entire length of the seam (Figures 1 and 2). It can be concluded from the nature of propagation that it is a hot crack (Figure 3). This is confirmed by observation of the seam immediately after welding at a temperature of 700°C using a video observation system. To all appearances, the cause of the cracks is axial segregation of harmful impurities, as a result of which a low-melting film of molybdenum oxides formed at the grain boundaries, causing a considerable decrease in strength of the boundaries at high temperatures [4]. Thus, the melting point of molybdenum trioxide MoO3 is only 801°C, whereas molybdenum dioxide MoO2 sublimes at a temperature above 1030°C with partial decomposition into Mo and MoO3, moreover on the MoO2-MoO3 phase diagram the solidus temperature ranges from 780 to 818°C [8], so it can be said that there is retention of the liquid interlayer at the oxygen-enriched grain boundaries at comparatively low temperatures. The melting point of molybdenum nitride is somewhat higher, at 1750°C, which may also have a negative influence on resistance to hot cracking with increase in nitrogen concentration at the grain boundaries. Intercrystalline peripheral cracks are present on the surface of the weld seam, and they develop for similar reasons.