Oxidation high-alloy steels

An important iadustrial use of NaH involves its in situ formation ia molten NaOH or ia fused eutectic salt baths. At concentrations of 1—2% NaH, these compositions are powerful reducing systems for metal salts and oxides (5). They have been used industrially for descaling metals such as high alloy steels, titanium, zirconium, etc. 

Sulphates, which form part of the ash from the combustion of many fuels, are not harmful to high-alloy steels, but can become so if reduction to sulphide occurs. This leads to the formation of low melting point oxide-sulphide mixtures and to sulphide penetration of the metal. Such reduction is particularly easy if the sulphate can form a mixture of low melting point with some other substance. Reduction can be brought about by bad combustion, as demonstrated by Sykes and Shirley , and it is obviously important to avoid contact with inefficiently burnt fuels when sulphate deposits may be present. Reduction can also be brought about in atmospheres other than reducing ones and the presence of chlorides or vanadium pentoxide has been shown to be sufficient to initiate the reaction. It has also been shown that it can be initiated by prior cathodic polarisation in fused sodium sulphate. The effect of even small amounts of chloride on oxidation in the presence of sulphate is illustrated in Fig. 7.33 . 

Sodium hydride is used as a reducing agent in organic synthesis, for reduction of oxide scale for metals and high-alloy steel, and as a hydrogenation catalyst. 

Cr203 In the oxidation of high-alloy steels and other high-temperature alloys, Cr203 is the most important constituent of protective oxide films. Its formation is... 

Lei] Leitner, J., Dubsky, J., Had, J., Hanousek, F., Kolman, B., Volenik, K., Metastable Chromium-rich Oxide Formed During Plasma Spraying of High-alloy Steel , Oxid. Met, 54(5-6), 549-558 (2000) (Experimental, Morphology, Phase Relations, 23)... 

Some data on scale failure have been determined experimentally and are illustrated in Fig. 2-26 for tensile stresses, which is regarded as the more critical situation. Figure 2-26 a shows the situations for an alumina scale on a high alloy steel and for a chromia former. Figure 2-26 b shows the results for nickel oxide on nickel. The data are plotted versus the applied strain rate at a variety... 

Schematic of the morphology of metal-dusting attack on high-alloy steels with pitting at unprotected (oxide free) sites. The pits are filled with a mixture of "metal dust" (white) and carbon powder (black). On the surface of the attacked site graphite filaments (carbon "nanotubes") grow catalyzed by the presence of carbide or metal particles at their tips.

Vapor pressure values of the metal oxides and hydroxyls potentially formed from (low or high alloy) steels at 850°C as a function of temperature and for different oxygen and water vapor contents (ChemSage calculations), (a) Dry air, (b) nitrogen-based environment with 10% oxygen and 10% water vapor, and (c) pure water vapor. 

Zirconium oxide abrasive. High stock removal on mild, stainless, and high alloy steels. 

High density tungsten alloy machine chips are recovered by oxidation at about 850°C, foUowed by reduction in hydrogen at 700—900°C. Typically, the resultant powders are about 3-p.m grain size and resinter readily. There can be some pickup of refractory materials used in furnace constmction, which must be controUed. This process is important commercially. Eor materials that may be contaminated with other metals or impurities, the preferred recovery process is the wet chemical conversion process used for recovery of tungsten from ores and process wastes. Materials can always be considered for use as additions in alloy steel melting. 

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