Scaling high-alloy steels

Calcium hydride is prepared on a commercial scale by heating calcium metal to about 300°C in a high alloy steel, covered cmcible under 101 kPa (1 atm) of hydrogen gas. Hydrogen is rapidly absorbed at this temperature and the reaction is exothermic. 

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. 

Unalloyed steels can be used in air up to 550°C and low-alloy steels up to approximately 600°C. The applicability of high-alloy steels is determined by the alloy contents, with special importance attached to Cr, Si, and Al, as demonstrated in Figures 20.48,20.55, and 20.56. Water vapor and carbon dioxide in air generally worsen the scaling behavior of steels. The resistance of steels in water vapor is of particular importance in steam boilers and heat exchangers. It has been investigated in the literature at temperatures up to 800°C. 

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... 

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

Since the paper by Pilling and Bedworth in 1923 much has been written about the mechanism and laws of growth of oxides on metals. These studies have greatly assisted the understanding of high-temperature oxidation, and the mathematical rate laws deduced in some cases make possible useful quantitative predictions. With alloy steels the oxide scales have a complex structure chromium steels owe much of their oxidation resistance to the presence of chromium oxide in the inner scale layer. Other elements can act in the same way, but it is their chromium content which in the main establishes the oxidation resistance of most heat-resisting steels. 

An interesting case of ash attack is encountered with valves in engines powered by high octane fuels containing lead compounds. These compounds are deposited from the gases as mixtures of lead oxide, sulphate and bromide, and can cause serious scale-fluxing effects with high-alloy valve steels. 

The Pb-Na alloys with high Na content (71-89 at % Na = 21-47 wt% Na) are made on a laboratory scale in a steel bomb in an electric furnace at 420°C by fusing the components , and Pb-Na alloys with lower Na content are prepared by adding Na metal lumps to molten Pb at the lowest possible T using a steel crucible . Fireclay crucibles are also used . 

Iron oxide scale Oxidation of iron or low-alloy steels at temperatures >570 C (wustite stable) leads to a scale composed of an inner thick layer of wustite, Fei j,0, and two outer thinner layers of magnetite Fe3 04 and hematite Fe203. The disorder of Fei yO has been described in Sect. 6.2.2.1.3, its very high-iron vacancy concentration being the reason for fast outward cation diffusion and rapid growth. 

Depending on the requirements, most frequently the L6, 52M (52100), or 01 alloyed steels are used for rolls. D2 tool steel is used for products requiring tight tolerances (products such as automotive, appliance, furniture, aerospace, and others) and for hot-rolled steel with scale. When extreme high-wear resistance is required, other materials such as CPMVIO or the like can be used. 

The heat-resistant steels are weldable by the usual processes, with arc welding preferred over gas fusion welding. For the ferritic steels, the tendency to grain coarsening in the heat affected zone has to be kept in mind. The application of austenitic filler metals will lead to better mechanical properties of the weld connection than those of the base metal (however, with respect to the scaling resistance, different thermal expansions of the ferritic and austenitic materials may be a problem). Filler materials should be at least as highly alloyed as the base metal. In sulfurizing atmospheres it is advisable to use ferritic electrodes for the cap passes only in order... 

Minimills. During the final decades of the twentieth and the first decade of the twenty-first century, a minimill revolution transformed the world steel industry. The central component in a minimill is the electric arc furnace, where scrap metal is melted, purified, and processed into various steel products. Because minimills do not have to make such intermediate products as pig iron, they can minimize expenses for raw materials. New technologies in minimills allow the manufacture of a wide variety of products, from low- and high-carbon steels to specialized alloy steels. In 1970, minimiUs accounted for about 10 percent of U.S. steel production, but by 2010 they were responsible for more than 60 percent. These gains were due not so much to expansions in scale but to the development of more and better steel products. 

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