High-carbon steels

The most important application of chromium is in the production of steel. High-carbon and other grades of ferro-chomium alloys are added to steel to improve mechanical properties, increase hardening, and enhance corrosion resistance. Chromium also is added to cobalt and nickel-base alloys for the same purpose. 

These solutions can contain as much as 60 percent (by volume) hydrochloric or sulfuric acid, depending on the shape of the steel and type of steel (high carbon or low carbon) to be cleaned. The solutions are used until either the acid content is too low (Baume gravity scale), or the iron content is too high (titration technique). By the time it is ready to be discarded, pickling solutions can have very high iron concentrations (60 g/L or 0.5 Ib/gal total dissolve iron). These solutions pose one of the most difficult waste disposal problems for the steel industry. There are a few other industries that can use the solutions (mainly for the acid content), but the volume of the solutions can literally flood the market. 

Low carbon steel Mild steel Medium carbon steel High carbon steel ... 

The corrosion rate of steel in carbonic acid is faster than in hydrochloric acid Correlations are available to predict the rate of steel corrosion for different partial pressures of CO2 and different temperatures. At high temperatures the iron carbonate forms a film of protective scale on the steel s surface, but this is easily washed away at lower temperatures (again a corrosion nomogram is available to predict the impact of the scale on the corrosion rate at various CO2 partial pressures and temperatures). 

Fig. 6. (a) The effect of sub2ero cooling on the hardness gradient in a carburized and quenched 3312 steel where (e) is oil quenched from 925 to 20°C and ( ) is cooled to -195°C. The initial quench to 20°C does not convert all of the austenite to martensite because the high carbon content in the surface region lowers the temperature below 20°C. Subsequent cooling to -195°C converts most of the retained austenite to martensite, raising the hardness, (b) The... 

Ferrous foundries consist of two types steel foundries in which electric furnaces (EAF and induction) are used, and iron foundries in which hot-blast cupolas and/or electric furnaces are used. Electric furnaces use virtually 100% scrap charges. Cupolas are shaft furnaces which use preheated air, coke, fluxes, and metallic charges. Scrap is over 90% of the metallic charge. Cupolas accounted for about 64% of total iron foundry scrap consumption in 1994 and electric furnaces accounted for about 34%. The balance was consumed by other furnaces, such as air furnaces. Iron foundry products have a high carbon content and the scrap charge usually contains a high percentage of cast iron or is used in combination with pig iron. 

Feedwater treatment is designed to protect the feedwater system and, to some extent, the boiler. Most systems contain carbon steel piping. Carbon steel corrosion (Fig. 23a) is considerably slower at a pH between 9.0 and 11.0. In aH-ferrous feedwater systems, the preferred pH range is therefore 9.2 to 9.6, although some systems are operated at a pH as high as 10. In systems where copper alloys are present, high concentrations of ammonia accelerate corrosion of the copper alloys. In those systems the preferred pH is 8.8—9.2. 

Parameter Carbon and low—medium aHoy steels High speed steels Cast cobalt alloys Carbides Cemented Coated Ceramics Polycrystalline cBN Diamond... 

Iron carbide (3 1), Fe C mol wt 179.56 carbon 6.69 wt % density 7.64 g/cm mp 1650°C is obtained from high carbon iron melts as a dark gray air-sensitive powder by anodic isolation with hydrochloric acid. In the microstmcture of steels, cementite appears in the form of etch-resistant grain borders, needles, or lamellae. Fe C powder cannot be sintered with binder metals to produce cemented carbides because Fe C reacts with the binder phase. The hard components in alloy steels, such as chromium steels, are double carbides of the formulas (Cr,Fe)23Cg, (Fe,Cr)2C3, or (Fe,Cr)3C2, that derive from the binary chromium carbides, and can also contain tungsten or molybdenum. These double carbides are related to Tj-carbides, ternary compounds of the general formula M M C where M = iron metal M = refractory transition metal. 

For high carbon steel, keep mild as possible. 

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