High-alloy irons

Table 4-15 lists base materials Elliott has tested. This list, which is continually being expanded, includes low alloy steels, high alloy iron base, nickel base, cobalt base materials, and odiers. Table 4-16 shows some of the coatings Elliott has tested. The list indicates die supplier, coating designation, and major components of the coating composition. 

Fig. 7.19 Scaling resistance of high-alloy irons (after Hallett ). Curve a Niresist (Fe-14-6Ni-2-lCr-7-2Cu). Curve b Silal (Fe-5-7Si). Curve c Niresist (Fe-14-4Ni-4-8Cr-7-4Cu). Curved Nicrosilal (Fe-4-6Si-22-5Ni-2-5Cr). Curve e Nicrosilal (Fe-4-9Si-22-8Ni-4-6Cr).

In gray iron, most of the contained carbon is in the form of graphite flakes, dispersed throughout the iron. In ductile iron, the major form of contained carbon is graphite spheres, which are visible as dots on a ground surface. In white iron, practically all contained carbon is combined with iron as iron carbide (cementite). a very hard material. In malleable iron, the carbon is present as graphite nodules. High-alloy irons usually contain an alloy content in excess of 3%. 

Vanderpan (1982) recommended the use of Ni-hard as a material to cast the impellers and liners of coal handling slurry pumps. For certain high pH applications due to acidic water, or in the case of high-salt mixtures, special high-alloy irons may be used instead of... 

Applications. Alloy 20 is a highly alloyed iron-base nickel-chromium-molybdenum stainless steel developed primarily for use in the sulfuric acid-related processes. Other t5fpical corrosion-resistant applications for the alloy include chemical, pharmaceutical, food, plastics, synthetic fibers, pickling, and FGD systems. 

Soft magnetic materials are characterized by high permeabiUty and low coercivity. There are sis principal groups of commercially important soft magnetic materials iron and low carbon steels, iron—siUcon alloys, iron—aluminum and iron—aluminum—silicon alloys, nickel—iron alloys, iron-cobalt alloys, and ferrites. In addition, iron-boron-based amorphous soft magnetic alloys are commercially available. Some have properties similar to the best grades of the permalloys whereas others exhibit core losses substantially below those of the oriented siUcon steels. Table 1 summarizes the properties of some of these materials. 

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. 

Tube material includes any that can be formed into a coil, but usually copper, copper alloys, and stainless steel are most common. The casing or shell material can be cast iron, cast steel, cast bronze, fabri-catea steel, stainless, and other high-alloy materials. Units are available with pressure vessel code conformance. 

If the amount of metal removal by erosion is significant the surface will probably be continually active. Metal loss will be the additive effect of erosion and active corrosion. Sometimes the erosion rate is higher than that of active corrosion. The material selection judgment can then disregard coirosion and proceed on the basis of erosion resistance provided the corrosion rates of aetive surfaces of the alloys considered are not much different. As an example of magnitudes, a good high-chromium iron may lose metal from erosion only a tenth as fast as do the usual stainless steels. 

D02 G.E. Duvall, D.E. Davenport, and J.J. Kelly, Metallurgical Effects of Explosion-Induced Shock Waves, in Research Seminar on High Nickel Alloys for High Temperatures, Iron-Nickel Alloys, Stainless Steels (The International Nickel Co., New York, 1960). 

Tube O.D. Carbon Steel High Alloy Steel (750) Low Alloy Steel (850) Nickel-Cooper (600) Nickel (850) Nickel-Chromium-Iron (1000) Alum mum Almninmn Alloys, Copper Copper Alloys, Titanimn Alloys at Code Maximmn Allowable Temperature... 

Figure 4-227. Magnetic flux-lines representation in a highly permeable iron alloy core.

Butler, G., Stretton, P. and Beynon, J. G., Initiation and Growth of Pits on High-purity Iron and its Alloys with Chromium and Copper in Neutral Chloride Solutions , Br. Corr. J., 7, 168 (1972)... 

The individual characteristics and uses of the basic grades of the austenitic irons are given in Table 3.55. The major uses for these materials occur in the handling of fluids in the chemical and petroleum industries and also in the power industry and in many marine applications. The austenitic irons are also used in the food, soap and plastics industries where low corrosion rates are essential in order to avoid contamination of the product. Ni-Resist grades Type 2, 3 or 4 are generally used for such applications but the highly alloyed Type 4 Ni-Resist is preferred where low product contamination is of prime importance. 

The high-chromium irons undoubtedly owe their corrosion-resistant properties to the development on the surface of the alloys of an impervious and highly tenacious film, probably consisting of a complex mixture of chromium and iron oxides. Since the chromium oxide will be derived from the chromium present in the matrix and not from that combined with the carbide, it follows that a stainless iron will be produced only when an adequate excess (probably not less than 12% of chromium over the amount required to form carbides is present. It is commonly held, and with some theoretical backing, that carbon combines with ten times its own weight of chromium to produce carbides. It has been said that an increase in the silicon content increases the corrosion resistance of the iron this result is probably achieved because the silicon refines the carbides and so aids the development of a more continuous oxide film over the metal surface. It seems likely that the addition of molybdenum has a similar effect, although it is possible that the molybdenum displaces some chromium from combination with the carbon and therefore increases the chromium content of the ferrite. 

Many salts which are corrosive towards unalloyed iron because of their tendency to hydrolyse to release acid, e.g. calcium and zinc chlorides, are not dangerous to high-chromium irons. The more corrosive salts, typified by aluminium sulphate and ferric chloride, are, however, corrosive to high-chromium irons. Hot aluminium sulphate solutions can give corrosion rates greater than 1 27 mm/y although cold solutions corrode the alloys at rates not exceeding 0-127 mm/y. 

Ferric chloride solutions are particularly aggressive to high-chromium irons. Rates of attack greater than 12 mm/y have been recorded for a 25% solution at 20°C. The useful resistance of the alloys to mine waters which contain this salt is probably because the concentration involved is very much lower than this. 

Corrosion-Erosion Resistant High Chromium Alloy Iron ... 

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