White cast irons, wear

White cast iron is brittle and difficult to machine. It is made by controlling the composition and rate of solidification of the molten iron so that all the carbon is present in the combined form. Very abrasive- and wear-resistant, white cast iron is used as liners and for grinding balls, dies, and pump impellers. 

Brittle materials have an additional mode of abrasive wear, namely, microcracking or microfracture. This occurs when forces applied by the abrasive grain exceed the fracture toughness of the material. This is often the predominant mode of severe wear for ceramic materials, and is active in materials such as white cast irons. 

Prevention of abrasive wear is possible through proper material selection and the use of surface engineering treatments. A number of material families have demonstrated good resistance to abrasive wear. They are typically hard materials that resist scratching, and include ceramics, carbide materials, alloyed white cast irons containing hard chromium carbides (see Fig. 8), and hardened alloy steels. 

Wear plates, for example, white cast iron or manganese steels... 

White cast iron Iron carbide compound. By reducing the carbon and silicon content and cooling rapidly, much of the carbon is retained in the form of iron carbide without graphite flakes. However, iron carbide, or cementite, is extremely hard and brittle, and these castings are used where high hardness and wear resistance are needed. 

Low-alloy white cast iron. Low-alloy white cast iron has improved toughness and wear resistance. The main limitation is that a better performance or a longer life must justify its extra cost. 

Figure 14-5. Influence of contact pressure at various rubbing speeds on the wear rate of hardened alloy cast iron lubricated with white oil. Unpublished data by A. Dorinson. 

Ferritic nitrocarburizing is a subcritical heat treatment process, carried out by either gaseous or plasma techniques, and involves the diffusion of carbon and nitrogen into the ferritic phase. The process results in the formation of a thin white layer or compound layer, with an underlying diffusion zone of dissolved nitrogen in iron, or alloy nitrides (Ref 17). The white layer improves surface resistance to wear, and the diffusion zone increases the fatigue endurance limit, especially in carbon and low-alloy steels. Alloy steels, cast irons, and some stainless steels can be treated. The process is used to produce a thin, hard skin, usually less than 25 xm (1 mil) thick, on low-carbon steels in the form of sheet metal parts, powder metallurgy parts, small shaft sprockets, and so forth. 

Bimetallic cylinder n. In most modern extruders and injection machines, the barrel is lined, by centrifugal casting from the melt, with any of several white irons containing chromium and boron carbides and having hardnesses near Rockwell C65. Aster finish-grinding and polishing, the liner, about 1 mm thick, provides excellent resistance to wear or corrosion or both, depending on the formation. The best known trade name is XALOY . 

A large number of materials are available for the screw and barrel. Most extruder barrels in the U. S. have a liner, which is centrifugally cast into the barrel. The barrel liner is made of a wear-resistant material, often boron-stabilized white irons with a Rockwell C hardness of about 65 containing iron chromium boron carbides. Bimetallic barrels provide better wear resistance than nitrided barrels as reported, for... 

Corrosion can have very detrimental effects on the wear resistance of certain white irons. If the matrix wears out and exposes the carbides to a corrosive environment, they may quickly deteriorate, causing a further loss of wear resistance. Manufacturers of slurry pumps have gone beyond the specifications of ASTM A532-87 to increase the chromium contents to 30% with 50% chromium carbides in a martensitic matrix. These materials have found applications in phosphate matrix and phosphoric acid pumping. These new alloys are intermediary between white irons and superalloys. Walker (1990) reported that the use of these alloys leads to a substantial increase of the wear life over conventional white iron castings (Ni-hard, 27% high chrome). 

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