Argentum Solutions, Inc.
Sterling guidance on corrosion and materials degradation
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David C. Silverman |
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The term cast iron identifies a large class of ferrous alloys that are intended to be cast to shape from the liquid state rather than formed in the solid state. These alloys tend to have low melting temperatures, are very fluid when molten, do not form surface films when poured, and undergo lesser amounts of shrinkage during solidification and cooling. They tend to have relatively low impact resistance and ductility. Cast irons are usually alloys of iron that contain more than 2 wt% carbon and 1 to 3 wt% silicon. Wide variations in properties can be achieved by varying the balance between carbon and silicon, by alloying with various metallic or non-metallic elements, and by varying the heat treating process. Cast irons vary greatly in machinability. There are four main classes of cast iron, white cast iron, gray cast iron, ductile cast iron, and malleable cast iron. Within these classes are divisions determined by microstructure. For example, there are ferritic gray irons, pearlitic gray irons, ferritic ductile irons, and ferritic-pearlitic ductile irons. From a corrosion standpoint, internal galvanic cells can sometimes be established between the carbon-rich and carbon poor areas of the alloys. Such cells when formed can adversely affect corrosion resistance and lead to selective leaching of iron. Machinability ranges from white cast irons which are usually extremely difficult (or impossible) to machine to malleable cast iron which tends to be among the easiest metals to machine. There is some dependence of machinability on the microstructure subclass within each cast iron class. Cast iron can often be satisfactorily cut without using a cutting fluid. A cutting fluid is commonly used because such usage can enable the achieving of higher metal removal rates. The following discussion is divided according to the type of cast iron. White Cast Iron White cast iron is formed when the carbon in solution in the molten iron does not form graphite upon solidification. The carbon remains combined with iron, often as massive carbides. The result is a hard and brittle material that produces white crystalline fracture surfaces. White cast irons containing carbide stabilizing elements such as chromium, molybdenum, and vanadium may have hardness values as high as a Brinell hardness of 700 (Rockwell C hardness of 62.5). White cast irons are usually not discussed in terms of machining because of their brittleness. Gray Cast Iron When the composition of the iron alloy and the cooling rate of solidification are appropriate, a substantial portion of the carbon content separates out of the liquid to form flakes of graphite. When a piece of the solidified alloy is broken, the fracture appears gray because of the exposed graphite. The flake graphite in the microstructure gives gray cast iron a number of unique properties. The alloy can be machined easily at a hardness conducive to good wear resistance. It resists galling under conditions in which the film of lubricant is insufficient to maintain a complete liquid film. Gray cast irons are often referred to in terms of class 20 to 60. This designation is an ASTM designation and refers to differences in mechanical properties. Grade 20 is ferritic gray cast iron, Grade 25 is pearlitic-ferritic gray cast iron, and Grade 30 and above is pearlitic gray cast iron. Gray cast irons have compositions of 3.25 to 3.5 wt% carbon, 0.6 to 0.9 wt% manganese, 1.8 to 2.3 wt% silicon, less than 0.12 wt% phosphorus, and less than 0.12 wt% sulfur. Carbon levels tend to decrease slightly with increasing number. There is some variation depending on application requirements. The machinability of most gray cast iron tends to be superior to other types of cast iron. The graphite serves as a lubricant for the cutting tool and to introduce discontinuities that serve as chip breakers. Ductile Cast Iron Ductile iron which may also be called nodular cast iron or spherulitic-graphite cast iron is similar to gray cast iron in composition. During casting of ductile cast iron, the graphite is forced to nucleate as spherical particles instead of as flakes. This nucleation is caused by the addition of small amounts of magnesium, sodium, or cerium to the molten iron. Ductile iron combines high strength with ductility. Machinability and corrosion resistance are about the same as for gray cast iron. Heat treatment is used to enhance properties. In ductile iron, the tiny balls of graphite serve as chip breakers and lubrication, similar to their role in gray cast iron. Classification is in terms of mechanical properties and can appear as an ASTM specification. The composition of ductile iron tends to be similar to that of gray cast iron, about 3.5 to 3.8 wt% carbon, 2.0 to 2.8 wt% silicon, 0.3 to 1 wt% manganese, less than 0.08 wt% phosphorus, less than 0.02 wt% sulfur, and, additionally, 0.03 to 0.05 wt% magnesium. There is some variation depending on application requirements. Malleable Cast Iron Malleable cast iron is a cast ferrous metal that is initially produced as white cast iron. The iron is reheated to convert the carbon-containing phase from iron carbide to a nodular form of graphite called temper carbon in addition to ferrite. The temper carbon tends to form small compact particles. Ferritic malleable cast iron has a ferrite structure into which are embedded the fine particles of temper carbon. A rule of thumb is that machinability of ferritic malleable cast iron is 25% better than that of free-cutting steel (e.g. leaded steel). This number is not an absolute but provides a sense of relative ease of machining. Actual data on machining are hard to find because machining is seldom a problem. Pearlitic or decarburized structures just below the surface can adversely affect machining. The surface can be hardened to Rockwell C values above 60. The composition of malleable iron is about 2.3 to 2.59 wt% carbon, 0.31 to 0.74 wt% manganese, and 0.97 to 1.45 wt% silicon. Grades tend to be specified in terms of mechanical properties via ASTM, ANSI, or MIL specifications. ALLOY CAST STEELS Alloy cast steels are considered to be those casting alloys based on the iron-carbon-silicon system that contain one or more alloying elements added to enhance properties. Usually, the alloying elements only affect machinability through their effect on the relative stability of the iron, iron-carbide, and graphite constituents in the microstructure. Two exceptions are additions of phosphorus and tin because they directly interact with the iron matrix forming new compounds. When phosphorus is present at levels above 0.15 to 0.20 wt%, iron phosphide (steadite) can form. This intermetallic material is extremely hard and abrasion resistant. Its presence may reduce tool life relative to that normally expected for the cast iron unless special precautions are taken to alter the iron matrix in contact with the steadite from pearlite to ferrite. The presence of tin (0.05 to 0.15 wt%) has the opposite effect. Its presence tends to increase tool life by reducing hardness. | |||
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David C. Silverman, Ph.D. - Primary Consultant |