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Gray iron is one of the oldest cast ferrous products. In spite of competition from newer materials and their energetic promotion, gray iron is still used for those applications where its properties have proved it to be the most suitable material available. Next to wrought steel, gray iron is the most widely used metallic material for engineering purposes. For 1967, production of gray iron castings was over 14 million tons, or about two and one-half times the volume of all other types of castings combined. There are several reasons for its popularity and widespread use. It has a number of desirable characteristics not possessed by any other metal and yet is among the cheapest of ferrous materials available to the engineer. Gray iron castings are readily available in nearly all industrial areas and can be produced in foundries representing comparatively modest investments.
Gray iron castings can be produced by virtually any well-known foundry process. Surprisingly enough, in spite of gray iron being an old material and widely used in engineering construction, the metallurgy of the material has not been clearly understood until comparatively recent times. The mechanical properties of gray iron are not only determined by composition but also greatly influenced by foundry practice, particularly cooling rate in the casting. All of the carbon in gray iron, other than that combined with iron to form pearlite in the matrix, is present as graphite in the form of flakes of varying size and shape. It is the presence of these flakes formed on solidification which characterize gray iron. The presence of these flakes also imparts most of the desirable properties to gray iron.

 
  Casting Processes  
 

Several molding processes are used to produce gray iron castings. Some of these have a marked influence on the structure and properties of the resulting casting. The selection of a particular process depends on a number of factors, and the design of the casting has much to do with it. The processes using sand as the mold media have a somewhat similar effect on the rate of solidification of the casting, while the permanent mold process has a very marked effect on structure and properties.

Green sand molding is frequently the most economical method of producing castings. Until the introduction of high-pressure molding and very rigid flask equipment, dimensional accuracy has not been as good as can be obtained from shell molding. If green sand molds are not sufficiently hard or strong, some mold wall movement may take place during solidification, and shrinkage defects develop. Although castings up to 1000 lb or more can be made in green sand, it generally is used for medium to small size castings. For the larger castings, the mold surfaces are sometimes sprayed with a blacking mix and skin dried to produce a cleaner surface on the casting. This procedure is often used on engine blocks.

To withstand the higher ferrostatic pressures developed in pouring larger castings; dry sand molds are often used. In some cases, the same sand as used for green sand molding is employed, although it is common practice to add another binder to increase the dry strength.

The shell molding process is also used for making cores which are used in other types of molds besides shell molds. Its principal advantage is derived from the ability to harden the mold or core in contact with a heated metal pattern, thus improving the accuracy with which a core or mold can be made. In addition to the improved accuracy, a much cleaner casting is produced than by any other high-production process. Although the techniques and binders for hot box and the newest cold box processes differ from those used for the shell molding process, the principle is similar in that the core is hardened while in contact with the pattern.

Centrifugal casting of iron in water-cooled metal molds is widely used by the cast iron pipe industry as well as for some other applications. With sand or other refractory lining of the metal molds, the process is used for making large cylinder liners.

For some types of castings, the permanent mold process is a very satisfactory one, and its capabilities have been described by Frye[16]. Since the cooling or freezing rate of iron cast into permanent molds is quite high, the thinner sections of the casting will have cementite. To remove the cementite the castings must be annealed, and it is universal practice to anneal all castings. The most economical composition of the iron for permanent mold castings is hypereutectic. This type of iron expands on solidification, and, because the molds are very rigid, the pressure developed by separation of the graphite during freezing of the eutectic ensures a pressure tight casting. Since the graphite occurs predominantly as Type D with very small flakes, permanent mold castings are capable of taking a very fine finish. For this reason, it finds extensive use in making valve plates for refrigeration compressors. The process is also ideally suited for such components as automotive brake cylinders and hydraulic valve bodies. Although the predominantly Type D graphite structure in permanent mold castings with a matrix of ferrite have much higher strength than sand castings of comparable graphite content, the structure is not considered ideal for applications with borderline lubrication. The castings perform very well, however, when operating in an oil bath.

Unless some special properties are desired and are obtained only with a particular casting process, the one generally selected yields castings at the lowest cost for the finished part.

 
  Mechanical Properties of Gray Iron  
 

Properties of principal interest to the designer and user of castings are: resistance to wear; hardness; strength; and, in many cases, modulus of elasticity. Some of the relationships between these properties are quite different for gray iron as compared with steel. The variable relation between hardness and tensile strength in gray iron appears to confuse the engineer when most of his experience may have been with other metals.

The excellent performance of gray iron in applications involving sliding surfaces, such as machine tool ways, cylinder bores, and piston rings, is well known. The performance in internal combustion engines and machine tools is remarkable when one considers the ease of machining gray iron. Gray iron is also known for its resistance to galling and seizing. Many explanations have been given for this behavior, such as the lubricating effect of the graphite flakes and retention of oil in the graphite areas. This is very likely true, but it is also possible that the graphite flakes allow some minor accommodation of the pearlite matrix at areas of contact between mating surfaces. It is seldom possible to obtain perfect fits, and, ordinarily, high spots in mating metal surfaces may result in high unit pressures causing seizing.

The Brinell hardness test is the one most frequently used for gray iron, and, whenever possible, the 10-mm ball and 3000-kg load is preferred. If the section thickness or area to be tested will not withstand the 3000-kg load, a 1500-kg load is frequently used. The hardness values obtained with the lower load may differ appreciably from those obtained with the higher load, and this possibility is pointed out in ASTM Test for Brinell Hardness of Metallic Materials (E 10-66). For gray iron, the difference in hardness values may be as great as 35 BHN, and, if a difference exists, it is always lower for the lower load. Since in most cases the Brinell hardness test can be considered a nondestructive test, Brinell hardness is used as an indication of machinability, resistance to wear, and tensile strength. For light sections, such as piston rings and other light castings having a small graphite size, the Rockwell hardness test is often satisfactory.

The Brinell hardness test is actually a specialized compression test and measures the combined effect of matrix hardness, graphite configuration, and volume of graphite. The Brinell hardness of gray iron with an entirely pearlitic matrix may vary from as low as 148 to over 277 depending on the fineness of the pearlite and to a greater extent on the volume of graphite present. Over this range of hardness, the actual hardness of the pearlite may vary from about 241 to over 400 Knoop hardness as determined by microhardness measurements.

Virtually, all specifications and standards for gray iron classify it by tensile strength. The tensile strength of gray iron for a given cooling rate or section size is very much dependent on the amount of graphite in the iron. The carbon equivalent value for the iron will give a close approximation to the amount of graphite present. The tensile strength is also very much influenced by cooling rate, particularly through the eutectic solidification interval, and is generally related to section size. In recognition of the effect of section size on strength, ASTM Specification A 48 not only classifies the iron by strength but also requires selection of the size of the test bar in which the strength is to be obtained.

 
  Heat Treatment of Gray Iron  
 

Although the majority of gray iron castings are used in the as-cast condition, gray iron is heat treated for a variety of reasons, such as to relieve residual stresses, improve machinability, increase the hardness of the surface either through induction or flame hardening, or harden the entire section through an oil quench and draw treatment. Recommended practice for such heat treatments and the results obtained will be found in handbooks dealing with cast iron, particularly, ASM Metals Handbook[22]. The graphite structure cannot be changed by heat treatment. although the graphite may increase in volume if a pearlitic iron is completely ferritized, in which case, the graphite is usually deposited on the flakes originally present. The matrix however is quite responsive to heat treatment just as in the case of steel.

 
  Machining of Gray Iron  
 

Of the widely used ferrous materials for construction purposes, gray iron for a given hardness level is one of the most readily machinable. Gray iron is free cutting in that the chips are small and easily removed from the cutting area. Furthermore, there is little difficulty with the chips marring the finished surface. The free cutting behavior is a result of the randomly distributed graphite flakes which interrupt the continuity of the matrix. Although gray iron is very successfully machined without coolants, they may be found necessary if high machining rates and close tolerances are desired. The coolant not only helps in chip removal but also controls the temperature of the casting, which is necessary for close tolerance work.

 
 
GRADE PROPERTIES APPLICATIONS

GREY CAST IRON

 

FC300

Having a dense, fine grain pearlitic matrix structure, this material possesses exceptionally high physical properties and may be surface harden by induction heat treatment. It is recommended for heavy casting with soundness and solidity. Crankshafts for compressor, forging dies, drawing tools and marine cyliner heads. Heavy machine-tool beds, presses and frames, saddles and racks, chucks. Heavy gear wheels and high-pressure valve bodies
FC250 This is a general utility material combining tensile, toughness, impact, self-lubricating and wear resisting properties togther with high damping capacity. It is excellent for engineering castings with high mechanical properties required. Machine- tool beds, head stocks, tables, press frames, bed plates. Flywheel, diesel engine parts, pulleys, hydraulic housings, and valves.
FC150-200 This material is designed principally fit high mach inability and damping capacity with normal tensile strength. It is very suitable for general applications. Machine- tool beds, head stocks, tables, press frames, bed plates. Flywheel, diesel engine parts, pulleys, hydraulic housings, and valves.
 
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