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Class Notes From Rajan Mital

Cast Iron
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ABSTRACT:
Gray Cast Iron is the most versatile of all foundry metals. The high carbon content is responsible for ease of melting and casting in the foundry and for ease of machining in subsequent manufacturing. The low degree or absence of shrinkage and high fluidity provide maximum freedom of design for the engineer. By suitable adjustment in composition and selection of casting method, tensile strength can be varied from less than 20,000 psi to over 60,000 psi and hardness from 100 to 300 BHN in the as-cast condition. By subsequent heat treatment, the hardness can be increased further.
If the service life of a Gray Cast Iron part is considered to be too short, the design of the casting should be carefully reviewed before specifying a higher strength and hardness grade of iron. An unnecessary increase in strength and hardness may increase the cost of the casting as well as increase the cost of machining through lower machining rates.

Detailed notes :
Gray Cast Iron is one of the oldest cast ferrous products. In spite of competition from newer materials and their energetic promotion, Gray Cast 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 Cast Iron is the most widely used metallic material for engineering purposes. Gray Cast Iron is one of the most easily cast of all metals in the foundry. It has the lowest pouring temperature of the ferrous metals, which is reflected in its high fluidity and its ability to be cast into intricate shapes. As a result of a peculiarity during final stages of solidification, it has very low and, in some cases, no liquid to solid shrinkage so that sound castings are readily obtainable. For the majority of applications, Gray Cast Iron is used in its as-cast condition, thus simplifying production. Gray Cast Iron has excellent machining qualities producing easily disposed of chips and yielding a surface with excellent wear characteristics. Gray Cast Iron castings can be produced by virtually any well-known foundry process. Surprisingly enough, in spite of Gray Cast 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 Cast 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 Cast 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 Cast Iron. The presence of these flakes also imparts most of the desirable properties to Gray Cast Iron.

Composition:
Gray Cast Iron is commercially produced over a wide range of compositions. Foundries meeting the same specifications may use different compositions to take advantage of lower cost raw materials locally available and the general nature of the type of castings produced in the foundry. The range of compositions which one may find in Gray Cast Iron castings is as follows: total carbon, 2.75 to 4.00 percent; silicon, 0.75 to 3.00 percent; manganese, 0.25 to 1.50 percent; sulfur, 0.02 to 0.20 percent; phosphorus, 0.02 to 0.75 percent. One or more of the following alloying elements may be present in varying amounts: molybdenum, copper, nickel, vanadium, titanium, tin, antimony, and chromium. Nitrogen is generally present in the range of 20 to 92 ppm.
The concentration of some elements may exceed the limits shown above, but generally the ranges are less than shown.
Carbon is by far the most important element in Gray Cast Iron. With the exception of the carbon in the pearlite of the matrix, the carbon is present as graphite. The graphite is present in flake form and as such greatly reduces the tensile strength of the matrix. It would be impossible to produce Gray Cast Iron without an appropriate amount of silicon being present. The addition of silicon reduces the solubility of carbon in iron and also decreases the carbon content of the eutectic. The eutectic of iron and carbon is about 4.3 percent. The addition of each 1.00 percent silicon reduces the amount of carbon in the eutectic by 0.33 percent. Since carbon and silicon are the two principal elements in Gray Cast Iron, the combined effect of these elements in the form of percent carbon plus 1/s percent silicon is termed carbon equivalent (CE). If the cooling or solidification rate is too great for the carbon equivalent value selected. the iron may freeze in the iron-iron carbide metastable system rather than the stable iron-graphite system, which results in hard or chilled edges on castings. The carbon equivalent value may be varied by changing either or both the carbon and silicon content. Increasing the silicon content has a greater effect on reduction of hard edges than increasing the carbon content to the same carbon equivalent value. The most common range for manganese in Gray Cast Iron is from 0.55 to 0.75 percent. Increasing the manganese content tends to promote the formation of pearlite while cooling through the critical range. It is necessary to recognize that only that portion of the manganese not combined with sulfur is effective. Virtually, all of the sulfur in Gray Cast Iron is present as manganese sulfide, and the manganese necessary for this purpose is l.7 times the sulfur content. Manganese is often raised beyond 1.00 percent, but in some types of green sand castings blowholes may be encountered.
Sulfur is seldom intentionally added to Gray Cast Iron and usually comes from the coke in the cupola melting process. Up to 0.15 percent, sulfur tends to promote the formation of graphite. Somewhere beyond about 0.17 percent, sulfur may lead to the formation of blowholes in green sand castings. The majority of foundries maintain sulfur content below 0.15 percent with 0.09 to 0.12 percent being a common range for cupola melted irons. If the sulfur is decreased to a very low value together with low phosphorus and silicon, tougher irons will result.
The phosphorus content of most high-production Gray Cast Iron castings is less than 0.15 percent with the current trend toward more steel in the furnace charge; phosphorus contents below 0.10 percent are common. Beyond 0.20 percent phosphorus a decrease in machinability may be encountered. Phosphorus contents over 0.10 percent are undesirable in the lower-carbon equivalent irons used for engine heads and blocks and other applications requiring pressure tightness. For increased resistance to wear, phosphorus is often increased to 0.50 percent and above as in automotive piston rings. At this level, phosphorus also improves the fluidity of the iron and increases the stiffness of the final casting.
Copper and nickel behave in a similar manner in cast iron. They strengthen the matrix and decrease the tendency to form hard edges on castings. Since they are mild graphitizers, they are often substituted for some of the silicon in Gray Cast Iron. An austenitic Gray Cast Iron may be obtained by raising the nickel content to about 15 percent together with about 6 percent copper, or to 20 percent without copper.
Chromium is generally present in amounts below 0.10 percent as a residual element carried over from the charge materials. Chromium is often added to improve hardness and strength of Gray Cast Iron, and for this purpose the chromium level is raised to 0.20 to 0.35 percent. Chromium also improves the elevated temperature properties of Gray Cast Iron.
One of the most widely used alloying elements for the purpose of increasing the strength is molybdenum. It is added in amounts of 0.20 to 0.75 percent, although the most common range is 0.35 to 0.55 percent. Best results are obtained when the phosphorus content is below 0.10 percent, since molybdenum forms a complex eutectic with phosphorus and thus reduces its alloying effect. Molybdenum is widely used for improving the elevated temperature properties of Gray Cast Iron. Since the modulus of elasticity of molybdenum is quite high, molybdenum additions to Gray Cast Iron increase its modulus of elasticity.
Vanadium has an effect on Gray Cast Iron similar to molybdenum, but the concentration must be limited to less than 0.15 percent if carbides are to be avoided. Even in such small amounts, vanadium has a beneficial effect on the elevated temperature properties of Gray Cast Iron.
There are beneficial effects of relatively small additions of tin (less than 0.10 percent) on the stability of pearlite in Gray Cast Iron. There was extensive use of tin in automotive engines. Its use is particularly helpful in complex castings wherein some sections cool rather slowly. It has been found that additions of up to 0.05 percent antimony have a similar effect. In larger amounts, these elements tend to reduce the toughness and impact strength of Gray Cast Iron. Normally, nitrogen is not considered as an alloying element and generally occurs in Gray Cast Iron as a result of having been in the charge materials. At higher nitrogen levels the graphite flakes become shorter and the strength of the iron is improved.

Mechanical Properties of Gray Cast 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 Cast Iron as compared with steel. The variable relation between hardness and tensile strength in Gray Cast Iron appears to confuse the engineer when most of his experience may have been with other metals.
The excellent performance of Gray Cast 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 Cast Iron. Gray Cast 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.
The Brinell hardness test is the one most frequently used for Gray Cast Iron, and, whenever possible, the 10-mm ball and 3000-kg load is preferred. 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.
Virtually, all specifications and standards for Gray Cast Iron classify it by tensile strength. The tensile strength of Gray Cast 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.

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