Metallurgy of Gray Iron
MacKenzie in his l944 Howe Memorial Lecture referred to cast iron as "steel plus graphite." Although this simple definition still applies, the properties of gray iron are affected by the amount of graphite present as well as the shape, size, and distribution of the graphite flakes. Although the matrix resembles steel, the silicon content is generally higher than for cast steels, and the higher silicon content together with cooling rate influences the amount of carbon in the matrix. Gray iron belongs to a family of high-carbon silicon alloys which include malleable and nodular irons. With the exception of magnesium or other nodularizing elements in nodular iron, it is possible through variations in melting and foundry practice to produce all three materials from the same composition. In spite of the widespread use of gray iron, the metallurgy of it is not clearly understood by many users and even producers of the material. One of the first and most complete discussions of the mechanism of solidification of cast irons was presented in 1946 by Boyles[2]. Detailed discussions of the metallurgy of gray iron may be found in readily available handbooks[3-7]. Iron Casting The most recent review of cast iron metallurgy and the formation of graphite is one by Wieser et al[8]. To avoid unnecessary duplication of information, only the more essential features of the metallurgy of gray iron will be discussed here.
Composition
Ductile Iron Casting 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. For these reasons, inclusion of chemical composition in purchase specifications for castings should be avoided unless essential to the application. The range of compositions which one may find in gray 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.
Grey Iron Casting 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 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 is possible to produce all grades of iron of ASTM Specification for Gray Iron Castings (A 48-64) by merely adjusting the carbon and silicon content of the iron. It would be impossible to produce gray 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 iron, the combined effect of these elements in the form of percent carbon plus 1/s percent silicon is termed carbon equivalent (CE). Gray irons having a carbon equivalent value of less than 4.3 percent are designated hypoeutectic irons, and those with more than 4.3 percent carbon equivalent are called hypereutectic irons. For hypoeutectic irons in the automotive and allied industries, each 0.10 percent increase in carbon equivalent value decreases the tensile strength by about 2700 psi.
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. Silicon has other effects than changing the carbon content of the eutectic. Increasing the silicon content decreases the carbon content of the pearlite and raises the transformation temperature of ferrite plus pearlite to austenite. This influence of silicon on the critical ranges has been discussed by Rehder[9].
The most common range for manganese in gray 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 iron is present as manganese sulfide, and the manganese necessary for this purpose is 1.7 times the sulfur content. Manganese is often raised beyond 1.00 percent, but in some types of green sand castings pinholes may be encountered.
Sulfur is seldom intentionally added to gray iron and usually comes from the coke in the cupola melting process. Up to 0.15 percent, sulfur tends to promote the formation of Type A 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. Collaud and Thieme report that, if the sulfur is decreased to a very low value together with low phosphorus and silicon, tougher irons will result and have been designated as "TG," or tough graphite irons.
The phosphorus content of most high-production gray 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. Phosphorus generally occurs as an iron iron-phosphide eutectic, although in some of the higher- carbon irons, the ternary eutectic of iron iron-phosphide iron-carbide may form. This eutectic will be found in the eutectic cell boundaries, and 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 iron. An austenitic gray 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 as shown in ASTM Specification for Austenitic Gray Iron Castings (A 436-63).
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 iron, and for this purpose the chromium level is raised to 0.20 to 0.35 percent. Beyond this range, it is necessary to add a graphitizer to avoid the formation of carbides and hard edges. Chromium improves the elevated temperature properties of gray 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 iron. Since the modulus of elasticity of molybdenum is quite high, molybdenum additions to gray iron increase its modulus of elasticity.
