An MCDP Staff Report
(Click here to see the story as it appears in the Sept./Oct. issue of Metal Casting Design & Purchasing.)
Because of their combination of properties, castability and cost effectiveness, gray and ductile iron are the dominant metals (based on weight) produced in metalcasting facilities today. In 2012, the U.S. produced 4.48 million metric tons of ductile iron castings and 4.3 million tons of gray iron castings. When sourcing, designing and purchasing ductile and gray iron castings, a number of variables must be considered when choosing between the two. This comparison of gray and ductile iron will help designers and purchasers decide which alloy is suitable for a specific cast component.
Gray iron is the preferred material when seeking low-cost, complex geometries with strength and high density. Gray iron can be used for a wide range of applications, from table legs and bases to large fountains and building facades. Gray iron is particularly good for applications in which vibration damping is required, such as engine blocks and frames for manufacturing equipment. For higher strength applications, ductile iron can be used.
Ductile iron, because of its strength and ductility, often is specified in severe applications. Like gray iron castings, ductile iron is used in a wide range of applications, from pumps, compressors valves and fittings to diesel engine parts and oil field machinery.
Table 1 shows specifications, characteristics and applications for both gray and ductile iron. Table 2 shows the ranges for elements in gray and ductile iron.
Gray Iron
Gray iron derives its properties from flake graphite in its microstructure. Its unique attributes include excellent machinability at hardness levels that produce superior wear-resistant characteristics, the ability to resist galling and excellent vibration damping. When the composition of the molten iron and its cooling rate are suitable, the carbon in the iron separates during solidification and forms interconnected graphite flakes. The graphite grows edgewise into the liquid and forms the characteristic flake shape.
The properties of gray iron also are influenced by the relative hardness of the metal matrix, which is the iron that surrounds the graphite. Microstructural properties are primarily controlled by the carbon and silicon content of the metal and the cooling rate of the casting. The desired properties should be specified rather than the factors that influence them.
The most significant markets for gray iron include powertrain parts for transportation equipment, farm and construction equipment, diesel engine components, pumps, compressors, and valves and fittings. Centrifugally cast pipe is also a major application for gray iron alloys.
The mechanical properties of gray iron are determined by its chemical composition, processing technique in the metalcasting facility and solidification and cooling rates. The amount of graphite present, the length of the flakes and its distribution in the matrix directly influence the properties of iron. Basic strength and hardness are determined by the metallic structure in which the graphite occurs.
Graphite has little strength or hardness, so it decreases these properties of the metallic matrix. However, graphite provides several valuable characteristics to cast iron, including:
The lower strength grades of gray iron can be produced consistently by simply selecting the proper melting stock. Iron castings in the higher strength grades require closer control of their processing and composition. Alloys can be added to gray iron to increase its strength and hardness in the as-cast condition. The metal also can be heat treated to a low hardness to increase machinability, but this decreases its strength.
An important influence on gray iron properties is the effective section thickness in which it is cast. The thicker the metal and the more compact the casting, the slower the liquid metal will solidify and cool in the mold. In gray iron, slower solidification of the casting will produce a lower hardness.
Alternately, iron that is cast with thin sections will solidify more rapidly. A casting with varying section thicknesses exhibits differences in hardness between the thick and thin sections. These casting structure variations produce differences in mechanical properties.
Ductile iron, also referred to as nodular iron or spheroidal graphite iron, was patented in 1948. After a decade of intensive development in the 1950s, ductile iron had a phenomenal increase in use as an engineering material during the 1960s, and the rapid increase in commercial application continues today.
An unusual combination of properties is obtained in ductile iron because the graphite occurs as spheroids rather than as individual flakes as in gray iron. This mode of solidification is obtained by adding a very small but specific amount of magnesium to molten iron. The allowable content of minor elements that can interfere with the graphite spheroid formation in the base iron is severely restricted. The added magnesium reacts with the sulfur and oxygen in the molten iron and changes the way the graphite is formed.
Ductile iron can be a low-cost alternative to wrought steel products because of its elevated strength-to-weight ratio, castability and machinability. Many products that were produced from machined or fabricated steel in the past are cast in ductile iron today. Ductile iron exhibits a linear stress-strain relation, a considerable range of yield strengths and ductility. Castings are made in a wide range of sizes with thin or thick sections.
Ductile iron castings are used in a number of automotive applications, including crankshafts, camshafts, exhaust manifolds, piston rings and cylinder liners. In the earth-moving, mining and agricultural industries, ductile iron castings are used for couplings, hydraulic valves, sprockets wheels, track and backhoe components, structural brackets and rollers. General applications include hydraulic cylinders, machine frames, mill rolls, tunnel segments, rubber molds and mandrels.
The common grades of ductile iron differ primarily due to the matrix structure that contains the spherical graphite. Distinctions between grades are the result of differences in composition, in the cooling rate of the casting or as the result of heat treatment. Minor differences in composition or the addition of alloys may be used to enhance the desired microstructure.
Hardness: Hardness is the most commonly determined property of metal because it is a simple test, and many useful properties are directly related to hardness. Within a class or type of gray iron, hardness is a good indicator of engineering properties, but this relation is not useful between types of gray iron because differences in graphite structure have more of an effect on tensile properties than on hardness.
Specifying the hardness at a designated area on each casting is an excellent method of establishing testing consistency in production. Compression strength correlates well with hardness for all types of iron. Hardness usually gives a good indication of tool life in machining. The hardness of gray iron (shown in Table 3) cannot be compared directly to the hardness of other metals for an indication of properties such as machinability or wear resistance. In a particular size or type of casting, gray iron also can be designated by its Brinell hardness.
Because of the minimum influence of ductile iron’s spheroidal graphite on mechanical properties, hardness is a useful test for ductile iron and can be directly related to other properties. The relationship between tensile properties and hardness is reliable with a typical microstructure and chemical analysis.
Tensile Properties: Gray iron’s tensile strength, yield strength, ductility and modulus of elasticity can be established by a conventional test, as specified by the standard ASTM A-48. Although yield strength and ductility may be measured, they are seldom determined or specified. Its modulus of elasticity is not constant, as in the case of steel, and varies with the class of iron and type of graphite loading.
The tensile strength of gray iron is influenced by both the normal elements present in plain iron, such as carbon, silicon, phosphorus, sulfur and manganese, and the presence of alloying additions and trace elements.
Ductile iron’s commonly established tensile properties are tensile strength, yield strength and percent elongation. The minimums for these properties typically are established by the specification or implied by specifying the hardness of the casting. Because of the nominal and consistent influence of spheroidal graphite, the tensile properties and the Brinell hardness of ductile iron are well correlated. The relationship between tensile properties and hardness depends on microstructure.
Ductile iron combines the processing advantages of gray iron (low melting point, good fluidity and castability, and ready machinability) with many of the engineering advantages of steel (high strength, ductility and wear resistance), which allows for higher material properties as tensile and yield strength than gray iron. Table 4 shows the properties for various grades of ductile iron.
Damping Capacity: The relative ability of a material to absorb vibration is evaluated as its damping capacity. The quelling of vibration by converting the mechanical energy into heat can be important in structures and devices with moving parts. Components made of materials with a high damping capacity can reduce noise such as chatter, ringing and squealing, and minimize the level of applied stresses. Because it can cause unsatisfactory operation or even failure, vibration can be a critical factor in the operation of machinery.
Gray iron exhibits an exceptionally high damping capacity. For this reason, it is ideally suited for machine bases and supports, engine cylinder blocks and braking components. The damping capacity of gray iron is considerably greater than that of steel or other kinds of iron. The graphite flakes in gray iron have less influence on its compression properties than on tension properties. Gray iron’s compressive strength is typically three to four times more than its tensile strength.
The damping capacity of ductile iron is well below that of gray iron. (See Table 5.) For example, if gray iron and ductile iron have a similar composition, the relative damping capacity of gray iron is 1.0, while ductile iron is 0.14. Still, ductile iron compares favorably versus steel alloys in terms of damping capacity, especially in automotive applications such as shaft gears where this increased capacity can reduce noise.
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