Cast Iron

Cast irons may often be used in place of steel at considerable cost savings. The design and production advantages of cast iron include:

Low tooling and production cost

Good machinability without burring

Ability to cast into complex shapes

Excellent wear resistance and high hardness (particularly white cats irons)

High inherent damping capabilities

The properties of the cast iron are affected by the following factors:

Chemical composition of the iron

Rate of cooling of the casting in the mold (which depends on the section thickness in the casting)

Type of graphite formed (if any)

Types of Cast Iron:

Major types of cast iron are shown in Figure 1.

Figure 1. Types of Cast Iron

Gray Cast Iron:

Gray cast iron is by far the oldest and most common form of cast iron. As a result, it is assumed by many to be the only form of cast iron and the terms "cast iron" and "gray iron" are used interchangeably. Unfortunately the only commonly known property of gray iron- brittleness- is also assigned to "cast iron" and hence to all cast irons. Gray iron, named because its fracture has a gray appearance. It contains carbon in the form of flake graphite in a matrix which consists of ferrite, pearlite or a mixture of the two. The fluidity of liquid gray iron, and its expansion during solidification due to the formation of graphite, have made this metal ideal for the economical production of shrinkage-free, intricate castings such as motor blocks.

The flake-like shape of graphite in Gray iron, see Figures 2 and 3, exerts a dominant influence on its mechanical properties. The graphite flakes act as stress raisers which may prematurely cause localized plastic flow at low stresses, and initiate fracture in the matrix at higher stresses. As a result, Gray iron exhibits no elastic behavior but excellent damping characteristics, and fails in tension without significant plastic deformation. The presence of graphite flakes also gives Gray Iron excellent machinability and self-lubricating properties.

Figure 2. Graphite Flakes in Gray Cast iron

Figure 3. Photomicrograph of Gray Cast iron

Advantages of Gray Cast Iron:

Graphite acts a s a chip breaker and a tool lubricant.

Very high damping capacity.

Good dry bearing qualities due to graphite.

After formation of protective scales, it resists corrosion in many common engineering environments.

Disadvantages:

Brittle (low impact strength) which severely limits use for critical applications.

Graphite acts as a void and reduces strength. Maximum recommended design stress is 1/4 of the ultimate tensile strength. Maximum fatigue loading limit is 1/3 of fatigue strength.

Changes in section size will cause variations in machining characteristics due to variation in microstructure.

Higher strength gray cast irons are more expensive to produce.

Low Alloy Gray Cast Iron:

Enables gray cast iron to be used in higher duty applications without redesign or need for costly materials.

Advantages:

Reduction in section sensitivity.

Improvement in strength, corrosion resistance, heat and wear resistance or combination of these properties.

Disadvantages:

Higher cost.

Alloy additions can cause foundry problems with reuse of scrap (runners, risers, etc) and interrupt normal production.

Increase in strength does not bring corresponding increase in fatigue strength.

Cr, Mo and V are carbide stabilizers which improve strength and heat resistance but impair machinability.

White Cast Iron:

White cast iron is unique in that it is the only member of the cast iron family in which carbon is present only as carbide. Due to the absence of graphite, it has a light appearance. The presence of different carbides, depending on the alloy content, makes white cast irons extremely hard and abrasion resistant but very brittle. An improved form of white cast iron is the chilled cast iron.

Figure 4a. Photomicrograph of White Cast Iron

Chilled Cast Iron:

When localized area of a gray cast iron is cooled very rapidly from the melt, cast iron is formed at the place that has been cooled. This type of white cast iron is called chilled iron. A chilled iron casting can be produced by adjusting the carbon composition of the white cast iron so that the normal cooling rate at the surface is just fast enough to produce white cast iron while the slower cooling rate below the surface will produce gray iron. The depth of chill decreases and the hardness of the chilled zone increases with increasing carbon content.

Chromium is used in small amounts to control chill depth. Because of the formation of chromium carbides, chromium is used in amount of 1 to 4 percent in chilled iron to increase hardness and improve abrasion resistance. It also stabilizes carbide and suppresses the formation of graphite in heavy sections. When added in amounts of 12 to 35 percent, chromium will impart resistance to corrosion and oxidation at elevated temperatures.

Figure 4b. Photomicrograph of Chilled Cast Iron

Fast cooling prevents graphite and pearlite formation. If alloys such as nickel, chromium, or molybdenum are added, much of the austenite transforms to martensite instead of pearlite. The hardness of chilled cast iron is generally due to the formation of martensite.

Chilled cast iron is used for railway-car wheels, crushing rolls, stamp shoes and dies, and many heavy-duty machinery parts.

Ductile Cast Iron (Nodular Cast Iron):

This structure is developed from the melt. The carbon forms into spheres when cerium, magnesium, sodium, or other elements are added to a melt of iron with a very low sulfur content that will inhibit carbon from forming. The control of the heat-treating process can yield pearlitic, ferritic, martensitic matrices into which the carbon spheres are embedded.

Figure 5. Nodular (Ductile) Cast Iron and the spherical carbon embedded into the matrix.

Figure 6. Photomicrograph of Nodular Cast iron

The advantages of ductile cast iron which have led to its success are numerous, but they can be summarized easily-versatility and high performance at low cost. Other members of the ferrous casting family may have superior individual properties which might make them the material of choice in some applications, but none have the versatility of ductile cast iron, which often provides the designer with the best combination of overall properties. This is especially evident in the area of mechanical properties where ductile cast iron offers the designer the option of selecting high ductility, with grades guaranteeing more than 18% elongation (as high as 25 %), or high strength, with tensile strengths exceeding 120 Ksi. Austempered ductile iron offers even greater mechanical and wear resistance, providing tensile strengths exceeding 230 Ksi.

In addition to cost advantages offered by all castings, ductile cast iron, when compared to steel and malleable cast iron, also offers further cost savings. Like most commercial cast metal, steel and malleable cast iron decrease in volume during solidification, and as a result, require feeders and risers to offset the shrinkage and prevent the formation of internal or external shrinkage defects. Ductile cast iron offers significantly low shrinkage during casting. In the case of large castings produced in rigid molds, it does not require feeders. In other cases, it requires feeders that are much smaller than those used for malleable cast iron and steel. This reduced requirement for feed metal increases the productivity of ductile cast iron and reduces its material and energy requirements, resulting in substantial cost savings. The use of the most common grades of ductile cast iron "as-cast" eliminates heat treatment costs, offering a further advantage.

Ductile cast iron is used for many structural applications, particularly those requiring strength and toughness combined with good machinability and low cost. The automotive and agricultural industries are the major users of ductile iron castings. Because of economic advantage and high reliability, ductile iron is used for such critical automotive parts as crankshafts, engine connecting rods, idler arms, wheel hubs, truck axles, front wheel spindle supports, disk brake calipers, suspension system parts, power transmission yokes, high temperature applications for turbo housing and manifolds, and high security valves for many applications. The cast iron pipe industry is another major user of ductile iron.

Malleable Cast Iron:

If cast iron is cooled rapidly, the graphite flakes needed for gray cast iron do not get a chance to form. Instead, white cast iron forms. This white cast iron is reheated to about 1700^oF for long periods of time in the presence of materials containing oxygen, such as iron oxide. At the elevated temperatures cementite (Fe3C) decomposes into ferrite and free carbon. Upon cooling, the combined carbon further decomposes to small compact particles of graphite (instead of flake -like graphite seen in gray cast iron). If the cooling is very slow, more free carbon is released. This free carbon is referred to as temper carbon, and the process is called malleableizing.

Figure 7. Malleable Cast Iron

Figure 8 shows ferritic malleable cast iron, which has a ferrite matrix and the tempered carbon particles are embedded into the matrix.

Figure 8. Ferritic Malleable Cast iron

Figure 9 shows pearlite malleable cast iron, which has a pearlite matrix. By adding manganese to the structure, carbon is retained in the form of cementite.

Figure 9. Pearlitic Malleable Cast Iron

A wide variety of physical properties can be obtained by heating and cooling through the eutectoid temperature or by adding alloying elements. Slow cooling will cause the cementite to decompose and release more free carbon (temper carbon). Fast cooling will retain some of the cementite. The amount retained, will depend on the rapidity of cooling.

Malleable cast iron is used for connecting rods and universal joint yokes, transmission gears, differential cases and certain gears, compressor crankshafts and hubs, flanges, pipe fittings and valve parts for railroad, marine and other heavy-duty applications.

Advantages:

Excellent machinability

Significant ductility

Good shock resistance properties

Disadvantages:

The major disadvantage is shrinkage. Malleable cast iron decreases in volume during solidification, and as a result, requires attached reservoirs (feeders and risers) of liquid metal to offset the shrinkage and prevent the formation of internal or external shrinkage defects.