irons are cast white that is, their as-cast structure consists of metastable carbide in a pearlitic matrix. 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 900 to 930°C (1650 to 1700°F) for long periods of time (50 hours), in the presence of materials containing oxygen, such as iron oxide; after that time it is slowly (60 hours) cooled. Upon cooling, the combined carbon further decomposes to small compact particles of graphite (instead of flake like graphite as 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. The new microstructures can possess significant ductility and good shock resistance properties. Fig. 3.11 shows the microstructure of malleable iron.
Fig. 3.11 Graphite form in malleable iron structure.
Malleable cast iron is used for connecting rods and universal joint yokes, transmission gears, differential cases and certain gears, compressor crankshafts and hubs, flanges, pipefittings, and valve parts for railroad, marine, and other heavy duty applications.
e) Compacted Graphite Iron
In 1949 a now well known material called ductile iron was patented. At the same time that ductile iron was patented, a lesser known material called compacted graphite iron (CGI) was also patented, although it was only considered a curiosity at the time. This material has higher strength than gray iron and better thermal conductivity than ductile iron.
The graphite of CGI is in the form of relatively short thick flakes with rounded ends and undulating surfaces. In compacted graphite, the graphite does not have the same weakening effect as flake graphite in grey iron, but it is still continuous and gives greater thermal conductivity than the discrete graphite nodules in ductile iron. Fig. 3.12 shows the form of graphite in compacted graphite cast iron.
Fig. 3.12 Graphite form in compacted iron structure.
CGI has risen to an important status in the automotive industry, particularly since 1996. The material has been used for manufacturing parts such as brake disk, exhaust manifolds, engine heads, and diesel engine blocks. The superior strength characteristics of CGI as compared to gray iron, allows the manufacturing of engines with higher pressure operating combustion chambers, which means these engines are more efficient, having lower emissions levels. Also, thinner walls are possible, which means lighter engines.
f) Alloy Cast Irons
Alloy cast irons is a term that designates casting containing alloying elements such as nickel, chromium, molybdenum, copper, and manganese in sufficient amounts to appreciably change the physical properties. They include high-alloy graphitic irons and high-alloy white irons.
One of the most important alloy cast irons is high-alloy white cast iron. Alloy cast irons are an important group of materials whose production must be considered separately from that of ordinary types of cast irons. In these cast iron alloys, the alloy content is well above 4%; and consequently such metals cannot be produced by ladle additions to irons of otherwise standard compositions. They are usually produced in foundries especially equipped to produce highly alloyed irons.
The high-alloy white irons are primarily used for abrasion-resistant applications and are readily cast into the parts needed in machinery for crushing, grinding, and handling of abrasive materials. The chromium content of high-alloy white irons also enhances their corrosion-resistance properties. The large volume fraction of primary or eutectic carbides in their microstructures provides the high hardness needed for crushing and grinding other materials. The metallic matrix supporting the carbide phase in these irons can be adjusted by alloy content and heat treatment to create the proper balance between resistance to abrasion and the toughness needed to withstand repeated impact. The high alloy graphitic cast irons have found special uses primarily in applications requiring corrosion resistance or strength and oxidation resistance in high-temperature service.
Steel is an alloy whose major component is iron with carbon content between 0.02 and 1.7% by weight. The mechanical properties of steel make it an attractive engineering material, and its utility in complex geometries makes casting an appealing process. However, the high temperature required to melt cast steels (for example carbon’s and low alloy steel’s pouring temperature is 1565 to 1700°C (2850 to 3100°F) means that casting them requires considerable experience. At these high temperatures, melting steel oxidizes very quickly; so special procedures must be used during melting and pouring. The high temperatures involved present difficulties in the selection of mold material. Carbon and low alloy steels approach the limit of temperature and ceramic refractors and titanium and zirconium alloys go beyond it, creating special situations. So, it is easy to see the abuse that sand and ceramic molds are subjected to when pouring temperatures approach the refractory limits. Also, molten steel has relatively poor fluidity, and this limits the design of thin sections in parts cast out of steel. Several advantages of cast steels are that steel castings have better toughness than most other casting alloys; also, steel castings’ process properties are isotropic, and depending on the requirement of the product, isotropic behavior of the material may be desirable. Steel castings can be welded, but after welding, castings need to be heat treated to restore to them their mechanical properties.
3.4.3 Nonferrous Casting Alloys
The nonferrous metals include metal elements and alloys not based on iron. Many nonferrous metals are used as engineering materials. Such metals include aluminum, magnesium, titanium, nickel, copper, zinc, refractory metals (molybdenum and tungsten), and noble metals. Although they are more expensive and cannot match the strength of the steels, nonferrous metals and alloys have important applications because of their numerous positive characteristics, such as low density, corrosion resistance, ease of fabrication, and color choice. For example, copper has one of the lowest electrical resistances; zinc has a relatively low melting point; aluminum is an excellent thermal conductor, and it is also one of the most readily formed metals.
a) Aluminum and Aluminum Alloys
Pure aluminum is a silver-white metal characterized by a slightly bluish cast. It has a crystal structure that is face-centered cubic (fcc), a melting point of 660°C, (1220°F), and density of 2700 kg/m3 (163.55 lb/ft3). Aluminum is thermodynamically the least stable of the main engineering metals, but a lucky property of aluminum is the formation of a dense, highly protective alumina film only 1 mm in thickness. This film can be reinforced by anodizing, and can be destroyed by salt. Aluminum is one of the few metals that can be cast by all of the processes used in casting metals. When aluminum is alloyed with other metals, numerous properties are obtained that make these alloys useful over a wide range of applications. Main alloying additions are copper, magnesium, manganese (Mn), silicon, lithium, and zinc.
Depending on the use to which the alloy will be put, different metals will be mixed in with the aluminum. For example, high magnesium content yields superior corrosion resistance. High copper or zinc adds superior strength. A large number of aluminum alloys has been developed for casting, but most of them are varieties of six basic types: aluminum-copper, aluminum-copper-silicon, aluminum-silicon,
