Unit 1-2 Engineering Material

Metals

Metals are divided into ferrous and non-ferrous metals. The former contain iron and the latter do not contain iron. Certain elements can improve the properties of steel and are therefore added to it. For example, chromium may be included to resist corrosion and tungsten to increase hardness. Aluminum, copper, alloys, bronze, and brass, are common non-ferrous metals.

Materials in this group are composed of one or more metallic elements (e.g., Fe, Al, Cu, Ti, Au, and Ni), and often also nonmetallic elements (e.g., C, N, and O) in relatively small amounts. Atoms in metals and their alloys are arranged in a very orderly manner, and in comparison to ceramics and polymers, are relatively dense. With regard to mechanical characteristics, these materials are relatively stiff and strong, yet are ductile (i.e., capable of large amounts of deformation without fracture), and are resistant to fracture, which accounts for their widespread use in structural applications. Metallic materials have large numbers of delocalized electrons; that is, these electrons are not bound to particular atoms. Many properties of metals are directly attributable to these electrons. For example, metals are extremely good conductors of electricity and heat, and are not transparent to visible light；a polished metal surface has a lustrous appearance. In addition, some of the metals (e.g., Fe, Co, and Ni) have desirable magnetic properties.

While metals comprise about three-fourths of the elements that we use, few find service in their pure form. There are several reasons for not using pure metals. Pure metals may be too weak or too soft, or they may be too costly because of their scarcity, but the key factor normally is that desired property sought in engineering requires a blending of metals and other elements. Thus, the combination forms (alloys) find the greatest use.

Metal alloys

Metal alloys, by virtue of composition, are often grouped into two classes—ferrous and nonferrous. Ferrous alloys, those in which iron is the principal constituent, include steels and cast irons. They are especially important as engineering construction materials making up the largest proportion both by quantity and commercial value. Their widespread use is accounted for by three factors:

1) Iron-containing compounds exist in abundant quantities within the earth's crust.

2) Metallic iron and steel alloys may be produced using relatively economical extraction, refining, alloying, and fabrication techniques.

3) Ferrous alloys are extremely versatile, in that they may be tailored to have a wide range of mechanical and physical properties.

Steel

Steel is our most widely used alloy. Iron alloyed with various proportions of carbon gives low, mid, and high carbon steels. An iron carbon alloy is considered steel only if the carbon level is between 0.01% and 2.00%. For the steels, the hardness and tensile strength of the steel are related to the amount of carbon present, with increasing carbon levels also leading to lower ductility and toughness. However, heat treatment processes such as quenching and tempering can significantly change these properties.

The most common types of steels are plain low-carbon, high-strength low-alloy, medium-carbon, tool, and stainless. Stainless steel is defined as a regular steel alloy with greater than 10% by weight alloying content of chromium. Nickel and molybdenum are typically also found in stainless steels. A wide range of mechanical properties combined with excellent resistance to corrosion make stainless steels very versatile in their applicability. Figure 1-2 shows the plain carbon steels.

Figure 1-2 Plain carbon steels

Cast iron is defined as an iron-carbon alloy with more than 2.00% but less than 6.67% carbon.

Alloy steels

All alloys that are not iron-based are nonferrous. The significant nonferrous alloys and copper-based include copper, aluminum, titanium, and magnesium. Alloys of copper, possessing a desirable combination of physical properties, have been utilized in quite a variety of applications such as piping, utensils, thermal conduction, electrical conduction, etc. The alloys of aluminum, titanium, and magnesium are known and valued for their high strength-to-weight ratios and, in the case of magnesium, their ability to provide electromagnetic shielding. These materials are ideal for situations where high strength-to-weight ratios are more important than bulk costs, such as in the aerospace industry and certain automotive engineering applications. Figure 1-3 shows the alloy steels.

Figure 1-3 Alloy steels