Chapter 9 Characteristics of Metallic Materials

Objectives Four major classifications – ferrous,non ferrous, high temperature super alloy, and refractories. Physical and mechanical properties of metals. Two major ferrous metals – iron and steel. Techniques used to identify types of steel

Background Six of the top employment sectors listed in NAICS (North American Industry Classification System) account for half of all employment in manufacturing: primary metals, fabricated metal products, machinery, computers and electronics, transportation equipment manufacturing, and electrical equipment, appliances, and component manufacturing.

Structure of metallic materials Metals (fig 9-1, page 128) are useful in manufacturing because they can retain their essential properties when subjected to extreme hot and cold temperatures. Alloys, a blend of metals and other elements are used more frequently than pure metals to make products. Elements are substances that cannot be broken down or further reduced. These elements exist as atoms. Iron, a common element, has an atomic number of 26 (the number of protons in the atom’s nucleus) and its symbol is Fe. Copper has an atomic number of 29 (heavier than iron) and its symbol is Cu.

Physical Properties Four physical properties used to classify metals: weight, color, conductivity (electrical and thermal) and reaction of the material when exposed to heat. Lightweight, less dense materials (aluminum) have a more open or loose molecular structure. The brilliant color and reflection of platinum, gold, and silver are quite different from the color and shine of stainless steel, or the dull gray color and shine of iron. Copper and aluminum expand much more when subject to heat than cast iron or steel. Copper exhibits high thermal conductivity and is an excellent conductor of electricity. A superconductor is a material that is able to conduct electricity without exhibiting any resistance to the flow of current.

Mechanical Properties There are many times when mechanical properties are more important than physical properties. Tensile strength (the ability of materials to resist being pulled apart – tensile test) and hardness (the resistance to penetration – Rockwell test, Brinell test, or Charpy impact test) are examples of mechanical properties. Steel can be hardened through heat treating and can be softened through another heat treating process called annealing. Flexibility is measured using Young’s Modulus of Elasticity. Fatigue is measured using the fatigue failure test. With metal parts most breakage is caused by fatigue. Creep is the elongation of material that occurs when the material is exposed to elevated temperatures while under stress. The creep rate of steel can be reduced by adding elements such as nickel and chromium to the alloy. Plasticity refers to the ability of the to change shape and size as a result of force being applied. Ductility is the ability of the material to be formed plastically without breaking.

Classification of Metals Four types: ferrous metals, nonferrous metals, high temperature super alloys, and refractory metals. 1. Ferrous metals: those that contain iron. Most commonly used metals. Steel and cast iron are two types of ferrous metals most often used. Two basic types of iron: Wrought iron is tough and ductile – is easy to bend – used in ornamental iron work (low carbon content). Cast iron is hard and brittle (high carbon content). Used for engine blocks, machine frames, machine parts, and gear cases. Ductile cast iron is heat treatable and used for making crankshafts, camshafts, and connecting rods for engines. It can be arc welded. Steel: is an alloy of iron and carbon. Stainless steel is an alloy of iron with nickel and chromium.

Classification of Metals 2. Nonferrous metals: metals with no iron content such as aluminum, copper, lead, magnesium, nickel and zinc are called non ferrous metals. They are usually more durable since they are resistant to corrosion. Copper and its alloys are used in product applications that require superior electrical and thermal conductivity. Aluminum is lightweight, resistant to corrosion and a good conductor.

Classification of Metals 3. High Temperature Super alloys: are used in applications requiring material that can withstand extreme forces and severe oxidizing. They are used in the manufacture of rocket engines, turbines, jet engines, and space vehicles. In most cases the base metal is iron, nickel, or cobalt. In some product applications such as turbine vanes and blades, the parts are often coated with other materials to improve their environmental resistance.

Classification of Metals 4. Refractory metals: are high temperature metals such as niobium (Nb), tungsten (W), and molybdenum (Mo). Tungsten has the highest known melting point of any metal. These metals are used in products such as incandescent light filaments and welding electrodes. They are also used for tools, dies, rocket engines, gas turbines, and containers for holding and dispensing molten metal.

Nature of Industrial Stock Industrial stock is the product of primary manufacturing firms – mining, refining, and production of metals. Hard goods for sale to the consumer are the product of secondary manufacturing firms. The steel making industry shapes molten steel into solid ingots. Ingots are later rolled or extruded into various shapes. Tin plate (mild steel coated with tin) is used in the food industry for can making. There are different methods for identifying the different types of steel. One method is the Unified Numbering System (fig 9-10, page 137). Another method is by color code. Most manufacturers paint the end of the stock with one or more colors to indicate the particular type of steel. Another method is grinding off a small section of the stock and studying the pattern and color of the sparks (fig 9-11, page 138).

Summary Metals are useful in manufacturing because they can retain their essential properties when subjected to extreme hot and cold temperatures. Alloys, a blend of metals and other elements are used more frequently than pure metals to make products. Four physical properties used to classify metals: weight, color, conductivity (electrical and thermal) and reaction of the material when exposed to heat. With metal parts most breakage is caused by fatigue. Four types of metals are: ferrous metals, nonferrous metals, high temperature super alloys, and refractory metals. Ferrous metals: those that contain iron. Most commonly used metals. Steel and cast iron are two types of ferrous metals most often used. Nonferrous metals: metals with no iron content such as aluminum, copper, lead, magnesium, nickel and zinc are called non ferrous metals. High Temperature Super alloys: are used in applications requiring material that can withstand extreme forces and severe oxidizing. Refractory metals: are high temperature metals.

Home Work 1. What is an alloy? 2. What are the physical properties used to classify metals? 3. What is the cause of most breakage with metal parts? 4. What are the 4 types of metals? 5. What are non ferrous metals? Give examples.