7.1 Metal Alloys
7.1.1 Cast Iron
Cast iron is a common term that applies to high carbon iron alloys containing silicon. The common alloys include gray cast iron, white cast iron, malleable cast iron and ductile/nodular cast iron. Most cast irons contain carbon content between 3.0 to 4.5 wt%. The melting points of cast iron ranges from 1150 to 1300 °C, which is considerably lower than that of steels. Thus, they are easily melted and amenable to casting. Furthermore most cast irons are very brittle because of the high carbon content.
The cementite (Fe3C) is a metastable phase and is dissociated into ferrite
(almost pure iron) and graphite under some conditions. For most cast irons, the carbon exists as graphite and both microstructure and mechanical properties depend on composition and heat treatment. The important structural constituents of cast iron are graphite, ferrite, and cementite. These are the least expensive of the engineering metals. They can be alloyed for improvement of corrosion resistance and strength. These materials are brittle and exhibit practically no ductility.
7.1.1.1 White Cast Iron
When silicon content is less than 1% and the cooling rate is rapid, most carbon exists as cementite instead of graphite. This is called white cast iron. These irons are extremely hard and brittle because of the cementite inclusion. Thus it is used for applications that necessitate a very hard and wear resistant surface such as rollers, crushers, and grinding mills.
7.1.1.2 Malleable Iron
Heating white cast iron between 800 to 900 °C decomposes the cementite forming graphite into clusters. It has high strength and appreciable ductility or malleability. It is used in numerous applications in automobiles (such as connecting rods and transmission gears), locomotives, road machinery, pipefittings, valves, etc.
7.1.1.3 Ductile Iron
Ductile iron is also called nodular cast iron. Adding magnesium to the gray cast iron before casting produces a distinctly different microstructure and mechanical properties. The mechanical properties of ductile irons can be improved by heat treatments. It is used in applications such as automobile crankshafts, main shafts and rotors for machinery drive etc.
7.1.1.4 Gray Cast Iron
The carbon and silicon contents of gray cast irons lie between 2.5 to 4 wt% and 1.0 to 3.0 wt%, respectively. Because of the high silicon content, cementite decomposes into ferrite and graphite. The graphite exists in the form of flakes.
Gray cast iron is weak and brittle in tension. However its strength and ductility are much higher under compressive loads. They are very effective in damping vibrational energy. It is used as base structures for machines and heavy equipment that are exposed to vibrations.
7.1.1.5 High Silicon Cast Iron
High silicon cast iron is produced by increasing the silicon content in gray cast iron to about 14%. It is usually corrosion resistant in many environments. The notable exception is hydrofluoric acid. Their inherent hardness makes them corrosion resistant to erosion corrosion. A straight high silicon iron such as Duriron contains about 14.5% silicon and 0.95% carbon. This composition is suited to provide the best combination of
corrosion resistance and mechanical strength. The excellent corrosion resistance of high silicon irons is due to the formation of passive SiO2 surface layer, which forms
during exposure to the environment. The high silicon cast iron has a major application in pipelines and fittings.
7.1.2 Steels
Steels are basically iron-carbon alloys in which the carbon content is usually less than 1.0 wt%, along with the addition of other alloying elements. The mechanical properties of steels depend on its carbon content. Steels are classified into different types depending on their carbon content. They are plain carbon steels, low alloy steels and high alloy steels.
7.1.2.1 Plain Carbon Steels
Plain carbon steel contains carbon as the only alloying element. As the carbon content increases, brittleness increases. The mechanical properties of this steel are controlled by the carbon content. The plain carbon steels are further classified as low- carbon (0.3%), medium-carbon (0.3 to 0.6%) and high-carbon (0.6 to 1.0%). Low carbon steels are used in automobile body panels, tin plate, and wire products. Medium carbon steels are used in shafts, axles, gears, crankshafts, couplings, and forgings. High carbon steels are used in spring materials and high strength wires.
7.1.2.2 Low Alloy Steels
Low alloy steels contain alloy content less than 8%. Carbon steel is usually alloyed individually or in combination with small quantities of chromium, nickel, copper, molybdenum, phosphorus, and vanadium to produce low alloy steels. Better mechanical properties can be obtained by increasing the alloy content, but the most important advantage is better corrosion resistance to atmospheric corrosion. Strengths are higher than ordinary carbon steel. These materials can be used for stampings, forgings and boiler plates.
7.1.2.3 Stainless Steels
The most important advantage of stainless steels is their resistance to corrosion and good mechanical properties at atmospheric and elevated temperatures. Chromium is the main alloying element and the steel should contain at least 11%. Chromium tends to form passive film and exhibit excellent resistance to many environments. Stainless steels form pits when exposed to chloride environments. The stainless steels are classified into three groups based on their structure, composition, and characteristics. They are austenitic stainless steels, ferritic stainless steels, and martensitic stainless steels. Austenitic stainless steels are also called as work hardening steels. The strength and hardness of this steel can be improved by cold working. The most predominant alloying elements are chromium and nickel, which is usually in the range of 16 to 26% and 6 to 22%, respectively. Austenitic stainless steels are the most corrosion resistant because of the high chromium contents and the nickel addition. They are produced in the largest quantities. The applications include chemical processing equipment. Ferritic stainless steels contain 11.5 to 27% chromium and the carbon content is usually low. These stainless steels are also called non-hardening steels because they cannot be hardened by heat treatment. These steels find applications in automotive industry. Martensitic stainless steels are also known as hardened alloy steels. Chromium content will be usually in the range of 11.5 to 18%. The mechanical properties can be improved by quenching. These steels find major applications in oil and gas industry.
7.1.3 Aluminum and its Alloys
Aluminum alloys are a mixture of base metal aluminum with one or more alloying elements, which can be metallic or non-metallic (such as silicon). The aim of alloying is to enhance the properties of the base metal, (e.g. its strength and corrosion resistance, etc). Aluminum has a specific gravity of 2.7. Aluminum alloys have a strong resistance to corrosion because of an oxide skin that forms as a result of reactions with the atmosphere. This corrosive skin protects aluminum from most chemicals, weathering conditions, and even many acids. However alkaline substances are known to penetrate