Engineering materials may be broadly classified under the headings of ferrous metals, non-ferrous metals, plastics.
A ferrous metal is one which contains iron. To the engineering this means either a steel or a cast iron. In both cases the other essential ingredients is carbon, but it is the quantity and form of carbon which govern the type of material. Cast iron usually contains between 2.5 and 4% carbon. This is more than for steels which contain up to 1.7% carbon and require much critical control of percentage content.
Gray Cast Iron:
This is the most widely used of all cast irons and is thus often referred to merely as cast iron. The general properties of cast iron may be listed as follows:
1. Excellent fluidity in casting, enabling intricate shapes to be cast quickly and cheaply, eg. cylinder heads, motor frames, manifolds.
2. Weakness in tension, due to graphite structure.
3. High compressive strength and rigidity, making it the essential material for large machine-tools beds.
4. Low coefficient of friction. The graphite content gives grey cast iron self-lubricating properties, useful for application such as bearing blocks and piston rings.
5. Good machining properties.
6. Vibration damping properties, giving advantage in application such as gearbox housings.
The versatility of steel as an engineering material is due to its wide range of good mechanical properties and forming processes. Steels generally combine most of the following properties:
1. High tensile strength
2. High compressive and shear strength
3. Good ductility and malleability
4. Toughness and hardness
5. Moderately high thermal and electrical conductivity
6. High melting point
7. Good merchantability
8. Ease of heat treatment.
Since 1988 a new series of 'mandatory' European standards (EN = Europaischen Normen) has been created, to replace national standards, such as BS, DIN, SS and NF, throughout 18 countries of Western Europe.
This classified types of steel under EN numbers. It is the mechanical properties of the steel, and its reaction to heat-treatment, welding and machining. This classification system has now been replaced by another one.
The following are some common plain carbon EN steels, and it will be given under both classifications:
Cold-formed mild steel used for low duty nuts, bolts, studs, etc.
Hot rolled, general purpose mild steel. This is one of the most widely used of steels for mechanical and structure applications.
Medium carbon steel which has high strength and toughness, especially when heat-treated. Used extensively for keys, general forgings, crankshafts, connecting rods, etc.
This is a common case-hardening steel used mainly for wear-resistance applications such as camshafts, tappets, gears, gudgeon pins, etc.
High carbon steel used chiefly for leaf springs and coil springs.
American Iron and Steel Institute (AISI) standard:
The first two digits indicate the grades of the steels, while the last two digits give the nominal carbon content of the alloy in hundredths of a percent (0.xx%). Example: AISI 1060.
Typically, alloy steels are designated by distinct AISI (American Iron and Steel Institute) four-digit numbers. The first two digits indicate the leading alloying elements, while the last two digits give the nominal carbon content of the alloy in hundredths of a percent (0.xx%). Example AISI 13xx.
The DIN standard:
St 37 is a steel with ultimate strength of 370 N/mm2 . The application such that: St 34 for rods and levers, St 37 for car bodies, St 42 for connecting rods and crankshafts. St 60 for springs.