Identification of Metals
There is a wide variety of metals. In fact about three-quarters of chemical elements are metals. Each metal has its own set of characteristics. Because of this wide variability, metals have been much used in technology.
As a group, metals can be differentiated from other materials by the following characteristics.
- Metals are not organic substances (i.e. not made up of carbon compounds) like wood, plastics and rubber.
- Metals are excellent conductors of heat and electricity. This is unlike wood, plastics, rubber and ceramics.
- Metals are mined from the ground. In their natural state of occurrence, they are not as readily available as wood or ceramics. They often have to undergo complex processing procedures.
- With the exception of some very soft metals for example lead, metals are generally very strong.
- Metals produce a high-pitched sound when struck against each other. This is not so for wood, plastics, rubber and ceramics.
Physical Properties of Metals
The following properties distinguish a metal from another metal.
1) Colour or Degree of Brightness: Most metals have specific colours. For example aluminium is silvery white, gold is bright yellow, while zinc is bluish white. Some metals also shine more brightly than others. Tin for example has a bright silvery colour while lead is dull grey. The degree of brightness is also called lustre. Gold and silver shine very brightly.
2) Magnetic Property: Metals that contain some amounts of iron (e.g. steel) are attracted by magnets while those that do not contain iron (e.g. gold, copper) are not attracted by magnets.
3) Heat Conductivity: Metals generally conduct heat better than other materials used in technology. Metals conduct heat at different rates. Aluminium and silver for example conduct heat quite rapidly.
4) Density: Density is the quantity of matter, which a substance contains in its unit volume. It is calculated by the formula;
Density = Mass of substance/Volume of the substance
Equal volumes of different metals have different masses. For example, 1 cm3 of gold has a mass of 19.3 g while the same volume of copper has a mass of 9.0g.
5) Sound: Each metal gives a different sound when hit against another material. For example the sounds produced by brass, bronze or silver if each is struck against a block of wood or another hard object is unique.
6) Elasticity: This is the tendency of a body to regain its original shape after its shape has been temporarily altered. For example if a rubber band is pulled, it stretches but quickly returns to its original shape when the force causing the pulling action is removed.
Metals also behave in this manner. However, the changes in their shapes, otherwise called deformation, are not very noticeable without the use of the appropriate scientific device. The elasticity of each metal is different from that of another.
7) Ductility: This is the ability of a metal to be drawn out into a wire. Copper is a metal that is very ductile. Metals differ in their degrees of ductility.
8) Fusibility: Metals can melt, and while in the liquid or molten form become joined to other metals. This property is called fusibility. Some metals are more fusible than others.
9) Malleability: Malleability is the ability of a metal to be hammered or bent into desired shapes. Metals exhibit different degrees of this property.
10) Melting point: Although metals are in the solid state at ordinary temperatures, they however can melt and in fact, turn into liquid state if their temperatures are raised high enough. Every metal melts at its own specific temperature called melting point.
11) Brittleness: The tendency of a metal to break quite easily is called brittleness. Some metals crack and break easily while others do not. Cast iron for example is more brittle than steel.
12) Hardness: A metal is described as being hard if it is resistant to being dented and cannot be easily worked with tools. If a metal is hard, this does not mean that it is strong. White cast iron for example is hard but it breaks fairly easily.
13) Toughness: A metal is said to be tough if it exhibits strong resistance to cracking, breaking, stretching, bending and other forms of stresses. Toughness is one of the most. sought characteristics in metals, particularly those required to fabricate load-bearing parts. Steel is a very tough metal. Heating a metal causes it to loose part of its toughness.
14) Electrical conductivity: As a rule, metals are good conductors of electricity. However, individual metals still conduct electricity at different rates. Silver and copper are the best conductors of electricity.
Classification of Metals
Metals are classified into two main groups, namely; ferrous metals and non-ferrous metals.
1) Ferrous Metals and their Alloys
A ferrous metal is one, which is wholly or partly made up of iron. An alloy is a metal, which is a combination of two or more metals. If an alloy has a ferrous metal as one of its constituent metals, it is called a ferrous alloy. Wrought iron, cast iron and steel are examples of ferrous metals. Some grades of cast iron are Grey, Chilled, White and Malleable cast iron. Steel is principally made up of carbon and iron. Some grades of steel are mild steel (or low carbon steel), medium carbon steel and high carbon steel.
An example of ferrous alloy is steel. This is steel which has some element other than carbon added to it. Some types of alloy steel are silver steel, stainless steel, high speed steel and high tensile steel.
Magnets attract ferrous metals and their alloys because they contain iron. Iron is the element largely responsible for magnetism.
2) Non-ferrous Metals and their Alloys
All metals, which do not contain iron, are referred to as non-ferrous. Due to this absence of iron, non-ferrous metals are not attracted by magnets. Generally, nonferrous metals are not as strong as ferrous metals. However, they are much more resistant to corrosion. Corrosion in this instance is the wearing away of metal as a result of chemical action. Examples of nonferrous metals are tin, copper, phosphorus, zinc, manganese, lead, and aluminium. The majority of metals are non-ferrous.
In the course of designing and eventual manufacture of an object, the technologist may not find a single metal having all desired characteristics. This situation usually warrants the need to use an alloy. An alloy combines the characteristics of its constituent metals.
Some examples of non-ferrous alloys are the following.
- Bronze: This is made up of copper and tin. If phosphorus or aluminium is added, it is called phosphorous bronze or aluminium bronze respectively.
- Brass: Copper and zinc make up this alloy.
- Duralumin: This is composed of copper, aluminium, manganese and magnesium. It is light in weight and quite strong.
Most non-ferrous alloys contain copper.
Forms of Metals
To ensure standardization in design and construction, ferrous and non-ferrous metals are manufactured in certain regular cross-sections and lengths. These are called forms of metals. It is mainly the cross-section, which distinguishes one form of metal from the other.
Some forms of metals listed below are:
- Round section: These include rods, pipes, round bars.
- Square section: These are square bars and square pipes, etc.
- Channel section: This is like a square or rectangular shaped section, which is open on one side.
- Sheets, strips and plates: Sheets are easily bent while plates are not. Strips are of short width.
- Others such as wires, I-sections T-sections also exist.