Read the text and write an abstractMetal working processes Traditionally, many metal tools were made by heating iron bars in a fire,
called a forge, until they were red hot or white hot. The metal was then worked –
in other words, shaped by hammering it. Working metal using compression (for
example, hammering) is also called forging. The same basic technique is still in
use today, especially with steel. However, large, automated machines are now
used. Metal is often worked (or forged) when hot (hot forged), but may also be
worked when it is cold (cold forged). A common forging technique is drop forging, where a heavy hammer is
dropped onto a piece of metal. A die fixed to the hammer compresses the metal
into the required shape. Rollers can also be used to apply compression, with or
without heat, to produce hot rolled or cold rolled metal. Forging also increases the hardness of metal. This is called work hardening.
Metal becomes work hardened because its structure is changed by compression.
The same result can be achieved without hammering or rolling – and therefore
without changing the component’s shape – by shot – peening. This involves firing
small metal balls (metal shot) at the surface of components (when cold), at high speed.
After components have been shot-peened, their surface is significantly harder.
Casting, sintering and extruding metal
Metal can be formed into shapes using heat and pressure. Casting involves
heating metal until it becomes molten and pouring it, or forcing it under pressure,
into a mould called a die. Instead of being cast, metal components can be formed
by sintering. This is done by using metal powder instead of molten metal.
The powder is placed in a die and compressed into a solid mass. It is then heated
(though not melted) until it becomes sintered – that is, the powder particles join
together structurally, due to the heat. Metal can also be shaped by extruding it into
long lengths. Extrusion involves heating metal until it is molten, then forcing it at
high pressure through a shaping tool – also called a die – to form bars or tubes, for
example. At the same time, the metal cools and becomes solid.