Forming, working and heat-treating metal

A Casting, sintering and extruding metal

Metal can be formed into shapes using heat and pressure. Casting involves heating metal until it becomes molten (liquid) 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.

B Working metal

Traditionally, many metal tools were made by heating iron bars in a fire, called a forge, until they were red hot or (hotter still) 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.

20.1 Decide whether the sentences below are true or false, and correct the false sentences. Look at B opposite to help you.

1. Metal must always be heated before it can be forged.

2. When referring to metals, the terms working and / forging mean the same.

3. A common reason for forging metal is to increase its hardness.

4. One way of forging metal is by heating it and then rolling it.

5. Metal can only be rolled after it has been heated to a high temperature.

6. When metal is drop forged, it is subjected to compression.

7. Metal can only be work hardened by the process of hot forging.

8. Shot-peening is a hot forging technique used to work harden metal.

 

20.2 Make correct sentences using one part from each column. Look at C opposite to help you. The first one has been done for you.

	If a metal is precipitation hardened,	it is held at a high temperature for a time	making it harder, but more brittle.
	When metal is annealed,	it is heated within a gas	to improve its hardness without reducing its elasticity too much.
	If metal is quenched, this means	it can also be described as age hardened,	to harden only the metal near. the surface.
	When a metal is tempered,	its temperature is allowed to decrease gradually	because it is heated for a long time.
	If a metal case is hardened,	its temperature is reduced rapidly,	in order to make it more elastic and less brittle.

 

20.3 Replace the highlighted expressions in the report extract with alternative words and expressions from A, B and C opposite. Sometimes there is more than one possible answer.

The first stage in manufacturing the blades for the cutting tools is to form them into an approximate shape by (1) a process of squeezing molten metal through a die. Before the blades have cooled, they are then (2) hammered while still at a high temperature - a process which not only flattens them into their final shape, but also ensures the metal becomes (3) harder as a result of the hammering action. The blades are then (4) cooled quickly in water. Finally, they are (5) bombarded with small metal balls in order to further increase their surface hardness.

 

Over to you

Think of a type of steel component that needs to have specific properties. Suggest different ways of obtaining these properties by forging of heat treating the steel.

Material formats

A Raw materials for processing

Generally, raw materials are materials which need to be processed before they are used - for example, melted and cast in a mould. Common formats of raw material are:

■ powder: quantities of very fine (small) particles, such as cement powder

■ pellets: larger, standard-sized pieces of material, typically pea-sized to egg-sized, intended to be melted for forming in moulds - for instance, plastic pellets

■ fibres: very fine, hair-like lengths, such as glass fibres.

When steel and other metals are produced, they are made into blocks called ingots, which can subsequently be melted and cast. Very large steel ingots are called blooms. One standard size for steel blooms is 630 mm x 400 mm x 6 m. Steel can also be supplied in smaller blocks, of various sizes, called billets.

 

B Formats of processed materials

Materials are frequently supplied ready for use in the formats described below.

■ Bars are long lengths of solid metal with a relatively small cross-sectional area. These can be round bars (or rods) which have a circular section. They may also be square bars, with a square section, and flat bars, with a flat, rectangular section. A bar is generally made of metal, but a rod can be made of any material.

■ Sheets are flat, wide and thin - for steel, thinner than about 3 mm. Other materials supplied in sheets include plastic, glass and wood. However, sheets of wood are often called boards. When sheets of metal (or metal sheets) are delivered in large quantities, they can be supplied in rolls called coils.

■ Plates are flat pieces of metal that are wide, but thicker than sheets (for steel, thicker than 3 mm). Non-metals, such as glass, plastic or wood, are not usually called plates; even if these materials are thicker than 3 mm, they are usually called sheets.

■ Structural steel sections are made from rolled or extruded steel, and produced in a variety of section shapes. I-sections, with profiles in the shape of the letter I, are common examples. (See Appendix V on page 106 for types of structural section.)

■ Tubes are hollow, not solid. The most common types are round tubes, but square tubes and rectangular tubes are also produced. Pipes are specifically for carrying liquid or gas.

o A pipe is therefore just one type of tube.

■ Wires are thin lengths of metal with circular sections, consisting of one strand - that is, a long, thin, single piece of material. They are usually supplied in coils. Several wires can be combined to form a cable. An electrical wire is a single conductor covered with insulation. The conductor can be a single wire (called a solid wire) or several strands of wire grouped together (called a stranded wire). An electrical cable has several conductors, separately covered with insulation, grouped within a second outer layer of insulation.

 

Notes:

Tubes can also be called tubing - for example, steel tubing.

21.1 Decide whether the sentences below are true or false, and correct the false sentences. Look at A opposite to help you.

1. Raw materials are often intended to be melted or mixed.

2. Powder particles are smaller than pellets.

3. Pellets do not require further processing.

4. A steel bloom is a type of ingot.

5. Steel billets can be cut into smaller sized pieces called blooms.

 

21.2 Complete the descriptions below the photos using the words in the box. You will need to use some words more than once. Look at B opposite to help you.

 

bar	cable	flat	rod	sheet	stranded	wire
bloom	coil	plate	round	solid	tube

 

 

D component features