Metals for A level Unit 1

1. Unit 1 Metals
Low carbon steels
These are also called mild steels. They are the
cheapest and most widely used group of
steels. Although they are the weakest of the
steels, nevertheless they are stronger than
most of the non-ferrous metals and alloys.
They can be hot and cold worked and
machined with ease.

2. Unit 1 Metals
Medium carbon steels
These are harder, tougher, stronger and more costly than the low
carbon steels. They are less ductile than the low carbon steels and
cannot be bent or formed to any great extent in the cold condition
without risk of cracking. Greater force is required to bend and
form them. Medium carbon steels hot forge well but close
temperature control is essential. Two carbon ranges are shown.
The lower carbon range can only be toughened by heating and
quenching (cooling quickly by dipping in water). They cannot be
hardened. The higher carbon range can be hardened and
tempered by heating and quenching.

3. Unit 1 Metals
High carbon steels
These are harder, stronger and more costly
than medium carbon steels. They are also
less tough. High carbon steels are available as
hot rolled bars and forgings. Cold drawn high
carbon steel wire (piano wire) is available in a
limited range of sizes. Centreless ground high
carbon steel rods (silver steel) are available in
a wide range of diameters (inch and metric
sizes) in lengths of 333 mm, 1 m and 2 m.
High carbon steels can only be bent cold to a
limited extent before cracking. They are
mostly used for making cutting tools such as
files, knives and carpenters’ tools.

4. Unit 1 Metals

5. Copper
Pure copper is widely used for electrical conductors and switchgear
components. It is second only to silver in conductivity but it is much
more plentiful and very much less costly. Pure copper is too soft and
ductile for most mechanical applications.
For general purpose applications such as roofing, chemical plant,
decorative metal work and copper-smithing, tough-pitch copper is
used. This contains some copper oxide which makes it stronger,
more rigid and less likely to tear when being machined. Because it is
not so highly refined, it is less expensive than high conductivity
copper.
There are many other grades of copper for special applications.
Copper is also the basis of many important alloys such as brass and
bronze, and we will be considering these next. The general properties
of copper are:
• relatively high strength
• very ductile so that it is usually cold worked. An annealed
(softened) copper wire can be stretched to nearly twice its length
before it snaps
• corrosion resistant
• second only to silver as a conductor of heat and electricity
• easily joined by soldering and brazing. For welding, a
phosphorous deoxidized grade of copper must be used.
Copper is available as cold-drawn rods, wires and tubes. It is also
available as cold-rolled sheet, strip and plate. Hot worked copper is
available as extruded sections and hot stampings. It can also be cast.
Copper powders are used for making sintered components. It is one
of the few pure metals of use to the engineer as a structural material.

6. Brass
Brass is an alloy of copper and zinc. The properties of a brass alloy
and the applications for which you can use it depends upon the
amount of zinc present. Most brasses are attacked by sea water.
The salt water eats away the zinc (dezincification) and leaves a
weak, porous, spongy mass of copper. To prevent this happening, a
small amount of tin is added to the alloy. There are two types of
brass that can be used at sea or on land near the sea. These are
Naval brass and Admiralty brass.
Brass is a difficult metal to cast and brass castings tend to be
coarse grained and porous. Brass depends upon hot rolling from
cast ingots, followed by cold rolling or drawing to give it its
mechanical strength. It can also be hot extruded and plumbing
fittings are made by hot stamping. Brass machines to a better finish
than copper as it is more rigid and less ductile than that metal.
Table 1.2 lists some
typical brasses, together with their compositions, properties and
applications.

8. Tin bronze
As the name implies, the tin bronzes are alloys of copper and tin.
These alloys also have to have a deoxidizing element present to
prevent the tin from oxidizing during casting and hot working. If
the
tin oxidizes the metal becomes hard and ‘scratchy’ and is
weakened.
The two deoxidizing elements commonly used are:
• zinc in the gun-metal alloys.
• phosphorus in the phosphor–bronze alloys.
Unlike the brass alloys, the bronze alloys are usually used as
castings. However low-tin content phosphor–bronze alloys can
be extensively cold worked.
Tin–bronze alloys are extremely resistant to corrosion and wear
and are used for high pressure valve bodies and heavy duty
bearings. Table 1.3 lists some typical bronze alloys together with
their compositions, properties and applications.

10. Aluminium
Aluminium has a density approximately one third that of steel.
However it is also very much weaker so its strength/weight ratio is
inferior. For stressed components, such as those found in aircraft,
aluminium alloys have to be used. These can be as strong as steel and
nearly as light as pure aluminium.
High purity aluminium is second only to copper as a conductor of
heat and electricity. It is very difficult to join by welding or soldering
and aluminium conductors are often terminated by crimping. Despite
these difficulties, it is increasingly used for electrical conductors
where its light weight and low cost compared with copper is an
advantage. Pure aluminium is resistant to normal atmospheric corrosion but it
is unsuitable for marine environments. It is available as wire, rod, cold-rolled
sheet and extruded sections for heat sinks.
Commercially pure aluminium is not as pure as high purity aluminium and it
also contains up to 1% silicon to improve its strength and stiffness. As a result
it is not such a good conductor of electricity nor is it so corrosion resistant. It is
available as wire, rod, cold-rolled sheet and extruded sections. It is also
available as castings and forgings. Being stiffer than high purity aluminium it
machines better with less tendency to tear. It forms non-toxic oxides on its
surface which makes it suitable for food processing plant and utensils. It is also
used for forged and die-cast small machine parts.