2. Further treatments
page 11
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Interactive graphic of sliding layers in alloy
Interactive graphic of sliding layers in alloy
Picture 2.7 The ions in an alloy. The larger, alloying ion disrupts the regular lattice of the pure metal.
Alloying
What does alloying do?
Pure metals are rarely used in manufacturing because they are too soft. Usually, other elements are added to the molten metal so that the resulting solid is harder and has other desirable properties. An exception to this is is in the manufacture of car bodies which are made from new steels that are nearly pure iron.

Adding larger ions
In the alloy, some of the added ions may be larger than most of the ions making up the metal lattice. They disrupt the regular arrangement of ions and make it more difficult for the layers to slide over each other. This makes the alloy harder and less malleable and ductile than the pure metal (in which the layers slip over each other more easily).

Adding smaller atoms
Smaller sized atoms can also have a significant effect on the alloy structure. In steel, for example, atoms of non-metals such as carbon and nitrogen can fit into holes between the iron atoms. This also distorts the metal lattice and makes it more difficult for the layers to move over each other.

Photo of steel products
Picture 2.8 Many of these car parts use carbon steels.
Carbon steels
The percentage of carbon has a dramatic effect on the properties of the material and therefore on the uses for which it is suitable:
Type of steel Percentage of carbon Properties Uses
Low carbon
(mild steel)
0.07 – 0.25 Easily cold worked Car bodies
Medium carbon 0.25 – 0.50 Wear resistant Rails and rail products: couplings, crank shafts, axles, gears, forgings
High carbon
(carbon tool steel)
0.85 – 1.2 Strong and wear resistant Cutting tools
Railway lines
Cast iron 2.5 – 3.8 Easy to cast but brittle Pistons and cylinders
Photo of rails
Picture 2.9 Railway tracks are made from high carbon steel. Some heavy duty crossings are made from manganese steels.

Alloy Steels
The addition of small amounts of other metals to make alloy steels changes the properties of the material even further and means that a steel can be manufactured that has exactly the right properties for its purpose.

Alloying element Properties given to steel Uses
Cobalt High magnetic permeability Magnets
Manganese Strong and hard Heavy duty railway crossings.
Molybdenum Maintains high strength at high temperature High speed drill tips
Nickel and chromium Resists corrosion Surgical instruments
Titanium Increased hardness and tensile strength High speed tool steels, permanent magnets
Tungsten High melting temperature, tough Cutting and drilling tools
Vanadium Strong, hard Tools
Question 2-5.
a) Why are alloys used in manufacturing rather than pure metals?

b) Which of the following types of steel are paper clips made out of?
(i) low carbon steeel     (ii) medium carbon steel    (iii) high carbon steel

c) What alloying element might be found in a steel chisel?


Summary                   Close
  • metals can be alloyed to tailor their properties
  • steel is an alloy of iron and carbon; the amount of carbon determines its properties
  • steel can be alloyed further by adding other metals to tailor its properties