Engineering - Steels & Iron
In today's society Steel is very important. It has many uses from surgical equipment to bridges. Plain Steel is a mixture of Iron and Carbon. It is not an alloy, because an alloy is a mixture of two or more metals. However most Steels that you are familiar with such as Stainless Steel, are alloys because they also contain minute quantities of other metals to give the Steel its properties.
Iron containing less than 2% Carbon is classed as Steel.
Iron Ore exists in four forms :
The two most important ores are :
The Production of Iron
Iron Ore is first mined, then graded and crushed. Coke and Limestone are then added. Following this the ore is Sintered. Goto Metallurgy if you want to learn more about the most commonly used mining methods.
Sintering : The ore powder is first compacted, then heated to a temperature well below the melting point of the metal. This results in the waste being removed.
After Sintering the Iron Ore mixture gathers into ball shaped masses called Agglomerates.
The Iron Ore mixture, which has been Sintered, is then taken to the Blast Furnace to be converted into Iron metal. Click The Blast Furnace to learn more.
Most Iron is taken straight from the Blast Furnace to the Steel Mill, but some is poured into buckets called Pigs. This Iron is called Pig Iron and is used to make Cast Iron.
The molten Iron is about 95% pure and contains some Sulphur, Phosphorous, Silicon and Carbon as impurities.
The Production of Steel
There are two stages in the production of Steel.
Iron from the Blast Furnace contains around 4% Carbon, (which as we know is too much for Steel ), and sometimes up to 3-4% Sulphur, Phosphorus, Manganese and Silicon. Refining removes these unwanted impurities by heating them and bringing them into contact with Oxygen to form oxides.
There are two main process used to manufacture Steel, (click on them to learn more) :
Cast Iron Moulding Methods
Traditionally Cast Iron was moulded by hand in sand. This is called Sand Casting. In modern Engineering there are a variety of moulding methods. Two common types are shown below.
Shell Moulding : Patterns are manufactured from Steel or Aluminium. A polymer mixed with sand is then poured over the hot patterns and allowed to solidify and cool. When the cooled shell is removed from the pattern, the molten Iron can then be poured into it. The Shell is then removed to reveal the solidified Cast Iron. Shell Moulding is also used for Alloy Steels.
Centrifugal Casting : Molten Iron is fed into a mould rotating at high speed. Centrifugal force acting on the molten metal forces it into the mould. The result is hollow castings.
How Iron Cools
When Iron cools slowly Graphite Flakes or Rosettes form causing weakness in the Iron. Quick cooling results in Cementite forming, resulting in strength in the Iron.
A problem arises when cooling a casting with varying thickness. Different sections will cool at different rates, resulting in the casting having varying degrees of strength. In order to overcome this problem large metal plates are placed in the mould near to the thicker sections to act as heat sinks. The result is the casting having a uniform strength.
Classification of Cast Iron
Cast Irons have between 2% and 4% Carbon. The Carbon can be present as :
Grey Cast Irons
Malleable Cast Irons