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 :

  1. Haematite
  2. Magnetite
  3. Limonite
  4. Siderite

The two most important ores are :

  1. Haematite : (Fe2O3) Colours range from, steel grey, to coal black, but Haematite always has red streaks running through it.
  2. Magnetite : (Fe3O4) Contains about 72% Iron. Black and shiny in colour, and with a metallic lustre. Magnetite has strong metallic properties.

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.

  1. The extraction of Iron from Iron Ore.
  2. The refining of Iron in special furnaces.

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) :

  1. The Basic Oxygen Process
  2. The Electric Arc Furnace

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 :

  • Graphite Flakes - weaken the structure
  • Cementite - also known as Iron Carbide, (Fe3C), strengthens the structure.
There are two main types of Cast Irons.
  1. Grey Cast Irons
  2. Malleable Irons

Grey Cast Irons
  • Alloy of Iron, Carbon and Silicon.
  • Grey in colour, hence name.
  • Most have Pearlitic matrix, giving good strength.
  • Where machinability is important a Ferritic matrix is best.
  • It can be cast into very intricate shapes.
  • Good resistance to sliding wear, because of Graphite Flakes.
  • Machinability, generally very good.
  • Can be surface hardened.
  • Grey Cast Iron moulding is an economic method of manufacture.

Malleable Cast Irons
  • Have some of the properties of Grey Cast Irons and some of the properties of Mild Steels.
  • Tougher then Grey Cast Iron.
  • Less ductile than Mild Steel.
  • Fairly soft.
  • Machines very well.
There are 3 types of Malleable Cast Irons, based on fracture appearance when broken.
  1. Whiteheart : In the as cast state it has a high Carbon content, consisting of Pearlite and Cementite. Is hard and brittle. Easy to cast in thin sections, which have a tough core. Structure of high tensile Pearlite, surrounded by soft ductile Ferrite. Whiteheart has a Tensile Strength of about 400N/mm2. This is roughly the same as Mild Steel.
  2. Blackheart : Structure has graphite in nodules, ( little round knotty lumps), in Ferrite, over the whole section. The presence of the graphite causes the material to machine easily. The Tensile Strength of Blackheart Malleable is about 300 - 330 N/mm2.
  3. Pearlitic : Made from the same white Iron as Blackheart but different heat treatments give different appearance.