If we consider the primary metal of steel, which is iron, there are a number of features that we can observe immediately about the metal:

It is stable at room temperature.

It is magnetic.

It can have a shiny finish. In other words, it polishes to a bright shine.

It has a high density (it is heavy).

It is ductile (it can bend or be shaped easily).

 Consider a straight vertical line as being an iron and temperature line combined. If we look at the line in the vertical configuration at room temperature, the iron is stable.

  To understand what happens to iron when heat is applied, you must first know about ferrite, a condition in iron that has a number of inherent properties at room temperature such as a large grain size, a low hardness, good ductility and is easily machinable.

  The ferrite condition exists in a particular crystal form and will exist at low temperatures. Iron is therefore made up of millions of tiny crystals much like sugar or salt. Those crystals are bound together in what is called a lattice structure.

 The ferrite condition of iron can be compared to water, or H2O to be more precise. H2O exists in three forms: ice, liquid and vapor. Each of the three phases or conditions is still H2O, but they each exist in a different crystal form.

  Similarly, ferrite is one of the conditions of iron.
As heat is applied to iron, a number of things begin to happen to the shiny piece of steel that is being heated. The oxygen in the air will begin to react with the shiny surface of the iron to form Iron Oxide. As the temperature is increased, the reaction between the iron and oxygen becomes more aggressive, and a visible product begins to form on the surface of the iron, which is known as scale.

  Once the steel reaches an approximate temperature of 1,350xF, a change in the structure takes place as well as the phase. The phase changes from ferrite to austenite. In addition to this, the crystal structure changes from body-centered cubic to face-centered cubic.

  An indication of the change occurring is that the ion will lose its magnetic property, which can be seen by testing the surface of the steel with a magnet. Protect your hand and fingers when testing for the loss of magnetism from the hot steel .

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