Heating of steel does not produce a constant rate of increase of its temperature but rather through an uneven flow. Initially the temperature may show a steady rise after which it may stop absorbing heat for a moment and after a short period of temporary halt the temperature again starts rising normally. During this short pause, the metal absorbs heat which instead of heating further, initiates certain transformations in the metal. The temperature at which this change occurs is termed as the “critical" or “arrest" points.
The diagram shows equilibrium conditions, where the critical points are illustrated over an increasing temperature. The line AEB indicates lower critical points while the line DEC the higher critical points.
However it is important to remember that though steels may vary in their carbon compositions, their lower critical point is the same = 730 degree Celsius.
At this temperature pearlite ceases to exist and laminae of other compositions like ferrite and cementite dissolve to form a solid structure called austenite, which shows non-magnetic properties.
The proportion of pearlitee and cementite may be such that the mixture produces a solid solution at lower temperatures. The solidness may be more than that obtained with other mixtures of steel, like pearlite and eutectoid.
On heating steel a little above the critical point temperature and then cooling suddenly produces no effect over its structural form. The quick cooling restricts the formation of pearlite, however austenite may change into a different atomic structure called “Martensite", which exhibits extreme hardness and brittleness.
In case the cooling is not done drastically, then the procedure may transform austenite into another form called Troostite, which is not so hard but is very tough.
If the cooling procedure is made at a further slower rate then the transformation results “Sorbite", a finely grained, strong and ductile structure.
The above discussion shows how the degree of hardness may be varied by observing different rates of heating and cooling.
Image: By Karann