Iron-carbon phase diagram is shown in Fig. 5.4. Steel undergoes definite internal changes when subjected to temperatures above its critical range. If the steel cools naturally from this temperature it returns to its normal condition similar to that found after normalizing. Time needs to be allowed during cooling cycle so that the internal changes that occurred during heating have time to reverse.
• If the time needed to modify the internal changes is not allowed, the properties of steel change on cooling.
• Critical points are designated as Acx, Ac2 and Ac3 for heating and Ar3 Ar2 and Arx for cooling. These letters were taken from French language.
A = Arrent (stop).
C = Chauffage (heating)
r = Refroidissment (cooling)
Thus, Acx = stop heating at the number 1 critical point
Arx = stop cooling at the lower critical point.
Fig. 5.4 I ron-carbon phase diagram
5.1.3 Critical Range
If a piece of SAE-1030 steel is heated its colour will change though the temper colours up into red range becoming more and more brighter as the temperature increases. At 723°C, the colour will remain constant for a short time even though the heat is being supplied. Upto this point the metal will expand at a uniform rate proportionate to the temperature.
At the Acx point the expansion stops and the material begins ro shrink until to Ac3 point (813°C) is reached. At this point on, the material will start expanding again to its normal expansion rate.
When the steel is heated to or beyond Ac3 point it becomes nonmagnetic. The critical point Ac3 falls as the carbon content increases.