Heat treatment of steel causes a rearrangement of its grain structure, resulting in a harder, tougher product. To accomplish any change in the grain structure, the steel must be heated to what is known as its critical point or range, whereupon a change will occur in the metal at the lower edge of this critical range and again at its upper edge. The desired change to refine the grain structure occurs within this range, between the low point and the high point in temperature. This critical range varies somewhat for different types of steel, the higher the carbon content is the lower is the temperature of the critical range for that steel.
As this critical range is rather narrow, 50 to 75 degrees, the heat must be closely controlled, as upon entering this range from its lower point the steel assumes the finest grain structure possible for it, but upon passing through it to a higher temperature the grain changes to a coarser structure and a little more heat burns the steel. If the steel is heated above its critical point, but is not burned, it may be restored by allowing it to cool slowly and then reheating to its critical range again, upon which the fine grain structure is restored.
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