Why we do Vacuum Heat Treatment?

The air we breathe contains a number of elements that can react with metals under the proper conditions. Moisture, oxygen, carbon dioxide and hydrogen are present in significant amounts in our atmosphere. Each can react to varying degrees with many different metals. While many of these reactions occur to only a small extent at room temperature, they are often greatly accelerated in the presence of heat. Consider the example of a piece of polished metal held over a heat source. It will eventually turn blue or black as the elements in the atmosphere react with the hot metal. We try to minimize the extent of these reactions during heat treating. The reactions cause changes in the surface properties of the metal that may result in a heat treated component with a surface(edge) that is much harder than the rest of the component. To minimize these undesirable reactions, the source of the reactive elements, air, must be eliminated from the heat treating environment.