Enlarge this picture

A vacuum system (Fig. 1) provides a space in which the pressure can be maintained below atmospheric pressure at all times. The primary advantage of vacuum heat treatment is its versatility. In almost all cases it provides a “safe” environment with respect to the surface of the components being treated, is self-contained and uses cycles/recipes that can be reproduced consistently. When not in use, like an electric light, it is simply turned off. When turned back on, minimal conditioning time is required.

A principal difference between vacuum heat treating and all other forms of heat treatment is the absence of or the precise control of surface reactions. In addition, vacuum processing can remove contaminants and, under certain circumstances, degas or convert oxides found on the surface of a material.

The word vacuum comes from the Latin “vacuus” meaning empty or “vacare” meaning to be empty. When we think of an empty space, what comes to mind is something entirely devoid of matter. Such a space does not exist, nor can it be produced. In practical terms, a vacuum must be considered a space with a highly reduced gas density. In heat treating, gas molecules and contaminants are removed from a vacuum vessel using a pump. Air (Table 1) is the most important of all gases to be eliminated since it is present in every system.

Common Vacuum Units

Enlarge this picture

In vacuum applications, pressure is commonly measured in torr (U.S.) or millibar (Europe and Asia). A torr is 1/760th of atmospheric pressure. In other words, atmospheric pressure – the pressure all around us – at standard temperature (0ºC) and pressure (sea level) is 760 torr or 1013 mbar.

One of the mystifying things about vacuum and vacuum furnaces, especially in the U.S. heat-treating industry, is the confusing way in which vacuum units are used. Devices installed on furnaces often measure in different units, which force us to speak in terms of microns, torr, millitorr, millimeters of mercury, millibar, bar, Pascal, inches of water column and inches of mercury! This is extremely confusing, especially to those who are not familiar with vacuum terminology. If at all possible, try to stay with one unit of measure, converting everything to that common base. Conversions between common vacuum units (Table 2) are available from a number of sources.