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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

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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.

A Little Vacuum Theory

A gas is a collection of molecules in constant motion. The higher the temperature, the faster these molecules move. As one might expect, the motion of gas molecules stops or dramatically slows down at or near absolute zero. As molecules speed up with an increase in temperature, there is an increase in their kinetic energy (or energy of motion). Molecular collisions occur between molecules. If contained, these molecular collisions against the walls of their container result in a pressure rise (which always occurs in a closed container when a gas is heated). In other words, pressure is simply the force per unit area that a gas exerts on the walls of its container.

In 1811, Avogadro determined that a mole of any gas occupies a volume of 22.4 liters and contains 6.02 x 10²³ molecules at standard temperature and pressure. To create a vacuum in any closed vessel, therefore, some of these gas molecules must be removed.

At atmospheric pressure, 1 cubic centimeter (1 cc) of air contains 2.69 x 10¹⁹ molecules all moving around in a random motion. This results in a fantastic number of collisions. The mean free path between molecules (or the average distance a molecule can travel before colliding with another molecule) is 2.6 x 10^-6 inches. So, if we pump a 1 cubic centimeter volume down to 1 x 10^-3 torr (one micron), which is a vacuum level commonly used in heat treating, we still have a whopping 4 x 10¹³ molecules remaining!

How then can this be an acceptable condition for heat-treating, especially when about 20% of the remaining molecules are oxygen? The answer is that the mean free path increases tremendously, thus reducing the probability of molecular collision with each other, and since the number of molecules (density) has been reduced, fewer collisions occur with the surface of the workpiece. This results in even fewer collisions of oxygen molecules with the work surface and no visible oxidation forms.

For example, if we pumped down to 1 x 10^-9 torr, or one millionth of a micron, the number of molecules per cubic centimeter is decreased to 4 x 10⁷ and the mean free path is 5,000,000 cm (over 30 miles!). The path of the molecules is now limited by the walls of the vessel and not by the collisions between molecules. Flow, as we know it, doesn’t exist. This is the reason for the relatively large opening and piping used for diffusion pumping systems. If the opening to the diffusion pump weren’t that large, any molecules that migrated into the pump suction stream would rebound and move to other parts of the chamber.

Vacuum Level

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The more molecules that are removed from a vessel, the better the vacuum. The quality of a vacuum is described by the degree of reduction in gas density – that is, gas pressure. In the field of heat-treating, we distinguish four different vacuum levels or quality of vacuum (Table 3). The heat treatment of steel is carried out in three of these: rough, fine and high. The majority of applications are processed in the fine vacuum range. By contrast, at 200 miles above the Earth, the vacuum in space is 1 x 10^-8 torr. At 400 miles it is 1 x 10^-10 torr, and in deep (or outer) space it is 1 x 10^-16 torr.

Next Time

Part two of this series will teach us all about how gases behave in a vacuum environment, look at the equations needed to explain their behavior and explore what happens when we pump down a vacuum vessel.