Chapter 15 Introduction to Nonferrous Metals 335 Vacuum Chambers and Pumps A vacuum chamber is never comp letely void of air, and the pressure in a vacuum chamber depends on how well the air is pumped out. Most industrial vacuum chambers report pressure in torr, or millimeters of mercury, in a manometer. One standard atmosphere pressure is equal to 760 torr, or 760 mm (29.9″) of mercury. A vacuum chamber consists of a leak-free container with well-sealed access doors, electric heating elements on the inside of the furnace, and pumps to remove gases from the chamber while the workpieces are heated. Figure 15-14 shows a vacuum heat-treating chamber being loaded. For vacuum processing where some surface oxidation and discoloring are not significant issues, a mechanical roughing pump moves gas out of the chamber to achieve a pressure of approximately 10–3 torr. For processing reactive metals or in situations where the surface must remain free of oxidation, oil diffusion pumps, in sequence with roughing pumps, reduce the pressure below 10–5 torr to achieve a high vacuum, or hard vacuum, Figure 15-15. At pressures below 10–3 torr, gases do not behave like a fluid. Instead, the gas molecules behave as individual particles. A wave of oil vapor will sweep gas molecules along with it and concentrate the gas at higher pressures, just as a handful of large marbles rolled across a floor will sweep a few small marbles along with them. A mechanical pump can then remove the gas at pressures above 10–3 torr. Vacuum systems require very good seals at all openings. The door seal for the cover in Figure 15-14 is a neoprene rubber O-ring with a low–vapor pressure grease. For very high vacuums and seals that reach over 212°F (100°C), the O-ring is fully annealed copper. The copper deforms slightly when the cover is clamped in place, sealing the joint from leaks. Cleanliness of the flange at the O-ring contact is absolutely necessary. Small bits of dirt can prevent an adequate O-ring seal, allowing air to leak into the chamber and contaminate the workpieces inside. Warping of a flange due to uneven thermal cycling may cause severe leaks, preventing operation. The pumps must be designed to handle severe outgassing as parts and materials are heated in the furnace, so that the pumps remove all the released gases and keep the pressure low. Alternatively, the heating rate must be reduced so the pumps can handle the outgassing load. If the pumps fail to operate fast enough, the roughing pump will “choke” and the pressure will rise, allowing the workpiece metal inside the chamber to react with the gas. The reason for using the vacuum chamber will be lost. The technician working near the pumps may be the only person alerted by the “choking” sound. PRACTICAL METALLURGY PRACTICAL METALLURGY Solar Atmospheres Figure 15-14. The operator loads a tray of parts to be stress relieved into a vacuum furnace. Note the door seal surface and the latching mechanism to keep the door seals firmly in place during operation. Note the thermocouple leads at the front end of the tray, to assure that the temperature at the center of the tray reaches the specified temperature. Copyright Goodheart-Willcox Co., Inc.