Typical Liquid Storage System
In many vacuum brazing applications, it is deemed necessary to use an atmosphere gas inside the vacuum furnace, perhaps to quench components following a vacuum-brazing run, or to perhaps build up a partial-pressure atmosphere inside the furnace to prevent the outgassing/volatilization of higher vapor-pressure metals, or perhaps merely to allow gaseous conduction of heat from part to part being brazed.
Whenever a gas is introduced into a vacuum furnace for a brazing operation, I’m always very concerned about the dewpoint of that gas, since dewpoint represents moisture in the gas, and moisture represents the presence of oxygen. In vacuum brazing of aluminum, moisture molecules present their own issues to the brazing process, in addition to their oxidizing characteristics.
A sometimes overlooked aspect of the entire vacuum brazing process is the piping used to bring in a gaseous atmosphere to the vacuum furnace. The piping may originate outside the brazing shop’s walls, starting at a large bank of gas-cylinders, or from a large cryogenic liquid-tank sitting on a concrete pad out in the yard.
In any event, the gaseous atmosphere which begins its journey from the tank or gas-cylinder out in the yard is in the “driest” condition (lowest dewpoint) it will see in its journey to the vacuum furnace. The piped-atmosphere can pick up significant amounts of moisture, and other contaminants, throughout the entire length of the piping system used to bring it to the furnace. It is very important that the entire piping system be as clean, and as leak-tight as possible, to insure that the gas used in the vacuum-brazing chamber is as dry as possible (must be – 60F/-50C or drier for effective brazing).