Please note, however, that water (H2O) also represents the presence of oxygen, and oxygen is an element that can react with metals to form oxides on the metal surface being heated up in that furnace. Thus, to prevent oxidation of metals during a furnace brazing process, it is important to keep oxygen away from the part being brazed, which means controlling the amount of moisture present in any gaseous atmosphere being used in a vacuum-furnace during that brazing process. Gaseous atmospheres are indeed introduced into vacuum furnaces for rapid-cooling after brazing, or may be introduced into the vacuum chamber in order to build up a partial-pressure to prevent outgassing of certain elements from the metals being brazed, or from the brazing-filler-metal (BFM) itself. Gases may also be introduced into “multi-bar” vacuum furnaces for purposes of perhaps heat-treating parts during or following a brazing run.
The dewpoint of a gas can be measured fairly easily by the use of a dewpoint-meter (also called a hygrometer), which, as the name suggests, is an instrument that will measure the amount of moisture in a gas and provide that information via readouts, either as part of the furnace control instrumentation (as shown in Fig. 1), or perhaps as a separate, portable dewpoint-meter that can be connected to a special port in the atmosphere-piping system to the vacuum furnace (as shown in Fig. 2).
Three very important things to know about dewpoint control:
1. Dewpoint Must Be Measured at the Furnace, Not at the Gas Source
Receiving a certification from your gas-supplier stating that the atmosphere (gaseous or liquid) they have delivered to you has “such and such” dewpoint, is actually quite meaningless to your brazing operations. What is important, instead, is the dewpoint of that atmosphere when it is inside the furnace chamber itself where the brazing is taking place – after it’s been piped from the outside tanks all the way to the furnace. The piping and pipe-fittings can have a major effect (either negative or positive) on the dewpoint of an atmosphere gas being piped from a liquid tank or other external source of gas. This will be the topic of another article soon.
2. If You Are, in Fact, Putting a Gaseous Atmosphere into Your Vacuum Chamber, You Must Know Its Dewpoint
If a gaseous atmosphere is being used in your vacuum furnace brazing process, perhaps for outgassing-suppression, or for rapid cooling, etc., or for multi-bar HT/brazing, then the dewpoint of that gas MUST be measured – at the furnace — and provided to the furnace operator, or the brazing process is NOT being properly controlled! I am often amazed by brazing shop personnel who do NOT know the dewpoint of the gaseous-atmosphere being used inside their furnace. They merely “assume” its okay. That is a very poor brazing practice.
Remember, dewpoint represents moisture. Moisture represents oxygen. Oxygen at brazing temp causes oxidation of metals. Unfortunately, brazing filler metals (BFMs) don’t like to bond to, or flow over, oxides on metals! Thus, if you don’t accurately monitor, and control, the dewpoint of the gaseous atmosphere you are using in your brazing operations, you can easily ruin your brazing.
Therefore, the dewpoint of any brazing-atmosphere used must be known in order for the brazing process to be “in control”. There are a number of good manufacturers of dewpoint equipment available in the marketplace. Remember, if a gaseous atmosphere touches a brazed component at any time during the entire vacuum-brazing cycle, from loading to unloading, know (and control) the dewpoint of that gas.