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).
To achieve this, the gas-piping used should be:
1. Certified gas piping
2. All joints should be soldered, brazed, welded, or O-ring sealed
3. Run piping underground if possible
4. Insulate piping if possible
Let’s take a look at these more closely, and see why each of these items is very important to follow.
1. Certified gas piping
It is not acceptable to merely go down to the hardware store and purchase standard copper or steel piping for this purpose. It should be ordered as piping that is certified for use in piping gaseous atmospheres. Unfortunately, in today’s world, when piping is manufactured and supplied from many different locations around the world, there are pipes/tubes being manufactured and sold in which the walls of the pipes/tubes are not sound, and sometimes contain some occasional porous spots in their walls that can allow gases to leak right through that section of the piping/tubes. Such poor quality piping is completely unacceptable for use. The only way to insure sound piping is, in my opinion, to buy certified gas piping.
2. All Joints should be soldered, brazed, welded, or O-ring sealed
A number of brazing shops have run piping from the cryogenic liquid tanks stationed out behind their plants, using standard threaded plumbing-type fittings to make their pipe connections. Unfortunately, such connections, even if “sealed” using Teflon tape or “dope” can open up with time, allowing leakage of air (thus oxygen) into the piping system from the surrounding air, even when the piping system is already transmitting another pressurized gas (e.g., 150-psi internal pressure of argon or nitrogen, etc. in the piping).
Oxygen molecules see any gaseous piping system as a “vacuum”, and wants to enter that piping system to help “equalize” the amount of oxygen in the pipes with the amount of oxygen on the outside of the pipes. To us, the pressure inside the pipe may have significance, since we can feel the pressure if it is directed at us. But to atoms and molecules of oxygen, the pressure that we feel, as large beings, is meaningless to them, since they are only tiny atoms. All that the atom or molecule of oxygen knows is that there is little, if any, oxygen inside the piped argon (or nitrogen, etc.) gas, and the tiny atom of oxygen wants to equalize the amount of oxygen inside the pipe with the amount of oxygen outside. So, the oxygen atom goes right in, through any leak path, and will contaminate that gas in its effort to equalize the amount of oxygen inside the pipe with the amount of oxygen on the outside of it. Internal pressure of the gas inside the pipe has no real meaning at all to the molecules of oxygen. As an example of this, it is not uncommon for gas-bottles, such as nitrogen cylinders at the gas-dealers shop to be leak-tested prior to delivery of the cylinders, even though those bottles have been pressurized to about 3000 psi. However, if that supplier does a leak check by, for example, spraying a soapy solution onto the tightly sealed threaded “spud” on top of the cylinder prior to delivery of the cylinders, and discovers that a bubble has developed at that location, then the supplier knows that yes, atoms of contaminating gases (such as oxygen) have migrated upstream against several-thousand pounds of internal pressure in that tank, because those contaminating atoms saw the inside of that tank as a “vacuum” to its type of atom.
3. Run gas piping underground.
By running the atmosphere piping up over driveways, or along the outside of building walls, or up onto a building roof and then down from the roof into the portion of the factory where the vacuum furnace is located, that piping and its fittings/connections will be subject to large temperature variations from day to night and from winter to summer. These combined forces of heating/cooling lead to extensive expansion/contraction issues with those pipes and fittings/connections, causing the joints in those fittings/connections to open up with time, and lose their seal.
Instead, if the piping is run underground from the gas-tanks, it can be protected from such temperature extremes, and minimize (or eliminate) any such sealing failures.
4. Insulate the pipes
Whether the piping is run underground or above ground, wrapping the pipes with insulation can be a distinct help in preventing temperature extremes from affecting the piping and all its connections.
CONCLUSIONS: It stands to reason that the optimal way to insure that a protective atmosphere (either in liquid or gaseous form), can be most protected from losing it’s high-quality if it is piped to its destination using high-quality (certified) gas piping materials, being sure that all connections are leak-tight by joining them using only soldering/brazing/welding procedures (no threaded plumbing fittings), then running the pipes underground to their destination. The additional use of pipe-insulation wrappings would be a plus.
Even with such care, there is typically still a slight loss of dewpoint in the gas when transferred from its original storage tank to the vacuum furnace. But such losses are minimal when compared to the significantly large dewpoint losses (contamination) when piping is done as cheaply and as fast as possible by merely buying some non-certified, standard piping procured from large national housing-supply chains, then installing it with regular threaded piping fittings, and running the pipe up over driveways and roofs, and then dropping them down into the building from high side-walls or roofs.
Be wise. If you are planning to use a dry non-oxidizing atmosphere in your vacuum furnace for brazing, heat-treating, quenching, etc., do your best to be sure that atmosphere reaches your vacuum furnace in the driest condition possible, by taking the time and spending the money to properly install the gas-piping system from your external gas-tank source all the way to the vacuum furnace.
Dan Kay – Tel: 860-651-5595: – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/
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