When heat treating or brazing in vacuum, the vapour pressure of the constituents in the materials being processed can be a very important consideration. 

The vapour pressure of a material is that pressure exerted at a given temperature when a material is in equilibrium with its own vapour.  Vapour pressure is a function of both the material and the temperature.  Figure 1 shows the approximate vapour pressure curves for a variety of metals and compounds.  The area to the left of each curve represents the conditions of temperature and pressure under which the material exists as a solid.  The area to the right of each curve represents those conditions under which the material exists as a gas (or vapour). BY JEFF PRITCHARD

In vacuum processing, metal surfaces remain very clean and free of oxides. When these near-perfect surfaces are in contact with other surfaces, certain elements have a tendency to interact between the surfaces through solid state diffusion. Therefore, a major consideration when selecting both hearth and load fixturing materials for vacuum heat treating is the possibility of solid state diffusion between different materials in contact at high temperatures. Solid state diffusion of certain elements can cause the formation of a lower melting point alloy called a eutectic. For example, solid state diffusion between carbon and nickel can begin to occur at temperatures as low as 1165ºC (2130ºF) and cause local melting, also known as eutectic melting. BY JEFF PRITCHARD

The air we breathe contains a number of elements that can react with metals under the proper conditions.  Moisture, oxygen, carbon dioxide and hydrogen are present in significant amounts in our atmosphere.  Each can react to varying degrees with many different metals.  While many of these reactions occur to only a small extent at room temperature, they are often greatly accelerated in the presence of heat.  Consider the example of a piece of polished metal held over a heat source.  It will eventually turn blue or black as the elements in the atmosphere react with the hot metal.

In most cases, the heat treater tries to minimize the extent of these reactions during heat treating.  The reactions cause changes in the surface properties of the metal that may result in a heat treated component with a “skin” that is much softer (or harder) than the rest of the component.  To minimize these undesirable reactions, the source of the reactive elements, air, must be eliminated from the heat treating environment.  Sometimes this is done by replacing the air in a heat treating chamber with a non-reactive atmosphere such as nitrogen, argon or other gas mixtures.  This is often referred to as controlled atmosphere heat treating.  Another alternative is to heat treat in a bath of non-reactive molten salt.  However, these environments still contain some very low levels of residual impurities so metals heat treated in a controlled atmosphere or salt usually exhibit a small amount of discoloration. BY JEFF PRITCHARD

A residual gas analyzer or RGA for short is a compact mass spectrometer, designed for use either in the laboratory or out on the shop floor. These devices are often mounted for in-situ use on a vacuum furnace. RGA’s are typically designed for process control and contamination monitoring in vacuum systems.

Applications for residual gas analyzers include distinguishing leaks from outgassing, fingerprinting the process background, detecting helium and determining the effectiveness of gas line purging. A typical RGA gas analysis can reveal how much of a particular species is present either in the vacuum vessel or in the pump manifold. RGAs are used in most cases to monitor the quality of the vacuum and easily detect minute traces of impurities in the low-pressure gas environment. These impurities can be measured down to 10^-14 Torr levels, possessing sub-ppm detectability in the absence of background interferences. RGAs can also be used as sensitive in-situ, helium leak detectors. With vacuum systems pumped down to lower than 10^-5 Torr for checking of the integrity of the vacuum seals and the quality of the vacuum to detect air leaks, virtual leaks and other contaminants at low levels before a process is initiated.

Thermal spray processes like air plasma spray and High-Velocity Oxygen Fuel (HVOF) are usually thought of as being used primarily for applying protective coatings to new parts. While new part applications do indeed constitute the majority of their use, there are also a wide variety of repair techniques that employ thermal spray processes. VAC AERO has been a leader in developing repairs for aircraft structural components and gas turbine engine parts using thermal spray processes. An example of a structural component repair involves a flap track from a popular turboprop aircraft.  As the wing flaps of this aircraft are extended and retracted during landing and take-off, rollers run along the surfaces of a series of components known as flap tracks.

An example of a structural component repair involves a flap track from a popular turboprop aircraft.  As the wing flaps of this aircraft are extended and retracted during landing and take-off, rollers run along the surfaces of a series of components known as flap tracks.  For the original flap track design, the manufacturer applied a nickel-based electroplated coating to protect the roller wear surfaces. However, after a certain period of service, the electro-plated coating was worn away and significant wear also occurred in the base metal substrate beneath. VAC AERO was tasked with coming up with a method of re-building the worn substrate and applying a more durable coating to the wear surfaces. VAC AERO’s solution was to use the HVOF process to apply a tungsten carbide coating over the worn area.  This coating was used for both the restoration of the damaged substrate and as a wear resistant overlay. Because this application requires high bond strength and fatigue resistance, several coating compositions were tested before the ideal candidate emerged. The development of the proper spray parameters also required significant effort to ensure the coating adhered properly, particularly where it “feathered” out at the edges of the overlay.  VAC AERO’s repair technique was subsequently approved by the aircraft manufacturer.

The purchase of a vacuum furnace involves a considerable capital investment.  As a result, the question of buying a used furnace at a lower cost than a new furnace is a fairly common one.  However, there are a number of potential issues with used equipment that should underscore the warning “buyer beware”. To begin with, good used vacuum furnaces are a rare commodity.  When they do appear on the market, they don’t last long.  Many of the best are purchased through industry networking and never reach the general market.  Still, there are numerous dealers of used furnace equipment with inventories posted on their websites.

Most used furnaces are sold on an as-is, where-is basis with no manufacturer’s warranty.  If a decent used candidate is located, there are a few very important items to investigate before purchasing.  One of the biggest and most difficult to detect problems with used furnaces is the condition of the water jacket in the vacuum chamber.  The life of a properly maintained vacuum chamber can be well over twenty years.  However, in situations where the furnace cooling system has been connected to an untreated water supply, water jacket blockages from mineral build-up can begin to appear in as little as three years.  Beyond dissecting the vacuum chamber, there are no fool-proof methods for detecting blockages.  Ultrasonic testing is sometimes used but can be expensive and unreliable.  The presence of blistered or discolored paint on the outside of the chamber is a good indication of hot spots due to blockage.  Perhaps the best approach is to avoid altogether used furnaces more than twenty years old.  If water jacket blockage problems arise after purchase, the only sure solution is re-lining the chamber at considerable time and expense. BY JEFF PRITCHARD

Component parts come in all shapes and sizes. To meet this demand vacuum furnaces have been designed to accommodate many standard workload configurations. Despite the almost limitless choices, some common sense rules apply. It is important to recognize that loading arrangements generally fall into two classes: weight limited and volume limited. In either case, when loading parts in furnace baskets or onto racks the goal is often to maximize loading efficiency. One must also be concerned with proper part spacing, that is, how parts are situated within the load for optimal heat transfer (e.g. line of sight heating), support and stability of the load at temperature, temperature uniformity, and heat extraction during quenching so as to achieve the desired metallurgical properties and minimize distortion.