For heat treating purposes, "quenching" can be defined as the rapid cooling of a metal to impart some desired property such as hardness.

Different metals and alloys require different quenching rates to achieve their optimum properties. Regardless of equipment design, gas quenching in vacuum furnaces involve the same basic principles. The gas quenching process normally consists of the following sequence of events. First, the power to the heating elements is shut off. Next, the furnace chamber and quench loop are backfilled with a non-reactive gas, commonly nitrogen or argon. READ MORE

There are several factors to be considered in preparing workpieces for vacuum heat treating or brazing. Cleanliness of the workpieces and baskets or fixtures is very important.

They must be free of oil, dirt, machining lubricants or other contaminants prior to being loaded into the furnace. Some lubricants contain sulphur compounds which can adversely affect the alloys being heat treated. Inadequate cleaning can also cause staining and discoloring of the end product or result in poor braze alloy flow. Contaminants with high vapour pressures will evaporate during heating causing loss of vacuum. The vapours may eventually condense on colder surfaces in the furnace only to re-vaporize to cause contamination problems in subsequent runs. READ MORE

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 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).

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.

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.

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 turbo-prop 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.

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.

By J.E. Pritchard, S. Rush, A. Kiela

VAC AERO International Inc.

Abstract

Application of sacrificial coatings has long been used to reduce rotor-shroud clearances in gas turbine engines. Materials normally used for these coatings include sintered metal-powder segments, sintered metal-fibre segments, metallic honeycomb (filled and unfilled), elastomers and thermally sprayed abradable coatings. Thermally sprayed coatings offer advantages over the other materials, including direct application, easy removal and repair, variety of coating materials and good performance. New abradable thermal spray coating materials have been developed for performance in industrial turbine engines at operating temperatures up to 980ºC. Results are presented from laboratory evaluations of these coatings by burner rig and hot abradability testing.

For maximum fuel efficiency, many gas turbine engine designs depend on sacrificial coatings to tighten internal clearances between moving parts. An extra gap of .005" between the rotating blades and the engine casing can increase fuel consumption by as much as 0.5%. As fuel comprises more than half of direct operating costs, this waste can be significant. Engine efficiency largely depends on close clearance between blades and casing. Clearance can be affected by a number of engine operating variables, including casing expansion and contraction, loading due to maneuvering, thrust, gust, stall, vibration and manufacturing tolerances. An industrial turbine engine manufacturer was experiencing unsatisfactory results with the ring segment coatings used to maintain rotor-shroud clearance. Because of poor abradability, the coatings caused excessive wear on the tips of the turbine blades. The engine manufacturer and VAC AERO agreed to work cooperatively to develop an improved abradable coating for these applications. Read More

When joining aluminum for aerospace electronics, brazing often is the most practical choice for creating a continuous all-metal joint interface. Because of its light weight and excellent thermal conductivity, aluminum often is the material of choice for assemblies that house or cool airborne electronics. These complex assemblies often are manufactured from numerous individual components that must be joined. There are many ways to join aluminum including mechanical fastening (screws, rivets, etc.), adhesive bonding, welding and brazing. The selection of a joining process must be based on a careful analysis of the service requirements and the materials involved.