Vacaero

Manufacturers of heat treating and brazing vacuum furnaces and controls, complete hot zone and vacuum furnace retrofits, thermal spray coatings, plasma, HVOF and paint coating services.

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Fixture Design for Vacuum Processing

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Good fixturing and loading practices are essential elements in achieving proper heat treating results and long equipment life.

Fixture materials and design must be appropriate for the processing application.  Maintenance of fixtures is equally important.  The possibility of reactions between the workpieces and baskets or fixtures must also be considered.  High temperature sintering of the workpieces to themselves or the fixtures can occur.  Eutectic melting can also occur when certain chemical compositions come into contact at high temperature.  Selection of a fixture material is influenced by cost, service environment and compatibility with the workpiece and furnace hearth. BY JEFF PRITCHARD

Oil Diffusion Pump Controls

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Oil diffusion pumps remain in popular use in the vacuum heat treating industry, possibly one of the few applications remaining for this type of high vacuum pump in the western world. The main reasons for their continued use are their longevity the lack of other options. When your process requires a pressure below that of a mechanical pump or mechanical pump and Roots pump combination a secondary vacuum pump has to be used. These are oil diffusion pumps, turbomolecular pumps and possibly cryogenic pumps.

Turbomolecular pumps are limited in physical size due to the high rotor speeds needed to create molecular flow into the pump mechanism; and both “turbos” and “cryos” are very susceptible to process contamination. Large cryos are often used in vacuum coating applications but, as far as I am aware, not in vacuum heat treating applications. I think that many oil diffusion pumps are still used for industrial and some scientific applications in the eastern parts of the world where the cost of a turbomolecular pump is still very high based on the local costs of doing business.

Vacuum Furnace Quenching Systems: External versus Internal

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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. The quench blower then activates, forcing the gas through quench nozzles located circumferentially in a manifold that is part of the hot zone and into the hot load. As the gas passes over the load, it picks up heat. The hot gas then exits the main chamber and travels through the quench piping to a water-cooled heat exchanger, where it is cooled. After exiting the heat exchanger, the cooled gas is drawn back through the blower to be recirculated through the chamber in a continuous cooling loop. BY JEFF PRITCHARD

Metallographic Preparation of Titanium and Its Alloys

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Experiments were conducted using three-step preparation procedures for titanium and its alloys.  For CP titanium and alpha-titanium alloys, use of an attack-polishing agent in the third step was required to obtain good results.  The experiments defined optimum surfaces for each step and operating conditions.  Two-phase, α-ß alloy specimens are significantly easier to prepare than a single-phase α specimen. The method does yield perfect polarized light response with α-phase alloys, such as commercial-purity titanium.

Titanium and its alloys have become quite important commercially over the past fifty years due to their low density, good strength-to-weight ratio, excellent corrosion resistance and good mechanical properties.  On the negative side, the alloys are expensive to produce. Titanium, like iron, is allotropic and this produces many heat treatment similarities with steels.  Moreover, the influences of alloying elements are assessed in like manner regarding their ability to stabilize the low temperature phase, alpha, or the high temperature phase, beta.  Like steels, Ti and its alloys are generally characterized by their stable room temperature phases – alpha alloys, alpha-beta alloys and beta alloys, but with two additional categories: near alpha and near beta.

A Layman’s Guide to Understanding The Theory of Gases

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The movement of gases is an important and interesting subject but one often dismissed as a topic best left to scientists. However, the Heat Treater needs to know something about the basic nature (theory) of gases and in particular how they behave in vacuum. The main difficulty is that too much theory tends to become a distraction. Our focus here will be to better understand what goes on inside a vacuum furnace.

One definition of a gas is that it is simply a collection of molecules in constant motion (Fig. 2). The higher the temperature, the faster these molecules move, and as one might expect, the motion of gas molecules stops or dramatically slows down at or near absolute zero (0°K). As molecules speed up with an increase in temperature, there is an increase in their kinetic energy (or energy of motion). Molecular collisions occur between molecules and if contained, these molecular collisions against the walls of their container result in a pressure rise (which always occurs in a closed container when a gas is heated). In other words, pressure is simply the force per unit area that a gas exerts on the walls of its container.

Cleaning Practices prior to Vacuum Heat Treating

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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. BY JEFF PRITCHARD

Case Study: The Benefits of High Pressure Gas Quenching in Dimensional Control

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Highly distortion prone gearing (Fig. 1) was the subject of an investigation into the dimensional changes which result from utilizing either oil or high pressure gas quenching following a low pressure vacuum carburizing process. For comparative purposes, the gears in question were also atmosphere gas carburized and plug quenched, which is standard practice for these geometries. Full production loads (Fig. 2) were run using two (2) different carburizing methods (atmosphere, vacuum) in combination with free quenching in either oil at 75°C (165°F) or high pressure gas (nitrogen) at 11 bar.

Gears were taken from multiple locations throughout each load for analysis. Parts for metallurgical evaluation were selected from the center of each load. Multiple areas on each part were then analyzed for microstructure, case depth, and hardness (surface, profile, core). Dimensional checks (out of round, gear tooth profiles) were conducted on the gears before and after heat treatment. For brevity, only a portion of the complete test program is presented here (see Reference 4 for more detail).