In a typical industrial vacuum system it is possible to trap pockets of atmospheric air when manufacturing the system and each time a cyclic system is unloaded and reloaded. One example is  fixtures that are bolted to brackets on the chamber wall. If the securing bolts are placed in blind tapped holes, gas can be trapped in the bottom of the hole. With a regular bolt in a blind tapped hole, the only escape for gas molecules is along the thread form where there is space in the root and crown areas of the thread. If a blind hole is unavoidable, bolts that have a hole through them will allow the air to escape easily during the evacuation part of the process. There is a company in the USA that specializes in these types of bolts and screws. Using tapped through holes eliminates this problem.

If a fixture is held in place with a bolt and a nut or nut and washer, air can be trapped in the space around the bolt in the clearance hole. Again the air will slowly bleed away during the evacuation and extend the pump down time. Even mating surfaces can trap air under them which will cause a slower evacuation because the trapped air will only release slowly. Another example of trapped gases in the heat treat industry is when porous insulation materials are used. Air will be adsorbed into the porous material, which has a very large internal surface area, at atmospheric pressure and will then be slow to release during the evacuation due to low conductance of gas from the very tiny spaces in the material.. By Howard Tring

Over the years, many people have asked if we could recommend good books on the subject of Vacuum Heat Treatment. The following list includes books that we have found particularly useful  with respect to the scientific and practical aspects of vacuum, heat treatment, metallurgy and material science. Enjoy..

As you can tell from the list, some books are classics which have stood the test of time, some are relative newcomers, but all share the common trait that they are used each and every day by those of us who work in the fields of vacuum, heat treatment and metallurgy. The readers are encouraged to offer suggestions as to their favorite and most useful texts. By Dan Herring

ASTM Committee E-4 on Metallography began a program to develop a test method for rating the nodularity of graphite in ductile iron in 1986. The initial effort centered upon using the sphericity equation to assess the shape. However, an interlaboratory study showed that the perimeter measurement varied with the magnification used. A perimeter-free shape factor based on the maximum Feret’s diameter was determined to be magnification independent and reliable.

Additionally, two types of “convex perimeters” were proposed over the ensuing years, but they were demonstrated to be highly biased towards yielding high nodularity ratings regardless of the irregularity of the particle’s shape. The study reported here compared the use of the maximum Feret’s diameter in the shape equation to the mean Feret’s diameter, 100X vs. 200X, a minimum shape factor limit for a particle to be a nodule of 0.5 vs. 0.6, and calculation of an area-based vs. a number-based % nodularity. By George Vander Voort

Burlington, April 14, 2014 – The production-scale, 2-bar VAH3436 horizontal vacuum furnace with work zone dimensions of 24” wide x 24” high x 36” is designed to process aerospace parts under high-vacuum and partial pressures with excellent process control and is rated at a normal operating temperature of 2400°F (1316°C). The hot zone is comprised of curved graphite elements and the heavy duty hearth is constructed with quickly removable rails of pure molybdenum designed to support a uniformly distributed load of 1000 lbs at 2400°F.  The vacuum chamber and door are constructed of carbon steel and feature a double-wall, water-cooled design. The furnace operating system is based on VAC AERO’s versatile HC900 interactive hybrid control package with SCADA and complete network integration capabilities and remote monitoring and control. The furnace is expected to run at full capacity next month.

As stated last month, in Part 1, this article talks only about one and two stage “medium vacuum” oil sealed rotary vane vacuum pumps that can produce a catalog ultimate vacuum of about 1 x 10^-2 Torr (0.01 Torr or 10 microns) for a one stage model and about 1 x 10^-3 Torr (0.001 Torr or 1 micron) for a two stage model..

Smaller vacuum pumps such as those used in the heating, ventilating and air conditioning industry (HVAC) are not included as they are often only for intermittent use and do not have the design features built into the laboratory sized continuous running vacuum pumps used in industry and science. Larger rotary vane vacuum pumps, ones that require ball or roller bearings to support the weight of the rotor are not included either. Although they have many similar features to the laboratory sized vacuum pumps, they also have a variety of options to suit different applications. By Howard Tring

There was a crack in the helmet which is not visible in this image (some associated damage can just be seen in the lower left side of the helmet visor). The crack was opened and the fracture began at a streak with mostly intergranular fracture and then propagated by cleavage as shown below.

Note the intergranular fracture in the center foreground. The walls show transgranular cleavage the propagated from the intergranular origin. Next to the fracture, we see a region of columnar grains at the surface with a small region of finer, more equiaxed grains below and the very coarse columnar grains blow that, as shown below. By George Vander Voort

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. By Dan Herring

On a warm, moist day, our earth’s atmosphere will contain a significant amount of moisture in it.  During the night, when the sun has gone down, this atmosphere will become cooled, and will not be able to hold onto the amount of moisture (water) that it could when it was warm, and so, some of that moisture will condense out onto the grass in the form of “dew”.  Then, during the following day, when the sun heats the air up once again, the dew will evaporate from the ground.

It is well known that the warmer the gas, the greater will be the amount of moisture that gas can hold. At any given point in time, all gases will have what is called a “dewpoint”.   The “dewpoint” of any gas is the temperature to which that gas must be cooled to get the first droplet of moisture to condense out of that gas (assumed to be at one standard atmosphere of pressure).  The less the amount of moisture in that gas, the cooler must be the temperature to which that gas must be cooled in order to get the first condensation to occur.  Based on that fact then, it will be understood that the lower the dewpoint of a gas, the drier (lacking moisture) is that gas. by Dan Kay

This article talks only about one and two stage “medium vacuum” oil sealed rotary vane vacuum pumps that can produce a catalog ultimate vacuum of about 1 x 10^-2 Torr (0.01 Torr or 10 microns) for a one stage model and about 1 x 10^-3 Torr (0.001 Torr or 1 micron) for a two stage model.

Oil sealed rotary vane vacuum pumps are used in the vacuum heat treating and vacuum furnace industry as a holding pump at the exhaust side of the oil diffusion pump. It keeps the exhaust line pressure low enough to prevent stalling of the oil diffusion pump while the larger mechanical pump is roughing (evacuating from atmospheric pressure) the main vacuum chamber. In the early days, pre-second world war, there were also oil sealed rotary cam pumps designs, such as the “world famous Cenco ‘Hyvac’ two stage vacuum pump and the Nelson Pump Co. ‘Nevaco’ vacuum pump (a). Although the Hyvac pumps and other Cenco models are still manufactured today by HyVac Products, PA, the Nelson Pump Company was bought by Ace Pump, TN, and their vacuum pumps are no longer made. By Howard Tring

Burlington, March 14, 2014 – The VAH5050 HV-2 horizontal vacuum furnace with work zone dimensions 36” wide x 36” high x 50” deep is the first VAC AERO furnace to be installed at this facility and will replace an old existing furnace. The hot zone is comprised of curved graphite elements and the heavy duty hearth is constructed with quickly removable rails of pure molybdenum designed to support a uniformly distributed load of 3000 lbs at 2400°F. The lightweight design’s low thermal mass enables faster quenching and longer life. The furnace is equipped with an external recirculating inert gas quenching system and includes gas storage and dual loop cooling systems. The furnace operating system is based on VAC AERO’s versatile HC900 interactive hybrid control package with SCADA and complete network integration capabilities and remote monitoring and control.