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Looking after Mechanical Vacuum Pumps

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This article is written for vacuum pumps such as the oil sealed rotary piston pumps used on many heat treating and vacuum furnace applications. The same information would also apply to the oil diffusion holding pump if it is used. This pump may be either a vee belt driven pump or a direct drive pump. The holding pump is used to keep the oil diffusion pump evacuated below the critical backing pressure when the main pump is in roughing mode.

All mechanical vacuum pumps need maintenance and the pump manufacturer usually lists the basic checks needed in the pump operation manual. This will vary with the application that the pump is used on but, at a minimum, will include the following: check oil level daily or weekly, depending on the application and use, change oil and check the shaft seal area for leaks every 6 months and inspect the exhaust valves and gas ballast valve seals every 12 months.

Delineation and Measurement of Grain Size by EBSD

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Grain size measurement by electron backscattered diffraction (EBSD) has several unique advantages over the traditional measurement of etched specimens by the light optical microscopy (LOM) approach as defined in ASTM E 112. This is most evident when trying to measure the grain size of twinned face-centered cubic (FCC) metals where two major problems are encountered.

First, in many cases, it is difficult to reveal a very high percentage of the grain boundaries by etching. Secondly, nearly all etchants for twinned FCC metals do reveal the twin boundaries and one must ignore the twin boundaries when measuring the grain size by LOM. The notable exception to this experience is electrolytic etching of the 300 series of austenitic stainless steels where Bell and Sonon’s aqueous 60% nitric acid [1], using a platinum cathode and a voltage no greater than 1.5 V DC will reveal nearly 100% of the grain boundaries and virtually none of the twin boundaries. Another significant problem that affects EBSD results somewhat more than LOM etching results is the greater difficulty in preparing the highly ductile FCC metals to the perfection needed to get a very high percentage of the pixels to be indexable. Specimen preparation [2-4] is a very critical step in getting a very high percentage of indexable pixels in the field of view. This is not a trivial matter.

Loading Practices for Vacuum Processing

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Once a good fixture design has been developed, careful consideration should next be given to the loading of the workpieces.

Heating in a vacuum depends mostly on the transfer of energy through radiation from the elements to the load.  For uniform heating and cooling, it is important that the workpieces are not shielded by one another.  Pieces within the load should be evenly spaced to ensure even exposure to radiation.  The size, shape and high-temperature strength of the workpiece should also be considered during loading.  Alloys with complex shapes and relatively low strength at heat treating temperatures may distort during processing.  In some cases, it may be necessary to support these components with specially designed fixtures. BY JEFF PRITCHARD

Inlet Filters for Mechanical Vacuum Pumps

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This article discusses inlet filters that are used on oil sealed mechanical medium vacuum pumps such as rotary vane and rotary piston pumps typically used on vacuum furnaces and, for smaller pumps used for many laboratory and light industrial applications. One of the downsides of any trap is that it will eventually require servicing. Many vacuum system operators prefer not to use traps for that reason. If the correct traps are used and maintenance is planned, the downtime and service costs can be kept in line.

There are four types of inlet filters used on vacuum pumps used in laboratories and in light industrial applications: foreline traps, catchpots, dust traps and vapor traps. The first, foreline traps, are used to prevent contamination coming out of the vacuum pump; and the other three are used to prevent contaminants from entering the vacuum pump. Foreline traps – This type of trap is to prevent oil vapor that moves out of the pump inlet under low pressure conditions when the gas is in molecular flow. That would be at a pressure lower than about 0.1 Torr or 100 microns. The ultimate vacuum of an oil sealed vacuum pump is reached when the hot oil in the pump starts to evaporate. Under these conditions some molecules of oil vapor will backstream from the pump inlet toward the vacuum system. Although back streaming of oil vapor occurs in larger pumps as well, it can be more critical in smaller vacuum systems where the piping is shorter. Instruments such as mass spectrometers, electron microscopes and ultra-high vacuum systems can be contaminated if oil vapor reaches them so most of these instruments use foreline traps. If these instruments become contaminated it can take several days to clean them out and return them to operation.

Difficulties Using Standard Chart Methods for Rating Non-Metallic Inclusions

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Untitled-1Over the years, ASTM Committee E-4 on Metallography has conducted interlaboratory test programs to evaluate the precision and bias associated with measurements of microstructure using proposed and existing test methods. ASTM decided in the late 1970s that all test methods that generated numerical data must have a precision and bias section defining the repeatability and reproducibility of the method. Defining bias associated with a test method is difficult unless there is an absolute known value for the quantity being measured and this is not possible when microstructural features are being measured. This paper shows the results for an interlaboratory test using Method A, “worst field” ratings of inclusions in steels by ASTM E-45. The results from 9 people who were reported to be qualified, regular users of the method revealed consistent problems of misclassification of inclusions types and a wide range of severity ratings for each specimen.

ASTM E45 was created in 1942 and was based on an earlier (1, 2) chart developed by Jernkontoret in Sweden. The charts were designed to determine the size, distribution, number and types of indigenous inclusions (naturally occurring particles that form before or during solidification due to limited solid solubility for O and S) in steels. Originally, E45 included 3 charts, Plates I, II and III, but now there are two, Plates 1r and II. Plate 1r replaced Plates I and III after these charts were measured (3) and corrected in the creating of the image analysis method for making E45 JK inclusion ratings (4, 5) published as E1122 in 1992, which was incorporated into E45 in 2006. The JK chart, the original Plate I, categorized indigenous inclusions as: sulfides (type A), aluminates (type B), silicates (type C) and globular oxides (type D), although the classification was stated to be only by morphology. There were thin and thick categories of each based on their thickness (or diameter for the D types) and the severity ratings varied in whole increments from 1 to 5. Plate III was similar but the severity limits were in 0.5 increments from 0.5 to 2.5.

Vapor Pressure and Evaporation in Vacuum Furnaces

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Knowledge of vapor pressure and rates of evaporation of various materials is valuable information for those operating vacuum furnaces, whether we are heat treating or brazing at high temperature and low vacuum levels or dealing with outgassing at very low temperatures and pressures.

When we think about a solid or liquid in a sealed vessel, we find that, even at room temperature and atmospheric pressure, there are molecules that leave the surface and go into the gaseous phase. The gas phase thus formed is called a vapor. The process of forming a vapor is known as evaporation and the rate of evaporation is determined by the temperature of the substance involved. In time, some of the evaporated molecules will, in all likelihood in the course of random movement, strike and stick to the surface of the vessel. This process is known as condensation and the rate of condensation is determined by the concentration of gas molecules (that is, the pressure of the evacuated gas). Eventually, the number of molecules leaving the surface of the substance is equal to the number returning to it (that is, the evaporation rate equals the condensation rate) and we have dynamic equilibrium. The (partial) pressure at which this occurs is known as the vapor pressure of the substance.2 Below this pressure, surface evaporation occurs faster than condensation, while above it, surface evaporation is slower.

Revealing Prior-Austenite Grain Boundaries

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Revealing the prior-austenite grain boundaries in heat treated steel is probably the most difficult, and frustrating task, faced by the metallographer or metallurgist. Grain boundaries, regardless of the type, are generally impossible to see in cast metals, as they solidify dendritically and segregation is present and often substantial. After deformation and annealing, if recrystallization occurs, grain boundaries in the product may be visible, but they are not necessarily prior-austenite grain boundaries.

In a deformed, partially recrystallized specimen, it is usually possible to see both recrystallized and non-recrystallized grain boundaries. But, prior-austenite grain boundaries are those of the steel when it was austenitized prior to quenching and tempering. If the steel’s microstructure is fully martensitic after hardening, or contains some retained austenite or lower bainite, the prior-austenite grain boundaries may be revealed. They can often be revealed in specimens isothermally processed to obtain fully lower bainitic microstructures; but they cannot be revealed if the transformation microstructure consists of upper bainite, pearlite and/or ferrite. Composition also is important in trying to reveal the prior-austenite grain boundaries, as is the tempering temperature. In general, steels with low carbon contents and low phosphorous contents are very difficult subjects.  This article summarizes the state-of-the-art in revealing prior-austenite grain boundaries.