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Archives for October 2014

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.