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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Archives for February 2015

Temperature Uniformity Surveys

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All furnace equipment used for heat treating should be properly instrumented and periodically tested for uniformity. The temperature uniformity within the furnace must be regularly surveyed.  The frequency of surveying is largely dependent on the type of equipment in use and its previous history in accuracy and reliability.  Exact survey frequencies should be determined from applicable processing specifications.  However, quarterly temperature uniformity surveys are fairly standard.

The purpose of the uniformity survey is to determine the range of temperatures present at different locations in the furnace under normal operating conditions.  A furnace is normally qualified through an initial comprehensive survey.  This involves determining temperature variations by surveying at the maximum and minimum operating temperatures and at a series of intermediate temperatures not more than 167ºC (300ºF) apart.  After initial qualification, periodic surveys can be taken, usually on a quarterly basis.  Unless otherwise specified, periodic surveys can be performed at a single temperature that rotates between the minimum, mid-range and maximum operating temperatures of the furnace.  Uniformity surveys are also performed after any major repair to the furnace or when the operating integrity of the equipment may be in question. BY JEFF PRITCHARD

Calculating Evacuation Time

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When a vacuum system is designed it is often necessary to select a mechanical vacuum pump or pump set that will evacuate the chamber and associated piping in a certain amount of time. In laboratory or research situations that may not be as necessary as in a production environment where the time to complete a process has quite a lot to do with the cost of the manufactured part. In addition, the cost of the vacuum system has to be taken into consideration as well. Larger pumps may reduce the evacuation time, but also are more expensive. There has to be a balance between all the parameters.

There are two simple methods for calculating evacuation time; one for a rotary vacuum pump, vane or piston, on its own, and a second for larger volumes when a vacuum booster pump may be used. Both methods give good results for simple vacuum systems where the mechanical vacuum pumps are located close to the chamber and the chamber is relatively empty.

Use of the Scanning Electron Microscope in Failure Analysis

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Commercial introduction of the Scanning Electron Microscope (SEM) in 1965, and its subsequent rapid development and implementation in metallographic laboratories, has had a profound influence on failure studies. The chief advantage of the SEM is its great depth of field in comparison of the light microscope. Observations can be made over a much wider range of magnifications including those above the range for light microscopes (and those below the range of TEM replicas). Examination of fracture features by SEM is much simpler than through the study of replicas with the Transmission Electron Microscope, TEM. A further advantage of the SEM is the chemical analytical capability of spectrometers that can be attached to the microscope, Energy-dispersive Spectrometers (EDS) being the most common.

The study of fractures with the unaided eye has been practiced since antiquity chiefly for controlling the quality of metals production. R. A. F. de Réammur may have been the first to examine and publish drawings of fractures examined at high magnifications, at least 100X, in 1722. R. Mallet appears to have been the first to link fracture appearance to service performances in a study of failed cannon barrels published in 1856. Adolf Martens may have been the first to study both fracture surfaces and the underlying microstructures in 1878 followed by the first description of fracture surface features in 1887 when he showed that these lineal features could be traced backward to identify the fracture origin.