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Archives for July 2015

The Use of Tungsten in Vacuum Furnace Applications

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Tungsten is used in vacuum furnaces when there is a need for structural integrity at elevated temperature and/or in situations where other materials may degrade, such as when lower melting point eutectics are a concern. One example of its use in is roller rail assemblies in which graphite wheels are positioned between molybdenum rails using tungsten axles.

Tungsten (chemical symbol W) is a member of the family of refractory metal (Mo, Nb, Re, Ta, W) and has the highest melting point and vapor pressure of this group. Due to this unique property, it is commonly used as a material of construction in specific areas of vacuum furnace hot zones operating above 1315ºC (2400ºF). Tungsten can also be used for heating elements given that it has the highest duty temperature, typically 2800°C (5075°F). In practice, this rating is often downgraded as it is for all heating element material choices. Tungsten will become brittle, however, if exposed to oxygen or water vapor and is sensitive to changes in emissivity. In general, tungsten is resistant to corrosion below 60% relative humidity.

Measurement of the Volume Fraction of Beta Phase in Naval Brass

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Measurement of the amount of phases or constituents in metals and alloys is probably the most commonly performed quantitative microstructural test. The amount present is usually referred to as the volume fraction, although it is rarely expressed as a fraction but usually as a percentage. The volume fraction, or VV, in stereological terms, is the volume per unit volume of the phase or constituent. However, there is no simple direct way to measure the volume fraction. Instead we measure the area fraction, AA, a lineal fraction, LL, or a point fraction, PP, which can be measured and correlate with the volume fraction: VV = AA = LL = PP (1).

Areal analysis was first described by Delesse, a French geologist, in 1848. As the minerals were rather coarse in size, he could measure the area fraction of the grains of interest compared to the total two-dimensional area. As microstructures are rather fine in size, this is not a simple method to perform manually. Delesse suggested that a linear ratio of dimensions could also be used, but he thought that the accuracy would not be as good and did not try to develop a lineal analysis method. Rosiwal, a German geologist, was the first to publish a lineal fraction method in 1898 to assess the volume fraction. The point counting method to assess the volume fraction came much later and was proposed by Thompson in 1933, by Glagolev in 1933 and by Chalkey in 1943 – each working in different countries and different fields of science.