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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Dry Lubricants for Vacuum Service

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Lubricants in vacuum applications include wet and dry lubricant types (Table 1), greases and oils. So-called “wet” lubricants tend to stay wet on the surface to which they are applied, while dry lubricants go on wet but dry as they are applied. In general solid particulates do not stick to dry lubricants but they do not tend to last as long as wet lubricants and as such need to be reapplied. By contrast, greases adhere better than oils and tend to last longer. Oil is preferred where the lubricant needs to be circulated.

The major disadvantage of conventional liquid lubricants is that they have relatively high vapor pressures (= 1.3 x 10-4 Pa at room temperature) and surface diffusion coefficients (= 1 x 10-8 cm2/s) with low surface tensions (in the order of 18 – 30 dyne/cm) and can volatilize or creep away from areas of mechanical contact resulting in high friction, wear or mechanical seizure. In addition, their volatility can cause issue with achieving proper vacuum levels and/or depositing on component part surfaces. The presence of other gaseous species in a vacuum environment (e.g., water vapor, oxygen, carbonaceous gases) can cause the force of adhesion between metal surfaces joined by liquid lubricants to be so strong that the joined areas can only be separated by fracture.

Hot Zone Repair, Re-build and Retrofit

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Besides catastrophic damage due to mechanical abuse, eutectic melting or braze alloy spillage, vacuum furnace hot zones will deteriorate over time as a result of the repeated thermal cycling to which they are exposed. 

The life of a hot zone may vary widely depending on operating conditions.  Furnaces that are operated constantly in aggressive environments (eg. sintering) or consistently at temperatures over 1370ºC (2500ºF) may require fairly major hot zone maintenance as often as once per year.  Hot zones in furnaces used for standard heat treating operations can be expected to last from 5 to 7 years. BY JEFF PRITCHARD

The Interlamellar Spacing of Pearlite

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The interlamellar spacing of pearlite is a very important microstructural parameter for steels containing pearlite, and becomes more important as the pearlite content increases towards a fully pearlitic microstructure. As the amount of pearlite in ferrite-pearlite microstructures increase, so does the strength, but toughness and ductility decrease. For a fully pearlitic steel, as the interlamellar spacing becomes finer, strength, toughness and ductility all increase. Consequently, in structure-property correlations it is important to measure the interlamellar spacing. This paper reviews procedures for performing such measurements. Due to the fineness of the spacing, either SEM images or TEM images of replicas or thin foils can be utilized. The range of spacings in a given specimen will be much narrower if the pearlite in the steel was formed isothermally rather than transformed over a range of temperatures, as in as-rolled or normalized steels.

Maintaining Oil Diffusion Pumps

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Even as turbomolecular vacuum pumps have displaced most small laboratory sized oil diffusion pumps these days because of perceived ease of use and cleanliness, most high vacuum heat treating furnaces still rely on a large oil diffusion pumps to generate the pressures below about 10-3 Torr needed for many metal conditioning processes.

The main reason for this is that turbomolecular vacuum pumps have a physical size limit due to the high rotational speed of the rotor. That size limit is around 320 mm or 13 inches inlet diameter and may vary a small amount from manufacturer to manufacturer. In many cases the pumping speed may not be high enough as it is directly related to the inlet size of the pump. Metal can disintegrate at very high speed, so the tip speed of the rotor blades has to be within the safe limit. Turbomolecular pump rotors have to move faster than the speed of the gas molecules they are pumping in order that the rotor blades can deflect the gas molecules downwards in the pump mechanism. The second reason that turbomolecular pumps are not used in many metal treating systems is they cannot tolerate any particulate matter entering them. They must only be used on clean vacuum systems.

Metallographic Imaging Modes

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5 wsThe reflected light microscope is the most commonly used tool for the study of the microstructure of metals. It has long been recognized that the microstructure of metals and alloys has a profound influence on many of their properties. Mechanical properties (strength, toughness, ductility, etc.) are influenced much more than physical properties (many are insensitive to microstructure).

The structure of metals and alloys can be viewed at a wide range of levels – macrostructure, microstructure, and ultra-microstructure. Microstructural examination should always begin with the light microscope progressing from low magnifications to higher magnifications, followed by the use of electron instruments, as needed. In the study of microstructure, the metallographer determines what phases or constituents are present, their relative amounts, and their size, spacing, morphology and arrangement. The microstructure is established based upon the chemical composition of the alloy and the processing steps. A small specimen is cut from a larger mass (for example: a casting, forging, rolled bar, plate, sheet, or wire) for evaluation.

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.