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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Pressure Relief Valves

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One of the most critical components on any vacuum furnace is the pressure relief valve. While its function is clear, the fact that it needs to be inspected – and tested is either not well understood or simply ignored. Normally positioned atop a vacuum furnace it is in an area that is not always conducive to maintenance, and complicated in many instances by the fact that only the manufacturer can service them. 

What is a Pressure Relief Valve? A pressure relief valve is a safety device designed to protect a vacuum furnace from over-pressurization. An overpressure event refers to any condition that would cause the pressure to increase beyond the specified design pressure (the so-called maximum allowable working pressure or MAWP). The pressure relief valve is an integral part of the safety system provided on most vacuum furnaces. Vacuum vessels, including evacuated chambers and associated piping, pose a potential hazard to personnel and the equipment itself from collapse, rupture, or implosion.

What is the Purpose of a Vacuum Pump

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To successfully process component parts in a vacuum furnace, we need to create and control the “atmosphere” surrounding the work. In general, applications run in vacuum furnaces can be broken down into five main (5) categories: 1) Processes that can be done in no other way than in vacuum; 2) processes that can be done better in vacuum from a metallurgical standpoint; 3) processes that can be done better in vacuum from an economic viewpoint; 4) processes that can be done better in vacuum from a surface finish perspective and, 5) process that can be done better in vacuum from an environmental perspective.

A principal difference between vacuum heat treatment and all other forms of thermal processing is the absence of, or perhaps better stated, the precise control of surface reactions. In addition, vacuum processing can remove contaminants, and under certain circumstances degas or convert oxides found on the surface of a material. Typical vacuum applications include industrial, food and packaging, coatings, analytical and medical technology, solar, semiconductor technology and research and development. In the heat-treating industry typical processes involve: Brazing, Hardening, Annealing, Case Hardening (e.g. carburizing, nitriding), Sintering, Tempering and Special Processes (e.g. degassing, diffusion bonding). 

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.

The Use of Molybdenum in Vacuum Furnace Applications

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Vacuum furnace hot zones are manufactured using materials that can withstand temperatures in the range of 1315ºC (2400ºF) and higher. Of the various types of refractory metals in use, none is more common than molybdenum.

The popularity and widespread use of molybdenum in vacuum furnaces is due to the wide range of properties that it exhibits, namely: high melting point, 2620ºC (4748ºF), low vapor pressure, high strength at elevated temperature, low thermal expansion, high thermal conductivity, high elastic modulus, high corrosion resistance, and elevated recrystallization temperature, between 800º – 1200ºC (1470º – 2190ºF). Mechanical properties of molybdenum are influenced by purity, type and composition of any alloying elements and by microstructure. Properties such as strength, ductility, creep resistance and machinability are enhanced by additions of alloys such as titanium, zirconium, hafnium, carbon and potassium along with rare earth element (La, Y, Ce) oxides.

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

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.