The buyers guide to vacuum furnaces: 10 things to know #1

The buyers guide to vacuum furnaces: 10 things to know #1

Excellence and appropriate equipment can be decisive for your company production. The vacuum technology has been established over the last few years, demonstrating how a rational initial investment results in concrete achievements in a short time.
In the previous article, Do I really need a vacuum furnace?, we analyzed the reasons that should motivate a company to equip itself with a vacuum furnace.
So if now there is clear the "why", in this and in the next article we will see the "what", we will find out what are the 10 essential elements to evaluate with respect to the dynamics of an oven that uses the vacuum system: once known, your choice regarding vacuum heat treatment will be definitely easier.

Vacuum technology is increasingly associated with innovative applications in the most advanced research fields: think for example of accelerators, synchrotron light beams and high-density plasma. Industrial processes have long exploited the vacuum to increase accuracy in results and optimize processes. I want to mention microelectronics and optoelectronics, the mechanism of superconductivity and uses in the field of metallurgy, in aeronautics and space. The vacuum technology is based on some basic knowledge that, regardless of the final application, are essential for achieving the desired optimal result: I’m talking about a standardized system of components, measures, practical rules, which are common to absolutely different plants. These are basic concepts and having a general and clear idea will be of great help for your next steps and, above all, for a careful choice of your next vacuum heat treatment furnace.

The development of vacuum furnaces was started with the aim of obtaining better extinguishing pressures, in order to broaden the range of treatable materials and reduce cycle times: this led to a revision in the meaning of the concept of vacuum technology, creating the conditions for the adoption of different component parts. Today it is not complicated to identify the elements that differentiate modern heat treatment plants from classic vacuum systems.
But let's go step by step. I lead you to a quick examination of the three aspects that remain basic and unchanged, that is:

  • welding
  • pumping and vacuum measurement systems
  • leak detection.

I will complete this first part by touching three other key factors for vacuum heat treatments, whose progress has been substantial in recent years, above all thanks to sophisticated research in the aeronautics sector. I'm talking about:

  • materials
  • sealing elements
  • components: flanges, valves, tightenings, etc.

Welds

In the manufacture of the vacuum furnace, the weld is the simplest junction, because it is codified by use and by precise rules (see the tables by CERN, ASME, etc.). These rules are justified by the potential danger of the presence of joints: for this reason, the encoder has imposed that the welding seam is simple and carried out on one side, while on the opposite surface a piecewise welding is allowed. This prevents the formation of microcracks difficult to identify.

Ask yourself then: what should be considered in the design phase of a furnace for heat treatment under vacuum?

First of all, that the weld does not generate deformations or important tension states, to be prevented by creating the necessary drains. Then, that the thicknesses of the joints are of the same order of magnitude. Finally, excess material must be made available to facilitate the joining without excessive filler material, avoiding any overheating.

Pumping and vacuum measurement systems

A vacuum furnace must be equipped with a pumping system , which has the crucial purpose of bringing the system to the final voids required by the heat treatment.
But the vacuum heat treatment furnace is generally a clean system, with well degassed surfaces and with no substantial amount of water vapor to be expelled. If the parts to be treated have been carefully cleaned beforehand, traces of condensates that may generate sludges or suspensions in the oils of the diffusion pump or of the rotary press will be minimal. Considering the cycle times (relatively long compared to those of a metallizer), a considerable reduction in the size of the pumping unit can be achieved thanks to the application of this precautionary measure.

For the different vacuum levels, the vacuum measurement heads remain:

  • the diaphragm pressure gauge with mechanical indication
  • the measurement head for the thermal conductivity (thermocross and Pirani)
  • cold cathode ionization heads (Penning).

On the subject of roughing pumps take a look at the article Roughing pump in high-vacuum furnaces for beginners.

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Leaks

Since always, the main cause of system degradation and lack of production in the course of vacuum heat treatments is traced back to leaks originating during operation. In fact - due to aging of the sealing elastomers, either due to the wear of dynamic seals, or due to overheating caused by a reduced flow of cooling water - it happens that there are leak situations: being promptly reported by the instruments, they must be interpreted by the operators with due attention.
If this aspect is of complex management, much can be done a priori to ensure that the plant already in place has no losses, usually caused by:

  • microporosity of the sheets
  • welding defects (in the "vacuum chamber" containment vessel or in the heat exchanger)
  • defects in the dielectric resins of feedthroughs
  • discontinuity of static seals
  • incorrect assembly
  • non-compliant design.

The leak detection operation must therefore be carried out with highly sensitive instruments on the insulated parts, in the various assembly and pre-assembly phases.

For more information on this topic, read Maintenance procedures: leak detection in vacuum furnaces.

Materials

Here I am at the crucial point. What are the materials to consider in the project? And which ones are obsolete?

The obligation to comply with the standards for pressurized equipment, the use of higher thicknesses, the need for strong welding and avoid areas with high temperature concentration: it has long been proved that light alloys and stainless steels are not materials that meet these needs.

Follow me in this quick overview on the materials for each part.

During the design and dimensioning of the vessel, both the maximum working pressure and the maximum temperature of the gases in contact with the wall during the extinguishing circulation must be considered.
In the realization of the thermal chamber it must be taken into account possible openings of the insulating screen, from which the radiation can escape. Although a rare case may appear, the designer will tell you that the three power supply passages of the resistor are sources of openings and that it should not be excluded that irregularities of the insulator - from damage or consumption - can be illuminating.

What to do?
Opt for high refractory and high thermal conductivity steels.

The insulating screen must be made of rigid graphite fiber wafers, with an extremely low heat transfer coefficient. Molybdenum is now eliminated, being a fragile material with great affinity for oxygen at low temperatures; its oxides also have a high vapor pressure.
The rigid carbon fiber (of which graphite is an allotropic state) is the material of excellence for stiffeners and wafer frames. A long series of tests, aimed at optimizing the thermal treatments of aeronautical parts, has been decisive in demonstrating that there is no chemical combination between graphite and metal: therefore, the use of graphite and the elimination of ceramic fibers (strongly hygroscopic, with a tendency to form chemical bonds with water vapors) are made possible today by the exceptional resistance of this material.
Graphite is also the base of the heating elements, with flat and extended radiating surfaces. The only precaution is to avoid important consumption of graphite by eliminating reducing partial gases (Hz, NH3, etc.).

Sealing elements

The sealing element of the vacuum heat treatment oven must be able to withstand both positive and negative pressures.

My advice?
Carefully study the O-Ring seals cavities. The movements of the gasket must be avoided in the passage from vacuum to pressure. And, if the sealing surface is large enough to put strain on it, suitable seals (for example with double lip) should be studied, able to adapt to a vacuum seal request with a section of the element and pressure seal with the other section. Ask that the specific loads, which can reach several tens of kg/cm2, are also carefully calculated.

Furthermore, in the event of:

  • seals for rotating shafts: it may be required to vary the seal oil pressure to compensate for pressure drops in the chamber.
  • held on axial movements: it is good that the motor fluid is made of a neutral gas to avoid possible blows of air during the command.

Components

Finally, here’s an often underestimated element: the components. I'm talking about valves, flanges, clamps, measuring heads, etc.: all of them will be sized to withstand pressure from outside in of 1 atmosphere.
An incautious use of such components, in addition to the serious danger for the personnel present in the installation area of the oven, can irreparably compromise a high-value vacuum technology plant.
Imagine the scenario: the catastrophic event would occur in the moment of maximum temperature of the vacuum furnace, with open screens, and with gas under pressure inside. This is enough to induce you to put this element in the checklist of the vacuum technology furnace project.

Conclusions

I hope I have been clear and useful in illustrating these first factors. But the list does not end like this: in the next part we will find out what are the other aspects to consider in the commission phase of the vacuum furnace.
Make this list your own and you will be the ideal interlocutor of the designer!

This is the first of two articles, stay tuned for the next one.

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See also

Do I really need a vacuum furnace?

Here are the parameters on which to base the choice of a vacuum furnace. Read our tips on vacuum heat treatments.

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