Published on 6/11/2018
Categories: Insider tips

Do I really need a vacuum furnace?

Do I really need a vacuum furnace?

In which cases and why is it worth getting a vacuum furnace?

A company that wants to play its cards best, in terms of investment and yield, already knows that it must resort to the advantages offered by vacuum heat treatments. And this is in the short-medium term and in the long term. Because with a vacuum furnace the marriage is destined to last. So let's understand how to choose the ideal partner for years to come, starting with applications.

Who needs a vacuum oven?

The vacuum furnace finds its most common field of application in the heat treatments of some steels. Rapid tool steels, steels for hot or cold working tools, martensitic stainless steels, precipitation hardening steels, etc.: a vacuum heat treatment furnace has long been indicated as the recommended solution for working at top with these steels. It is shown that, with this type of system, the quality of the treatment is maximized, due to the minor deformations, and a great economic advantage is obtained.

There are many fields, ranging from the vacuum sintering of metal powders or ceramics to the vacuum brazing of aluminum alloys to continue with high temperature brazing, in which technological avant-garde stands out. In these sectors, the decision to use a vacuum furnace is linked to the possibility of implementing an advanced development production process, for which the focus was on high-yield plants.
Other typical applications are connected to a use in the nuclear or aeronautical field: here the requirements used to choose the type of furnace are based on highly technical parameters.
To sum up this succinct review of application fields, it can be stated that, for everybody, the question is not whether to buy a vacuum furnace, but what parameters make a heat treatments plant the ideal solution for your needs.

So, let's see what to consider to make a weighted choice.

Let's talk about power and isolation

I start immediately with a firm statement: the attempt to compare two or more systems to understand which is the most convenient is a stressful and non-productive operation. It is better to avoid this route and choose roads that guarantee the greatest success.
Your assessment must be based on a series of criteria, with respect to which it is essential to have clear ideas and, as it is not a test, our consultants will always be available to help you. What I will try then to do, entering into the merits of these aspects, is to explain how we can deepen the knowledge of the product offered to you.

The heating power of the vacuum oven is, of course, one of the elements not to be overlooked. It is often the case of two plants which, with the same functions, have different powers; the consumption of energy per cycle is not involved, since it is almost identical for both systems, but rather the possibility of having faster and therefore shorter cycles. It will be your assessment in terms of efficiency and timing, to establish which to prioritize.
Likewise, two furnaces with the same useful size and an insulated "hot zone" with graphite wafer panels of identical thickness (for example 40 mm), could have different insulation capacities, with divergent energy consumption at the same cycle, or an extremely varied life span. This is because the panels production process plays an essential role in their insulation capacity. The longest fibers and the presence of an external protection, with a sacrificial layer in flexible graphite foil or an abrasion resistant coating, are, in fact, fundamental in determining a greater effectiveness and longevity of the panels. This result is also found in the presence of a very fast flow of the extinguishing gas.

But we are still about insulation of your future vacuum heat treatment furnace. The fixing system of the insulating panels is another delicate aspect, since it must respond to the need to prevent the kinetic energy of the extinguishing gas from bending the panels. Insufficient fixing would be responsible for the damage that the gas could create to the insulator, infiltrating the external surface of the panel, that is in the panel portion in contact with the frame structure.

Always pay attention to this aspect: for example, systems for fixing panels that use CFC (Carbon Fiber Composite) screws are more resistant than those in Molybdenum. The optimal fixing system must guarantee a perfect adhesion to the metal support structure of the panel, even after many cycles, without undergoing a relaxation due to the thermal effect.

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Heat exchanger and vessel

Let's continue with the review of the key factors for the choice and let's talk about the heat exchanger. What affects its duration and its performance?

  • The load losses of the gas that passes through it;
  • the validity of the welds on the joints for the different materials (TIG welding or filler brazing);
  • corrosion on joints;
  • the vibrations caused by the crossing of the gas.

It is certainly advisable to analyze the load losses of the circulating gas, as a function of the path made by the heat exchanger (curves in the path or narrowings). The extinguishing gas pressure, in fact, can be the same in two similar furnaces, but the speed is different even if the impeller is identical.

Pay attention to the presence of an inverter. At the same pressure of the impeller and the impeller engine, the use of the inverter would allow you to obtain higher speeds of the impeller in the high temperature phase, when the density of the gas is lower, while lower speeds (equal to the plate of the engine) at lower temperatures.

In order to obtain a high cooling rate of the load, both the size of the heat exchanger and the material with which it is constructed have significant effects (construction steel, stainless steel, copper).
With the same maximum operating and temperature pressures, even if declared with a variety of codes (ASME, PED, GOST, ML, etc.), vessel performance is a factor of difficult comparison, especially if it is compared alone, between two units of different manufacturers.
For example, the circulation of the cooling water in the jacket can be conveyed to affect all the surfaces of the vessel subjected to thermal loading; in the event of incorrect distribution, obstructions may occur in the circuit due to the deposition of salts in areas that are not sufficiently cooled.

The role of the pumping unit

Also, on the pumping unit a couple of clarifications have to be made, which help in the estimation. Often, the producers' own catalogs fly over some technical aspects, which can not be overlooked by industry experts.

In fact, with the same flow rate, the type of pump can be very different; for the pre-vacuum phase the pumps can be rotary or vane pistons, but have a different life depending on the oil contamination. The longevity of the pumps, in this case, is determined by the release of the load in the treatment of solvents or powders. The Roots type intermediate pumps can have a direct coupling of an electric motor-pump shaft, or a coupling by means of the hydrodynamic coupling or, again, through the opening valve, to facilitate the recirculation of the gas under pressure. The emptying times vary according to the different executions.

A vacuum furnace, to be used, needs several groups:

  • a regulation and control panel (with or without PLC for safety checks, even with software suitable for the Customer)
  • a cooling water system (designed to calibrate consumption and meet the cooling needs of the furnace and the various pumps)
  • a gas supply system (N2, Ar, H2, etc.) with sophisticated pressure control
  • the dew point for measuring the purity of the gas, etc.

If you want more information on this subject, read: Roughing pump in high-vacuum furnaces for beginners.

Conclusions: the floor to the consultant

I have shown you what are the key points that must guide the choice of the vacuum furnace. Surely you have deduced that the game is to calibrate the weight of the various factors and get to determine a graduated scale of priorities.

The time has come to deepen, with one or more interviews with the supplier, all the aspects, especially those considered critical, for example:

  • maximum operating temperature of the oven
  • load cooling speed, assuming the size of the pieces it contains (kg of load, diameter / height / shape of the treated material and type of steel)
  • amount of operating shifts
  • amortization of the investment
  • ancillary costs of the plant (water circuit, electrical power required, layout and characteristics of the gas tanks).

Determining the vacuum heat treatment furnace that best meets the company's production needs is an interesting and important process: choosing the right business partner will affect both the quality of the product purchased and the advice provided at the time and in the years to come.

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