Do I really need a vacuum furnace?

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

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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.

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?

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Additional resources:

• 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.

Heat Recovery Steam Generators (HRSG) - GE Vernova

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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But does not that require gas quenchable steel?

No it doesn't, because any quenchant can be approximated with pressurised gas/vacuum furnace by tweaking process parameters.

For example you can introduce the gas at higher pressure and achieve a faster quench (simulating water) or introduce it slowly and simulate air quench. Big furnaces also have gas heat exchangers and fans, they can control the speed of cooling of the gas too.

There are air quenched steels, but they don't even need any pressurised gas/vacuum furnace. You can pull it out of the furnace, put it away and it will quench in free air.

What i have now is "silver steel" that i think is pretty much the same as O1 tool steel in the US.

Not quite

Silver steel I believe is water hardening steel (for example EU 1., US 115CrV3), while O1 is an oil hardening steel.

And i was thinking the vacuum was good for isolation as well. Because it will be no convective heating going from the hot part to the tube and endcaps, just a little radiative heating. So i could get away with making the end caps out of aluminium, and maybe even just use borosilicate glass for the tube.

At higher temperatures more and more energy is given off as radiation. At yellow heat (820C) quite a bit of energy is lost as radiation. Consider how hot your hand gets in front of a radiative heater that glows yellow. It's definitely not going to be "a little radiative heating". Borosilicate glass will be fine for the tube as long as the heat is not going to be on too long. The end caps... Hmm, they will get hot if near the part, but the other side will be in air so it could be OK. I would worry more about aluminium expansion ratio being different than your glass and cracking the tube.

I would forgo the vacuum in the first place. This way the whole thing becomes a lot easier, also with gas quench you can tweak a lot if process parameters.

I think i will try to make it to final dimension before heat treatment and see how it turns out. And if it gets warped i will have to buy some inserts to try hard turning.

I would love to hear how it turns out. I think that's the first time I hear anyone trying a vacuum quench in oil. I'm curious myself.

I thought "silver steel" was "drill rod" - which is to say the entire class of O, W, A - x hardenable steels. I am quite certain that it is used that way in at least some casual talk.

GsT

Indeed, the term is often used incorrectly

Silver steel is another name for W1, 1. or 115CrV3. It is a water hardenable steel for cold work(paper punches, tools that don't get too hot generally) . This steel has over 1% of carbon, some manganum, and no molybdenum. It makes it's appearance a lot more "silvery" than O steels for example. Also in contrast with O steels it is offered(at least here in EU) primarily in form of rods including some quite thin(is that why "drill rod" term is used? ) . One of its properties is very good quench cracking resistance.

O type steels( O1, 1. etc. ) or are also cold work steels, but there is a lot less carbon and additional molybdenum makes their appearance more grey. Also they have far better dimensional stability during heat treatment. That's why they are(used to be?) used for precision tools like v blocks etc. O steels crack a lot more easily that's why they are considered oil hardening steels.

As for A steels like A2. Unlike O and W they are chromium steels. They are also used for cold work, but they are considered "premium" over O and W steels due to many excellent properties. Some are, good abrasion resistance, very low dimensional stability etc.

About that "drill rod" term. I don't know where it came from. Perhaps from W1 being available in small rod sizes? In general any hardenable steel can be used for drills, but hot work steels will make much more resilient drills!

So as you can see there is one commonality between all W, O, A steels. That is they are all cold working steels. This means they will get annealed over a certain temperature considered low so in a way you could refer to them all as a group, but at the same time they have very different properties. So in a thread beginners might read I thought it useful to describe them briefly.

I've done heat treatment with a tube furnace (alumina ceramic tube) in vacuum, but not for 'oxidation', rather
because the alloy was a vacuum-killed specialty item (lessens the creep under stress by eliminating
nitrogen). For anti-oxidation, just cover the part with boric acid.

I wonder if anyone has a good way to get rid of that crust formed by boric acid on parts. I used it to prevent decarbonisation of my parts when I was making my "renzetti" blocks. It formed a glass like surface on the parts that was very difficult to remove (especially in threads which I packed with a lot of it..). In the end I had to run a carbide tap slowly through those threads to get rid of it. I would use it only for smooth parts it can be flaked off easily.

My recommendation for preventing oxidation is a thin stainless steel bag and a piece of paper inside. One can buy very thin stainless sheet to make them.

I've done heat treatment with a tube furnace (alumina ceramic tube) in vacuum, but not for 'oxidation', rather
because the alloy was a vacuum-killed specialty item (lessens the creep under stress by eliminating
nitrogen). For anti-oxidation, just cover the part with boric acid.

From what i have seen both boric acid (or any anti scaling compound) and stainless steel foil with paper packing gives varying degrees of hard to remove surface layers. I want the part to be shiny right out of the heat treatment with no post processing.

If the end of the pipe has a good finish i think i can solve both differential thermal expansion of glass and aluminium and any over pressure issue by making a silicone face seal so the aluminium end cap can move relative to the glass tube, and if there is any over pressure the end cap can just pop off the tube. And make both the induction heater and the release mechanism remote controlled so i can keep a distance in case anything happens. The part is small so i think the induction coil will be able to heat all of it without moving it up and down in the tube.