Vacuum furnace vessels can be grouped into so-called “hot-wall” and “cold-wall” designs, the latter being far and away the most common. In this article, the cold-wall vacuum furnace is the main topic I’m going to discuss, with particular focus on furnaces for thermal tempering treatments.
Let’s start right from the hot-wall furnaces, to understand the reason why modern vacuum furnaces are - almost without exception - cold-wall furnaces. Afterwards we will take a closer look at how a cold-wall furnace works, seeing the significant benefits of using this type of furnace, and below you will find important tips to help you operate your vacuum furnace.
Hot-wall furnaces: how they work
The early designs were hot-wall furnaces. For convenience, the hot-wall furnace is defined as a vacuum furnace, which does not differ substantially from a furnace working in an inert gas atmosphere, generally in nitrogen. In fact, the effect of the vacuum pump is to discharge the air contained in the retort and then refill it straightaway with gas. In the past, instead of using a vacuum pump, the gas was made to flow in order to eliminate the air contained therein. This furnace cannot generally work in a vacuum at the process temperature, since the ferrule would implode, unless it was oversized, something that is not usually feasible.
The hot-wall furnace consists essentially of a resistor in air, insulated with a considerable thickness of ceramic refractory, a heat-resistant retort made of steel or in the best case an Ni based alloy for higher temperature use (700 – 750 °C), a sealed door, where the seal is cooled by water circulation, an overall support structure and a blower that circulates air outside the ferrule to cool the furnace and load. An impeller inside the ferrule agitates the gas by forced convection of the heat that the ferrule transmits to the load.
More sophisticated vacuum plants also feature a gas circulator outside the ferrule with a water-gas exchanger. The hot gas is extracted from the ferrule and reintroduced cooled. This serves to cool the load more quickly. It takes an extremely long time to cool the load with this device as well.
Cold-wall furnaces: how they work
Cold-wall tempering furnaces are vacuum furnaces for low temperature use (Tmax 750 - 800 °C), and feature a water-cooled vessel, thermal chamber with graphite wafer insulation, convection impeller and load cooling impeller inside the vacuum chamber. For particular requirements (e.g. aerospace sector) they can reach extremely high vacuum levels, similar to vacuum furnaces for high/very high temperatures.
Normally the vacuum required entails the use of a primary pump and a booster. The thermal chamber, containing the hot zone confinement system, the resistor and the load support base, is constructed in accordance with aerospace criteria. This means that both the insulation and the resistor are made with a light mass, which affects the thermal calculation as a fraction of the load mass. Compared to hot-chamber furnaces, the vessel contains the cooling system (high flow rate impeller and exchanger). The load cooling gas flow is actuated when doors, one at the head and one at the tail, are opened.
The furnace has a compact geometry (generally horizontal development). The control panel programs the various phases of a cycle or multiple successive cycles, guaranteeing thermal uniformity (typically +/- 5°C). A system of thermocouples on the load sends the temperature measurements to a recorder. The number of thermocouples on the load may be significant.
In the initial phase the operation includes a period of emptying in heat up to temperatures designed to eliminate not only the air gases, but also vapors of water or residual oils from the processing or washing of pieces. This is followed by the introduction of neutral gas (Ar, N2) to accelerate the load heating process. Unlike what generally happens with hot-wall furnaces, once the treatment value is reached the furnace can be evacuated again at the annealing temperature, or convection continued.
The last phase is load cooling. This is a very fast operation. Finally at the end of the thermal cycles (from 1 to 3) the furnace can be opened to extract the load. This operation is allowed when the load temperature has reached the external temperature. As a result of the cycle set, the vacuum-treated material will have a mate (clear) appearance.
Material treated in this way features no coloration due to surface oxide films typical of materials treated in hot-wall furnaces, or similarly carbonaceous crusts due to oils, which require removal. The use of this type of furnace allows identical replications of cycles conforming to aerospace standard AMS 2750 REV. E and is suitable for obtaining NADCAP certification.
After examining the relative advantages of the cold-wall furnaces, let us now turn our attention to some important tips to help you operate your vacuum furnace.
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Vacuum furnaces: what you need to know to operate it
The vacuum furnace requires a different environment from that we are accustomed to seeing in “conventional” treatment departments. The environment must be “clean”. Operators must wear clean garments and white gloves. Vacuum-treated pieces must not be handled without such protection. A gloveless hand would leave an imprint on the pieces that would create a rust stain immediately. Pieces must then be packed in plastic envelopes and sealed after filling with nitrogen.
The operators are important in the loading-unloading and start-up phases of the cycle. The cycle must not be assisted. During the loading phase, the operators have to check initially that there are no breakages or deposits inside the furnace, introduce the load with the due caution so as not to cause damage, and do the same during unloading. During the start-up phase, they have to conduct the checks already introduced automatically for the plant’s seal. In the event of alarms, they must be able to understand whether the problem allows the alarm to be silenced or not and to ask for assistance if considered necessary. The plant can be guaranteed for years. The vacuum protects against wear but not against lack of human preparation.
Heating processes in vacuum are used in the industry for decades, since they offer a range of benefits that allow in special applications to obtain high quality products.
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