Welcome back! This is the second part of the article devoted to the vacuum furnaces hot zones, providing information that you need to be able to make a conscious choice on the most economical and best performing hot zone based on losses and overall power costs. In the first part, we went through the graphite-based hot zone design by analyzing its specific characteristics and problems. In this second part, we’ll deal with the all-metal design, whilst constantly keeping an eye on energy-consumption.
Let’s then go straight into the peculiarities of all-metal hot zones, focusing on their strengths compared to the graphite design, how the reflecting shields actually work, with particular attention to Molybdenum coating, highlighting its strengths and weaknesses.
All-metal hot zones: an energy, cost saving solution
In an all-metal hot zone, the shielding consists of molybdenum, tungsten or stainless steel. Molybdenum (Mo) is typically used in conventional all-metal hot zones for vacuum furnaces. For the sake of simplicity I anticipate that molybdenum alloys are used conventionally up to 1600 °C both for the resistor and the insulation; tungsten alloys are used for higher temperatures on commercial installations.
All-metal hot zones are used in high demand industries where sensitive materials are processed, such as aerospace, electronics and medical. There are heat treatments that require a particularly clean environment or extreme vacuum levels. There may be different reasons: in some cases the chamber’s graphite could interfere with the process, resulting in unwanted carburation of the pieces treated. In other cases, the load could be particularly sensitive to the presence of residues in the oxygen or hydrogen atmosphere (which could lead to embrittlement of the pieces), and so graphite wafer degassing during the cycle could be damaging. In these circumstances, the user should opt for all-metal heating chambers (shields and resistor).
Despite a slightly higher initial investment, all-metal hot zones provide the most economical solution thanks to the following properties:
- rapid heating and cooling;
- shorter pump-down times;
- higher ultimate vacuums;
- uniform heat distribution.
How do reflecting shields work in a metal hot zone
In a vacuum heat transfer can be reduced substantially by multiple reflecting shields.
A shield is defined as a surface that blocks the transmission of radiation when there is high thermal conductivity and low emissivity. The ability to form a barrier for the hot zone is increased if the shielding is provided by a set of Molybdenum (Mo) minimal-thickness sheets where the innermost sheet in the chamber, which therefore faces the hot zone, is coupled with a given number of similar parallel sheets and where the outermost sheet faces the cold wall of the vacuum vessel. The minimal thickness is required to reduce the heated metallic mass, and does not alter the shielding effect compared to a thicker sheet.
The higher the temperature, the more numerous the metallic sheets. The lower the material’s emissivity, the more effective the shield and the lower the energy loss. As well as having the capacity to withstand high temperatures, the molybdenum sheet possesses the fortunate property of having very low emissivity. This feature leads manufacturers to use full molybdenum shielding for the outermost and less hot surfaces as well. This shielding will have the lowest energy loss.
Metallic furnaces also feature certain interesting characteristics as regards the load cooling rate.
The heating chamber’s ultimate shield, the one facing the vessel’s water-cooled wall, has the same temperature as the hot zone at a higher temperature than the equivalent surface of a graphite chamber.
The presence of refractory insulation material and a considerable resistor mass, both in graphite, tends to make the graphite furnace’s hot zone slower during cooling, whilst the cooling rates reached in the all-metal hot zone are greater, at least at the highest temperatures, due to the shields’ higher temperatures but smaller masses. In certain applications this feature leads the heat-treater to opt for a metallic hot zone furnace not so much due to the final vacuum but due to this speed characteristic.
So far I have addressed positive technical issues. But what are the negative aspects that affect the all-metal hot zone? We are now seeing the disadvantages of an all-metal hot zone lined up with molybdenum sheets.
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All-metal hot zones: the drawback of molybdenum sheets
Molybdenum has a number of properties that the user should bear in mind. When it reaches the operating temperatures the material becomes brittle and can no longer be dismantled after the first heatings. Any attempt to handle the material inevitably causes it to break up!
In addition molybdenum tends to form oxides when oxygen is present (also at low temperatures) and such oxide has a greater emissive power. Any loss of vacuum creates this unwanted effect. A stringent procedure is necessary, before authorizing the start of the heat cycle, to ensure that the installation is free from losses.
Major non-repairable damage may be caused by the load striking the shield.
Material "colouration" due to the presence of traces of oxygen or material evaporating from the pieces alters and reduces the shielding conditions and also compromises the ability to achieve the thermal uniformity required by the specifications.
It should be mentioned that a metallic hot zone installation requires a greater level of skill and care.
By contrast these problems are resolved in graphite hot zones with simple maintenance operations, for breakages, and with cleaning cycles for deposits of evaporated material on the wafer surfaces.
So, the moment of choice has come! Graphite-based or all-metal hot zone design for your next vacuum furnace?
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