Q: I’m using a high temperature vacuum furnace at 2500F. What type of material do you suggest using and is there any advantage of using a metal over a ceramic?

A:   Under vacuum conditions with little to no oxygen present, there are some metallic options for fasteners which include molybdenum and tantalum. These metals are considered referactory metals and could be used up 2900F and 3600F for molybdenum and tantalum fasteners respectively. In addition to metals, alumina and zirconia ceramics could be used as well.

The advantages of each come from what is most important and the conditions of your application. For example, if quenching is involved, you might not want to use ceramic fasteners due to the thermal shock which will cause failure. However if having low thermal conductivity through the fastener is important, then ceramics are a good choice. If the fasteners are to hold higher loads, the molybdenum and tantalum metals might be more robust.

Vacuum furnaces offer engineers a unique processing environment: high temperatures,
controlled atmospheres, and rapid cooling. This low contamination environment is perfect for brazing, sintering, and heat treatment. Due to the constant extreme temperatures (typically 1,100–1,500°C), choosing the right fastener material can be a challenge. This brief paper will give you an overview on what high temperature materials will work best for your vacuum furnace fastener needs. As always, if you have a specific concern, contact us, for material analysis by one of our engineering team members.

Molybdenum Fasteners
Molybdenum is the ideal fastener material for use in a vacuum furnace – delivering super high strength at extreme temperatures. Because molybdenum bolts will oxidize rapidly in oxygen, the vacuum environment with little to no oxygen is perfect. In addition, molybdenum has a low coefficient of thermal expansion and a high level of thermal conductivity. It can also be “doped” or formulated into an alloy with titanium and zirconium, known as TZM, where it can exceed its typical strength and temperature limitations. Molybdenum has a usable temperature limit of approximately 2900°F/1600°C.

Tantalum Fasteners
Typically, tantalum fasteners are used in vacuum furnaces for sensitive electronic applications where contamination is an issue. This is due to the inertness of tantalum. Also, tantalum is useful when treating other tantalum containing parts or when heating to extreme temperatures above 3090°F/1600°C. Tantalum remains ductile at high temperatures, whereas molybdenum becomes more brittle. This is an advantage when cycling furnaces – often Tantalum can withstand temperatures of up to 2000°C in a vacuum and inert atmospheres.

Alloy 330
Though Alloy 330 cannot withstand temperatures as extreme as tantalum and molybdenum, it also serves a purpose in vacuum furnaces as it is good for metal fixturing and jigs. A high temperature steel, Alloy 330 is resistant to oxidation and offers good strength at high temperatures up to 2100°F /980°C. It is perfect for furnace applications where oxygen or air is present. In addition, Alloy 330 fasteners deliver a much more economic option when temperatures are high, but not extreme.

What About Ceramics?

Though ceramics have usable temperature limits well within the range of a vacuum furnace (alumina 3000°F / 1650°C and zirconia a 4350°F / 2400°C), the quenching cycles and thermal shock of a vacuum furnace can cause ceramic breakage. In addition, metal shielding has better emissivity back into the furnace. Emissivity is a techy word that means it reflects heat and radiation. However for certain applications, ceramic fasteners can play a role in vacuum furnaces. Here ceramics are used as fixturing at very high temperatures.