High temperature fastener for furnace up to 3000F

Q: I am looking for a suitable fastener for use in a high heat furnace that is running at 2000F but the actual hearth wear flames go up to 3000F. The furnace is not under vacuum and has a normal atmosphere. We are finding that the 304 bolts are failing rapidly and we tried Inconel bolts which are doing better but still failing in about 2-3 weeks. Do you have any recommendations?

A: At those temperatures in an oxygen rich environment you need to consider ceramic fasteners. As long as your loads are not too high and thermal shock is not an issue, I think this would be your best option. Since your hearth can get up to 3000F, I would recommend using Alumina ceramic bolts which provide thermal resistance to 3000F. In addition, Alumina is almost inert to corrosion including acids, solvents, salts and organics.

Cost comparison of Zirconia vs Alumina screws

Q: Why does Zirconia cost more than Alumina?

A: 1) Raw material cost: Aluminia is far more abundant, whereas Yttrium oxide which is a stabilizer for Zirconia ceramic, is a rare earth element.

2) Shaping costs: Due to its higher density it takes approximately 10x longer to grind down and shape Zirconia into parts.

3) Higher sintering temperatures: The thermal shock resistance for Zirconia is poor so it requires a more costly sintering process.

Zirconia vs alumina fasteners

Q: What advantages does Zirconia have over Alumina?

A: Usually for most high temperature or electric insulator applications where ceramics are needed, alumina ceramic fasteners are the most popular choice due to their properties and relatively affordable price compared to zirconia fasteners. However sometimes the extreme properties of zirconia fasteners are needed which include:

Higher material density provides greater wear and corrosion resistance
Higher strength
Higher temperature resistance

Bolts for high temperature vacuum furnace: Metal vs. Ceramic

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.