The idea of making a furnace that utilizes a fluidized particle bed principle for heat treating knives came to me some years ago, but only two years ago I and two of my friends started discussing the project and make moves toward producing a prototype.
I chose Inconel 600 for the material for the chamber, as its working temperature up to 2000F allows to use it for heat treating both carbon and stainless steel. From material cost standpoint, this is not an economical way of building a furnace, but other options for the chamber - SS304 and SS310 stainless steels came up short, as SS304 maximum working temperature is below temperature for heat treating stainless and semi-stainless steels, and SS310 precipitates carbides if used under 1800F.
Once we settled on material for the chamber, we turned our attention to a diffuser. The principle of a fluidized bath is to have a gas enter the chamber from below and to suspend (fluidize) particles of heated media (aluminum oxide) while the chamber is heated to a desired temperature. On contact with metal, fluidized aluminum oxide acts like a fluid and heats the metal uniformly and much faster then air. After some back and forth, we decided the diffuser to be made from Inconel 600, so the whole unit will be from the same material, TIG welded together.
I decided to use tube furnace heating elements, to maximize efficiency. After some minor tweaking of a furnace temperature controller, the fluidized particle bath furnace is up and running and will see a lot of use in the coming months and years.