Fluidized Sand Bath for heat treatment

[JHCC]

Apparently this technology has been around for a while, although I’ve personally never encountered it before.

A fluidized sand bath is a container full of sand (here, aluminum oxide rather than silica, to reduce dust) with compressed gas injected from below, which keeps the sand grains from packing together, making them behave more like a liquid than a solid. In this setup, that container is surrounded by an electric kiln that heats the bath to the precise temperature needed for a specific heat treatment step (e.g., 1,675°F/913°C for normalizing 52100). The result is the fine temperature control of a salt bath or a heat-treatment oven without the environmental hazards of the former or the warpage risk of the latter. 

While this is clearly beyond the needs or budget of the casual knifemaker, it’s nonetheless a fascinating advancement in heat treatment technology. 

On a side note, the use of a robotic pick-and-place machine to move the blade from the sand need to the quenchant is also fascinating, as it adds another level of precision to ensuring that the blade moves straight up and down, thus further reducing the risk of warpage. 

[JHCC]

Here’s another example, this one from Australia. 

This fellow is using nitrogen as his fluidizing gas, to eliminate the risk of scaling. 

Apparently a somewhat lower-temperature version of this is also used for parts cleaning in industry, as quieter alternative to tumbling. 

[J.P. Hall]

I've been toying with the idea of making one for quite a while. I've been using a PID-controlled gas forge for heat treating and its biggest limitation is evenness of heat. I was thinking of having a stand that could tilt it vertical to accept the tube. A dedicated setup would obviously be ideal, but I'd like it to be something that other people could build for a modular setup with their own forge. I'm imagining a coil of small diameter tube with small holes drilled in it sitting in the base of the main sand tube.

A no-weld version would be great for both testing and maintenance, as well as minimal barrier to entry. The main issue would be passing a small tube into the larger one while maintaining a seal, not unlike a bulkhead or tank fitting for pipe.