Making BluRam (phos-bonded, mullite-based refractory) Standoffs

In the process of building an electric heat treat oven (essentially a large top-loading kiln) it became evident that I would need to make custom standoffs to support the heating elements.

See the other thread at: http://forums.thehomefoundry.org/index.php?threads/heat-treat-oven-for-iron-build.1619/

They consist of a 3/4” (19mm) diameter BluRam shaft with a 10-24 SS stud protruding on one end and a Kanthal A 20 ga pin protruding from the opposite end. Kanthal for the pin was selected as it is quite heat resistant and SS for the stud as it should be less inclined to corrode. That would be important should the standoff need to be removed later for maintenance. The pin has a buried half loop to improve its purchase in the refractory and the stud has ground flats to resist unscrewing from the refractory. (It is likely the pin half loop and the flats on the stud are unnecessary, but it was easy to add them.)

For me the trick in making these was to ensure the refractory was densely compacted and the stud was nicely center and perpendicular to the end of the standoff. I first stumbled around trying a split mold. But it was very difficult to remove the column of refractory intact since it stuck vigorously to the mold. So, I ended up extruding the refractory after strongly pressing it into a plastic conduit mold. I used my milling machine to compress the refractory, position the pin and stud, and finally extrude the refractory from the mold. That method worked very well.

I then cured the extruded pinned standoffs at 205 degrees F for 4 hours and then fire them ramping over an hour to 1700F. The result is a very hard and very strong standoff that should easily withstand the temps to be encountered in the heat treat oven.

The key to success was adding thin plastic (paper cardstock probably would work as could aluminum foil) washers to aid separation of the refractory from the mold ends.

Here is 5 minute video showing a method I came up with that works pretty well. Suggestions welcome. My apologies for the pause midway while I directed the "videographer" to the appropriate part of the scene. It may not be obvious from the video that the steel cylinder that pushes the pin into the refractory has a hole in it to a depth of about 1" to allow the pin to be held by the cylinder. :



Denis

I decided to make a separate thread for standoff making to make the information more accessible to future searches that might be done.

 

For completeness: the cooking schedule that has worked very well for these standoffs is to dry them out at 205F for 5 hours and then rapidly ramp up to 1500 for half an hour and finish them at 1700 for 30 mins and then turn off the kiln. This results in an extremely hard and very fracture-resistant standoff. The pins and the studs are very tightly held with no play. The pins will be twisted around the heating element to encircle the element and secure it to the standoff. I have not tried other cooking schedules. I am sure many others would work. But, I know from prior experience that BluRam does not acheive full strength and hardness until about 1500 to 1700 F. And I know that rapid initial heating can cause some minor cracking of the refractory.

Denis

 

Today’s batch of no-longer-blue BluRam standoffs. They turn orangish when high fired.

Denis