Killroy1999
Mechanical
- Nov 9, 2013
- 6
My work uses Watlow Multi Cell heater rods in in its vacuum process, but they don’t come the way we want them. We have a supplier do the following: Straitening (they can come bowed ¼ in), Oxidation (increases the radiation heat transfer and looks black), along with other things. The problem is that we can do ~15 a month, but we need to do up to 360 a month.
The concept is to do batches of 3, and combine the oxidation and straitening step, which is where the real question is: what is optimum for heat straitening .935 inch Incoloy 800 rods that are ~50 inches long?
Correct me if I am wrong, but I thought that most straitening was done more or less cold through mandrels or in tension. In this case, the rods are heated to 950 degrees C (1742 C) continuously, so they would be at risk of relieving the strain and warping again.
They current way they straiten them is put them in large heavy iron clam shell molds and then screw the molds together, heat to 1050 C (1922 F) for a minimum of 30 minutes and cool over 4 hours minimum. The iron molds are big heat sinks, which makes heating and cooling slow, they leave scale on the rods and the screws are seized.
The oxidation step takes 24 hours at 1050 C (1922 F) in air atmosphere and cool over 8 hours, and when I say cool, I believe it’s a linear ramp down from 1050 C to room temperature over 80 hours.
So the concept, is to combine the steps by using a fixture to hold the rods strait, but minimizing the contact so the surface can oxidize. We know that fasteners will seize, so we don’t want to do that and we want thus use high temp, machinable ceramics so the heat does not sink out of the rod and the heat chamber.
The fixture will allow the rod expand ~1”. The concept is to have a ceramic donut at the top, middle and bottom because the rods come with a simple bow and are not wavy. The rods go into a fire brick Kiln 30 at a time with the new concept and hang from the top and since they are heater rods, they are heat themselves and are controlled with thermocouples that run down the center.
Any ideas about, what is happening to the Incoloy at those temperatures and suggested temperatures and times would be appreciated. My concern is that even if we make the fixture out of high temp stainless steel at a much larger, stiffer diameter 3+ inches, the fixture will warp over time because 1050 C is HOT.
The current requirement on the drawing is .030” perpendicularity to the vacuum flange and .020” straightness, but we agreed to relax that.
Thanks in advance.
