Mike Farmer
02-09-2012, 03:13 PM
I had to do an emergency shutdown on my 80 lb. electric melter just before Christmas. My wife needed surgery and we got a call from the surgeon she had selected asking if we could be in New Orleans the next day for a pre-surgery appointment. We live in Dallas so we had to leave that night. (The procedure she needed was new and only done in New Orleans or New York.)
My crucible was about half full of glass. I could have taken time to empty it, but we wanted to get on the road as soon as possible, so I just programmed the controller for my normal 2-day ramp down. I had a neighbor check on the furnace after 2-days. It seemed to be cool, but both the temperature controller and limit controller were showing errors. So I had her unplug the furnace to be safe. When I got home two weeks later, I discovered that both my thermocouples had opened up. I was using the heavy 8 gauge K-type thermocouples installed in alumina protection tubes. It appears the metal sort of crystallized over time and the thermocouples opened up. It is strange to me that they would both fail at the exact same time.
The crucible appeared to have survived. However, before firing the furnace back up, I decided to do a closer inspection using one of those extension mirrors to look at the sides closer to the bottom of the crucible. What I found was disappointing. There was a hairline crack. I pulled the crucible from the furnace and found the hairline crack appears to be just below the glass line and runs a little less than halfway around the crucible. It measures about 19 inches long. The crucible circumference is around 45 inches.
I have attached a couple of pictures to this post.
So finally to my questions -
Should I replace the crucible? Or should I try to slowly bring the furnace up and try to get a little more life out of this one, knowing there is some risk? Either way, I am planning on ordering a new one.
If I do decide to fire the furnace back up with this crucible, does anyone know whether or not you can see cracks in a crucible at working temperature? Would they show up as a dark line or brighter line?
If I can see a crack at temperature if it is there, it seems it might be worth the risk. I can bring the furnace up, use the remaining glass, inspect the empty crucible at temperature, and decide whether or not I can charge the furnace again.
Please let me know if anyone has thoughts on this or experience with a similar crack.
(The last attached photo shows part of the non-cracked region.)
My crucible was about half full of glass. I could have taken time to empty it, but we wanted to get on the road as soon as possible, so I just programmed the controller for my normal 2-day ramp down. I had a neighbor check on the furnace after 2-days. It seemed to be cool, but both the temperature controller and limit controller were showing errors. So I had her unplug the furnace to be safe. When I got home two weeks later, I discovered that both my thermocouples had opened up. I was using the heavy 8 gauge K-type thermocouples installed in alumina protection tubes. It appears the metal sort of crystallized over time and the thermocouples opened up. It is strange to me that they would both fail at the exact same time.
The crucible appeared to have survived. However, before firing the furnace back up, I decided to do a closer inspection using one of those extension mirrors to look at the sides closer to the bottom of the crucible. What I found was disappointing. There was a hairline crack. I pulled the crucible from the furnace and found the hairline crack appears to be just below the glass line and runs a little less than halfway around the crucible. It measures about 19 inches long. The crucible circumference is around 45 inches.
I have attached a couple of pictures to this post.
So finally to my questions -
Should I replace the crucible? Or should I try to slowly bring the furnace up and try to get a little more life out of this one, knowing there is some risk? Either way, I am planning on ordering a new one.
If I do decide to fire the furnace back up with this crucible, does anyone know whether or not you can see cracks in a crucible at working temperature? Would they show up as a dark line or brighter line?
If I can see a crack at temperature if it is there, it seems it might be worth the risk. I can bring the furnace up, use the remaining glass, inspect the empty crucible at temperature, and decide whether or not I can charge the furnace again.
Please let me know if anyone has thoughts on this or experience with a similar crack.
(The last attached photo shows part of the non-cracked region.)