Crucible Failure

Discussion in 'General foundry chat' started by Petee716, May 31, 2020.

  1. Petee716

    Petee716 Gold Banner Member

    I've been using budget graphite crucibles from Legend for years. I'm not a real frequent flier so that isn't saying much and most of my casting has been aluminum. I've thrown one away before because it had been ridden pretty hard and didn't ring anymore but today I actually had one fail in the furnace. It was a #10 (10kg bronze) and its one I was using for Everdur. It probably had 15 bronze pours in 10 sessions over the last 2 years and it was my 3rd heat of the day. I had just checked the temp and had just opened the lid to get ready to lift and thought I'd better top it off. I was pouring my bell and didn't want to come up short. So I gathered up my last few scraps I had, preheated them for a couple minutes and added them. I submerged them and a minute later noticed it wasnt any fuller. I shut down and sure enough I could see it running out the side about 2 inches from the top. The floor, plinth, and pedestal covered. I had put a drain in this furnace but it was long forgotten and not functional. I laid the furnace on it's back at an angle and let it run for an hour. Quite a bit came out but I've got some repair work to do.
    The graphite had been completely burnt out of the top 2" of the crucible. I guess I used it one time too many.
    20200531_145259.jpg 20200531_180939.jpg

    I got off my wallet and bought some better crucibles awhile back but I haven't even taken them out of the box except to inspect them when they arrived. I'm not sure if my current shank will fit but I'll soon find out.
    I still don't have anything bad to say about the crucibles. You buy budget, you get budget. They've always worked fine for aluminum.

  2. Any photos of the one that failed in better days?. I'd be interested in seeing if it's like my cheapo one: there appears to be very cheap crucibles that are all right for a few uses and priced accordingly in their country of origin that get sold for premium prices by importers. My cheapo that failed after ten melts was $26 USD from the factory but a lot more with freight to Australia.
  3. Melterskelter

    Melterskelter Gold Banner Member

    Ah, bugger, as they say down under, I think. Frustrating, but you salvaged the situation well. I can remember some chatter here about drain holes and if they work. I don’t think anyone could report one actually working as designed when the need arose. I know I put one in my first furnace. But, it wasn’t long before it was thoroughly plugged with debris from skimming etc. None since.

  4. Petee716

    Petee716 Gold Banner Member

    Pictured are crucibles of the same product line from Legend Mining: Budget Graphite Crucible. I don't know if they're imports or not.
    Left to right, brand new #18 (kg bronze)unused, #8 used for bronze 3 or 4 times, #8 used for aluminum 5 or 6 times, #18 used for aluminum about 10 times and then used for iron once (last week). The 4th one looked much like the 3rd one until last week. I'll probably use it once more for iron. It feels sound but its also coated with iron slag!lol. I also slipped in a picture of my bell clapper. There was at least one success today.(some arrowheads too.) 20200531_214848.jpg 20200531_214854.jpg

    I think that #18 is around $50 plus shipping. The #10 I lost today is about $32 plus shipping. The #10 is my most common heat and the new ones I bought are the same capacity. Again I cant remember which ones they are but I know they were a significant upgrade.

  5. The far right one looks like my USD$26 Chinese import that was destroyed by iron slag eating it. To be honest it was sold to me as aluminium/copper alloys only, so I suspect your one won't be long for this world with iron in it.\


  6. OMM

    OMM Silver Banner Member

    Interesting! I had a fail on my second melt. (It was only with aluminum.) The aluminum just lifted right out about an hour later. I made provisions for a drain hole, but I abandoned it. I sprinkled my floor with the sawdust of my brick and with a paintbrush worked at around the sidewalls 2" high. I don't know if this was helpful or not. But the fail just lifted right out.

    I do get a little bit of a dusty start up. But I hold my breath and I runaway and I do it outside.

    The IFB sawdust I’m using. And my fail.
    I was using the square plinth which I lift it out about five minutes after shut down.
    Last edited: May 31, 2020
  7. Petee716

    Petee716 Gold Banner Member

    I remember when you initially posted that picture. I think I commented at that time that my oldest crucible (the one I threw out recently) looked just like it. I may have broken it to look at the cross section, I don't remember. I pretty much just assumed that's what they look like when they get used for awhile. I'm guessing now that it is normal for the cheap ones you and I have been buying!
    I assume the graphite fully penetrates the entire cross section of the crucible when it's new. Mine, like yours, shows depleted graphite as you move up the crucible till it reaches zero. Mine was at zero 2" from the top. Although I saw a couple of seemingly isolated spots a bit lower down this underscores the conversation we had recently about the temp being so much higher in the upper part of the furnace than down low. The flame may be more oxidizing in the upper part of the bore just by the nature of the furnace design, hard to say for certain at this point but it makes some sense to me. So since we're seeing the same thing in the lifecycle of these cheaper graphites in an unbaffled bore (I assume your crucible failed before you introduced the disk) I can place a refractory disk in the furnace and monitor the lifecycle of my brand new one to see what happens. It might take awhile though. In the meantime Ill be buying better quality crucibles moving forward.
    My first crucibles were steel propane cans so I'm no stranger to aluminum floods!
    Smoothness of the surface, absence of significant undercuts, and probably your sawdust were contributors to your ease of removal, but aluminum in general is a lot more cooperative than bronze when it comes to letting go of refractory. You'll notice that the residual aluminum skin easily peels out of a cold crucible. The bronze coating positively doesn't do that at all. It looks like iron doesn't either. It sticks, so I'll have some chiseling and patching to do. At least most of it is off the floor. I may put a layer of sand on the bottom and bring the furnace back up to temp now that it's upright and let the bronze that's on the walls remelt and run back down instead of just trying to chisel it off cold. I did reopen the drain hole, so maybe I can try to drain most of it out first, but will definitely need to recoat the floor. My pedestal and plinth will need to be replaced as they are completely coated and I'd be haunted by this episode until I do.
    I probably won't be back to the furnace for a couple of days so time will tell.

  8. Hi Pete, that failure was with the temporary 4mm mild steel disc that proved the principle, it was shedding liquid iron oxide over the outside of the crucible (10 melts old). Black iron II oxide melts at 1377 ℃ / 2511 ℉ according to Wikipedia so the disc at the bottom of the chamber was at that temperature at least.
    Last edited: Jun 1, 2020
  9. Petee716

    Petee716 Gold Banner Member

    I got to inspect the furnace yesterday. It turns out that laying it over and getting the metal to run up the wall was probably the best possible option. Most of the floor was left with a skin coating of bronze. There's a solidified river of metal up the side from top to bottom that needs to be removed. I'll put a piece of sacrificial sheet metal on the bottom as a catch, heat up the furnace and scrape it downward. A piece of 16ga bent in a curve and welded to a steel rod should do the trick as a scraper. After retrieving the catch pan I'll sprinkle a coating of dry sand on the bottom and let it cook onto the bronze skin. That should give me some tooth to ram a new refractory coating onto. Fortunately my tuyere is still a good 2" off the floor so I've got plenty of room to add a 1/2" layer.
    I rammed a new pedestal last night. Fortunately I saved the fire extinguisher form from my first one. I also made a disk per my discussion with Mark to see if it helps to give me better heat distribution in the furnace. Its 7/8" thick and will leave about 1" gap around the periphery. I'll give them both a few days to air dry and then fire them over the weekend. (The disk is drying on a flat surface, not standing up as shown in the photo). I rammed a dog food can into the pedestal form so it would be hollow which left a 1-1/8" gap all around. It's funny how whenever I need 1-1/8" dowel stock my broom handle gets shorter! Lol. (It's not the first time).

    image.jpeg image.jpeg

    Tobho Mott and Mark's castings like this.
  10. Fingers crossed it all works to plan. What are the plinth dimensions?, I'm trying to get a picture of the chamber size around the plinth you end up with. Attached is a photo of my current plinth about 5" in diameter and height made from a plastic 2 litre bleach bottle. It should give some idea of what I'm using.

    plinth 1.jpg

    furnace bore.jpg
    Last edited: Jun 4, 2020
  11. Petee716

    Petee716 Gold Banner Member

    Plinth is 5.25" diameter x 5.5" tall. Top of tuyere is 4.5" from the floor. Disk is 8.125" and furnace bore is 10"
  12. Tobho Mott

    Tobho Mott Gold Banner Member

    I take it the dowels are meant to center the dog food can inside the outer form. Looks good. Will the disc just sit up on top of the new plinth?

  13. Petee716

    Petee716 Gold Banner Member

    Yes. The dowels only go up a couple inches and center the can. I may attempt to remove them before firing so they dont expand on heating and end up cracking the thing, but if they dont come out I'll have to just cross my fingers. Ill try some kind of slide hammer technique. The can will come out eventually during use.
    The disk will just sit on tlhe plinth because I'm not really sure if I'll be keeping it or not and I doubt stability will be an issue.

    Mark's castings and Tobho Mott like this.
  14. HT1

    HT1 Silver Banner Member

    I've used the Legend Budget Crucibles, I stopped when they Quite giveing the Kick Posteriaor Case lot discount . Now I use Salamander or Vesouvious (Sp?) if I locate one for a good Price.

    One thing, the Ring test is only done to A new crucible,
    DO NOT scrape a crucible clean, seriosly degrades the life , after a pour invert it, If you have seen My videos, I invert mine on the ingot molds, when it is cool enough to handle with Barrel Handler Gloves 1500F I put it opening down in a metal bucket with brickes in the bottom, and cover it, with the lid of the bucket or a pizza pan (yes metal Buckets are hard to find now) when you break it out for next use gently rub the interior with a gloved hand and shake out the loose stuff

    scraping out a crucible causes you to loose wall, dont do it just use a different crucible for each metal watch hot crucibles for areas glowing brighter then the rest, that is your thin spots,, if you follow me on facebook you will see a picture of a crucible I retired looks like it has an evil smile .

    The cracked spakled look is not good ever, yes I have done 30 heats out of one, but it is a bad crucible to start with a good crucible will be solid on the exterior for sure,

    if you new guys directly impinge the flame onto the furnace, you will see a physical wear mark on the crucible . fix your furnace and replace your crucible NOW,

    I have not had a crucible fail for a few years, its a pucker facter that is hard to get past ( remember this is coming from a guy burnt recently bad enough that they where not sure I would not loose a finger ) so take it serious , spend some money Love you all too much to read yout Obit

    V/r HT1
    Jason and Mark's castings like this.
  15. Jason

    Jason Gold

    Imagine if it broke while holding it with some kaowool!:eek::eek::eek:;)

    We've only had a few small, but rather serious burns around here.. Let's keep it that way!
    Note for the single guys>>>> This stuff always happens to five finger rosie and not the other hand!:eek::confused::(o_O
  16. My cheapo crucible failed after ten melts instead of up to 50 melts or so with a quality crucible, so there's no cost or time saving to be made, rather the opposite. I'm now using a silicon carbide Vesuvius made in Mexico and a clay-graphite Morgan Salamander Super made in India for iron. The cheapo was sold direct from the manufacturer as an import sample specifically as a 30% silicon carbide crucible, if it is then they have yet to master crucible making technology: you can see small chunks of clay material in the mix, looking almost like clay bricks. I agree with HT1 not to try and clean the inside of a cold crucible, I've seen too much of the inside wall come away stuck to the aluminium, I do scrape the inside when I skim to dislodge crud.

    Pete: your chamber looks like it'll be pretty similar to mine so I hope it works well and improves the oil burn, I'd like to think I've some up with something new but that's never happened before. My 2" thick disc has some hairline cracks so far in a circle around the plinth so it must be a thermal difference. The disc air gap can be slightly closed if there's a lot of flame coming out in one spot but it's very sensitive so an even gap is probably best.
  17. Petee716

    Petee716 Gold Banner Member

    Got it.
  18. Gippeto

    Gippeto Silver

    An interesting read and good info. Curious about a couple things though...

    This is the crucible I'm using;

    For initial treatment, I baked it in the oven..250F for 4 hrs. Then into the furnace and brought it to orange heat. Closed up the furnace and let cool.

    It has around 20 melts in it now...all aluminum, and has been placed back into the closed up furnace to cool each time. When cold, the aluminum (mostly oxides) peels right out scraping required.

    Just wondering if I'm fast tracking my way to a failure and should be changing how I do things. There is a light crazing on the exterior as with Marks failed crucible.


    Attached Files:

  19. That's the same crucible, it just hasn't gotten really hot yet.
  20. Gippeto

    Gippeto Silver

    No, not "really hot" when compared to iron temps.

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