Preheat crucible? Yes/No?

Discussion in 'Furnaces and their construction' started by Rotarysmp, Dec 8, 2020.

  1. Rotarysmp

    Rotarysmp Silver

    Hi Guys,

    Using graphite/clay crucibles in a gas/oil fired furnace for copper alloys and cast iron, is it better to first bring the furnace and crucible up to red heat before charging it with metal to melt, for the shorter time to melt, or does that just beat the crucible too much?

    Mark
     
  2. Rotarysmp

    Rotarysmp Silver

    Thanks Mark, so it seems I should not be charging cold.
    Mark
     
  3. FishbonzWV

    FishbonzWV Silver Banner Member

    First, let me state this is my personal opinion.
    To me, thermal shock happens on the preliminary firing in the furnace.
    I have adopted a slow warmup with a lazy flame with the pot charged loosely.
    The lazy flame is propane without the blower on while I setup the pouring area.
    This is usually 5 minutes and then I'll kick on the blower and ramp up the gas.
    The furnace, the crucible and the ingots all get to warm up slowly driving off residual moisture.
    Most all conversations about thermal shock seem to be centered around cool down.
    That, if you want to call it shock, occurs as soon as the pot is empty (while it's in your shank). After that, it's just loosing heat slowly.
    I don't return the crucible to the furnace until the foundry is broken down and the furnace is stored away.
    Since crucibles are the largest outlay of $'s in my foundry, I want to treat them kindly.
     
  4. You can charge the crucible when cold, just make sure the metal pieces aren't jammed in tight so they crack the crucible due to expansion coefficient differences. So long as the metal pieces are loose in the crucible it will be fine, all things being equal, you want to minimize the time the crucible is hot as graphite can burn and minimize exposure time to fluxes by adding to the molten metal just before pouring.
     
    Jason likes this.
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I do both. The initial charge is cold and as Mark mentions, just arrange things so chunks aren't lodged against the crucible walls. I should mention that having a resistive electric furnace, I have a slow enough ramp that I don't think there is any risk of thermal shock on the way up. My current furnace get's to 1800F in about 15 minutes but the crucible and charge lag.

    Preheating additional charge is a bit more problematic with a resistive electric. On subsequent melts, I just place a big hunk of ingot in the hot crucible which usually gets me >50% of crucible capacity and wait for that to melt. To add additional charge, if its a cold ingot, I only add about 20%-30% more to the melt at a time. If you drop 4 lbs of cold ingot into 4lbs of melt in an A10 for example, you can freeze the melt and risk breaking your crucible on the remelt. If I place the ingot over the vent/access hole in the lid with a wool around it, I can manage some preheat. Usually I have enough small stock, adding in several charges doesn't pose a problem.

    I'm not sure I totally get the reason for Morgan recommending charging the crucible hot. I think the theory is a large mass of cold metal in the interior cools the inner wall and can create a big gradient across the wall. In practice, I don't think that happens because the charge doesn't have great contact with the wall until it melts. Also, it doesn't seem like that shock can compare to what happens when you pull it out of the furnace and empty it, and wouldn't putting a cold charge into the hot crucible just cool the interior and create the same condition?

    I used to put my crucible back in the furnace for slow cool down but don't do so anymore. When I did, it made the thin foil aluminum skin oxidize and adhere to the crucible so I couldn't clean the crucible before next use. When I let the crucible cool in the ambient, this doesn't occur and I can lift the residual (aluminum) foil from the crucible wall with my finger and pull out the entire skin in one piece leaving the crucible spic & span for the next use though this phenomena is probably limited to aluminum duty.

    Best,
    Kelly
     
  6. dennis

    dennis Silver

    Perhaps this is a place for (restrained) "wool?" Put the pot down on a non-conductive plinth of sorts, then put the woolen "hutch" over it, more to prevent drafts than all else?
     
  7. Melterskelter

    Melterskelter Gold Banner Member

    Kelly, I can assure you this peel-out phenomenon does not occur for iron. It would be sweet.

    On the few occasions I’ve cast aluminum, I delighted in peeling out the skin from the crucible reserved for aluminum just as you describe. Delightful.

    For the first year of casting work, I tried the quick return to the hot furnace for slow cooling of the crucible, but I eventually quit based on the reasoning you posted here.

    Denis
     
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Do you charge your crucible cold for the first (iron) melt?

    Best,
    Kelly
     
  9. Melterskelter

    Melterskelter Gold Banner Member

    Yes, I load it with as much iron as it will hold and let er rip. Is that optimal? Don’t know.

    Added: my observations of crucible failure is that I’ve yet to have one crack. But they all erode due to exposure to heat, iron, and especially a reactive atmosphere. There is little loss of thickness below the usual level of molten iron. It is the exposed inch or so that seems to be slowly eaten away by time in the furnace. So, I fail to see how bringing the crucible up slowly would help. It might actually just extend the total exposure time and make things worse. But, if anyone has better info, I’d sure like to learn it.

    Denis
     
    Last edited: Dec 12, 2020
  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Maybe just to add to this comment and thread discussion, in aluminum duty with a good quality crucible, I think you can expect hundreds of melts as long as you don't have direct flame impingement, or create stress from thermal expansion of the charge. Refreezing the melt would be a special case of the latter. Relatively speaking it's just an easy life a far as crucibles go. It reminded me of the this thread and the only crucible I've had to replace. I listed this as a possible cause but looking back, I was doing multiple melts and in a hurry of course, and think it very likely I froze the half full crucible with a large chunk of cold ingot and it cracked upon remelt, or I'd probably still be on my original crucible. I didn't notice it when I was casting, but did the following day.

    http://forums.thehomefoundry.org/index.php?threads/cracked-my-a10.478/

    In iron duty the crucible temps are probably double and the combination of that with slag and metal contact much more aggressively attack the glaze and then crucible. Fluxes also can accelerate the degradation of a crucible but can be a necessary evil depending upon what you are melting. I do think initial seasoning prior to first use (or if it has set for a long period) to set the glaze and drive off moisture is good practice. There was a fella that posted here that had acquired a furnace with a crucible that had set outside for several years and on his first fire (presumably full heat) he literally shattered an A25ish crucible into pieces no larger than 4".

    What can you say, crucibles are a consumable, it's just a matter of rate.

    Best,
    Kelly
     
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I think your right there MS. It's probably the least of your worry when it comes to iron duty service.

    I have read this from multiple sources, that being it's best to heat full charges rather than partial. The only thing with that recommendation is you can't charge a crucible with solid metal so that it's full when it melts. Might be worth noting for holding for extended periods.

    Best,
    Kelly
     
  12. Melterskelter

    Melterskelter Gold Banner Member

    I once neglected to do the preheat seasoning routine recommended by Morgan prior to my very first use of a new crucible. That crucible lasted for only a few melts. That is the last time I will neglect that step! It matters.

    Denis
     
  13. dennis

    dennis Silver

    Reading the graphs, it seems that you want:

    1) initial climb to ~ 400 degrees F, to take two hours;

    2) intermediate, ~400 to ~ 1100 degrees, to take one hour;

    3) final, 1100 to 1800/2500, 1/2 hour (for 1800, and an hour for 2500.

    The initial run needs to go to 1800 F at a minimum, if I read Morgan's literature correctly - and it needs to take 3 1/2 hours, too.

    Sounds like a job for a hot-rodded kiln.
     
  14. Rotarysmp

    Rotarysmp Silver

    It would be difficult for me to get even close to that slow ramp up with my gas an oil fueled furnace. Even with my tiny 1/2" bunsen, I am pretty sure sure a single crucible heats up faster than that.
     
  15. HT1

    HT1 Gold Banner Member

    even in the Navy we could not do that, we threw it in the core oven overnight at 200, if someone was up late they turned it up 100 , first guy in the morning turned it up to 400, then it was just turned up to 800 (hot as the oven went ) , then it went in the furnace for a brass heat, which took about an hour to get it up to 2150 in an induction furnace,

    now days I throw it in the oven at 250 for for a couple of hours, then try and move it to the furnace at the end of the day after pouring , the radiant heat probably gets it up to 800, then next time it is brass at 2150 over 50 Minutes
    Best I can do


    V/r HT1

    P.S. Ive had some glaze up nicely, and some bubble, seem to last about the same, bubbly just looks gawdaweful
     
    dennis likes this.
  16. rocco

    rocco Silver

    The prescribed heating schedule is only practical for those who have an electric kiln with a multi stage ramp/soak pid controller, the rest of us are forced to be more pragmatic along the lines of what HT1 describes.
     
  17. Melterskelter

    Melterskelter Gold Banner Member

    I’m glad I have a programmable electric kiln. Set the parameters, press Start and check it in the morning.

    Denis
     
  18. Chazza

    Chazza Silver

    The only time I did not pre-heat my graphite crucible in 40 years it cracked!

    I only melt aluminium and always charge the crucible with small scrap to create a puddle, before adding large pre-heated scrap.

    Pre-heating only takes about 10 minutes on low flame and is cheaper than finding out the expensive way,

    Cheers Charlie
     

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