Degassing Lance

Discussion in 'Foundry tools and flasks' started by Al2O3, Dec 25, 2017.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The ID of the stainless tube is only 1/8". If the end of the lance failed, the initial pop would very likely spill or splash some aluminum. A continuous 5 psi through the tube may or may not depending upon how full the crucible is. FWIW, I think my initial set up before I balked was pressurizing the lance way higher than 30 psig because it was blowing the same hose off the lance repeatedly and I never even cam close to doing that in my bench testing. This only happened when I turned the Argon bottle off and then the downstream pressure climbed rapidly because it couldn't bleed off quickly enough through the lance. I've got a fix for this but it probably challenged the hot lance at a much higher pressure than my bench/trial test in the video. From what I saw on the bench testing, I'd say mechanical failure is very unlikely unless the lance becomes compromised by thermal shock or erosion from metal contact.

    Best,
    Kelly
     
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I tested the lance on a melt today. The metal for the melt was all cup and sprue stock from previous lost foam pours. The surface of the old cups tend to be pretty drossy so I thought they’d be good candidates to clean up.

    It would have been much easier to hang the lance in furnace lid with the plug and stop ring I built for it but I couldn’t get a good look at the metal nor video that way so I just did a run with the furnace lid open and the lance standing in the crucible. I’m glad I did because there were still little splatters of aluminum that left the crucible during insertion and removal. I think I’ll build a crucible hat to use with the lance because going forward I don’t want to splatter my nice new resistive coils and that will eliminate that possibility. Here’s what it looked like in the melt.

    10 Lance in Melt.JPG




    After the initial melt I skimmed the melt surface clean. The lost foam cup stock is kind of nasty and you can see what I initially drug off a 3/4 full A10 in the picture below. With the surface clean I inserted the lance with 3 psig Argon feed pressure for about 5 minutes. The second bit of dross on the right is what surfaced after degassing. Some of that might be from oxides forming at the surface so I think a crucible hat might be beneficial for reducing surface interaction too, but I’d say the degassing action certainly appears to have drug some additional oxides to the surface.

    11 Dross.JPG


    I don’t have a picture, but when I pull the lance from the melt, a layer of dross and aluminum formed on the backside of the lance. Most of it flicked right off with a couple swipes with my skimming spoon. The picture below shows what was left and a couple swipes with wire brush cleaned up the lance.

    12 Lance Post Metal Contact.jpg

    I got few ingots for my trouble which should be better quality metal than they started out from old cup and sprue stock

    13 Ingot.JPG

    All in all I’m very encouraged by how the lance seemed to work. Not sure what I think about the distribution of the surface bubbles yet but I’ll continue to use it and see how it manages life with metal contact. I need to make a cylinder cart for my Argon bottle too. What do you guys think?

    Best,
    Kelly
     
  3. OCD

    OCD Silver

    Interesting.

    Id make that lid so it goes half way down and over your crucible.
    I’ld also make the throat hole in the lid large enough so your lance shaft can lift out of it and the lid encapsulates the lance end and prevents splattering on the coils

    Is the lance strong enough to safely support the weight of the lid?
     
  4. PatJ

    PatJ Silver

    Looks like it is going to work out well.

    Did you section an ingot to check for porosity?
     
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Got started on the A10 crucible hat. Made a quickie mold out of XPS on my overarm.

    14 Crucible Hat Mold.JPG

    Mixed up a small batch of refractory and vibed it in. It weighs about 0.4 lbs and has 3/8” wall thickness.

    15 Crucible Hat Refractory.JPG



    It slides over the lance shaft like shown in the sketch below so it will lift on and off when you raise the lid. I can slide the plug over the shaft and have the lance and crucible cap a any height in the furnace by adjusting the stop collar on the lance shaft.

    16 Crcuible Hat Sketch.JPG

    We’ll see how it works out.

    Best,
    Kelly
     
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Not yet.

    K
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I made it like shown in the sketch above. Minimizes weight and seals off well. The crucible is 6 1/2" OD and the furnace bore is 10" so may as well not have a furnace lid at all if you would try to lift out the crucible hat. The vent hole is about 4".

    Yah, easily. The hat is only ~0.4bs.

    Best,
    Kelly
     
  8. OCD

    OCD Silver

    I was thinking,

    If the lance is still discharging argon during removal to clear the discharge holes the lance head would be spitting molten metal on your coils during removal.
    Hence, my thoughts about the “hat” lid to act as a shield of the spitting metal during removal.
     
  9. Negativ3

    Negativ3 Silver

    Looking good Kelly,

    Pouring an ingot before and after degassing, from the same melt would be interesting from a porosity inspection point of view.

    You "could" have a timer and a gas valve/solenoid, similar to the type in your favorite welder, control on/off gas with the lance in the crucible...
     
  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Good thought, but might get a bit bigger/heavier than I was thinking though. It would need to clear the pouring spout too. If the crucible isn't full, that sort of happens naturally, but there is still residual metal and dross riding on top the lance disc when it is extracted. I can probably just turn off the gas before removing it. Sort of doubt there'd be much intrusion into the lance through those tiny holes and not sure why it wouldn't just remelt if so. Time will tell.

    Best,
    Kelly
     
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Thanks -3. You can see gross defects but not sure I could tell the difference without the assistance of high magnification (which I don't have) and/or etching. I've only had one instance when I cut up or machined my lost foam castings that I've actually found a void that could be seen with the naked eye. It takes a very lustrous polish too. I have a lot of parts to machine at the moment so I'll do some experimenting. I'm pretty confident Argon degassing yields benefit but quantifying it is bit more of a challenge. One test might be whether degassed melts produce liquid tight castings (without impregnating them) such as my automotive water neck castings.

    I was wondering what you thought of the video of the vibrated refractory and whether you had proceeded with acquiring a vibrator? That refractory was mixed at the recommended 5% water content and was very dry. With a fragile mold like the polystyrene and only 3/8" wall it would have been impossible to fill the mold without the vibe.

    Best,
    Kelly
     
  12. Negativ3

    Negativ3 Silver

    A pressure test over a period of time would also go a long way to proving casting integrity.

    No movement on my foundry setup for a while, a few other projects to make action on before I get back to it. Definitely on the list.
     
  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I demolded the crucible hat. It has a couple bug holes but came out fine. I wiped down the polystyrene with Carnuba wax to make it release easily. Could probably improve on the bug holes if I used a surfactant like propylene glycol.

    17 Demold Crucible Hat.JPG 18 Demolded Hat.JPG

    Did a test fit and all looks well

    19 Hat Fit.jpg

    So into the furnace it went for cure cycle.

    20 Cure Hat.jpg

    Best,
    Kelly
     
  14. ESC

    ESC Silver Banner Member

    I turned up a degassing lance like oldfoundryman. The bubbles in water looked about like your test. I made a pour with some newly broken up scrap so did not test it, maybe next time.

    IMG_20180121_175032.jpg
     
  15. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Looks good ESC. I see some pipe fittings and a bolt there that look to be steel. Is the diffuser steel or iron? Are they bare or coated? I know Olfoundryman coats most of his metal contact tools with Boron Nitride to slow reduction of the lance and potential metal contamination. How big is the diffuser diameter compared to the crucible? Do you preheat it before use? Should be interesting to see how it ages and withstands use.

    Best,
    Kelly
     
  16. ESC

    ESC Silver Banner Member

    Kelly, I used pipe fittings for the water test, but have some carbon rods I will drill to protect a stainless tube. The head is cast iron and heavier than I wanted, but I can always remove material or cast some heads that are lighter.
    You don't need magnification to see the effects of the hydrogen porosity. Just cut a piece and polish if on the face of the cut. I have been more concerned about getting a useable casting than avoiding it, but I'm getting to the point where I will use virgin scrap for the good stuff and sprues and rejects for fixtures or core plates where it doesn't matter as far as function is concerned. I will pursue the wand because it would be nice to be able to recover some of the aluminum losses.
     
  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I'd like to hear more about that ESC. The only way I know of actually measuring Hydrogen porosity is in molten state. Most of those sensors isolate a known gas and allow H2 to diffuse in from the melt and then they measure the change in how the gas sample conducts heat, refracts or absorbs light, etc etc....but they are expensive. I have counted defects/sq in under magnification but that is just gross flaw population and though most flaws are likely H2 induced, certainly not all. On most small parts (a couple pounds or less), if you can see flaws with your naked eye, it's severe porosity. You also need to be lucky enough to section where the flaws exist which isn't always luck, if it happens to be the location of a crack or leak.

    I sectioned and polished several of my 1 1/2" OD, 1/4" wall water neck castings and they looked great to the naked eye. However, they (most) will still sweat liquid. These melts were not degassed and the metal was subjected to many re-melts and presence of H2 porosity is a certainty as is with just about all aluminum casting I see here and AA it's just matter of degree. I don't expect to eliminate it with the lance, just reduce it and will still plan on vacuum impregnating parts that need to be more or less hermetic. Even if you do a good job on your melt, you still need to fill the mold and solidify and there are ample opportunities to add H2 there as well, especially in sand casting. I've also found luster of surface finish can be somewhat of an indicator but only for known alloy and temper.

    Best,
    Kelly
     
  18. ESC

    ESC Silver Banner Member

    Campbell mentions that the only gas soluble in any significant amount in aluminum is hydrogen so the defects that I get are attributed to it. Delving further it appears that the successful elimination of hydrogen to be one of those secrets of the aluminum smelting industry that has not been shared. He considers the chlorine and Florine based degassing methods as just stop gaps and with hazards of their own. He doesn't think the use of rotary the degassing lance is a surefire method since he uses some of the microscopic testing you mentioned and the duration of the degassing might also serve to create additional contamination when the bubbles burst on the surface. He mentions oxide bifilms as one result of that and that may be what I see.
    I'll see if I can get a closeup of some of my pieces.
     
  19. DavidF

    DavidF Administrator Staff Member Banner Member

    I remember seeing a video where the melt had a vacuum drawn on it to remove entrapped gasses before the pour. Kelly, with your current set up do you think something like that would be possible??
     
  20. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    This much is agreed. If you have gas porosity in aluminum it's going to be H2.

    Like I mentioned, though the probability is high that casting defects are H2 porosity, it is certainly not the only source of defects....even voids. There are plentyof other mechanisms and depending upon the alloy and mold conditions, other elements in the metal the readily combine with O2 to form oxides are among the most common, thus the popularity of filters.

    Sort of surprising because rotary degassers are prevalent and there is a lot of information on their effectiveness which can be measured in real time. IMO, the only alternatives would be vacuum casting or gas blanketed processes but I don't think those are worth it unless you have ceramic shell or perm molds.

    It's part of why I built the crucible cap because it creates an inert gas blanket between the melt surface and the cap which takes very little gas flow to maintain and hopefully prevent formation surface oxides. The other reason of course is H2 degassing which I obviously believe will have benefit or wouldn't bother with the added complexity. I expect improved results but certainly not perfect. You reach a point where you're picking the fly-poo out of the pepper especially if you're sand casting (with scrap metal no less) and just need to accept a level of flaw population, quality, reduction of material properties, and design for it.

    I'll do same.....eventually.

    If you mean with the crucible cap, I don't think so. The problem is how to make a vacuum seal that can survive the temperature. I think the exterior furnace shell needs to become the vacuum jacket and then you can thermally isolate the sealing surfaces. That's easy enough to do but you still need to be able to transfer the melt to the mold with both at vacuum which complicates matters quite a bit. If you are thinking just processing the melt under vacuum or degassing it as if it were a batch of investment and then pouring at atmospheric pressure, that seems more practical. If you can get the melt into a vacuum chamber and quickly to high vacuum, it wont loose heat except radiantly so just a modestly insulated wall might do for that as a holding chamber. Seems to me the only advantage over inert gas degassing and blanketing would be elimination of the cost of the expendable inert gas.

    Best,
    Kelly
     

Share This Page