Furnace measurements.

Discussion in 'Furnaces and their construction' started by Ironsides, Nov 26, 2017.

  1. Tobho Mott

    Tobho Mott Gold Banner Member

    I let a couple of emails go out saying they were from Tobho Mott when I first started my YouTube channel too. The solution was to disconnect Google plus from my yt account (still have no idea what google plus is) and tell YouTube not to send me email. Which was not easy to figure out how to do! I do still get notification emails when channels I sub to have a new video come out. It probably would have been easier to make a new gmail/google account that really is called Tobho Mott to create my channel under, and keep my normal one for dealing with people i know "in real life".

    Jeff
     
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Agreed. I think Jeff did a fine job on his heads and got good castings but it doesn't mean they are free from H2 porosity.

    There's a difference between a solute and just entrained air or gas. The first is more chemical in nature and the second is mechanical. H2 is the about the only gas that has any degree of solubility in molten aluminum. You can bubble air, N2, and Ar through molten aluminum and it won't dissolve into the melt in any appreciable amount and buoyancy is the mechanism for how it leaves the melt. The reason commercial foundries use rotary degassers is they work much better than other methods. The de-gassing mechanism is diffusion and is governed by Fick's law, but the reason it works is the forces that drive diffusion are greater than the weak bonds that hold the gas in solution, so they come out of solution into the other insoluble gas and leave the melt through buoyancy of the gas bubbles. To make this work well, you need to create as much surface area as possible between the degassing gas and the molten metal because this exposes the maximum amount of H2 to a gas bubble that has a stronger attraction than the metal. Rotary degassers create almost a froth and agitate the melt and it can be orders of magnitude more effective than merely exposing the melt to big bubbles for a couple minutes.

    It doesn't mean you cant have good castings but H2 porosity is just a fact of life and you can reduce it but for our purposes you won't eliminate it, especially in sand casting. Things that help:

    • Use virgin ingot, or the closest thing to it you can manage, and do not use re-melt.
    • Heat the melt as quickly as possible and only to the lowest needed temp, and reduce the time of melt to the least possible exposure time. -Solubility of H2 increases with temperature.
    • Degas, with finest and most well distributed means you can devise. I didn't mention chemical degassing pills but they work on the same principal and some chemically combine more readily with H2.
    • Will a cap and inert gas blanket help? To a degree yes, particularly if is a fairly close fitting cap, and practically speaking you find yourself needing to hold your melt for a while. However, water vapor will still diffuse into the gas blanket, dissociate on the melt surface, and the H2 dissolve into the melt....but it may slow the process down.
    • Some contend a neutral flame reduces H2 porosity. Though it has other beneficial affects of reducing oxides I don't believe it does anything for reducing H2 porosity. The reason being that even if incomplete combustion of fuel doesn't introduce H2, free water in the air is being introduced in large amounts with combustion air because H20 is just along for the ride.
    If the reason Baking Soda bubbles is because it's liberating H20, it isn't helping reduce Hydrogen porosity...it's increasing it. If it is somehow chemically dissociating or combining with something in the melt to create a buoyant gas that H2 can diffuse into and be carried away by, it could help, I just don't understand what that reaction would be.

    In a former life I developed aircraft gas and fluid controls systems. I worked with some of the best casting houses in the country to solve production problems and learn how to become a better casting designer. Best metal quality by far was vacuum cast lost wax investment and shell castings (our production levels never justified permanent mold). Some of the wall thickness were as low as .090" and those castings and the people that made them were very impressive....but we still had a scrap rate. Trying to control the properties to this degree in castings was a humbling experience.

    My automotive water neck manifolds that I have posted about here on HF are 1/4" wall and I consider that thin walled for home foundry work. The first ones I made were porous to the extent they will (very) slowly sweat warm ethylene glycol through the walls at 8-14 psig. It's hydrogen porosity and maybe some contribution from lost foam process but I remelted some of the metal ten times or more. Even so it didn't look much different under magnification than most of the commercial sand castings I analyzed back in the day. If you think you have dense castings, try to make something that you can leak test with low molecular weight gases and it will bring you down to Earth. On the bright side, vacuum impregnating the castings is very effective much easier solution and in industry cheaper than the scrap rate of trying to control the casting process to the gnats eyebrow.

    Best,
    Kelly
     
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  3. myfordboy

    myfordboy Silver

    A veiwer once sent me this explanation why sodium carbonate works

    "I am a chemical engineer, and I know sodium carbonate immersed in the liquid metal, dissociates into carbon dioxide and sodium oxide. Dioxide rises and drags all the impurities that float and also part of it is combined with the hydrogen dissolved in the molten metal and removes it from there.
    Part of oxide remaining is dissociated into oxygen and sodium. The oxygen, is also combined with hydrogen and remove it from the metal. The resulting pure sodium is dissolved and changed (improving it) metallographic structure of metal"
     
  4. Robert

    Robert Silver

    Kelly- That is an excellent explanation of how degassing works. Very well put.
    Can you elaborate on the role of water vapor in porosity? Does the water actually dissociate to create H2? Is there some other mechanism beyond H2 production from incomplete combustion? I would assume it is high temperature electrolysis like you see in the Nukes industry.
    Have you had any experience with measuring porosity based on density? How accurate is that metric? I would think you would have to be pretty accurate in your volume measurement, perhaps not achievable at home.
    Thanks
    Robert
     
    Last edited: Dec 10, 2017
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I’m aware this has been a bit of an emotive subject and debated in the past. I’m not a chemist. I had a friend that worked for Aluminum Company of America (ALCOA) and he worked with alloys for the casting and foundry industry and the explanation I gave about the mechanism for removing H2 actually came from him.

    I watched Ironsides video and from an experimental perspective don’t find it to be quite as conclusive.

    One obvious observation is the samples were cast in a sand mold which itself is a potential source of gas and the machine sample was taken at the surface in contact with the mold media. I think it would have been more interesting to compare the interiors of the samples.

    I’m also not certain I would conclude that CO2 can be used to degas but if there is water in the CO2 I’d change suppliers or buy a grade that is free from such. I think the comment about any dissociation of O2 would burn the aluminum isn’t so, because there is free oxygen in contact with molten aluminum at many points during the casting process which is why there are oxides.

    Just because Na2CO3 doesn’t dissociate from (molten aluminum) heat alone doesn’t mean it doesn’t when combined with another substance….like it does in water at room temperature for example. I just don’t understand what the reaction with aluminum would be.

    I will say when this was debated on AA I too tried a sample from the box which bubbled when immersed but then heated a sample and got no noticeable action when it was immersed in the melt.

    Here’s an article I found and saved sometime ago that is a good read. H20 does dissociate.

    http://www.totalmateria.com/Article83.htm

    I think that is impractical for the reasons you state. I’ve only examined samples under high magnification and counted flaws per square unit. I've mechanically tested samples but even that is very hard to do because so many other things affect mechanical properties. I have done failure investigations on castings where it’s clear that porosity was present at the point of failure and created the stress riser that precipitated the failure. The most common scenario was where you had a thick section in the critically stressed portion of the part.

    The basic issue is H2 is soluble in molten aluminum and not much of anything else you find in ambient air is. H2 doesn’t naturally exist in significant amounts because it readily combines with a so many things. In most places, there is plenty of water available in air and that is the primary source. H2 isn’t soluble in solid aluminum and that’s the problem we fight because it comes out of solution as the metal solidifies and forms porosity.

    Best,
    Kelly
     
    Al Puddle likes this.
  6. Robert

    Robert Silver

    That's a great reference article. Good discussion here! Am I to conclude that the most effective method for degassing in a home foundry (aside from the preventative bullet points above) is chlorine tablet degassing?

    Robert
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I was starting to feel a bit remiss for hi-jacking the thread but since Ironsides started the thread and took it this direction, let's go with it ;)

    I haven't used the tablets. Sure seems like you could get a lot of duration and control with a good gas wand. If you can make very fine bubbles it should work well and potentially use less gas.

    I'm noodling around a lance build similar to the one Oldfoundryman built in his Youtube video. His are steel discs coated with boron nitride. I thought I'd try to cast mine as one-piece in refractory with a plastic mesh filter screen suspended in the refractory disc and then see if it will burn out to create the fine holes around the perimeter of the disc. You can buy the plastic mesh with wire size from .003"-.015". I was thinking either .003" or .005".

    Best,
    Kelly
     
  8. Ironsides

    Ironsides Silver

    myfordboy

    I would be interested to know how you came up with the idea of using sodium carbonate, was it your own idea or did you copy of someone else?

    After reading your explanation of how sodium carbonate dissociates into pure sodium metal, you need to watch my video and see what happens when pure sodium metal hits molten aluminium.


    You also need to pay attention to what method the forum administrator (Kelly) recommended in another thread on this forum. read point 4 post 8 http://forums.thehomefoundry.org/index.php?threads/porosity-problem.161/

    Also read post 27 in this thread. He will not use or recommend your method of degassing.

    You need to watch olfoundrymans video about degassing aluminium and while you are there ask olfoundryman why he does not use sodium carbonate to degas his aluminium.
     
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I think the primary purpose of salt fluxing aluminum is to remove drossing oxides, not H2 gas. The two salts mentioned above, will barely even melt at common aluminum pouring temps let alone dissociate into their constituent elements. NaCl melts at 801C, KCl at 770C, and Aluminum melts at 660C. The common home foundry practice is to form 50/50 by volume Eutectic by melting the two salts. The resulting Eutectic then melts at a lower temp 540-590C (depending upon the accuracy of your mix) than either of the two individual salts which is lower than the aluminum melting temperature.

    Commecial foundries use fluoride salts but those are less commonly available for the home gamer whereas one can buy NaCl and KCl at grocery stores in table and water softener salt or even as a 50/50 mixture in the so-called light salts but the latter are just mixtures of the two, not Eutectics; you need to melt them for that.

    An interesting thing is the oxide layer on top an aluminum melt is said to form a barrier which can slow H2 entering the melt. These oxides and others are often referred to as cover fluxes which are also commercially available. So another thing to add to the list above to reduce H2 porosity would be to add a cover flux or at least not skim oxides until just before pouring, which I think is common practice for most.

    The reason I’m not convinced that Na2CO3 (sodium carbonate) degasses aluminum is because I know it doesn’t melt at molten aluminum temps and if it won’t even melt, it’s not going to dissociate due to heat alone and it would need to chemically react with something in the melt. I described my one and only experience with it above and as near as I could tell when it was dry it merely floated to the top of the melt and didn’t change state at all and although I’m not a chemist, this is why I doubted that it breaks down into its elemental constituents as Myfordboys subscriber had suggested. The other reason I’m skeptical about Na2CO3 is it’s very inexpensive to buy in bulk and if it was an effective degasser it seems certain industry would prefer it over rotary degassers because that equipment is much more complex, expensive to purchase, operate, and maintain.

    I will say that although the salt Eutectic I mention is a poor man’s flux, when I add it to the melt I get a small amount of multicolored flames at the surface and oxides/drosses form on the surface where there were none before. So I conclude it is gathering the oxides/dross to the surface and working. What the flames are I don’t know. I only salt flux when I’m using a lot of remelt and sprues, usually from previous lost foam pours and the cup metal is very drossy from the previous pour. If I have clean metal I don’t salt flux, but do want to get into the habit of employing a good degassing process, in my case with Argon because I already have it on hand as a welding gas.

    All of these salts have a strong affinity for water. I keep mine in a tightly sealed container but even that only delays the inevitable. I remember my foundry friends would never keep too much stock of their degassing tablets because they said they had a very short shelf life because water vapor.

    I think it’s an interesting discussion and try to keep an open mind on these things. I guess the one question I would have for those who advocate the use of Na2Co3 would be why do you think it works? Is it just because it bubbles or has there been some analysis of samples?


    Best,
    Kelly
     
  10. Peedee

    Peedee Silver

    I have used foseco Coveral 11 (Exothermic granular powder on melts) the intention is to help gather dross, reduce oxygen and hydrogen absortion. To be fair most of my casting didn't need great consideration for porosity> I've also got a stash of the chlorine based tablets but they are very hygroscopic (as mentioned for other materials above). The chlorine based tablets have been removed from sale due to the HCl gases produced as I understand.
     
  11. Ironsides

    Ironsides Silver

    Peedee I used those tablets a long time ago, all I can say is don,t breath in the fumes and do it in a well ventilated area. So because of the fumes I stopped using them.
     

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