Tale of a smallish foundry furnace

I don't think the bubbles exist in the melt. The Hydrogen is soluble in Al and dispersed throughout the molten Al. The higher the melt temp the more H2 that can be dissolved in the melt. The bubbles only form as the melt cools because the Hydrogen comes out of solution as temperature drops and forms H2 gas and thus the bubbles. The melt is quite viscous at that point and resists buoyancy of the bubbles.....and thus hydrogen porosity.

I agree that a rich mixture can be a source of hydrogen but no matter how lean you tune the burner there will always be free H2O in the combustion air from the ambient so though you can help but can't eliminate it with combustion mixture. Unfortunately, the level of dissolved hydrogen at the point of solidification will persist in the next re-melt, until it is removed because even though the hydrogen will start to come out of the melt if the atmosphere has lower partial pressure of hydrogen, it's excruciatingly slow so you need de-gassing agents or inert gas to clean it up in any sensible time frame.

Being the Gippetto is located in Alberta, and it is cold there now, there air can't old much water at those ambient temps so he may actually be able to make a noticeable improvement on his next melt with good burner tune. Spending the minimum amount of time in molten state is helpful unless you can blanket/shield your melt with inert gas or cover flux. Hydrogen is not soluble in Al2O3 (aka aluminum oxide, dross) which can act as a diffusion barrier to hydrogen so don't skim dross off your melt until you are ready to pour.

Gipetto, I confess I don't always degass because it's not always worth it....depends on the part. When I degas, I use Argon because I have it on hand as welding gas. N2 should be fine. But bubbling gas through a simple tube will isn't very effective (bt better than nothing) and you will need to do it for quite some time to have a material affect and all that time your melt will be (rapidly) cooling. Have a look at the discussion in the thread below.

http://forums.thehomefoundry.org/index.php?threads/degassing-lance.204/

..and you can read the whole thread but here is the post that get right downtoo it.

http://forums.thehomefoundry.org/index.php?threads/porosity-problem.161/#post-2226

I don't follow you here Bonz. If the last ingot of the melt has been poured and leveled out, what's left to skim, assuming there is something to skim?

Best,
Kelly

 

Ahh, now I understand now (but still don't agree ). You were referring to putting ingots into the new melt instead of pouring ingots from the melt.

How do you know the bubbles you see are H2? That phenomena could be explained as simply as surface tension and entrained air. I can't say that an H2 bubble from a freshly deposited ingot near the surface never escapes the melt but I think I can say with great certainty that you can't effectively degas simply by remelting. Can some H2 be lost? Perhaps but if that really happens, why doesn't all the H2 just collect and buoyantly leave in the first instance of the melt.....? Because the forces that drive hydrogen into solution are stronger than buoyancy and the forces that attract H2 molecules to themselves.

One thing I wouldn't debate is whether a rich burner tune could be a rich source of H2 and become a further aggravation to the problem but there will be an equilibrium at which point no additional hydrogen can be dissolved into the metal, and that is a function of temperature and availability of H2 which is abundant in combustion air. I've have a close friends that owned a commercial foundry for many years. They made some impressive castings and many tons of them per year. I watched them on a daily basis battle hydrogen contamination. They used virgin ingot as feed stock, induction furnaces that obviously are not fuel fired and still had to resort to things like chemical degassers, rotary degassers, inert gas blanketing, and sophisticated analysis equipment. One thing that is fortunate, very few gases other than hydrogen are soluble in aluminum.....thank goodness.

While we're on the subject of experience, as everyone knows, I have an electric furnace. Before I built it I thought that could be a liability because the furnace atmosphere would be air. I have surprisingly little (but certainly not zero) problem with porosity. The biggest advantage is I don't put a very large volume of air (and H20!) through my furnace. There is no way around this with a fuel fired furnace if ambient air is the source of combustion air. The H20 in that air is along for the ride and many, many, furnace volumes are being exchanged with the burner volumetric flow rate during the course of a melt.

The other pleasant surprise with my electric was the furnace atmosphere seems to become oxygen depleted. My furnace is pretty tight because I use 1/4" fiber paper for gasketing. When I was melting wheel weights, I would get very large flashes when I opened the furnace from all the crap I was cooking off the wheel weights. For that to happen, there must not have been enough O2 available in the furnace atmosphere to combust the byproducts and then when I open the lid.....vv-ooom. It happens when I melt oily or painted aluminum too.....just much more subtle.....a small flash on the surface of the melt that is short lived. I do degass when my melt stock is dirty metal.

Can only help but don't expect hydrogen free metal. Folks always discourage using wrought metals instead of recycled castings which is good advice accept if you are trying to make machining blanks. Problem is, it's surprisingly difficult to do if your aim is achieving the properties of wrought stock.

That is time well spent. Martin's videos are very good and he is a very experienced foundry man. From corresponding with Martin, in addition to the boron nitride wash, his diffuser head is iron not steel and iron does considerably better with molten aluminum contact. Molten metals act like solvents to other metals. It's why you can place a chunk of copper in molten aluminum and even though the melt temp is way below that of copper, it dissolves. Happens to steel too, just much more slowly. The washes act as a barrier to metal contact and also prevent wetting. You'll get build up on your tools due to wetting and eventually the oxides/dross will tenaciously adhere without barrier coating.

I'd save your time on this one. You'd be better off to just heating it in air and developing a good oxide layer before use. If metal quality on your mind, I'd suggest investing in a clay graphite crucible too.

Two cups of coffee down the hatch....I'm fully caffeinated and off to the shop! Good luck with round two.

Best,
Kelly

 

Hadn't appreciated you used the siphon burner the for first melt. What are you using to measure temperature? Was 16 minutes from a complete cold start?

The quote above was from the first melt where you observed the porosity. Did you measure temperature before you poured this melt? If so what was the reading?

Good deal Al. Will be interested in the answers to the Qs above.

Best,
Kelly

 

Melting aluminum with that siphon burner running at full tilt is sort of like mouse hunting with an elephant gun.

If the aluminum was superheated in the first melt and in a rich atmosphere, that creates near ideal conditions to dissolve hydrogen into the melt. The only thing that would create more ideal conditions for such would be high humidity atmospheric/combustion air (but being in Alberta in December, I suspect that's not the case) and increased exposure time of the melt. If the aluminum had been remelted a number of times, you would likely be starting out with more H2.

If the first melt overshot and ran up to say 1800F (which would be easy to do with that oil burner) or more that can dramatically increase the amount of Hydrogen that can be dissolved in the aluminum.......sort of in keeping with Bonz' comments about fast melt times. There will always be H2 in the aluminum, it's just a matter of controlling to acceptable levels.

You'll be happy you have that siphon burner if you decide to tackle higher melt point metals.

Best,
Kelly

 

3 gal/hr is lot to throw at that size furnace. I think that may have been what PatJ was getting at when he suggested that burner might surprise you....well tuned oil burners are beastly. It's all good learning. -Backing off is easier than throttling up. I'm sort of surprised you can make your steel crucible survive with that bearing down on it.

So with the furnace and a couple burners in hand, what kind of casting do you have planned?

Best,
Kelly

 

Agreed and that's why I was saying lean tune won't eliminate the issue. Depending upon how you define the issue of course. If the porosity in the photos is Hydrogen, I think most commercial foundries would consider that to be a quite severe case compared to their target levels. The comment about H20 in ambient air is still very relevant when you consider the burn, though not stoichiometric will still have an air-fuel ratio of ~12:1-16:1 (10% excess air). So if you're pumping >10x the amount of air than fuel through your furnace and you go look at the possible partial pressure of water available at 90F versus 20F atmosphere, I think you'll see what I mean.

If the concern is just cosmetic, then subjective assessment is just fine......if the method for achieving it is repeatable. But in that test, you could seeing nothing more than the difference in gas in the melt at one pour temperature versus another.....or as Joe said, some other source of defect. I'd hazard a guess, that you could pour six ingots into a muffin tin and may see similar variation from the first to last ingot.....even if the non-stick coating hasn't already been cooked off the muffin tin .

I like that slotted rotary lance and think I could easily build one of those. If my present lance craps out I may build one. Mine seems to work well (which is also subjective since I have no elaborate analysis equipment and seldom needed anyway) and I can hang my present lance from the furnace lid above the melt to preheat it. My biggest objection to my present lance is the size of the head or would-be impeller. It's larger than the hole in my furnace lid and I need to remove the clamp that suspends it before I can extract it from the lid....which is a bit cumbersome at temperature. The slotted impeller shaft like in Joe's example would insert and extract through my hole in the lid of my electric furnace (and even the crucible hat) and I could hang it from my overhead arm. One other thing on degassing, typical degass times range from 5-15 minutes with rotary degassers, and they have fantastic dispersion within the melt so it may not be realistic to expect the same results from inert gas popping out the end of an open tube stirred in the melt for a minute.....but we typically deal with pretty small melts and better than nothing.

Good discussion but sort of hijacking Al's thread....back to regularly scheduled programming.

Best,
Kelly