Cleaning up the scrap

I'm not a fan of steel (stainless steel is still steel) crucibles but it really depends on your casting objectives and depending upon that, they can be very practical. The reason is because of iron contamination. Molten aluminum is solvent, it will absorb Fe, and that can/will significantly reduce the mechanical properties of an aluminum casting. It's a very well known, documented, and measurable affect to professional foundry men.

That said, if the castings aren't critically stressed parts, it probably doesn't matter. Other than cosmetics, the same can probably be said for porosity.

The manual for your furnace at the MIFCO site says pot melting was a feature popular with high school programs because of visibility and ease of handling the melt. It may be helpful when it comes to reducing exposure to H2 at the expense of Fe contamination.

Some will say the metal crucibles form an oxide layer that prevents/reduces Fe contamination. I don't (fully) accept that. Why would commercial foundries go to the trouble of coating their ladles if that was so? There does seem to be some favor to using cast iron pots versus steel. I suspect this is just due to durability because not sure there is anything magical about cast iron versus steel when it comes to its solubility and reduction through molten metal contact. I see no appreciable difference in convenience handling a clay graphite crucible versus a steel crucible but I could make steel crucibles for $0.

I just do what I do because I'm trying to make the best castings possible.....albeit with lost foam which can have its own set of issues in regard to casting quality. I'd use a small sprue and not get concerned about pour speed. The fill rate of your lost foam castings is dictated by the rate the foam pattern evaporates, not how fast you pour.

Bottom line Gary, people here and elsewhere get decent results with fuel fired top vented furnaces, and I think it is most likely your porosity problem is the result of super heated metal, in a H2 rich environment, with the latter possibly caused by burner tune and availability of atmospheric humidity.

Best,
Kelly

 

You have a number of different threads started on the same/similar subjects. Some remarks about the features of your furnace, and others about porosity.

I understand you are not melting in the lid pot but the reason your furnace has the unique lid features is because it's a combination crucible and pot melting furnace. These/yours are C series which are considered "soft metals" rs, melteat least with respect to the pot melting mode. The B series furnaces for have conventional gas venting through the lid.

http://mifco.com/foundry-furnaces/high-speed-melters/

There was some discussion in one of your other threads about the side vent potentially being a superior approach for heating and combustion. -I don't think so. It only has side exit for exhaust gas because when it is being used as a pot melter, the gas would need to vent elsewhere other than the lid, because that's where the pot resides. There is also mention in the manual that when melting aluminum in a crucible, you need to dial the burner back 50% because at full tilt the velocity is actually high enough to aeresolize the light aluminum and expel it through the vent. If the furnace is able to achieve this, I suspect can aggressively agitate the melt surface and that combined with a rich burn and humid at could contribute to a very gassy melt.

Now it sounds like you place the crucible very near the lid and continue to use the side vent......which if it works well for you....fine, but I see no mention of that in the manual as the preferred embodiment or a method for controlling or preventing H2 porosity, and it doesn't seem like it's preventing the issue for you.

The manuals MIFCO manuals are an interesting read. The first ten pages or so talk about many subjects that are often discussed here at THF such as crucible tempering/annealing, correct burner atmosphere for various metals and how to achieve the state of tune, method for charging crucible, suitability of various types of crucible material, suitability of various sources of scrap metal (including iron), etc. Keep in mind fellas, MIFCO has been at it for a long time. Here's a link to the manual.

http://mifco.com/B & C Manual-2014.pdf

Ughh. That looks bad. High temp cement can be a relative expression. Did this high temp cement come from the local hardware store by chance? There are many pages in the MIFCO manual pertaining to the furnace refractory maintenance and patching. Once you have used a low refractory repair material it will be hard to remove it and it may have fluxed the base material. Might want to investigate MIFCOs relining kit for the lid.

Best,
Kelly

 

Good to hear Gary. Hopefully that will carry through to your next cast parts. Good luck when that time comes!

Best,
Kelly