Incomplete casting: wrong runners ?

I just made an iron casting of an old sundial made of brass. It is 15cm (6") diameter and 4mm thick.
I used the brass one as a pattern and added two risers opposite the sprue which did not rise at all, so during the pour I thought bad news.
I made two gates from the sprue towards the actual mold, each about 3cm (1 1/4") wide and made a larger cavity under the sprue. The temperature appeared to be OK, the metal looked lustrous and clean and the furnace temperature was well over 1400 C.
Probably the gates are too narrow, should I add more gates or should I pou the sprue right in the center ?

 

I think you poured very cold metal as evidenced by the rounded advancing edge of metal. You need to be 200 to 300 hundred degrees hotter. That thin flat part should not need risers. I do not think larger gates are the answer. For that small part a gate 1/8" high and 1" long will be plenty if your metal is hot.

Give it another go. Not sure how you are measuring pouring temp.

Denis

 

Thats what I was thinking too. Looks slightly on the colder side so it froze before migrating into all the spaces. That rounded edge is a pretty good indication of being too cold from what I've seen.

 

Indeed, during pouring the pour suddenly interrupted for a second and there was a blob of sludge in the further fluid liquid iron. At that moment the iron which was already in the mold froze already, inhibiting further metal flow. Probably an incomplete molten part, as I added some chunks later because not all did not fit initially in the crucible.
I measured the temp with a thermocouple beside the crucible (which was 1480 C and still rising) and the crucible was slightly cooler (less bright) but not much. I had to use my infrared pyrometer to measure on the slag layer.
Next time I'll wait slightly longer till the outside (the crucible) temp gets over 1500 C, In previous sessions that happened successfullt.

Here the pour: https://www.metallab.net/jwplayer/video.php?f=/clips/others/Test+iron+cast+20201025.mp4

No risers ? But then I should at least make venting holes to evacuate air to allow metal flowing in ?

 

It does indeed sound like the melt was just barely or maybe incompletely into the liquidus phase if there was gloppy metal as you poured. That would suggest that your melt was in the range of 2100 degrees. For a good pour you need to be getting around 2550 to 2600F. That means the metal should be essentially WHITE HOT. The surface should be roiling and it should not skin over with scum after skimming. The furnace temperature should also be white hot and needs to be white hot for about 15 minutes or so for the average melt before the crucible will begin to catch up to that temp.

As far as running the metal into that small flat and thin plaque, I really think if your iron is hot it will zoom into that area in seconds. Here's why. On several of my castings I routinely rubn iron 18 inches through a runner 1/4" thick and 1/2" wide. Given a chance iron will form flash several inches into a space only paper thin.

No risers are needed as the part is a pancake and the shrinkage of the metal can easily be accomodated by the two flat surfaces simply moving together a shade. A chunkier part can not so easily make up for the shrinkage and risers are needed.

Try making two molds. Make one with a single gate 1" wide and 3/16" thick, no vents and no risers. Then on the second vent it, use risers and see how it goes. But for a riser to be effective it will have to be pretty thick at its contact---a thin contact will freeze off long before the riser solidifies and the before the mold freezes.

Vents can never hurt unless they are too large. You will see that they cause shrinks if they are 1/8" diameter or larger. I usually vent with a bike spoke 1/16" diameter to avoid shrinks.

Those thoughts are what I think is true.

Don't be discouraged by a few bumps in the road as I think everyone of us has found them and, despite best efforts, continues to find them.

Denis

 

I made a new mold. Mow with longer runners and multiple gates and only narrow venting holes. No risers.
Drag = left, cope = right. so the sundial pattern faces upward.

 

Counterflow error ???

Now I checked the temperature, inside the furnace space it was 1510 C (with an S thermocouple), it was really white hot, without dark goggles it was too blinding. A few minutes before the pour I added some ferrosilicon and stirred with a steel bar and did not feel any solid blobs. Then I measured with the infrared pyrometer the slag on top of the metal which was 1430 C ( 2600 F) and before pouring the metal showed as a nice mirror.
Pouring was quicker now, but I think the multiple gates were the problem. The number of gates delayed the metal flow, speeding up the premature freezing of the metal, particularly between 12 and 1 o'clock on the dial (closest to the sprue).

Next time a single large sprue in the center ?

 

You should have made it with all that gating... that is the average thickness of our cookware. The first casting misran at the opposite side of gating which is typical on a pressurized system and cold iron. The second misran at the sprue because the iron runs to the end of the runner first and then froze off. Both should have poured this casting. The issue is probably with temperature or possibly chemistry related. You can make a taller cope and that will help add velocity to get the iron in faster. Also I do not see the vents connected to the casting...one vent opposite the sprue should suffice. You can try adding some phos to the iron to get the fluidity up. Should see some finning and such if temp and chemistry is right unless you have immaculate patterns.

 

Cold metal. I would not go chasing esoteric answers when cold metal is the problem --- 99.5% likelihood. A single small vent or none should be enough. A single gate should be enough. Add a second gate if you feel you must. Most of all, get your metal hot. Find some way to measure the metal itself. Measuring the furnace and the slag tells you little about the temperature of the metal.

Using all those gates allowed "dealers choice" for the metal to decide which way to run at any given time. So it could sluggishly start seeping at all those ports and cooling at all those points. A single port would provide a brisk flow and a single advancing front of liquid metal. I can't remember seeing anyone use so many gates on such a small item. Two would be the reasonable max. Most folks would just carve the runner and gates in the drag but not both cope and drag.

Added: You have not mentioned FerroSilicon. Adding it always to your melt a a rate of 1 ounce per fifty pounds will improve fluidity a little. Too much is not better----actually bad. Use 1 ounce to fifty pound ratio. Your melt looks like about 5 to 7 pounds. 3 grams FeSi will be just right. And to do anything at all it has to be added not more than 5 minutes prior to pouring and should be stirred a few brisk stirs into the already skimmed crucible. I usually add it less than a minute prior to pouring and never more than 3 minutes.

Denis

 

Here is Ironsides, likely the most experienced iron caster on the forum, Casting an item of much greater diameter and similar thickness. Note he used only two gates, no vents, and had “perfect” results. There is much to be learned watching his videos.

http://forums.thehomefoundry.org/index.php?threads/making-a-house-number.1324/

Denis

 

I know that video channel Luckygen1001, that man is excellent and he is really a master in iron casting.
Well, I also have made succeeded iron castings of only a few mm thickness with a single sprue and one of them is a hemispheral bowl of 7.5cm (3") diameter and 4mm thickness (like the sundial), and it filled with iron without problems. Strangely, the first attempt of that bowl in brass failed....
To my experience, cast iron is cleaner than brass, as the latter produces much dross floating on the metal due to the zinc.

Anyway, next attempt still pending as I had to patch some cracks in the furnace wall, but that was easy. The Fermit coated Kaowool holds excellent in this extreme heat.

EDIT:

Measuring temperature of the metal is very difficult. I cannot dip the S (or any) type thermocouple into the bath as it will dissolve. And the infrared pyrometer reading on clean metal will give different values. I can adjust the emission factor but don't know that of a polished cast iron (that is actually how it looks like when the slag is removed) surface.

And I did use Ferrosilicon, just a small blob, like 1% in mass.

 

The polished look you described sounds how it looks when it is around 2200F to 2300F degrees here. It looks similar to that if you slag a trough right before it freezes up. I saw somebody do that once and it was froze up by the time he got back to the pour control. I would say most of the time I iron solidies at right around 2190 F. I think I would just try to increase the temp by 150 to 200 and it will be fine. And like Melterskelter said less gating should pour it fine.

 

It did not freeze anyway when I removed the slag and stirred. I could stir the iron without problems with a thin steel bar and it was perfectly fluid. When I open the lid, the polished surface disappears because of oxidation.

I don't use a phone cam (too poor quality) but a Sony RX100.
BTW, the yellowish tinge of the hot metal looks like videos of the steel industry, so temperature should be not far from that. If it were as cold as 2200 F (1200 C) then it would freeze and look much darker than the white hot 1500 C thermocouple tip.

 

Yes...our is in a trough about four feet long in an open atmosphere while slagging and the polished look does not last but a few seconds after it is skimmed before the skin on top is formed. Post some updates when you can. This is the type of defect I deal with on a daily basis with thin wall castings. Some of our are 15" long and have .140" to .150" wall thickness. Very tough to pour without causing a lot of turbulence and getting inclusions. Just to add to gating a little...for reference we poured a 17" skillet with a single ingate that was 2.5" long and had a contact point of .085" thick. It is a little thicker and we eventually changed to a depressurized system because of inclusions but it ran that way for over a decade.