Segmental Thin Plastic Refractory/Wool Furnace Build 14" Dia Bore

Just a little follow up. This furnace build has been very durable. It is now 18+ months later and who knows how many iron melts have occured---likely 75 or so. But the lining has held together pretty well. The area where the flame impinges did eventually thin and sag, so I wire brushed the area and slapped on a half inch or so patch about 10 by 8 inches and I continue to use the furnace. The lid has required no maintanence. The Blu-Ram has hugely outlasted any of my other furnaces. I will be relining this furnace in the next month or two most likely. No complaints about that.

Denis

 

And one other Blu-Ram note: The material requires no "special handling" to get a very good result. For instance, yesterday I pounded some out into a 1/2 to 5/8" thick sheet and used it to "plaster" an area of my furnace that gets direct flame impingement and has therefore thinned over time. All I did (and this might not be "right" according to the manufacturer) was use a wire brush in my angle grinder to clean any carbon and crud off the area in question. Then I used a chip brush and brushed on sone water which seem to soak in a nd disappear instantly. Then I painted on some water onto the to-be-applied side of the Blu-Ram and pressed it on thinning down the edges. Today I started up my furnace and ran it normally. I did not otherwise preheat or dry the Blu-Ram which was still moist when I lit the furnace.

I also made a new plinth yesterday but fired it to 1500 in my kiln prior to installation.

Here is a pic I took soon after pouring. The patch is the slightly lighter orange roughly 6X8" area to the left of center and just above the plinth top.

The prior plinth was installed in June and is in direct line of fire from the burner. The old one was cracking pretty bad and getting close to failure after a lot of melts (60 or 70 I would guess) which I considered pretty good service. I wonder how a silicon carbide crucible upside down might work as a plinth.

Do other folks have methods of plinth making that are very durable?

Denis

 

I doubt anyone else here is working their kit any harder than you. 60 or more iron heats from any plinth taking direct contact seems pretty darned reasonable. Maintaining a sacrificial coating, say Satanite, zircon-type wash, or ITC 100 might have some positive effect, but the straight-up Blu-ram is tough as nails and might be as good as it gets in a continually cycling environment.
Pete

 

Pete,

I should have given a bit more information about the failure of the plinth. It was not eroding at all. In fact, it is very well glazed with the melted and dripped-down coatings of the various crucibles that have rested on it. Their coating melts some each heat and maybe a couple or 3 tablespoons of it drips or oozes down onto the plinth. As I lift off a crucible I usually see long glassy strands of coating/binder hanging from the crucible. It is that melted glassy stuff the tries to make the plinth hang onto the crucible.

The evident cause of my plinth failures is that eventually cracks develop and propagate. After quite a few heats there may start to be visible multiple cracks and, at first, I wondered if the plinth would collapse. But it seems that the interlocking of the fragments and the glaze seems to keep it all hanging together until some largish pieces start to separate. By that time the plinth will only survive a another heat or two. Thermal cycling is probably the main cause of the cracking and the way I fire my furnace is pretty abrupt. I also need to leave it open after a melt to cool the chimney enough that I can remove it. Closing it all up would likely be less punishing to the plinth and the liner. I may see if I can work on that with a tweak to my chimney handling process.

Incidentally, my “solution” to the plinth that sticks firmly to the crucible and wants to lift with it is to mud the base of the plinth in with Blu-Ram. I just pack 1/2” base of the stuff around the outside of the plinth extending to the vertical walls. That one-time treatment and a fairly thick (1” wall thickness) plinth provides enough hold down to avoid the lift off. Having a plinth stick to my crucible is a real nuisances. Of course I also use two thicknesses of cardboard under the crucible, but that wasn’t always a completely reliable solution.

 

I was out at the foundry today. Before I lit 'er up, I photographed the plinth to show the amount of glaze/binder from the crucible that dripped onto the plinth with a single 2-hour firing. I think the glaze protects the plinth quite effectively from erosion and probably even helps hold it together when it cracks. You can imagine how much it is glazed after 60 or 70 firings. The pinkish areas are the new Blu-Ram unaffected by glaze and not much affected by heat and combustion. It gets grey to black with age and prolonged firings.


Because of the concerns I mentioned above about the role of thermal shock eventually cracking the plinth, I changed my post-firing practice to seal the furnace up better to slow the cooling. Who knows whether that will actually help. I doubt it will hurt. It does require me to handle my very hot chimney though. I use the high-temp gunnery mits suggested by HT1 and am careful to not let it touch unprotected skin as it is very hot indeed.

Denis

 

Hi Denis,

since i couldn't find it in the thread here:
What size is your burner tube and your vent hole?

Thank you very much

Regards Christoph

 

Christoph,

Sorry to be slow responding. I wanted to check the burner tube diameter. It is 2.5 inches and the vent is 6 inches.

Good luck with your build.

Denis

 

And a bit more follow up. The lid is starting deteriorate at the lower edge where the veritical steel shell is welded to the horizontal ledge that supports the lid segments. That may be partly my fault as I sometimes did not optimally maintain the wool gasket beteween the lid and furnace. But that is partly because the furnace lining is getting tired and I am going to rebuild the furnace this time with another inch of wool insulation, a stainless lid shell (to resist flame attack) and slightly improved segment design---at least so I think it is improved. I'll post more soon. Quite busy right now getting castings shipped and doing some annealing furnace work, work around the house etc...
I think the interior size won't change appreciably.

Denis

 

Hi Denis,

Thanks for the reply!

So things you've learned are:

• stainless lid, due to flame impigment
• instead of 2" wool you will upgrade to 3" wool insulation

has the segmented floor pieces proven to be good? or will one round piece do the trick?

Regards Christoph

 

There was a fundamental flaw lurking in the design of my lid lifting mchanism that impaired durability of the seal of the lid against the furnace. Because the seal was impeded, hot flame could repeatedly leak a couple points especially and that stream of hot gas heated the steel and that promoted its erosion at that point.

In use I place a 1 to 2 inch wide ribbon of 1" thick wool on the upper edge of the furnace to act as a gasket knowing it is good for maybe a half dozen melts and will need to be replaced. I glue it in place using Satanite on the gaskets lower surface only. Heat, compression, irregularity of the aging lid, and time all contribute to the gasket losing effectiveness at various spots. When I see lid leakage happening mid-melt I just grab a scrap of wool and tuck it into place as a literal stop-gap. Opening and closing the lid often displaces the temporary plug and it needs to be replaced a few times per melt. That is easy enough wherever it occurs on the circumference of the lid EXCEPT where the lid lifting arms attach to the lid.

The arms, being the same width as the vertical portion of the lid rim, prevent easy and complete tucking-in of the wool. And the arms tend to compress the wool that may extend a bit beyond the circumferential edge of the lid. So, that is a spot that tends to be patched less well than other areas and it also tends to be the very spot where leakage is most likely. Over time scaling of the steel of the lid rim has caused some erosion of the rim making leakage there even more likely and causing a spiral of decline.

So, next build which is currently in progress I will not have the lifting arms made of 4" wide flat bar connecting to the 4" wide lid rim. I will be sure there is a clear spce of at least 1" at the bottom of the lid rim. That will reduce both factors mentioned above. Secondly I am going to use a lifting mechanism that will provide a longer vertical travel. Then I will be using not just a flat gasket of wool but the wool will be chair shaped in cross section so that the lid sits on and inside a rim of wool. I have not worked out all the fine details. But I am convinced that the improved cross section of gasket will porvide a more effective and longer lasting seal. And, being stainless, the rim will be far less likely to scale and degrade as a result of inevitable temporary gasket point-failures.

Next time I am out at the foundry I'll try to get a couple of pics that will hopefull clarify what may be a bit hazy in my verbal description above.

Denis

 

Here’s a pic of the support piece joining the vertical lid ring and the area of burned-away steel. On the very short term I think I will weld in a patch so I can continue to melt until I have the new furnace done With the patch I won’t be under so much pressure to complete the new furnace

And I forgot to answer about the segmented floor pieces They have worked fine

Denis

 

thanks for the pictures.
How thick was the steel? how long did it take to corrode to this state? ( seems your furnace gets a lot of use )

Regards Christoph

 

I think the plain hot rolled steel was 16ga. But, once hot gas starts flowing over it (as opposed to simply abutting it) it scales away pretty fast. I suspect that hole gotten eaten in two or three sessions with gas escape for only a total of an hour or maybew two at most.

Denis.

 

I decided to repair my existing lid to get a little more life out of it and buy time while I make a new furnace and lid. THat way I will not be without a furnace for a period of time.
The support, as originally made, tends to impact the wool gasket between my furnace body and lid and the support also makes maintanence of the gasket more difficult in that area. So, on the repaired lid I cut back the support some to ease both problems. I cut out the eroded metal enough to have some intact margins to which I could stitich weld patches. I am not particularly proud of the patch appearance but I did not want to waste time making it "pretty." I welded in 1/8" thick base material where the base had been 16 gauge. I figure this lid might hold up for quite a few more melts. I was surprised to see that the wool just under the BluRam segments was still soft and in good condition. This despite the fact that the segments do not fit snugly. I did paint Satanite on the wool where it overlaid seams when I did the original build and that might have helped. And the BluRam has held up very well in the lid. It looks to be good for many many more melts.



Denis

 

as good as new

 

I put the repaired lid to the test today and it did just fine. Reestablishing the integrity of the rim all around made it seal better to the wool gasket. There were no leaks today. The patched area is outlined in Red. THe furnace is burning----no leaks. Yes!



I also used some BluRam to cram the recess between the metal ring that forms the horizontal lower lid ring and is welded to the vertical metal wall of the lid. Doing so closed a little crevice that hot gases could circulate in. I checked after my melt today and that crammed-in refractory had all stayed in place just fine. When I build my new lid I will make sure to bed the segments in BluRam so that the crecie is filled with refractory. I think that will significantly improve durability.

To install the BluRam today I simply worked a 1/2 pound to moisten it some and get it pliable and then jammed it into place. Then I got out my weed burner and heated the refractory enough to make it good and hard. Then I fired my furnace as usual. No special care concerning drying, gradual heating, etc seem to be needed for this refractory as opposed to castables.

Denis

 

Hi Denis,

Can you elaborate, what you mean with that?
i imagine your furnace and lid combo looks currently something like that:



The lid has a lower horizontal ring, on wich the segmented blue-ram rest's on. An between the lid and the main body, there is a wool gasket?

Thanks

 

Yes, you have it right. If you look at my photo above showing the red-outlined patch, you can see that is a 1" wide horizontal rim stitch-welded to the 4" high sidewall of the lid. There is (or was since much of it eroded) a similar ledge or rim on the bottom of the lid. The segments rest on that rim. But, the segments are up-angled as you show in your drawing. So there is a cranny between the steel ledge and the segments. It is into that cranny I crammed BluRam. I should have done that in the very beggining. But I think I was needlessly concerned about restricting movement of the segments due to expansion and contraction.

Denis

 

I'm pretty sure we've seen one guy's furnace lid have potential show-stopper issues (ie. getting itself jammed shut with a molten crucible inside IIRC) here due to expansion and contraction before... Might have been back in the alloyavenue days though. Maybe not the exact issue you were worried about, but it's probably good generally speaking that you considered these things!

Jeff

 

Two years after building the first segmental furnace I rebuilt one almost exactly the same as the first except for more insulation. The original had gotten a lot of cracks in the hot face and the hot face had sagged some at the base. I had slapped a good number of patches of BluRam on the hot face as well---especially where the flame impinged on the face. The net result was that the volume of the furnace had decreased particularly in the lower 3rd. As a result it seemed like the furnace was getting slower. I attributed this to the reduced combustion area in the lower third of the furnace and somewhat impeded swirl due to the irregularity of the cavity of the furnace. So, I broke down and started over.

This time I used 3 1" wraps of ceramic blanket and had to split the barrel to fit it over the hot face/ceramic blanket layup. But, this time I had a mig welder for welding the 20 gauge steel making that process fast and easy. I rebuilt the lid too though it had held up pretty well save for the minor repairs detailed above. I used 3" of insulation on it too.

For a plinth I decided to try using an inverted clay-graphite crucible. It was 9" in height so that raised the melting crucible well above the furnace floor.

This morning I fired it up the first time. I was extremely pleased with the results. I reduced fuel consumption and melt time from 28 liters in the old furnace to 21 liters and melt time proportionately. I actually overran my target temp as the melt was finished sooner than expected. So. fuel use might have been more like 19 or 19.5. I think the combination of higher plinth, thicker insulation, and better cavity shape and volume combined to improve performance. I think that except for the last maybe 3 to 4 months the old furnace performed almost as well with fuel burn more like 21 to 22 for the same melt.

I neglected to measure the skin temp of the furnace late in the melt, but I can say for sure that mid melt the skin was about 160 to 180 degrees where the old furnace even new would have been more like 500. That shows clearly improved heat retention and therefore better efficiency.

Denis