Recommended furnace refractory coating

Discussion in 'Furnaces and their construction' started by Fasted58, Jan 20, 2020.

  1. Fasted58

    Fasted58 Silver

    Looking for the preferred reflective and protective coating from heat and for tool use.

    Keg foundry build with minimum of 1.5" Inswool in walls, base and lid. Kast-o-lite 30 LI Plus hard face, 1" minimum in walls, more in base and lid. Propane burner, aluminum and aluminum bronze melts. May be higher melts later but none anticipated as of now.

    Considerations so far:

    Fiberfrax QF-180 (2300°). I have an ample supply from my old boiler job. It appeared to hold up well in our application, Cleaver-Brooks boilers but lesser temps than seen in propane furnaces. Use by date is 9/16 though.

    Satanite (3200 °). Original plan for coating in my planning stages but have to wonder after many YT videos. Seems like the 'go to' for many builders.

    ITC-100 HT (5000°). Great reflective qualities as per reviews I've seen. Durability?

    Compromises are understood, looking for the best qualities for this application. Cost is not a factor.

    TIA
     
  2. Jason

    Jason Gold

    How is your melt times so far? I don't have any experience with kastolite, only mizzou. No need with these better refractories for a coating unless you have high mass and it's taking forever to melt. A coating of cheap satanite wouldn't hurt. ITC is super expensive.
     
  3. Fasted58

    Fasted58 Silver

    This past year I've only managed to cure the refractory, should have mentioned that above, so no melt times to compare.

    Some standards across the boards and videos suggest there should be a 'coating' on the hard face. I dunno, I'm still the amateur yet.

    From the Satanite videos I'm not that much impressed.
     
  4. FishbonzWV

    FishbonzWV Silver Banner Member

    I'm curious
    What misgiving's do you have with Satanite?
     
  5. Jason

    Jason Gold

    I've got the satanite over the kaowool for my glory hole, but I wouldnt have my furnace made the same way. Just the way it is. I appreciate how tough Mizzou is. Maybe if I wasnt so tough on stuff and a little less clumsy I'd feel different.
     
  6. FishbonzWV

    FishbonzWV Silver Banner Member

    What do you do that's so tough on your furnace?
     
  7. Jason

    Jason Gold

    Dumb stuff.. Like drag tools across the face, I've dropped ingots in and missed the crucible.:rolleyes: And the best one is melted ingots that were way taller than the crucible. That resulted in flying liquid metal I found solidified on the floor next to the plinth.:( I dont need my furnace light or portable. Cheap wasnt on the list either. I just want it to last a long time with zero maintenance. I'm tired of fixing stuff.
     
    joe yard likes this.
  8. FishbonzWV

    FishbonzWV Silver Banner Member

    I don't see anything on your list that I haven't done.
    Inexpensive doesn't mean it's 'cheap'.
    Seven years, no maintenance.
     
  9. Petee716

    Petee716 Gold Banner Member

    If you have a keg with wool and an inch of Kastolite over it then you don't need any more protection. I used mizzou on my last build. I wanted kastolite but my supplier (free) only had mizzou on hand. I would have preferred kastolite because of the insulation value but the small amount of extra time it takes to get up to temp doesn't matter too much to me. I think mizzou is probably tougher than kastolite but in our application the difference would be splitting hairs. In any event, coating with another refractory for the sake of protection would be redundant. What you have is already tough as nails and probably tougher than satanite, I don't know. You'd just be adding more heat sink and weight for no gain in performance. I have thought about getting some satanite to fill and level my eroded plinths though.
    IR reflectance is another matter. I don't use any because I've never gotten off my wallet so what I can tell you is out-of-school. Three of the options I've seen are ITC-100, Metrikote, and Plistix IR. The last 2 are much cheaper than the ITC. That's all I know about it.

    Pete
     
  10. Melterskelter

    Melterskelter Gold Banner Member

    What do you melt and how often? That is an impressive durability record.

    Denis
     
  11. FishbonzWV

    FishbonzWV Silver Banner Member

    So far just Aluminium. Over 250 melts and last week 4 hours straight using it as a bare bones scrapper.
    How's yours holding up to iron?
     
  12. Melterskelter

    Melterskelter Gold Banner Member

    I melt iron in 50 to 60 pound batches using a diesel Hago Siphon burner.

    One clear observation for me is that any drips (inevitable) of iron or slag will rapidly melt down through the Satanite/Kaowool combo liner I am using. To a lesser extent it also attacks castables inversely proportional to their density with lightweight castable being quite vulnerable and dense fairly resistant. The sat/Wool lid broke down (gravity) after 20 or 30 melts and I replaced the lining with IFB coated with bubble alumina castable. The BA castable is very very tough but also expensive and available from only one seller that I can find. It also tends to flake off the IFB. So I am continuing to learn how to apply it.

    After nearly every melt I have to touch up areas of Satanite that have burned through or flaked off and/or patch in some wool. I feel like I am still learning the correct combo.

    I am building a new furnace from a cut-down 50 gal barrel that will use a combination of Satanite/perlite (my own crazy idea that I am experimenting with) with a 3/4” hot face of BA. But, the walls will be in 3 or 4 segments as I have learned that expansion and contraction WILL crack cylinders of castable no matter what. So mine will be pre-cracked. And I am going with a 16 bore and about 20” internal height as I have concluded allowing plenty of room for combustion improves net flame temperature.

    The design I realize is a new fork in the furnace design tree and may be miserable failure. But I think there is a decent chance that it will have the advantage of quite low mass and a very tough lining. Time will tell. I will report in more detail later as I am up to my ears in new pattern making, casting proven patterns (and machining and shipping them) and new furnace construction not to mention the normal day-to-day of grandparenting a 4 and 6 year-old etc.

    Denis
     
  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The flaking of a dense coating on IFB is tough problem to solve. I think there are two contributing factors. The first is cure shrinkage. I coated the IFB in my first electric furnace with high alumina mortar similar to Satanite and here was a test coupon. The warpage was induced just by air curing not firing. The dilution of the mortar to a brushable consistency further aggravates the cure shrinkage due to higher water content.

    The second is difference in thermal expansion of the two materials and the fact that the coating is the hot face which will always be much hotter than what's behind it, further aggravated by cyclic expansion and contraction. This works the interface of the two and the high alumina mortar is much stronger and wins that battle every time. It initially seemed to flatten out, (this may have been the IFB movement) when fired but continued to shrink when cycled and then eventually curl and sluffs from the IFB. This actually happened fairly slowly for me because it was an aluminum duty furnace that was only being heated to 1800F and it can be easily patched but each patch causes a little more attrition of IFB at the interface. I expect the consequences would be much severe in iron duty service.

    Dilute Mortar on IFB Warpage.jpg

    Even in a monolithic structure, there will be a big temperature gradient across the furnace wall and the interior surface will expand much more than the regions deeper in the wall. The faster you heat or cool the worse the gradient and temperature induced stresses. This coupled with the repeated cycling and thermal shock our furnaces see is undoubtedly the major contributor to cracking of the cylindrical refractory structures and also why commercial furnaces are brought to temperature slowly and avoid temperature cycling wherever possible. I think the segments are a good idea, maybe with a step, or shallow tongue and groove, or just concave convex edge to each segment joint. How will you hold them together? You might consider rings/hoops like my 14" bore furnace below. The cylindrical wall is 5/8" thick dense castable.

    4 Main Body and Bore.jpg

    Also, I would not recommend casting the base and cylinder as one piece as is often done because this just induces more stress into the refractory structure induced by uneven heating. I'd suggest just casting a retaining ring in a disc shaped base with some modest clearance (to the segmented cylinder outer wall) to capture the segments and allow the cylinder sit/float on the base and happily expand/contract as it pleases.

    Best,
    Kelly
     
  14. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    How thick do you figure your Satanite hot face to be? -Cylinder wall & Lid.

    Best,
    Kelly
     
  15. Jason

    Jason Gold

    This is EXACTLY what I do not need in my life. When I'm ready to turn the key, hit the switch or light it off, it better work first time or things get ugly.
     
  16. Fasted58

    Fasted58 Silver

    If I can increase IR reflectance and efficiency I'm all for it. Not expecting payback for it tho, I'm just an egg head like that.

    I'm at a starting point just after curing last fall w/ zero melts so this may be the best time to apply any coating. If I spent the money on ITC-100 that would not break the bank. As long as any coating doesn't detract from the current Kastolite I'd call it good. Coat it now or wait and see, I'd rather try the ITC-100 now.
     
  17. FishbonzWV

    FishbonzWV Silver Banner Member

    The lid is about 1/8" (two coats), the floor is 5/16", walls vary from 3/16" to 5/16" (the wool had pleat indentations and I filled those to smooth the walls).
     
  18. PatJ

    PatJ Silver

    This is the same conclusion that I have reached.
    My Mizzou hot face cracked into four or five large pieces.
    The pieces are still perfectly usable, solid, and serviceable, and I patched them back together with Plastic A, and the furnace works like new.
    So if I build another furnace, I will use segments, and just seal the joints with Plastic A.

    .
     

Share This Page