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  • #16
    Have done a little more studying of my companies refractory manual.

    Don't get chicken wire - max service temperature for carbon steel = 260 C - bet mild steel would be less. We only let steel fans run at a max of 270C.

    304 grade stainless is the next one on the list. Max service temp = 900C

    HOWEVER - 300 series stainless steels held between 500 and 900C become embrittled by the formation of an iron-chromium phase that forms over time.
    Takes time to form, doesn't really cause too much problem at temperature but becomes very brittle when the installation is turned off and cooled down.
    The trick would be to use the 304 stainless and consider 500C the maximum service temperature.

    You can buy stainless steel chicken wire apparently, but it probably would be just as easy to get the melt extract fibres that are made for the job.
    Last edited by wotavidone; 06-14-2018, 12:27 AM.

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    • #17
      The ME 446 fibres are rated for continuous service at up to 1100 C, about double the requirements for a WFO
      http://antec.com.au/media/products/s...ibre_range.pdf
      Kindled with zeal and fired with passion.

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      • #18
        Definitely use the ME446 fibres. I don't know if they suffer sigma phase embrittlement, but I do know the 300 series ones will.
        Edit - More research:
        "As with other high chromium ferritic stainless, 446 embrittles severely in the 700-1000°F temperature range (885°F embrittlement). 446 should not be used in this temperature range unless nearly complete loss of room temperature ductility may be tolerated."

        https://www.rolledalloys.com/alloys/...steels/446/en/

        It looks like the fibres will be OK at temperature (apparently the formation of the sigma phase has little effect at formation temperature) but will have very little strength during heating and cooling
        Last edited by wotavidone; 06-18-2018, 05:24 PM. Reason: Further info

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        • #19
          well I haven't had any luck being able to source the extract fibres, which is no suprise in New Zealand.
          I might just go down the cast homebrew route, and flag all reinforcement. Surely it has to be stronger than a clay dome used in an earth oven.. If it doesn't work I'll just have to cut my losses and find some bricks and re build it with them.

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          • #20
            This is just one site in NZ, but most commercial concrete suppliers or providers are familiar with metal extract fibers.

            http://www.canzac.com/products/in-sl...n-steel-fibre/

            Russell
            Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

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            • #21
              Thanks, I flicked them an email, and they are mild steel, $60 per bag and $50 to get post to my location. Worth it you think?

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              • #22
                I wouldn’t use the mild steel fibres, i’m sure they’ll rust. You would need to source them from a refractory supplier rather than a concrete supplier, or if they don’t stock them, would be able to direct you. You are correct in concluding that they’re not really required. Because of the form it can’t collapse. They are expensive and as you are planning on using homebrew it’s a bit like buying expensive wheels for a cheap car. They also are a bitch to handle, don’t call them needles for nothing. The addition of the fine polypropelyne fibres is probably more important.
                Last edited by david s; 06-14-2018, 09:40 PM.
                Kindled with zeal and fired with passion.

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                • #23
                  Well David, I learned something new from you about metal extract fibers that they can come in carbon steel as well. Never to old to learn something new.
                  Russell
                  Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

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                  • #24
                    Mild steel might rust depending on how deeply it is covered in lime and how quickly the lime carbonates. Mild steel will lose strength at the temperatures we run. So not worth the trouble and expense.
                    Finding the right refractory and reinforcing for pizza ovens is very frustrating.
                    I had >550 C on the dome of my oven the night before last. Great, but that puts it firmly in the zone where:

                    a) most castable refractories lose strength due to being too hot for the cements that gave it the set, but not hot enough (usually need 1100 C ) to fuse the refractory components into non reversible minerals.

                    b) stainless steel needles will experience sigma phase embrittlement.

                    c) it's far too hot to expect strength from mild steel.

                    Pizza ovens - too hot for ordinary masonry, not hot enough for real refractories.
                    maybe a mix of fibres is the go - mild steel for low temperature strength, stainless steel for strength at operating temperatures?
                    Last edited by wotavidone; 06-18-2018, 05:30 PM.

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                    • #25
                      Originally posted by wotavidone View Post
                      Definitely use the ME446 fibres. I don't know if they suffer sigma phase embrittlement, but I do know the 300 series ones will.
                      Edit - More research:
                      "As with other high chromium ferritic stainless, 446 embrittles severely in the 700-1000°F temperature range (885°F embrittlement). 446 should not be used in this temperature range unless nearly complete loss of room temperature ductility may be tolerated."

                      https://www.rolledalloys.com/alloys/...steels/446/en/

                      It looks like the fibres will be OK at temperature (apparently the formation of the sigma phase has little effect at formation temperature) but will have very little strength during heating and cooling
                      I think you might be making more of the embrittlement issue than practice bears out. Eg on dismantling my 9 year old mobile oven, the 304 ss sleeve that holds the flue pipe in place showed no deterioration or loss of ductility yet often had flame licking around it during firing on average every fortnight for 9 years, so it got pretty hot pretty frequently. Likewise I’’ve never heard of any oven owner report deterioration of their stainless flues, but almost certainly a galvanised steel flue will fail after around 12 months. I also have a thin stainless100x100mm angle to hold the fire on the side which has had countless firings where it gets red hot every time and yet still has no noticeable change in ductility, but has buckled badly. I’’m not suggesting your reference is wrong, but practice doesn’tn’t always agree with theory.

                      Regarding the 446 fibres, they were the recommended product for my application and given the range of available ss fibres, I presume the refractory engineers who recommended them have chosen the most suitable type for my application considering the various fibre characteristics.

                      Likewise the suitability of unfired refractory castable, practice proves it works where theory suggests it’s in problem territory. In practice, firing large heavy castings slowly to over 1000 C is too costly and time consuming and the unfired product proves adequate for our temperature range. The problematic range of 500-650 C is a problem and damage is more likely to occur if oven owners try to see how hot they can get their ovens and thereby push their ovens into this range where it is not possible to control the fire with wood firing in an empty chamber. Temperature fluctuations causing sudden thermal expansion of some of the materials. A greater problem is actually folk heating the materials at too quick a rate. The industry standard is not to exceed 100 C/ hr, but no WFO owner ever fires this slow. Again practice doesn’t always agree with theory and we do what we can get away with.

                      one more example is the homebrew which is comprised of Portland cement which begins to deteriorate at temps north of 300 C, and lime which is ok up to 500C. Theoretically it shouldn’t work, because we exceed these temps, but in practice it does and a fine job it does too, proving better than most high temp refractory mortars for our temp range.
                      Last edited by david s; 06-19-2018, 04:14 AM.
                      Kindled with zeal and fired with passion.

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                      • #26
                        The embrittlement thing won't be a problem at temperature, and you won't see an issue with it at lower temperatures either - unless you actually need some tensile strength to hold some rapidly expanding thing together. Which is the point I was trying to make - while the stainless steel is holding it altogether at temperature there is a good chance it will provide bugger all strength at 2 or 300 degrees.

                        Industry only does the 100C per hour thing on the curing ramp up. After that it's "Ain't that thing hot yet? We got metal to make, pour, dross, purify, refine, etc. See if you can get that burner hotter, gotta melt that speiss"

                        I've seen cast ovens where I can rub the refractory away with the palm of my hand.

                        Anyway, I guess my point is maybe we are looking in the wrong place when we look at industrial materials and just because a material is rated to 1100 doesn't mean it's particularly well suited to half that temperature.. The work has gone into making them suitable for 4 digit temperatures, and all most of the designers care about 400 to 500 degrees is getting past that zone once without cracking the installation, then not coming back down to that temperature until it's time for a reline.

                        A lot of mid temperature linings are actually brick in my game not castable.
                        I do think the homebrew is just about as good as any.

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                        • #27
                          I’d agree with much of that. The reason a slow temperature rise is safer on the refractory is because of uneven temperature impingement. In a WFO the crown of the dome gets way hotter than the base perimeter leading to uneven expansion. In a kiln loaded with wares the expansion is evened out more, but an oven is an empty chamber and the top of the dome gets the full blast of the flame. This is more of a problem in large castings, particularly if the dome is cast in one section. This is part of the reason most manufacturers, me included, cast the dome in a number of separate pieces. The other reason is that it makes the pieces more manageable, single large castings are really heavy. If done in one section expect at least one large crack. Although it’s really only cosmetic because the form of the structure prevents collapse. A slower firing allows the temperature to even out more.
                          Regarding weak powdery castable refractory it is either caused by inferior material, out of date product, not mixed properly, placement after the mix has started to go off, or the mix has been cut with another material, usually sand, in an effort to extend it as it’s expensive.
                          Last edited by david s; 06-19-2018, 05:18 PM.
                          Kindled with zeal and fired with passion.

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                          • #28
                            Well I went ahead and went with the homebrew mix 3:1:1:1 without the stainless needles. Built a sandcastle then covered it with newspaper strips and molded over that. I could have probably gone a bit wetter looking at the inside surface after removing the sand.
                            I used a ruler to slice through the wet mix once I had molded it in an attempt to give it some stress relief and give it some controlled cracks. Not sure if it will help at all, but time will tell. Two days after and the mix has dried hard with no visible cracks yet, obviously this will likely change once its heated.

                            One thing about using homebrew as a castable I learnt. The type of sand is important. After laying the firebrick floor I packed a boarder of homebrew mix around the bricks to lock them in place. In this mix I used find sand. It was easy to work with but when it dried there was a lot of LARGE cracks around the border, due to shrinkage.

                            When I cast the dome I used course sand and no cracks yet.

                            I probably should have positioned the dome further to the front of the support slab. I'm going to make an entrance with a chimney which will come forward so that's the reason I positioned it where I did but in hind site I don't think it needs to be that far back. Luckily I'm a lanky bugger so it shouldn't be an issue for me :P

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