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Refractory Issues for Heat Treat Furnaces

  • 1.  Refractory Issues for Heat Treat Furnaces

    SILVER MEDAL AWARD
    Posted 05-06-2021 13:16
    I am working with a group to put together a Road Map document pertinent to advance materials related to heat treating furnaces.  As part of this team, I am looking at specific issues regarding refractory materials found in the various heat treating furnaces.  This is a steel-centric focused effort, so we are not specifically looking at the heat treating of other materials (titanium, high-temperature alloys, aluminum, etc.).  What the goal of the exercise is ultimately headed toward is identifying the current status in the industry, what is needed in the near future, and what is needed more long term in regard to cost reduction, higher temperature capabilities, longer life/greater reliability of systems, recycling, and energy reduction.  As a start, we have kind of broken representative heat treating systems down to air systems (650-1250C), controlled atmosphere systems (amonia, hydrocarbon, etc.) (>650C, 760-975C, 975-1250C), vacuum systems (800-1250C).  I would also welcome feedback on these selections, current state of the art, and what you feel needs to be improved upon in the near term and more longer term time frames. 


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    James Hemrick
    Senior Research Staff
    Oak Ridge National Laboratory
    Knoxville TN
    (865) 776-0758
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  • 2.  RE: Refractory Issues for Heat Treat Furnaces

    Posted 05-07-2021 14:46
    Every time the cost of energy goes up, furnace users take the obvious steps to improve furnace thermal efficiency. One of these obvious issues is improved furnace tightness. That has led to repeated experiences with the same problems. One of these problems is "catastrophic oxidation" where lower gas flows allow buildup of volatile substances such as (for example) molybdenum trioxide. When enough builds up in the gas phase, liquid molybdenum trioxide may start to flux protective oxides, leading to very rapid oxidation attach. This attack often is localized, leading to a pitting form of metal loss. This has also been seen where product being heat treated rests on ceramic floors or piers. UNS S31254 alloy appears to be most susceptible to this problem. Other 6% and 4% Molybdenum stainless steels have also suffered from this. Rarely, but occasionally, the 2% Mo type 316 stainless steel has suffered this attack also. 
    I have also seen problems involving aluminum bearing alloys and tight furnaces. The aluminum can deplete all the available oxygen before a fully protective layer of alumina forms. The hot, aluminum-bearing metal then reacts with the nitrogen in the residual atmosphere, leading to severe nitriding and severe embrittlement.

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    John Grubb
    New Kensington PA
    (724) 448-5272
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  • 3.  RE: Refractory Issues for Heat Treat Furnaces

    Posted 05-08-2021 03:42
    This comment about Mo oxides is really interesting.  For some reason I remember class discussions about not wanting vacuum furnaces to run at too low of a value because of the vapor pressures of things like the ceramic heating elements.....

    As far as attack from these Mo oxides, there was a similar to my mind situation that occurred in high temperature boilers that were dual fuel, natural gas and fuel oil capable.  When a set of boilers that had been run nearly 100% on gas was temporarily converted to fuel oil, the oil in use turned out to have a significant content of vanadium.  The ash from the oil burning, deposited on heat transfer surfaces as vanadium pentoxides turned out to be able to flux the iron and iron-chrome oxides above a certain metal temperature, which occurred when the burning pattern within the boiler changed with the different fuel.  Even with the low-sulfur spec oil, severe metal wastage underneath the deposited ash occurred as a result.  The industry-standard solution, of which we had been unaware, was to add a magnesium compound to the fuel oil which would react out the vanadium compounds preferentially, protecting the steel alloy steam tubes.





  • 4.  RE: Refractory Issues for Heat Treat Furnaces

    Posted 05-09-2021 10:35
    Interesting discussion about N2 embrittlement of aluminum. Does it apply to small cast parts? The following applies to large (100,000 to 240,000 pound) aluminum melting furnaces. Is the N2 able to diffuse throughout the molten aluminum bath in the furnace or it there a limit to the diffusion depth (as with solid metals)? Is the entire volume of the cast aluminum embrittled? Does the preheat of aluminum ingots before hot rolling temper the N2 embrittlement? Also, most molten metal furnaces usually run 10% excess air (8% excess N2, 2% excess O2) to avoid the risk of creating carbon monoxide (flammable and potentially explosive). How would you see this much excess N2 & O2 affecting these reactions? At one site, we actually used fluxing tubes that originally fed 100% Cl2 into the bath and then changed to a 90% N2 / Cl2 gas mixture to reduce the volume of Cl2 being released. Would preheating of ingots cast from this process reverse the N2 embrittlement before hot rolling? ​

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    John Cline
    Asset Performance Manager
    Evansville IN
    (812) 867-2278
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  • 5.  RE: Refractory Issues for Heat Treat Furnaces

    Posted 05-09-2021 14:19
    Liquids undergo turbulent mixing and diffusion is rarely the controlling factor in transport of alloying elements or contaminants. 
    Pure aluminum, even when molten, forms a very protective surface oxide film and that oxide film probably prevents nitrogen transfer from the atmosphere, even in grossly oxygen-poor atmospheres. An expert in aluminum (which I am not) may be able to provide more information.
    My comments were specific to aluminum-bearing stainless steels. They may apply to aluminum-bearing non-stainless steels (such as Fe-Mn-Al), but probably do not apply to aluminum base metals and alloys.

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    John Grubb
    New Kensington PA
    (724) 448-5272
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